NOAA’s National Weather Service 

Your gateway to web resources provided through NOAA’s 

Advanced Hydrologic Prediction Service begins here: 

water.weather.gov 

Guide to Hydrologic 

Information on the Web 

Prediction Services 

NOAA/PA 200850

U.S. DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

National Weather Service

September 2015
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National Map 

This web page shows the flood status at all National Weather Service river forecast locations.

Regional Map 

This web page shows the flood status of river forecast locations in the selected region. Selecting a 

specific river/stream location provides access to localized information.

River or stream gauge is 

not operating.

Observations are not 

current.

River or stream level 

below flood stage.

River or stream level 

approaching flood stage.

Minimal or no property 

damage, but possibly 

some public threat.

Some inundation of 

structures and roads 

near stream – some 

evacuations of people 

and property.

Extensive inundation of 

structures and roads. 

Significant evacuations

of people and property.

Color of icon indicates 

category of flooding.

Clicking on any location brings up 

a regional map showing a NWS 

Weather Forecast Office (WFO) 

Hydrologic Service Area (HSA) 

containing that location.

Clicking on this tab 

shows locations where 

observations are colorcoded according to their 

current flood status.

NWS river forecasts and 

flood products are critically 

dependent on the USGS 

stream gaging network.

Clicking on this tab 

shows locations that 

are color-coded 

according to the highest 

forecast flood category 

through the entire 

period. Use the forecast 

timeline slider at the top 

of the map to adjust the 

forecast period. For 

example, clicking on the 

Day 1 tick mark shows 

locations that are colorcoded according to the 

highest forecast flood 

category through Day 1.

Clicking on this tab 

shows locations where 

observations are colorcoded according to their 

current flood status.

Color-coding is used on map 

locations to indicate flood status. 

Click on a colored box and only 

points in that flood category are 

displayed.

Drop-down menu allows users 

to display all locations, 

hydrograph only locations, 

hydrograph/probability forecast 

locations, inundation mapping 

locations, near or above flood 

stage locations, and at or below 

low water threshold locations. 

Clicking on this tab shows 

locations that are color-coded 

according to the maximum 

forecast flood category through 

the entire period. Because the 

period for which a forecast is 

prepared varies from RFC to 

RFC, the entire period generally 

varies from 2 to 10 days.

Use drop-down menus to show 

locations across different regions, 

including states, NWS WFO HSAs, 

NWS River Forecast Center (RFC) 

areas of responsibility, and USGS 

water resource regions.

Bottom row shows 

options available under 

the “Water” category.

Click this button to have the 

map automatically update 

every 5 minutes.

Top row shows options 

available on the NWS 

home page.

Navigation arrows allow 

users to move to 

neighboring WFO 

HSAs.

Use checkboxes to turn 

overlays off and on and slider 

bars to adjust the 

transparency of the overlays.

Click here to only display locations 

currently in flood.

Hover over this box to learn more 

about UTC time.

Squares 

indicate 

locations where 

short-term 

forecasts are 

available. 

Circles indicate 

locations where 

short-term and 

probabilistic 

forecasts are 

available. 

Flood category has not 

been defined.

Diamonds indicate 

locations where only 

observations are 

available. 

River or stream level at 

or below low water 

threshold.
Hydrograph 

Weekly Chance of Exceeding Levels 

Mouse over a specific river/stream location on a regional map to display the location’s five-character identifier, 

name, and hydrograph. Click on the river/stream location to go to the main hydrograph page. The 

hydrograph provides recent observations and for many points a forecast for the next few days. Tabs at the top 

of the hydrograph allow a user to display other graphical information for the river/steam location.

This graphic provides the probability of exceeding various river/stream levels during the next three 

months given the weather patterns in past years and the best available long-term forecasts. 

On the web, click on “About 

this graph” for description of 

this plot.

Color-coded flood 

categories for location being 

viewed, with stage where 

each category starts. Flood 

severity color scheme is 

consistent throughout these 

web pages.

Forecast trace – shows 

expected water level trend 

over next few days.

Vertical line marks the 

current viewing time.

Observed trace – 

indicates water level 

trends over past 

several days.

Scale for indicating 

water level in terms of 

stage in feet (ft).

Peak observed and 

forecast stages during 

time period covered by 

graph.

Each colored section of 

vertical bar represents a 

range of probabilities 

(e.g., 10% to 25% for 

the yellow bars). Range 

of stages corresponding 

to those probabilities 

can be found on the left 

scale of the graph (see 

example).

Bars are shown in 

weekly increments.

Provides explanation of 

the graph.

Example: For week of 9/13 

to 9/20, the bottom of the 

yellow bar shows there is a 

25% chance the stage will 

exceed about 7.7 feet and 

the top shows there is a 10% 

chance the stage will exceed 

about 8.5 feet.

Allows user to display 

probabilities in terms of 

stage, discharge, or weekly 

flow volume.

Scale for indicating water 

level in terms of flow in 

thousands of cubic feet per 

second (kcfs) or cubic feet 

per second (cfs) if available.

Latest observed stage 

with time and date. 

Flood stage is included 

if available.

Lines for flood category 

levels use the same 

color scheme shown on 

the flood hydrograph 

and other web pages.

Hover over this tab to select the type of 

probability information you wish to view.

Local time scale.

Coordinated Universal Time 

(UTC) scale.
Precipitation 

Directions for use of 

interactive features.

Select geographic 

features to display.

Interactive Snow Information 

Miscellaneous display 

control features.

This web page shows the digital precipitation estimates used in National Weather Service river forecast models.

This web page (www.nohrsc.nws.gov/interactive/) provides an interface to remotely sensed snow information.

Select display units 

(U.S., metric).

Select date and time to 

display.

Select type of data to 

display – e.g., snow 

water equivalent, snow 

depth, or snow water 

equivalent change.

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When box is red, clicking 

and dragging while 

holding down mouse 

button and then 

releasing zooms in on 

created rectangle area.

Control map by zooming 

in or out.

Map shows portion of 

U.S. currently being 

displayed.

Key to information being 

displayed.

Elevation key for base 

map.

Description of precipitation display 

and its features.

Allows download of digital data in 

shapefile or netCDF format.

Provides list of links to other 

precipitation-related web pages.

Area covered: River 

Forecast Center (RFC) 

area of responsibility, 

state, full CONUS + 

Puerto Rico, Weather 

Forecast Office (WFO), or 

USGS water resource 

region (defaults to full 

CONUS + Puerto Rico).

Select units for display.

User-selectable overlays.

Precipitation depth in 

selected units.

Third row shows

options available under 

“Precipitation” category.

Feedback from users is 

highly valued.

Latitude and longitude 

of cursor location. User 

can enter coordinates 

to recenter map.

Selectable timeframe: archived 

yearly, archived monthly, or 

archived daily data.

User can select year and month 

or year, month, and day 

depending on archive timeframe 

chosen.
Flood Inundation Map 

Describes stage where 

each flood category 

begins.

List of historical flood 

crests.

Flood category display

Current forecast display

This interactive web page shows the spatial extent of possible or expected flooding in a given area. It can be used to show 

if roadways and structures will be impacted by floodwaters. At the limited number of forecast locations where inundation 

maps are currently available, this web page is accessed by clicking on the inundation mapping tab on the hydrograph web 

page. In collaboration with partners, this product will be expanded to new locations. 

Set display: inundation 

levels, flood categories, 

or current forecast.

Slider for adjusting 

transparency of overlay.

Stage at inundation 

level currently shown on 

map.

Select 

background: 

imagery, street, 

topographic, USGS 

topographic, and 

light grey canvas 

maps.

Color scale for 

depth of 

inundation.

Menu for inundation 

levels shown on map: 

stage = 18.5 feet is 

highlighted in yellow.

Flood 

inundation at stage 

= 18.5 ft is shown 

on map.

Link to user guide 

video on YouTube.

Latest stage 

observation.

Download data 

and links to 

supporting 

documents.

Use tear-away to 

move map overlay 

controls.

Levee risk area 

and flood control 

structures overlays 

are displayed 

when flood 

inundation map is 

viewed for the first 

time.

Clicking anywhere on 

map displays a red 

marker. Water depth, 

latitude, longitude, and 

nearest geo-located 

address is provided for 

that location.
Experimental Long-Range River Flood Risk Map 

This interactive web page shows the long-range (3-month) risk of minor, moderate and major river flooding at locations 

where probabilistic forecasts are produced. The risk information is based on NWS Ensemble Streamflow (ESP) 

forecasts which are generated for thousands of river and stream forecast locations across the nation.

Clicking on this 

tab shows 

locations where 

there is a 

greater than 

50% chance of 

exceeding 

minor, 

moderate, and 

major flooding.

Click on the map 

to display a 

regional map of 

the WFO HSA 

containing that 

location. 

Allows user to 

choose values 

of greater than 

5, 10, 25, 50, 

75, 90, or 95% 

chance of 

exceeding river 

flood levels 

during a 3-

month period.

On a regional 

map, click on a 

point to view the 

Chance of 

Exceeding 

Levels During 

Entire Period 

graphic for that 

location.

Use the 

dropdown menus 

to view four types 

of regional maps, 

including state, 

WFO HSA, NWS 

RFC area of 

responsibility, or 

USGS water 

resource region. 

Allows user to select the 3-month period of interest. Through the 28th day of the current month, 

the 3-month period defaults to the current month, current month + 1 month, current month + 2 

months. After the 28th day of the current month, the 3-month period defaults to the current month 

+ 1 month, current month + 2 months, current month + 3 months.

The Advanced Hydrologic Prediction Service, or AHPS, is a program in 

NOAA’s National Weather Service designed to provide improved river and 

flood forecasting and water information. AHPS provides a suite of graphical 

and numeric products over the Internet to assist community leaders and 

emergency managers in making better life- and cost-saving decisions about 

evacuations and movement of property before flooding occurs.

To access the water information in this brochure, see:

water.weather.gov

Feedback on these web pages is welcome at:

weather.gov/contact 

NOAA/PA 200850