weather.gov

Site Map

News

Organization

Search

Current Flooding

Hydrologic Conditions

Historic Flood and Hydrologic Information

General Information

Home

This map is intended to depict locations where hydrometeorological conditions either enhance the potential for flooding or make it less likely. It depicts this potential in general, broad-brush terms.  It is possible that local conditions could vary from those shown on the map. National Weather Service offices in the area of interest should be consulted for more detailed or specific information on local conditions (see Forecasts & Outlooks).      Delineation of flood potential is based on an analysis of a number of factors, including soil moisture, streamflow and snow cover. In areas where soils are wet, where streams are running high and where there is a deep snow cover, the potential for flooding is likely to be elevated. In addition to these three elements, consideration is given to other factors, such as the effect of river ice, frozen soils, reservoir storage capacity, etc.  Conversely, where these indicators show dryness, flood potential will be reduced. All these quantities can be observed -- in effect, no forecast is necessary to make this assessment.     Unlike a traditional forecasts, there is no specific rise or fall forecast in flood potential classifications.  If wet conditions persist, either because of continued rain/snow or because no factors reduce the wetness, then concern over an elevated flood potential will also continue. On the other hand, lack of precipitation and/or snow melt in dry areas will maintain lower flood potential.  In many cases, the flood potential will evolve, either to less threatening conditions, or to worse conditions.     Flood outlooks provided by NWS forecast offices are the basis for the areas on the map identifying above average, average or below average flood potential.   'No Outlook' indicates areas for which there is no recently updated information.  These outlooks usually include some assumption about future precipitation and temperature (e.g., expected normal climatological patterns) and anticipated operational hydrologic changes such as reservoir releases and diversions for irrigation.     In contrast to outlooks, flood forecasts are short-term projections (up to 3 days) based on forecasted patterns of precipitation and temperature and usually provide specific, quantitative information on river levels and/or flow volumes. The uncertainty of flood outlooks and flood forecasts vary from season to season and location to location. The uncertainty of flood forecasts tends to be less than the uncertainty of outlooks due to their shorter lead times. This map DOES NOT assess short-term risk of flooding, particularly, flash flooding.      Flash flooding typically results within a few hours from localized, intense precipitation.  Flash flooding can occur in areas of low flood potential.  Indeed, it is possible to have flash flooding in a drought area.  In some cases, drought impacts can either continue or quickly resume after a flash flood event.  This paradox is due to the fact rains that can cause flash flooding happen so quickly that the soils do not have chance to absorb the water.  If flash flood runoff flows into streams that do not have reservoirs to capture the water, much of the rainfall leaves the area, having limited impact on drought conditions.     The National Hydrologic Assessment provides links to information from many sources, but the areas identified as being of concern for possible flooding are based almost exclusively on NWS sources. While the various links provide recent information from a number of providers (including non-NWS sources), this information is not always concurrent (i.e., updated at the same time). In addition to depicting different time periods, the quantities illustrated may also vary from one source to another.  For example, interpretation of the spatial distribution of snow depth may differ from a similar presentation of snow water equivalent, depending on the variation of snow density.  Variations may also arise from differences in measurement systems, e.g., in situ vs. remotely sensed observations.   Some indices such as soil moisture may be misleading because they may not adequately account for water stored in deep aquifers, snow on the ground or frozen ground.  Finally, because information comes from a variety of sometimes independent sources, in some cases, data in the various links might appear to be inconsistent. Therefore, care should be used when comparing information provided by the links on this site.  [See "Hyperlinks" section of NWS disclaimer .]     Included in Forecasts & Outlooks is information on future meteorological factors that could determine whether the potential for flooding might be realized. This information includes monthly and seasonal precipitation and temperature outlooks. Climatological outlooks can provide valuable information about possible future flooding. Areas with high flood potential that are projected to be wet in the upcoming periods warrant attention. Under some conditions, locations with deep snow cover that are in areas where climatological outlooks indicate temperatures are likely to be warm could be susceptible to snow melt flooding. Some Weather Forecast Offices include the effects of climatological outlooks in their flood potential outlooks, see  Forecasts & Outlooks for individual office information.  However, climatological outlooks provide information about average conditions over a period of time, and they say nothing about the details of specific weather systems that, in aggregate, make up the climatology. An area can receive above normal precipitation over a period of time due to a series of light to moderate rainfall events. Such events may not be sufficient to cause serious flooding, even at locations identified as having above average flood potential. A critical factor in any flood is that the rate of rainfall (and/or snowmelt) must be more than the hydrologic system can accommodate. Conversely, even in areas where the flood potential is not especially high, intense rain can result in flooding. Please see statement on information content, use and availability of this site.