About the Excessive Rainfall Forecast
The Excessive Rainfall graphic provides a forecast of the potential
for flash flooding across the continental United States.
As of October 5, 2004, the graphic displays the probability that
precipitation will exceed the flash flood guidance values issued by the River Forecast Centers (RFCs).
A closed contour with an arrowhead will delineate the probability forecasts, with areas of threat
defined to the right of the direction of the arrowhead.
On June 29, 2006, the probability categories were changed due to calibration studies conducted at WPC.
The calibration for the excessive rainfall graphic is based on the frequency of events for which
observed rainfall exceeded FFG values for a given risk category.
Three probability categories are defined:
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If the potential exists for precipitation exceeding guidance values, but the expected probability is less than 5%, WPC will place the words SEE TEXT over the threat area. This area will then be referenced in the excessive rainfall discussion. In addition, areas where precipitation is expected to exceed five inches will also be indicated. If conditions are not favorable or are not expected to become favorable for flash flooding then "Rainfall Not Expected To Exceed Flash Flood Guidance" is appended to the graphic. The graphics and associated discussion are issued four times per day, at 03, 06, 15, and 18 UTC. The valid times of the products varies as noted in the table below. The 03 and 15 UTC issuances are valid for 21-hour periods, while the 06 and 18 UTC issuances are valid for 30 hours. In addition, if significant changes to the outlook are necessary, the forecaster has the option to issue products at 00, 12, and 21 UTC. Note: The definition of exceeding flash flood guidance is broad. Flash flood guidance values depict the amount of rainfall necessary in a specific period of time to cause flash flooding over a given area. The River Forecast Centers typically issue guidance values for 1-, 3-, and 6-hour periods, and in some cases, 12- and 24-hour periods. WPC forecasters make a determination of the threat that precipitation will exceed any of the 1-hour, 3-hour, or 6-hour FFG values associated with the specific time interval in the valid time period of the forecast. This product is not intended as a specific forecast of flash flooding, but rather as a probabilistic indicator of rainfall amounts exceeding flash flood guidance over an area. |
Issuance and Valid Times
Product | Issuance Time | Valid Time |
94e | 03 UTC | 03 UTC - 00 UTC (21 hours) |
06 UTC | 06 UTC - 12 UTC (30 hours) | |
15 UTC | 15 UTC - 00 UTC (21 hours) | |
18 UTC | 18 UTC - 00 UTC (30 hours) | |
Optional 94e | 00 UTC | 00 UTC - 00 UTC (24 hours) |
12 UTC | 12 UTC - 12 UTC (24 hours) | |
21 UTC | 21 UTC - 00 UTC (27 hours) | |
QPFERD Discussion | Required 03, 06, 15, 18 UTC |
Same as associated graphics listed above |
Optional 00, 12, 21 UTC |