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CW3E Watershed Precipitation Forecasts
Supported by the California Atmospheric Rivers Program and U.S. Army Corps of Engineers FIRO Program

The 284 watersheds overlapping with Washington, Oregon, and California on this page are obtained from the USGS for the Hydrologic Unit Code (HUC)-8 size and their geometry is simplified/upscaled for plotting efficiency. The watersheds are shaded based on their mean areal precipitation (MAP) forecast as a 5-day total or 10-day total from different numerical weather prediction models. The MAP is derived by (1) identifying grid points (lat/lon pairs) on a 0.1-degree grid that are within each of the HUC-8 watersheds, (2) finding the precipitation at those points using bilinearly interpolation from the NWP model's grid, and (3) averaging the values across those lat/lon pairs. It is not a median and its value may be skewed. The number of grid points varies with as few as 4 points in the Cloverdale watershed in southeast Arizona to 240 points in the Rio De La Concepcion watershed in Southern Arizona.

Data Sources:
The GFS and GEFS data are obtained from the NOAA/National Center for Environmental Prediction via NOMADS and the ECMWF and EFS data are obtained from ECMWF by CW3E. The use of this data are limited for research purposes and are not intended to directly support decision making operations according to our disclaimer. The map is created using Leaflet Javascript with additional JavaScript interactions necessary to produce the map and its interaction with both the table and chart capabalities. The table and chart are visualized using Google Charts with data manipulated within the workflow mentioned above.

The mean areal precipitation (MAP) may not be representative of the distribution of precipitation within the watershed and may be skewed; however, it is derived from global NWP models that have relatively coarse resolutions of ~10-25 km that are not able to resolve some of the finer resolution variability due to topography within the watersheds anyways. We are currently developing forecasts at the HUC-10 (smaller basin) scale for California that will highlight this variability using higher resolution models (e.g., the West-WRF, HRRR, and the CNRFC forecasts).

The methodology, workflow, JavaScript, visualizations, and interactivity on this page were created by Jay Cordeira ( with support provided by Brian Kawzenuk ( Please direct any questions you might have to either of them.

The watershed forecast tools on this page were originally developed to support the California Atmospheric River (AR) Program with funding provided by the California Department of Water Resources during Phase 1 of the Program (award #4600010378) and during Phase II of the Program (award (#4600013361). Further development of these tools to expand the domain into the Pacific Northwest and added functionality focusing on watersheds involved with Forecast Informed Reservoir Operations (FIRO) has been supported by a combination of the California AR program and funding provided by the U.S. Army Corps of Enginners supporting FIRO (Phase 2) at CW3E (award W912HZ-19-2-0023).

Last updated 3/19/22.