The mission of NWS—an agency within the Department of Commerce’s National Oceanic and Atmospheric Administration (NOAA)—is to provide weather, water, and climate forecasts and warnings for the United States, its territories, and its adjacent waters and oceans, in order to protect life and property and to enhance the national economy. NWS is the official source of aviation- and marine-related weather forecasts and warnings, as well as warnings about life-threatening weather situations. In the 1980s and 1990s, NWS undertook a nationwide modernization program to develop new systems and technologies and to consolidate its field office structure. The goals of the modernization program were to achieve more uniform weather services across the nation, improve forecasts, provide more reliable detection and prediction of severe weather and flooding, permit more cost-effective operations, and achieve higher productivity. The weather observing systems (including radars, satellites, and ground-based sensors) and data processing systems that currently support NWS operations were developed and deployed under the modernization program. During this period, NWS consolidated over 250 large and small weather service offices into the office structure currently in use. The coordinated activities of weather facilities throughout the United States allow NWS to deliver a broad spectrum of climate, weather, water, and space weather services. These facilities include weather forecast offices, river forecast centers, national centers, and aviation center weather service units. The functions of these facilities are described below. 122 weather forecast offices are responsible for providing a wide variety of weather, water, and climate services for their local county warning areas, including advisories, warnings, and forecasts (see fig. 1 for the current location of weather forecast offices). 13 river forecast centers provide river, stream, and reservoir information to a wide variety of government and commercial users as well as to local weather forecast offices for use in flood forecasts and warnings. 9 national centers constitute the National Centers for Environmental Prediction, which provide nationwide computer model output and manual forecast information to all NWS field offices and to a wide variety of government and commercial users. These centers include the Environmental Modeling Center, Storm Prediction Center, Tropical Prediction Center, Climate Prediction Center, Aviation Weather Center, and Space Environment Center, among others. 21 aviation center weather service units, which are co-located with key Federal Aviation Administration (FAA) air traffic control centers across the nation, provide meteorological support to air traffic controllers. To fulfill its mission, NWS relies on a national infrastructure of systems and technologies to gather and process data from the land, sea, and air. NWS collects data from many sources, including ground-based Automated Surface Observing Systems (ASOS), Next Generation Weather Radars (NEXRAD), and operational environmental satellites. These data are integrated by advanced data processing workstations—called Advanced Weather Interactive Processing Systems (AWIPS)—used by meteorologists to issue local forecasts and warnings. The data are also fed into sophisticated computer models running on high-speed supercomputers, which are then used to help develop forecasts and warnings. Figure 2 depicts the integration of the various systems and technologies and is followed by a description of each. NEXRAD is a Doppler radar system that detects, tracks, and determines the intensity of storms and other areas of precipitation, determines wind velocities in and around detected storm events, and generates data and imagery to help forecasters distinguish hazards such as severe thunderstorms and tornadoes. It also provides information about heavy precipitation that leads to warnings about flash floods and heavy snow. The NEXRAD network provides data to other government and commercial users and to the general public via the Internet. The NEXRAD network is made up of 158 operational radars and 8 nonoperational radars that are used for training and testing. Of these, NWS operates 120 radars, the Air Force operates 26 radars, and the FAA operates 12 radars. These radars are located throughout the continental United States and in 17 locations outside the continental United States. Figure 3 shows a NEXRAD radar tower. ASOS is a system of sensors, computers, display units, and communications equipment that automates the ground-based observation and dissemination of weather information nationwide. This system collects data on temperature and dew point, visibility, wind direction and speed, pressure, cloud height and amount, and types and amounts of precipitation. ASOS supports weather forecast activities and aviation operations, as well as the needs of research communities that study weather, water, and climate. Figure 4 is a picture of the system, while figure 5 depicts a configuration of ASOS sensors and describes their functions. There are currently 1,002 ASOS units deployed across the United States, with NWS, FAA, and the Department of Defense (DOD) operating 313, 571, and 118 units, respectively. Although NWS does not own or operate satellites, geostationary and polar- orbiting environmental satellite programs are key sources of data for its operations. NOAA manages the Geostationary Operational Environmental Satellite (GOES) system and the Polar-orbiting Operational Environmental Satellite (POES) system. In addition, DOD operates a different polar satellite program called the Defense Meteorological Satellite Program (DMSP). These satellite systems continuously collect environmental data about the Earth’s atmosphere, surface, cloud cover, and electromagnetic environment. These data are used by meteorologists to develop weather forecasts and other services, and are critical to the early and reliable prediction of severe storms, such as tornadoes and hurricanes. Geostationary satellites orbit above the Earth’s surface at the same speed as the Earth rotates, so that each satellite remains over the same location on Earth. NOAA operates GOES as a two-satellite system that is primarily focused on the United States (see fig. 6). To provide continuous satellite coverage, NOAA acquires several satellites at a time as part of a series and launches new satellites every few years. Three satellites, GOES-10, GOES- 11, and GOES-12, are currently in orbit. Both GOES-10 and GOES-12 are operational satellites, while GOES-11 is in an on-orbit storage mode. It is a backup for the other two satellites should they experience any degradation in service. The first in the next series of satellites, GOES-13, was launched in May 2006, and the others in the series, GOES-O and GOES-P, are planned for launch over the next few years. In addition, NOAA is planning a future generation of satellites, known as the GOES-R series, which are planned for launch beginning in 2014. Unlike the GOES satellites, which maintain a fixed position above the earth, polar satellites constantly circle the Earth in an almost north-south orbit, providing global coverage of conditions that affect the weather and climate. Each satellite makes about 14 orbits a day. As the Earth rotates beneath it, each satellite views the entire Earth’s surface twice a day. Currently, there are four operational polar-orbiting satellites—two are POES satellites and two are DMSP satellites. These satellites are positioned so that they can observe the Earth in early morning, morning, and afternoon polar orbits. Together, they ensure that for any region of the Earth, the data are generally no more than 6 hours old. Figure 7 illustrates the current configuration of operational polar satellites. NOAA and DOD plan to continue to launch remaining satellites in the POES and DMSP programs, with final launches scheduled for 2007 and 2011, respectively. In addition, NOAA, DOD, and the National Aeronautics and Space Administration are planning to replace the POES and DMSP systems with a state-of-the-art environment monitoring satellite system called the National Polar-orbiting Operational Environmental Satellite System (NPOESS). In recent years, we reported on a variety of issues affecting this major system acquisition. AWIPS is a computer system that integrates and displays all hydrometeorological data at NWS field offices. This system integrates data from NEXRAD, ASOS, GOES, and other sources to produce rich graphical displays to aid forecaster analysis and decision making. AWIPS is used to disseminate weather information to the national centers, weather offices, the media, and other federal, state, and local government agencies. NWS deployed hardware and software for this system to weather forecast offices, river forecast centers, and national centers throughout the United States between 1996 and 1999. As a software-intensive system, AWIPS regularly receives software upgrades called “builds.” The most recent build, called Operational Build 6, is currently being deployed. NWS officials estimated that the nationwide deployment of this build should be completed by July 2006. Figure 8 shows a standard AWIPS workstation. Numerical models are advanced software programs that assimilate data from satellites and ground-based observing systems and provide short- and long-term weather pattern predictions. Meteorologists typically use a combination of models and their own experience to develop local forecasts and warnings. Numerical weather models are also a critical source for forecasting weather up to 7 days in advance and forecasting long-term climate changes. One of NWS’s National Centers for Environmental Prediction, the Environmental Modeling Center, is the primary developer of these models within NWS and is responsible for making new and improved models available to regional forecasters via the AWIPS system. Figure 9 depicts model output as shown on an AWIPS workstation. NWS leases high-performance supercomputers to execute numerical calculations supporting weather prediction and climate modeling. In 2002, NWS awarded a $227 million contract to lease high-performance supercomputers to run its environmental models from 2002 through September 2011. Included in this contract are an operational supercomputer used to run numerical weather models, an identical backup supercomputer located at a different site, and a research and development supercomputer on which researchers can test out new analyses and models.