NOAA National Water Model

The National Water Model (NWM) is a computer model of the water cycle across the continental US, southern Alaska, Hawaii, Puerto Rico and the US Virgin Islands.¹ It represents rain and snowmelt, water soaking into and moving through the soil, and flow down rivers and streams.¹ NOAA's National Water Center in Tuscaloosa, Alabama began running it operationally in August 2016.^5,6 Version 3.0 entered service in September 2023, built by a joint team from the Office of Water Prediction (OWP) and the National Center for Atmospheric Research (NCAR).⁴ NOAA's sheet on the model describes it as forecast guidance for over 3.4 million miles of rivers and streams across the United States and its territories.⁴

The National Weather Service issues official river forecasts at about 4,000 places in the continental US. The model adds guidance at millions of other spots that have none.¹ Its core is WRF-Hydro, a community hydrologic model supported by NCAR, with the Noah-MP land surface model handling soil, snow and plants.¹ Water moves over and under the ground on a 250 m grid and down river channels drawn from the National Hydrography Dataset.¹ Rain comes from radar and gauge observations (MRMS and Stage IV) and from weather forecast models (HRRR, RAP, NAM-Nest, GFS and CFS).¹ Measured flows from USGS and Army Corps of Engineers gauges are blended in, a step called data assimilation.¹ Over 5,000 reservoirs are included.¹ Since version 3.0 a coastal model, SCHISM, joins river flow to ocean surge and tide forecasts.^1,4 Together they give total water level, the combined height of river, tide and surge, along the East, Gulf and Pacific coasts, Hawaii, Puerto Rico and the US Virgin Islands.^1,4

The model runs several analyses, simulations of current conditions that look back between 3 and 28 hours. The forecasts start from them.¹ In the continental US a short-range forecast runs every hour and looks 18 hours ahead.¹ A medium-range forecast runs four times a day as six runs with varied inputs, an ensemble, reaching 8.5 to 10 days.¹ A long-range forecast also runs four times a day and builds a 16-member, 30-day ensemble each day.¹ Alaska's short-range forecast runs every three hours and looks 15 or 45 hours ahead.¹ Hawaii, Puerto Rico and the US Virgin Islands get 48-hour forecasts twice a day.¹

Output comes as NetCDF, a common file format for gridded science data.¹ Gridded files at 1 km hold land variables such as snow depth, soil moisture and evaporation.^1,3 Finer grids, 250 m or 100 m, hold ponded water depth and water table depth.^1,3 Point files hold streamflow and velocity for each river reach, plus reservoir inflow, outflow and water level.³ Coastal water level also comes in SHEF, a text format the Weather Service uses for station time series.³ To put the river output on a map, a separate ESRI file geodatabase gives the latitude and longitude of the centre of each reach.¹ The full output and some of the weather input files are on NOAA's NOMADS server and the NCEP FTP server.¹ A subset of the model's parameter files can be downloaded separately.¹ The NWPS API serves the model's streamflow output, and water.noaa.gov shows it on an experimental interactive map and image viewer.^1,8 NOAA also publishes GIS web services built on the model, such as streamflow anomalies and high water arrival times.⁷ In the continental US the high water thresholds come from a 44-year NWM v3.0 reanalysis, a rerun of the model over past decades.⁷

A flood inundation map shows where water has spread over the banks of a river or stream.¹⁰ NOAA's dynamic maps for the continental US come in three services, and the model feeds all three.¹⁰ One maps the latest NWM analysis, updated hourly, wherever flows pass a "High Water" threshold.^10,11 Another maps the five-day maximum from the medium-range forecast, updated every six hours.^10,11 The third routes official River Forecast Center forecasts downstream through the NWM stream network along about 110,000 river miles.¹⁰ All three draw the flooded area with HAND (Height Above Nearest Drainage).¹⁰ The method compares the water level in a stream with the height of the land around it.¹² Hawaii, Puerto Rico and the US Virgin Islands get a 48-hour maximum forecast map.¹¹ NOAA set September 2026 as the date for the flood maps to reach nearly 100% of the US population, and its FAQ says they now do.^11,12 Parts of Alaska, American Samoa and Guam remain outside the service area.¹¹

OWP has proposed an interagency Next Generation Water Resources Modeling Framework, NextGen, built on open-source code and shared standards.⁴ It would let different model formulations run in different regions, where the current model uses one formulation everywhere.⁴ OWP's sheet said a NextGen configuration would replace the operational model in 2024.⁴