NOAA’s National Weather Service 

Hydrologic Information on the Web 

A Manual for Users 

Red River Flooding, March 19, 2010 Credit: NOAA/NOHRSC

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Weather Service

April 25, 2011

Version 1.0
2

Hydrologic Information on the Web 

A Manual for Users

Table of Contents.......................................................................................................................Page

Introduction....................................................................................................................................3

Gateway to Hydrologic Web Resources.......................................................................................3

National Map – River Observations and River Forecasts Tabs.................................................3

Regional Map: River Observations and River Forecast Tabs..............................................5

River Location: Hydrograph Tab.................................................................................. 7

River Location: River at a Glance Tab ......................................................................... 9

River Location: Download Tab .................................................................................. 11

River Location: Inundation Mapping Tab .................................................................. 11

River Location: Weekly Chance of Exceeding Levels Tab........................................ 13

River Location: Chance of Exceeding Levels During Entire Period Tab................... 13

National Map - Precipitation Tab ............................................................................................15

State Precipitation Map......................................................................................................18

National Map – River Download, Other Information Tabs.....................................................18

National Significant River Flood Outlook .................................................................................19

Precipitation Frequency Data Server.........................................................................................21

National Snow Analysis...............................................................................................................27

Snow Information Heading – National Analysis.....................................................................27

Snow Information Heading – Interactive Maps.......................................................................29

Interactive Maps.................................................................................................................29

Time-series plots................................................................................................................31

Snow Information Heading – Interactive Maps.......................................................................32

Snow Information Heading – Airborne Surveys .....................................................................33

Snow Information Heading – Satellite Obs. ............................................................................33

Snow Information Heading – Forecasts...................................................................................33

Snow Information Heading – Data Archive ............................................................................33

Snow Information Heading – SHEF Products.........................................................................34

Observations Near Heading .....................................................................................................34

Science/Technology Heading - NOHRSC...............................................................................34

Science/Technology Heading – GIS Data Sets........................................................................34

Science/Technology Heading – Special Purpose Imagery ......................................................34

Other Links on National Snow Analysis Page.........................................................................34
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Introduction

NOAA’s National Weather Service (NWS) provides a wide variety of hydrologic and hydrometeorologic forecasts and information through the web. These web-based resources originate at

NWS field, national center, and headquarters offices and are designed to meet the needs of a 

wide range of users from someone who needs the five-day forecast for a river near his home to 

the technically advanced water manager who needs probabilistic information to make long-term 

decisions on allocation of flood mitigation resources or water supply. The NWS will continue to 

expand and refine all types of web products to keep pace with the demands of all types of users.

Gateway to Hydrologic Web Resources

The gateway to hydrologic forecasts and information is entered at http://water.weather.gov, or

through the NWS home page (http://www.weather.gov) by clicking on the Water tab at the top.

National Map – River Observations and River Forecasts Tabs

The starting point for obtaining NWS hydrologic products on the web is a national map showing

locations at which observations are available for river/stream gaging stations (Figure 1). This 

home page has a second row of five tabs below the tabs for the NWS main page, each linking to 

a specific map or menu: River Observations (default), River Forecasts, Precipitation, River 

Download, and Other Information. The displays and other information available under these five 

tabs are discussed in this and other sections to follow.

When either the River Observations (default) or River Forecasts tabs are clicked above the 

national map, river/stream locations are displayed which are color coded according to the flood 

status of their most recent observation or latest forecast as follows:

 No Flooding (green) – river/stream below level where flooding becomes a concern.

 Near Flood Stage (yellow) – flooding becomes a concern. River/stream is approaching

or is forecast to approach the gage height – flood stage – where flooding is defined to 

begin. Flood stage is an established gage height for a given location at which a rise in 

water surface level begins to create a hazard to lives, property, or commerce.

 Minor Flooding (orange) - minimal or no property damage, but possibly some public 

threat.

 Moderate Flooding (red) - some inundation of structures and roads near stream. Some 

evacuations of people and/or transfer of property to higher elevations.

 Major Flooding (violet) - extensive inundation of structures and roads. Significant 

evacuations of people and/or transfer of property to higher elevations.

The last three of these levels plus another not shown – record flooding – constitute the flood 

categories used by the NWS. While some degree of subjectivity was involved in determining 

flood stage and the boundaries between minor, moderate, and major flooding, these categories

have proven to be a valuable tool in describing the severity of flooding in all areas of the 

country. The upper portion of Figure 1is a sample map linked to the River Observations tab.
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The river forecasts tab is associated with a national map depicting forecast hydrologic conditions 

at river/stream locations (Figure 1, lower right). Color codes here are the same as those for river

observations, according to the maximum forecast stage during the next 48 hours. The forecast 

map will show fewer locations than the River Observations map, because river/stream forecasts 

(e.g., daily) are produced for fewer locations than observations are available for. NWS partners 

such as the U.S. Army Corps of Engineers and other users involved in river navigation, power 

generation, water management, emergency management, and similar water-related activities 

coordinate with the NWS on the river/stream locations, dams, reservoirs, and lakes which require 

routine forecasts. When high water or flooding occurs or threatens to occur, “non-routine” flood 

Figure 1. Sample home page for the NWS hydrologic web presence, showing default River 

Observations display and inset showing map linked to the River Forecasts tab.
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forecasts are issued for additional locations. Click here to view the current national river 

observations map and here to view the current national river forecasts map.

In the narrow dark blue area immediately below the tabs and above the national map, a white 

pull-down menu can be used to change the display of river/stream locations from all locations 

(default) to:

 Hydrograph Only Locations – hydrographs for individual river/stream locations 

provide a graphical overview of past and future conditions at a river/stream gage. 

Selecting this option causes the map to show only the locations for which hydrographs 

are available. Click here to view the current national map of locations with hydrographs.

 Hydrograph/Probability Forecast Locations – specialized products with information 

on the probability of future river levels/flows are also available for selected river/stream 

locations. Selecting this option causes the map to show only the locations for which 

these probability forecasts and hydrographs are available. Click here to view the current 

national map of locations with both probability forecasts and hydrographs.

 Inundation Mapping Locations – at a small but increasing number of river/stream 

locations, specialized interactive displays depicting the amount of land inundated on each 

side of the river/stream at various stages are available. Selecting this option causes the 

map to show only locations for which these inundation maps are available. Click here to 

view the current national map of locations with inundation maps.

Regional Map: River Observations and River Forecast Tabs

Clicking on a point on the National River Observations or River Forecasts map brings up a

regional map covering an individual NWS weather forecast office’s (WFO) area of hydrologic

responsibility (known as a hydrologic service area, or HSA) plus the surrounding area (Figure 2). 

Click here to view the current version of the regional map shown in Figure 2. Most regional 

maps cover all or portions of two or more states. For larger states, the map may cover only a 

portion of one state.

Users have more control over features displayed on regional maps than on national maps. The 

default display on regional maps includes topographic background, river/stream gage locations, 

major rivers, other (smaller) rivers and streams, hydrologic service area boundary, state boundaries, names of river/stream gage locations, names of major rivers, and names of other rivers 

and streams – all of which can be toggled on or off. Other features which can be toggled on or 

off are county boundaries, highways, cities, major river names, and other river/stream names.

River/stream locations on regional maps are color coded according to their current flood status, 

using the same classification and color scheme as the National maps. Icons used for individual

river locations provide additional information on the products available for a location – if the 

icon is square, a hydrograph product is available for the location and if the icon is round, both

hydrograph and probability products are available for the location.

Like the national map, regional maps have tabs across the top allowing users to specify information provided at that scale. The first two tabs – “River Observations” and “River Forecasts 

Within 48 Hours” – correspond to the first two tabs on the national map with similar titles. 
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The default “River Observations” tab links to a regional map showing the flood status of the 

most current observation at river/stream locations. The “River Forecasts Within 48 Hours” tab 

map shows the flood status of the maximum forecast stage during the next 48 hours at river/

stream locations. Like the national map, fewer river locations will show on “River Forecasts

Within 48 Hours” maps than on “River Observations” maps.

Figure 2. Sample web page depicting river observation information at the regional level.
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The third tab above the regional map lead to maps of observed precipitation during the most 

recent 24 hour period ending at 1200 Coordinated Universal Time (UTC)1and the fourth tab to a 

menu for data download (described later).

River Location: Hydrograph Tab. Clicking on a color-coded river location on a regional 

“River Observations” or “River Forecasts Within 48 Hours” map will bring up a display of the 

observed/forecast hydrograph for that location (Figure 3, click here to view the latest version).

Hydrograph plots are divided into left and right portions, separated by a vertical blue dashed line. 

The vertical line represents the “base time” for which the hydrograph was created, with past 

observations plotted to the left using blue dots and forecasts for the future (if available) plotted to 

the right using green dots. The bottom scale for the hydrograph is labeled with dates and time. 

The river/stream stage2, or gage height, in feet, is indicated along the left vertical axis of the 

observed/forecast hydrograph. The flow (discharge) corresponding to the stage is provided 

along the right vertical axis in thousands of cubic feet per second. Some readers may be more 

familiar with flows in gallons per minute, but hydrologists and engineers decided long ago that 

the numbers were too large when flows were expressed in those units. You may notice that each 

stage on the left side corresponds to a single flow on the right, and flow rate increases with stage. 

This is true except in the rare case of near-peak flood flows on rivers in very flat areas.

Information Below Hydrograph. A small yellow menu below the hydrograph provides access to:

 A printable .pdf version of the hydrograph,

 A definition of a hydrograph along with explanations of key terms used in the display,

 A tabulation of all the observed and, if available, forecast data shown in the hydrograph,

 A downloadable XML file containing all observed and, if available, forecast data shown 

in the hydrograph,

 A description of how to automatically receive, via RSS; an XML-encoded version of the 

hydrograph you are viewing each time it is updated,

 The datum which stages are referenced to (NAVD 88 or NGVD 29), and

 A metadata (i.e., descriptive information on a location) table for the stream gage station.

Also below the hydrograph, a small pull-down menu to the right gives the user the option to 

display the hydrograph with a vertical scale that is close to the range of past, current, and future 

 

1

1200 UTC is 7:00 a.m. EST (8:00 a.m. EDT), 6:00 a.m. CST (7:00 a.m. CDT), 5:00 a.m. MST (6:00 a.m. MDT), 

4:00 a.m. PST (5:00 a.m. PDT), and 3:00 a.m. AKST (4:00 a.m. AKDT).

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Stage for a river or stream is provided in two ways – (1) height above a locally fixed level known as gage datum or 

gage zero and (2) absolute elevation above a fixed location in North America (at Father Point, Rimouski, Quebec, 

Canada) known as the North American Vertical Datum of 1988 (NAVD 88 – new standard) or an older datum 

known as National Geodetic Vertical Datum of 1929 (NGVD 29). 0.0 feet NAVD 88 or NGVD 29 is not the same 

as mean sea level (MSL), but they are fairly close (see http://tidesandcurrents.noaa.gov/datum_options.html for 

more precise definitions). In general, stages above locally fixed levels tend to be numerically smaller than elevations above NAVD 88 or NGVD 29, because absolute elevations are in the hundreds or thousands of feet in higher 

inland areas. Of course, this may not hold true for gages in low elevation areas near the coast.
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(if available) stages, or a vertical scale that is stretched out to include colored portions indicating 

the minor, moderate, and major flood categories as well as the record flood level. The former is 

the default display, but if a river/stream is already in flood or is forecast to be in flood, the 

hydrograph display will automatically include flood categories.

Figure 3. Sample hydrograph showing observed and forecast data.
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If the location currently being viewed has upstream and/or downstream gages, arrows labeled 

“Upstream Gage” and “Downstream Gage” can be found under the hydrograph and below the 

level of the yellow menu. This allows the user to review the hydrographs for river/stream 

locations in sequential order, which can be useful during a flood episode.

Still further below the hydrograph, a gage location map is provided with Google Map capabilities (Figure 4). To the left of this map, tables of historical crests and, if available, historical low 

flows are provided. This historical information is useful in comparing a current flood or low 

flow event to past events.

A list labeled “Flood Impacts” is provided below the gage location map and historical flood/low 

flow tables (Figure 4, lower half). This information describes impacts a river/stream will have at 

various upstream and downstream locations (e.g., roads, bridges, homes, commercial buildings, 

water treatment plants) at various stages. The lowest stage and associated impact is at the 

bottom of the list and the highest stage and most severe flood impact is listed the top.

Several tabbed areas are found below the flood impact list providing access to other kinds of 

information on the river/stream location (Figure 4). Each of these tabbed information sets can be 

expanded or collapsed by the user. Information sets under these tabs include ones entitled:

 “About this location,” which provides the same information obtained when clicking on 

“Metadata” in the yellow menu box; 

 “Additional Information,” which provides miscellaneous information on the river/stream 

gaging station such as photos of upstream and downstream areas; 

 “Resources,” which provides links to several web locations where additional information 

supporting the decision making process during hydrologic events can be found;

 “Collaborative Agencies,” which lists agencies at all levels of government with interests 

in hydrologic conditions at the location; and

 “NWS Information,” which contains the address of NWS weather forecast office (WFO) 

responsible for forecasts and warnings at the gaging station, a point of contact at the 

WFO, a disclaimer notice, and various NWS-related links.

River Location: River at a Glance Tab. Clicking on this tab allows the user to create a 

customized product for all or a subset of locations along a given river. Users select any 

combination of the following graphics and data to be included in this product: observed stage/

flow and forecast hydrograph for the point; numerical observed stage/flow and forecast values; 

flood impact information; chance of exceeding stage, flow and/or volume levels during entire 

(long term) period graphic; weekly chance of exceeding stage, flow, or volume levels graphic, 

location map; historical crests; low flow, low water impacts, low water records, and downloadable RSS/XML files. Some of these items, such as the probabilistic products, are not available 

for all forecast points. Therefore, the menu for selecting items to include in a customized product 

is not identical for all areas of the country. Click here to view the menu which appears after 

clicking on the “River at a Glance” tab for the Neuse River near Goldsboro location.
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Figure 4. Sample gage location map, flood impact descriptions, and tabs for various types of

information on a river/stream location. Note: bottom five tabs are collapsed to save space.
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River Location: Download Tab. Clicking on this tab brings up a menu providing for the 

download of several forms of information on the river/stream location, including an:

 Observed/forecast hydrograph image,

 Observed/forecast data and metadata on the forecast point in XML format,

 Criteria-based RSS feeds initiated by subscription, location map image,

 Weekly chance of exceeding levels image (discussed shortly),

 Chance of exceeding levels during entire (long term) period image (discussed shortly),

 Inundation map shapefiles,

 Inundation map KMZ files,

 Inundation map images, and

 Photos.

The number of options available varies by location. Click here to view the menu with the

options appearing after clicking the “Download” tab for the Neuse River near Goldsboro.

River Location: Inundation Mapping Tab. The inundation mapping interface (Figure 5) if

available for the location, is accessed through this tab. This feature is available for NWS forecast points where data sets known as flood inundation libraries have been developed through 

partnerships with Federal, state, and/or local agencies. The inundation mapping interface 

provides information on the spatial extent and depth of flood waters in the vicinity of the forecast 

point. It provides the ability to view inundation levels at stage (to the nearest foot) in the minor, 

moderate, and major flood categories. Figure 5 shows inundation at a stage of 21 feet for the 

Neuse River near Goldsboro. The 1 percent annual chance (100-year) flood, 0.2 percent annual 

chance (500-year) flood, and floodway zones may be displayed. From this interface, the user can 

also view maps of observed or forecast inundation levels based on current NWS river forecasts. 

The user has the option to use either a standard map background with basic features such as main 

roads and highways or a detailed background derived from areal photographs. Click here to 

view the current inundation map interface for the Neuse River near Goldsboro.

Additional information on the inundation mapping interface is available at:

http://newweb.erh.noaa.gov/ahps2/inundation/inundation_mapping_user_guide.pdf

Currently, a limited number of NWS forecast locations have a flood inundation mapping 

interface. A list of these locations can be found at:

http://water.weather.gov/inundation.php

As already mentioned, inundation map shapefiles, KMZ files and images may be downloaded by 

clicking on the “Download” tab along the top of an inundation map.
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Figure 5. Sample Flood Inundation Mapping Interface.
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River Location: Weekly Chance of Exceeding Levels Tab. Clicking on this tab brings up a 

graphical product showing the probability or chance of the maximum stage, flow or volume at a 

point on a river exceeding a particular value for consecutive 7-day periods in a 90-day interval. 

The vertical axis shows river stage or level measured in feet (ft) and the horizontal axis shows 

time. An individual multi-color vertical bar represents the exceedence probabilities for a 7-day 

period, each color representing a range of probability as defined in the key to the right of the 

graph. A sample display is provided in Figure 6. Click here to view the most recently generated 

Weekly Chance of Exceeding Levels product for the Neuse River near Goldsboro, NC.

River Location: Chance of Exceeding Levels During Entire Period Tab. Clicking on this tab 

brings up a graphical product showing probability of the river stage, flow, or volume (the user 

selects one of these) going above various levels during the forecast period labeled above the 

graph (usually 30 or 90 days). A sample display is provided in Figure 7. This graphical product 

Figure 6. Sample Weekly Chance of Exceeding Levels product.
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shows two sets of connected dots forming somewhat irregular traces which look somewhat like

curving lines. These two traces can be described as follows:

 Historical simulation (HS). A trace of small, blue dots indicating chances of a river 

going above the stages, flows, or volumes read along the vertical graph axis based on the

total range of past levels in the long-term historical record (e.g., covering next 90 days).

 Conditional simulation (CS). A trace of small, black triangles indicating chances of a

river going above the stages, flows, or volumes read along the vertical axis. This trace is

based on current weather and hydrologic observations plus the years from the historical 

record which are similar to the expected weather pattern as indicated in the long-term 

weather (climate) forecast (e.g., for next 90 days).

Chance of Exceeding Levels During Entire Period graphics may look a bit too technical and

abstract at first, but they provide a significant amount of information on the probability of hydrologic conditions reaching various levels of impact during the next 1-3 months, including:

Figure 7. Sample Chance of Exceeding Levels During Entire Period product.
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 If the black trace (conditional simulation) is below the blue trace (historical simulation) 

(as in Figure 7), future river/stream flows are expected to be lower than normal. If the 

black trace is above the blue trace, flows are expected to be higher than normal.

 If the left portion of the black trace is level (for higher probabilities), the river was 

already at that level at the start of the long-term forecast period.

 Noting the probability on the horizontal axis where the traces first intersect the flood 

level (stage) and enters the orange (minor flooding) area of the graph gives an indication 

of the likelihood of future flooding. The higher the probability where the traces intersect 

flood stage, the more likely flooding is during the period covered by the graphic.

In Figure 7, the historical simulation crosses flood level at a probability of about 45% and the 

conditional simulation crosses flood level at a probability somewhere between 20 and 10%. This

indicates the river was less likely to reach flood stage during the period than indicated by the 

historical record. Click here to view the most recently generated Chance of Exceeding Levels 

During Entire Period product for the same location (Neuse River near Goldsboro, NC).

When are forecasts in these web products updated?

The hydrologic models used to generate near-term river/stream forecasts are generally executed 

in the morning on a daily cycle. When river/stream conditions are below flood levels, the 

forecasts as seen in the hydrographs are issued at fairly regular times in the morning. However, 

during flood/high water situations, much more data is coming in and forecasters may need 

additional time to analyze observations and model output, causing the forecasts to be issued 

somewhat later than usual. Updates to the forecasts are issued more frequently during flood/high 

water situations. Updated long-term products such as exceedence probability graphics may only 

be generated once or twice a month after new long-term weather (climate) forecasts are issued.

National Map - Precipitation Tab

When the precipitation tab is clicked on the National map page, a map showing observed 

24-hour precipitation in the conterminous U.S. (CONUS) and Puerto Rico is displayed, along 

with a new third row of tabs immediately above the map and a large menu area with multiple 

selection options below (Figure 8). This national precipitation map is the default display for the

new third row of tabs and initially depicts observed precipitation for the most recent 24-hour 

period ending at 1200 Coordinated Universal Time (UTC). 1200 UTC is used because it marks 

the end of the "hydrologic day" – a standard used in river modeling – and it is also when many 

daily weather observations are taken in the morning. This map integrates 24-hour totals of 

multisensor precipitation estimates from the 12 NWS river forecast centers (RFC) serving the 

lower 48 states. Multisensor precipitation estimates are produced by integrating radar-based 

precipitation estimates, satellite-based precipitation estimates, and ground-based precipitation 

gage data. The default map is initially set to display at the CONUS scale, with an inset to the 

bottom right for Puerto Rico.

Several menus below the precipitation map giver the user control over the information displayed. 

The menu along the left side allows users to select to view current precipitation for the most 

recent 24-hour period (default), as well as totals for the last 7, 14, 30, 60, 90, 180 days, month to 
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date, year to date, and water year (starts Oct. 1) to date. The user can also display archived data 

by year/month, calendar year total, and water year total, with the option to display normal, departure from normal, and percent of normal for the time period covered by the archived data being 

viewed. Normals in the western U.S. were calculated using the PRISM analysis, a technique

developed by Oregon State University to account for variability of precipitation in mountainous 

areas. Menus further right provide control over the geographical area displayed, including the 

continental U.S. + Puerto Rico, individual states, regions of the country, NWS river forecast 

center areas, and NWS weather forecast office areas. A small menu at the far right controls 

whether data displayed in English (as used in U.S.A.) or metric units.

The other tabs in the third row immediately above the national precipitation map allow users to

interact with multisensor precipitation data in several ways:

Figure 8. Sample national scale observed (24-hour) precipitation display.
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 The “Download” tab allows current or archived precipitation data to be downloaded as 

gridded digital data in either shapefile or netCDF format.

 The “About NWS Precip Analysis” tab brings up an in-depth discussion on how the 

data is acquired and brought into the format seen in the display.

 The “Other Useful Information” tab brings up a condensed list of links to other related 

climate, water, and weather information.

 The “Survey & Feedback” tab provides instructions and links for various ways feedback 

can be provided to the NWS on the usefulness of information available through the 

precipitation web page.

Figure 9. Sample state scale precipitation display.
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 The “Regional/RFC Precip Data” tab provides access to the precipitation data sets from 

most RFCs that were contributed to this national precipitation web page. Links to experimental precipitation products being tested by the NWS may also be provided here.

State Precipitation Map

If any state in the national map is clicked on, an observed precipitation map for the state and its 

surrounding area is displayed (Figure 9). The color scheme and user interface is the same as that 

provided for the national level. Actually, for any data being viewed at the national level that can 

be specified using the menus under the map (e.g., last 7 days, departure from normal), clicking 

on any state will cause the corresponding map at the state level to be displayed which uses the 

same color scheme. Observed precipitation for an individual WFO area can also be displayed.

When is the precipitation map updated?

The data sets used to produce national, state, and weather forecast office scale precipitation maps

are updated on a daily basis. An update for the entire U.S. is usually ready by about 1600 UTC3.

National Map – River Download, Other Information Tabs

The River Download tab provides access to observed and forecast data for all locations in the 

CONUS and Puerto Rico in shapefile and KMZ formats as well as RSS/XML. Shapefile and 

netCDF formatted precipitation information is also available.

The Other Information tab provides links to a variety of other National Weather Service climate, 

water, and weather information, including water supply information, climatological information, 

snow information, drought information, and seasonal information.

 

3

1600 UTC is 11:00 a.m. EST (12 noon EDT), 10:00 a.m. CST (11:00 a.m. CDT), 9:00 a.m. MST (10:00 a.m. MDT), 

8:00 a.m. PST (9:00 a.m. PDT), and 7:00 AKST (8:00 a.m. AKDT)
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National Significant River Flood Outlook

The National Significant River Flood Outlook identifies areas where potential exists for 

significant river flooding over the coming 5-day period. It is not intended to depict all areas of 

minor flooding or small-scale events such as localized flooding and/or flash flooding. This 

graphical product is accessed at:

http://www.hpc.ncep.noaa.gov/nationalfloodoutlook/index.html

As it is used in this product, "significant flooding" corresponds to the moderate and major flood 

categories, which can be described as follows (see NWS Manual 10-950 - Definitions and 

General Terminology):

Moderate Flooding – involves some inundation of structures and roads near the river/

stream. Some evacuations of people and/or transfer of property to higher elevations are 

necessary.

Major Flooding – involves extensive inundation of structures and roads. Significant 

evacuations of people and/or transfer of property to higher elevations are necessary.

The National Significant River Flood Outlook covers the lower 48 states and is created by the

Hydrometeorological Prediction Center in the National Centers for Environmental Prediction by

joining together individual graphical outlooks from the 12 NWS river forecast centers serving

the lower 48 states. A separate significant river flood outlook for Alaska is provided by the 

Alaska-Pacific River Forecast Center (click here to view current outlook). Outlooks for both the 

lower 48 states and Alaska are posted on the Web once a day by approximately 2100 UTC 

(4 p.m. EST).

The National Significant River Flood Outlook provides links to the flood outlooks produced by 

individual RFCs. Each RFC’s significant river flood outlook links back to the National product.

Using the color scheme and patterns shown in Figure 10, flood potential is characterized in the 

National Significant River Flood Outlook according to the following criteria:

a. Possible: Hydrometeorological conditions indicate that significant flooding could occur. 

Such flooding is neither certain nor imminent.

b. 0Likely: Hydrometeorological conditions indicate that significant flooding can be 

expected during the outlook period.

c. Occurring/Imminent: Significant flooding is already occurring or is imminent during the 

outlook period.

The dates within the coming 5-day period when flood potential exists are annotated on the map

near each colored area where flood potential exists.
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Figure 10. Sample National Significant River Flood Potential Outlook for CONUS and Alaska.
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Precipitation Frequency Data Server

Civil engineers and others involved in the design of structures that will affect or be affected by 

runoff and streamflow caused by precipitation are concerned with extreme precipitation events. 

For some structures, extreme precipitation events as short as five minutes must be considered 

while others must take longer duration (e.g., multi-day) events into account. Statistical techniques have been used to calculate frequency estimates of precipitation intensities for a wide 

range of frequencies and durations. Such estimates are used in the design of a variety of 

structures from urban storm water drainage systems to dams and spillways.

In 1953, the National Weather Service began publishing precipitation frequency estimates –

commonly known as intensity-frequency-duration values – in hard copy Weather Bureau 

Technical Papers and Memoranda. After NOAA was formed in 1970, they were provided in 

NOAA atlases. The estimates in these documents became de-facto national standards for a wide 

variety of design and planning activities at the federal, state, and local levels as well as in the 

private sector.

Figure 11. Precipitation Frequency Data Server home page. 
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The Precipitation Frequency Data Server (http://hdsc.nws.noaa.gov/hdsc/pfds/; home page 

shown in Figure 11), is now the mechanism by which precipitation frequency estimates are 

provided by the National Weather Service. In general, the server provides these estimates in two 

formats:

1) New Format. For areas where recent statistical analyses have been performed to provide 

updated estimates for new volumes of NOAA Atlas 14, an interactive web capability is 

available which allows users to access digital output for any location interactively 

selected on a map (see example in Figure 12).

2) Old Format. For areas of the U.S. where updated estimates are yet to be derived, data are 

provided online in the form of scanned versions of the old Weather Bureau Technical 

Papers, Memoranda, and NOAA atlases (see example in Figure 14). Once the entire 

document appropriate to the location selected is retrieved by the server, the user must 

locate the map corresponding to the required frequency (e.g., 100-year) and rainfall 

duration (e.g., 24 hours), visually locate the point of interest on the map, and interpolate 

the precipitation depth from the isohyets (lines of constant precipitation depth).

In the national map on the Precipitation Frequency Data Server home page, areas where updates 

have been completed for a new volume of NOAA Atlas 14 are shown in dark blue, areas where 

updated statistical analyses have not yet been performed are shown in white, and areas where 

updates are currently in progress are shown in light blue. Precipitation frequency estimates in 

the new digital format are provided for any user-selected location in the dark blue areas of the 

map, while estimates for the white and light blue areas are provided in the old format.

If a user clicks on one of the dark blue states on the national map, an interactive (Google) map of 

that state and its surrounding area is displayed. Initially, a red selection cursor will appear at the 

center of the state map, along with a table of precipitation frequency estimates for that central 

location displayed immediately below. A user can then move the cursor to any desired location 

and zoom into progressively more localized areas with a standard slide mechanism. Across the 

top of the map, pull-down menus give the user control of whether precipitation depth or precipitation intensity frequency estimates calculated based on statistical analysis of partial duration or 

annual maximum series are displayed in English or metric units. The map and accompanying 

menus give the user four ways to obtain digital precipitation frequency estimates for a location:

 Manually enter a latitude and longitude (decimal degrees) of a desired location

 Select one of the historical observation locations from a pull-down menu. These

represent locations where a historical record of observations was available for use in the 

analysis conducted to derive the precipitation frequency estimates.

 Select one of the green historical observation location icons on the map (the “show 

stations on map” box must be checked and the map must be zoomed in at least one 

increment from initial display).

 Drag the red cursor to select any location on the map.
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Figure 12. New format – digital precipitation frequency estimates for a location produced by the 

Precipitation Frequency Data Server from a new volume of NOAA Atlas 14.
24

After one of these four ways has been used to select a specific location, a table of precipitation 

frequency (PF) estimates with corresponding upper and lower bounds of 90% confidence interval 

is displayed for the location as shown in the bottom half of Figure 12. As can be seen in this 

figure, this table is actually the default display associated with the first of three tabs (PF tabular). 

The tabular data can also be downloaded as comma-separated values (.csv) by selecting the type 

of data (mean estimates, upper bound, lower bound, or all) via a button beneath the table. The 

second tab, PF graphical, provides access to graphical representations of the same information 

provided under the first tab (Figure 13). The third tab, Supplementary information, provides 

access to the following:

 Document. The on-line volume of NOAA Atlas 14 describing the in-depth study that 

was conducted to derive updated precipitation frequency estimates for the project area 

encompassing the selected location.

 PF in GIS format. Spatially interpolated precipitation frequency estimates for a userselected frequency and duration (with upper and lower bounds of the 90% confidence 

interval) in GIS compatible formats.

 PF cartographic maps. State maps with colorized isopluvials (lines of constant depth) of 

precipitation frequency estimates for selected frequencies and durations.

Figure 13. New format – precipitation frequency estimates displayed in graphic form for the 

same location used in Figure 12. This is displayed by clicking on the “PF graphical” tab.
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 Temporal distributions of heavy precipitation. Files (.csv), viewable as spreadsheets in 

MS Excel, containing cumulative percentages of precipitation totals at various time steps 

which can be used to calculate the temporal distribution of precipitation frequency 

estimates for 6-, 12-, 24-, and 96-hour durations.

 Seasonality analysis. Graphs showing the monthly distribution of precipitation totals

(percent of total recorded events) for all stations in each climate sub-region that exceeded 

the precipitation frequency estimates for 50, 20, 10, 4, 2, and 1 percent annual 

exceedence probabilities for user-selected durations of 60 minutes, 24 hours, 2 days, or 

10 days.

 Rainfall frequency estimates. Rainfall (liquid precipitation only) frequency estimates for

60-minute, 2-hour, 3-hour, 6-hour, 12-hour, and 24-hour durations. These tables provide 

estimates for rainfall only, as opposed to the main precipitation estimate tables (as shown 

earlier in Figure 12), which apply to all forms of precipitation (e.g., including snow). 

Differences in these types of estimates will only be relevant for higher elevations.

 Time series data. Annual maximum series precipitation data are available if an observing 

station has been selected on the interactive map or from the pull-down list. Partial 

duration series data are currently only available for areas covered by NOAA Atlas 14, 

Volumes 1-3.

 Climate data source. Links to general information on data sites near the currently 

selected location which use the search engine of NOAA’s National Climatic Data Center 

(NCDC). User can select all stations within ± 30 minutes (1/2 degree) or ± 1 degree. Not 

all stations obtained through this search will have necessarily been used in the study to 

calculate precipitation frequency estimates.

Figure 14. Old format – national map with lines of constant depth for 100-year, 24 hour precipitation, scanned from Technical Paper No. 40 and provided via the Precipitation Frequency Data 

Server.
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 Watershed information. A link that can be used to obtain information on the watershed 

containing the location of interest from the U.S. Environmental Protection Agency (EPA) 

web site.
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National Snow Analysis

Snow and snow information is of critical importance to the Nation’s economy. Information on 

snowpack conditions is essential to support river, flood and water-supply forecasting, agriculture 

and forest management, recreation and tourism, and the commerce, industry and transportation 

sectors of the Nation’s economy. The National Snow Analysis (NSA) produced by the National 

Operational Hydrologic Remote Sensing Center (NOHRSC) in Chanhassen, MN, offers users a 

variety of web-based snow information available in near real-time (http://www.nohrsc.noaa.gov). 

The NOHRSC uses daily ground-based, aircraft, and satellite snow observations from all 

electronically-available sources coupled with numerical weather prediction models to drive a 

sophisticated snow model known as SNODAS (SNOw Data Assimilation System)4. SNODAS 

products are available in a variety of interactive map, text discussion, alphanumeric, time-series, 

and gridded formats through the NSA.

The home page for the National Snow Analysis is: http://www.nohrsc.noaa.gov/nsa/ (Figure 15). 

From the NSA page, users can access many forms of snowpack-related information including 

some of the observational data sets used in the snowpack modeling process. In the dark blue 

margin on the left side of the NSA page, a list of main categories and clickable sub-categories 

controls the current display. The following is a “tour” of NOHRSC products and services that 

are available via the clickable categories on the left margin of the NSA web page.

Snow Information Heading – National Analysis

The NSA home page is shown in Figure 15. This home page shows a large U.S. map above nine 

smaller maps depicting the following modeled snow parameters: snow water equivalent, snow 

depth, average snowpack temperature, snow water equivalent change, snow precipitation, snow 

melt, blowing snow sublimation, surface sublimation, and non-snow precipitation. The larger 

U.S. map is divided into 18 geographic areas. Snowpack conditions for each of these 18 areas 

(e.g., Northern Rocky Mountains) can usually be summarized on a regional basis. When a user 

clicks on one of these geographic areas in the large map, the small maps will display the region 

of interest. The user can access past information by selecting a year, month, day and region 

using the pull-down menu between the large and small maps. For any one of the nine maps, the 

user can animate the variable displayed. The “season” animation loops through daily data for the 

snow season, beginning on October 1 and proceeding to the present. The “two weeks” and “one 

day” animations loop through hourly data. The animations are helpful for visualizing broad 

spatial changes in snowpack characteristics over time.

The NSA page also includes a table to the right of the large U.S. map summarizing the most 

recent snow model analysis including percent of the U.S. (or selected geographic region) covered 

by snow and modeled snow depth and snow water equivalent statistics. A separate table below 

the maps highlights the top ten snowfall observations for the country or for a specified snow 

region.

 

4

SNODAS is an energy-and-mass-balance, spatially-uncoupled, vertically-distributed, multi-layer snow model run 

operationally at 1-km2 spatial resolution and hourly temporal resolution for the nation. Ground-based and remotelysensed snow observations are assimilated into the simulated state variables of the snow model.
28

Figure 15. Home page for the National Snow Analysis.
29

Below the maps are links to station snowfall, snow water equivalent and snow depth reports. 

Clicking on any one of those links will bring the user to a table with observations for whichever 

geographic region is selected in the maps above.

Near the bottom of the NSA page is a section describing the most recent snow data assimilation 

and a section describing upcoming airborne snow surveys. Both the assimilation description and 

survey schedule will update depending on which region is selected in the maps.

Snow Information Heading – Interactive Maps and Time Series

Interactive Maps

For users who need more spatial and temporal flexibility than what is provided in the default 

NSA display, the Interactive Maps link provides access to a powerful Web-based interactive tool 

(Figure 16) for accessing and viewing snow information. It is not practical to completely 

describe every capability of this interactive display, but some of its highlights include the 

following:

 Any of the following physical elements (data types) can be selected for display on the 

map: hourly and daily elements related to snowpack condition (e.g., hourly snow water 

equivalent, 24-hour change in snowpack temperature), daily and hourly elements driving 

snowpack conditions (e.g., surface air temperature), archived satellite snowpack 

observations, climatic data elements (e.g., freezing degree days), observational elements 

(e.g., total precipitation), and static NOHRSC model datasets, such as elevation and 

forest cover density.

 One or more overlays can be selected for display on the map, including hydrologic, 

political, civic, transportation and other features.

 A box labeled "Navigation tools” at the top of the page can be used to re-center the map 

and zoom in on a point of interest. These tools can be used to change the extent of the

map extent for the lower 48 states, Alaska, or a custom target. The zoom tool (box with 

the "target") is activated by default when the interactive page is brought up, and the user 

can draw a box on the map to zoom to that extent. Zooming can also be done with the 

"zoom control" toggle or by centering the map on latitude and longitude

 Once the user has zoomed into an area of interest, they may use the query tool to gather 

information about a point, such as an observing station. To query a point of interest, 

select the query tool (the button with an “i” and an arrow) from the menu in the upper 

right dark blue bar. When the query tool is active, the cursor will change from a plus sign 

to an arrow. When the cursor is hovering over a point that can be queried, the cursor will 

change to a hand icon. The default query leads to time series plots of station information 

(described below). However, the pull-down menu in the query box next to the query tool

can be used to query other elements such as flight lines, state and regional snowpack 

averages, basin snowpack averages and latest observations. The "latest observations" 

query option is activated to coincide with a selected "Latest Observation" from the 

"Select Physical Element" pull-down menu in the upper right-hand corner of the page.
30

Figure 16. Initial display for Interactive Snow Information interface.
31

 The "Redraw Map" button can be clicked on at any time when a new physical element or 

overlay is chosen for display. The map will not refresh automatically.

 A series of “Link to...” options which allows the user to bring up their area and most 

recent element of interest whenever they visit the page using a bookmark. A user can set 

up this customized link by zooming in on a desired area, highlight a physical element of 

interest (e.g. snow water equivalent), clicking on the “Link to latest page” option, and 

then making the new page a bookmark in their browser.

 At the very bottom of the dark blue margin on the left hand side is a series of “Link to...” 

options. If the user wishes to zoom into an area of interest and highlight a physical 

element of interest (e.g. snow water equivalent), they can click on the “Link to latest 

page” option and make the new page a bookmark in their browser. This allows the user 

to bring up their area and most recent element of interest whenever they visit the 

bookmarked page.

Time-series plots

 Once the user selects an observing station using the query tool, a page with time-series 

plots will appear. The user can select varying dates of interest at the top of the page. A 

user can also query a station by entering its station ID in the “Quick Query Links” box in 

the upper right corner of the Interactive page. A list of stations and their IDs can be 

obtained by clicking on the “Listing” option to the right of the query box.

 The time-series plots depict both modeled and observed snow information at a station. 

These plots include snow depth, snow water equivalent, snowpack temperature, 

snowpack density, surface snow sublimation and blowing snow sublimation. Station 

time series pages also include modeled and observed meteorological parameters such as 

air temperature, precipitation (snow and non-snow), wind speed, snow surface radiation 

fluxes and snow surface energy exchanges (click here to view a sample time-series plot).

 All data displayed in the time-series plots are available in tabular format and can be 

downloaded as a .csv file. Under the date range box, there is a pull-down menu that 

displays “All Images” as its default and is used to access individual plots, tabular data or 

.csv files. To change units from Metric to English, select the “Units” drop down menu.

 Time-series plots and associated text files for NWS River Forecast Center basin snow 

characteristics such as average snow depth, average snow water equivalent and percent 

snow cover can be obtained by using the “Get Time Series for Basin ID” option under the 

“Quick Query Links” in the upper right hand corner of the interactive page. Basin 

identifiers (ID) can be obtained by clicking on the “Listing” option to the right of each 

query option.

 The “Quick Query Links” also allow the user to search basins according to state, county, 

NWS County Warning Area (CWA), 6-digit US Geological Survey Hyrdologic Unit 

Code (HUC-6) or NWS River Forecast Center. These options are particularly useful for 

water managers from groups other than the NWS.
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Figure 17. Initial display for the National Snow Analysis 3D Interface.

Snow Information Heading – 3D Visualization

The 3D Visualization link of provides snow and snowpack data that can be projected onto threedimensional images of terrain (Figure 17). Three dimensional viewing allows users to visualize 

elevation and terrain impacts on snow distribution and snow events.

The Snow Analyses Overlays displayed through this capability are three-dimensional views of 

the same data one can see in the NSA two-dimensional maps. These overlays are provided in 

.kmz (Keyhole Markup Language, Zipped) files, which allow geographic data sets to be viewed 

using Internet-based, three-dimensional Earth browsers (e.g., Google Earth). Instructions for how 

to download appropriate software and view the 3D overlays are found near the top of the page. 
33

Snow Information Heading – Airborne Surveys

One of the key inputs to the National Snow Analysis is remotely-sensed snowpack data collected 

by NOAA aircraft. Airborne snow survey data collection is based on the relationship between 

gamma radiation emitted from the ground and the amount of water in the overlying snowpack. 

Naturally-occurring low level radiation is continually emitted from the soil from trace amounts 

of isotopes. Water, whether in the form of snow, ice or liquid, reduces (attenuates) the radiation 

that makes it into the open air. Sensitive detectors in the aircraft measure the radiation levels 

through the snow, compare the results from a previous snow-free flight and compute the amount 

of water contained in the snowpack. These flight lines are pre-selected to provide useful 

snowpack data to support NWS River Forecasting Centers.

The Airborne Surveys link brings up a web page with a photograph of the NOAA aircraft used to 

conduct airborne snow surveys and a short description of the program. Below the photograph is 

a table listing completed surveys for the current water year. At the bottom of the page is set of 

links providing access to the airborne snow survey schedule for the current snow accumulation 

period, aerial photos taken during surveys, an index and a map of flight lines, a link to soil 

moisture (snow-free) surveys, tables with historical snow survey information, a key to understanding Standard Hydrometeorological Exchange Format (SHEF) messages with airborne data, 

users guide for the Airborne Gamma Radiation Snow Survey Program (fairly technical), and 

information about the aircraft used in the collection of airborne snow data.

Snow Information Heading – Satellite Obs.

The Satellite Obs link provides access to graphical products depicting snow cover over the 

conterminous U.S., Alaska, and the Northern Hemisphere. These products are generated by the 

National Ice Center – an interagency center supported by NOAA, the U.S. Navy, and the U.S. 

Coast Guard.

Snow Information Heading – Forecasts

The Forecasts link provides access to numerous short- to long-term snow forecast products 

issued by entities outside the NOHRSC. The 3D Snowfall Probability Forecasts are derived 

from two-dimensional Hydrometeorological Prediction Center (HPC) data and have been 

reformatted by the NOHRSC to produce 3D files. Instructions for viewing the 3D files are 

directly above the file links. The rest of the products on this page, such as nationwide watches 

and warnings and river flood conditions, are produced by NOAA entities other than the 

NOHRSC.

Snow Information Heading – Data Archive

The Data Archive link provides access to past grid-based daily output from the SNODAS model 

archived at the National Snow and Ice Data Center. Documentation and instructions on how to 

use the archive are provided, and data is accessed via FTP.
34

Snow Information Heading – SHEF Products

The SHEF Products link provides access to text products containing basin averages for one of 

seven snowpack-related variables, encoded in SHEF, for any user-selected date and time. The 

products are produced for basins within each NWS River Forecast Center domain. The seven 

simulate snowpack-related variables are: snow water equivalent, basin snow-covered area, 

average snowpack temperature, snow melt, blowing snow sublimation, snow depth and snowpack surface sublimation. This product is primarily intended for NWS River Forecast Centers, 

but it is also made available to the public through this link. The SHEF Products display includes 

links which also allow users generate lists of Observation stations by WFO, CoCoRaHS station 

identifiers, and Stations with unknown metadata.

Observations Near Heading

This feature allows the user to request, for any city and state, the five closest observations of 

recent snowfall, snow depth, snow water equivalent, and total precipitation. The “Observations 

near” box will bring up a web page where the user can select a new location, time, date, year and 

English or metric units (click here for an example).

Science/Technology Heading - NOHRSC

The NOHRSC link brings up a mission statement for the NOHRSC and an overview of its 

functions. Following these are lists of documents and publications covering various aspects of 

the NOHRSC’s operations. For an in-depth overview of the SNODAS model, refer to “NOAA’s 

National Snow Analysis: Western Snow Conference, 2006 April” under the heading “NOAA’s 

National Snow Analysis (NSA).”

Science/Technology Heading – GIS Data Sets

The GIS Data Sets link brings up several tables listing geographic information system (GIS) files 

and instructions for downloading and decompressing the files. Available datasets include: 

(1) basin boundaries for each NWS river forecast center area, (2) datasets that are used on the 

NOHRSC interactive web page, and (3) rivers and streams in each of the NWS River Forecast 

Center regions.

Science/Technology Heading – Special Purpose Imagery

The Special Purpose Imagery link brings up thumbnails for various images created for special 

projects. These images, usually satellite-derived, are used to support NWS River Forecast 

Center operations, particularly during flood events.

Other Links on National Snow Analysis Page

Near the bottom of the left margin of the NSA page, links are provided for obtaining NOHRSC 

staff bios and contact information, links to NOAA snow climatology and other snow resources, 
35

help on using key features of the National Snow Analysis web pages (including frequently asked 

questions), and a link enabling users to submit comments.

On the bottom of the dark blue margin on the left side of the home page, links are provided to: 

(1) the National Operational Hydrologic Remote Sensing Center staff directory, (2) information 

on snow climatology and other snow resources, (3) the websites of related NWS offices and 

centers, NOAA centers, and external data providers, (4) a menu for obtaining help on using key 

features of the National Snow Analysis web pages (including frequently asked questions), and 

(5) a menu enabling users to submit comments.