The Role of Hydrology at NWS Tucson

The National Weather Service is best known for its role in meteorology and weather forecasting. However, the NWS is also involved in hydrology and river forecasting. The primary hydrologic duties of the NWS are to issue flood watches and warnings, flash flood watches and warnings, and water supply outlooks.

Flood watches and warnings are issued for mainstem rivers, such as the Gila and Santa Cruz Rivers in southeast Arizona. More specifically, watches and warnings focus on a particular river reach, usually associated with a flood forecast point. A forecast point is generally located at a USGS stream gage and has a defined bankfull stage and flood stage. A watch is issued when current conditions and forecast precipitation indicate a good possibility of flooding. A warning is issued when current conditions and observed and forecast precipitation indicate flooding is imminent.

Support for river forecasting in southeast Arizona is provided by the NWS Colorado Basin River Forecast Center in Salt Lake City. Hydrologists at the CBRFC use workstation computers to run models of snow accumulation and melt, runoff, routing and other hydrologic processes to forecast flows and stages at river forecast points. This information is used by the Tucson staff to issue public products, such as flood watches, warnings, and statements.

Click here for a list of river forecast points in southeast Arizona.

NWS forecast offices also issue watches, warnings and statements for flash flooding. As implied by its name, flash flooding typically occurs within 6 hours of a precipitation event. Flash floods are caused by very intense precipitation and are enhanced by steep slopes and impermeable surfaces (i.e. pavement). Flash flood watches are issued when meteorological conditions favor intense heavy precipitation. Watches are usually issued for multi-county areas. Flash flood warnings are issued when rainfall of an amount and rate sufficient to cause flash flooding is detected by rain gages, weather spotters, or radar. Warnings are issued for portions of counties to address the event(s) at hand. Specific flows or stages are not forecast for flash floods; rather, an area is warned within which flooding on washes and creeks is likely to occur.

In southeast Arizona, flash flood warnings for populated areas are issued when flows are expected to exceed a depth of 1 foot. This is done because numerous "low-water crossings" in the region and especially in the Tucson metro area put motorists in the path of rapid runoff in normally-dry washes. Flow 1-2 deep is capable of eroding roadbeds and/or sweeping away vehicles.

Water Supply Outlooks for various river forecast points and rivers in the Colorado Basin are issued monthly from January through April by the Colorado Basin River Forecast Center. These outlooks focus not on specific flow events but on the seasonal flow volume at a stream gage or reservoir. Water supply outlooks in Arizona represent a coordinated effort between the National Weather Service, Natural Resources Conservation Service, Bureau of Reclamation, Salt River Project, U.S. Geological Survey and local water district managers.