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Determining Precipitation Frequencies Over the Southern United States

A COMET Partners Project

As a result of the modernization of the National Weather Service (NWS) which began a decade ago, forecasters now are equipped with a variety of new tools and technologies which enable them to provide products and services in ways never before possible. Although the mission of the NWS has not changed, the capabilities of forecasters have been vastly enhanced. The end result is that the public will be provided with more accurate forecasts, with higher time and space resolution, and in a gridded-digital form that will enable them to utilize the products in new ways. In particular, forecasters now issue forecasts of the probability of precipitation (PoP) for 3-hourly time periods in the form of high resolution gridded fields. This is a significant increase in resolution over the 12-hourly PoPs that have been issued by the NWS for over 35 years covering much larger areas--county-sized regions or larger. The new software that the NWS uses to prepare these forecasts is called the Interactive Forecast Preparation System (IFPS) which runs in the AWIPS environment. The IFPS software allows forecasters to develop and communicate products such as the high resolution PoPs in gridded, graphical, and text forms.

A knowledge of the climatological likelihood of an event is an excellent starting point for a forecast. Climatological frequencies also provide a basis for assessing one measure of forecast skill. Jorgensen (1967) developed climatological precipitation probabilities for the U.S. for 6-, 12-, and 24-hour periods (using data from 1949-1964), publishing them in the form of maps. Jorgensen's work has served well for decades in providing important training materials for forecasters. However, frequencies are needed for 3-hour intervals, and using the longer period of data that now is available (e.g., from 1949-present).


This research is developing climatological guidance that will provide an essential foundation for forecasters as they initiate the higher resolution PoPs. To our knowledge, precipitation frequencies for 3-hourly periods, for a dense array of sites, has never been developed. Our objective is to extend Jorgensen's work to 3-hourly periods, and if resources permit, to re-derive climatological frequencies for the longer time periods using data for the most recent 30 or 40 years.

Specific Tasks

The data from which to derive the precipitation frequencies are available on CD-ROMs developed by NOAA's Forecast Systems Lab and NCDC which contain the NCDC Hourly Precipitation Data. We have developed software to extract the hourly precipitation data and compile the precipitation frequencies. The frequencies will be displayed in tabular and graphical forms. Maps showing horizontal distributions of the results also will be prepared.

Once these probabilities are calculated, they will be published in forms most useful to forecasters and the research community. The results already are on the web. The goal is for forecasters to easily integrate the results into the operational system which NWS forecasters use to produce their forecasts.

Undergraduate Students on this Project

John Sullivan
Matt Sitkowski

Click here to view the Precipitation Frequencies.

Click here to reach the COMET web page.

Click here to reach the National Weather Service--Tallahassee web page.

last updated July 9, 2006