the Excessive Rainfall Forecast
|The Excessive Rainfall graphics provide a forecast of the potential
for flash flooding across the continental United States.
As of October 5, 2004, the graphics display 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 graphics are based on the frequency of events for which
observed rainfall exceeded FFG values for a given risk category.
Three probability categories are defined:
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 Day 1 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.
On June 5, 2007, WPC began producing experimental Day 2 and Day 3 excessive rainfall forecasts. These forecasts were declared operational on February 11, 2008. Aside from the valid periods, there are several differences between these products and the Day 1 forecasts.
Day 2 and Day 3 excessive rainfall forecasts are based on Flash Flood guidance for Day 1 and quantitative precipitation
forecasts for Days 2 and 3. There is no Flash Flood guidance issued for Days 2 and 3. Flash Flood guidance will change
(increase or decrease) in response to future precipitation amounts. Increased variability in model guidance and the
inability of the models to resolve mesoscale features result in greater uncertainty forecasting excessive rainfall in the
day 2 and 3 period. WPC forecasters examine various deterministic and ensemble models looking for synoptic patterns
that favor organized areas of heavy rainfall and compare this to the Day 1 Flash Flood guidance before a threat region
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.
- Only two probability categories are defined - SLGT and MDT.
- No five-inch QPF areas will be indicated.
- There is no written discussion accompanying these products, but if an area is forecast, a text
representation of the threat region is created and can be accessed at the link below the graphic.
- Forecasts are issued only twice per day. See the table below for
information regarding issuance and valid times.
Issuance and Valid Times
||03 UTC - 00 UTC (21 hours)
||06 UTC - 12 UTC (30 hours)
||15 UTC - 00 UTC (21 hours)
||18 UTC - 00 UTC (30 hours)
||00 UTC - 00 UTC (24 hours)
||12 UTC - 12 UTC (24 hours)
||21 UTC - 00 UTC (27 hours)
||12 UTC Day 2 - 12 UTC Day 3
||00 UTC Day 2 - 00 UTC Day 3
||12 UTC Day 3 - 12 UTC Day 4
||00 UTC Day 3 - 00 UTC Day 4
03, 06, 15, 18 UTC
|Same as associated 94e graphics listed above
00, 12, 21 UTC
A Day 1 (94e) issued at 1500 UTC June 5, 2007 will be valid from
1500 UTC June 6 - 0000 UTC June 7, 2007.
A Day 2 (98e) issued at 0630 UTC June 5, 2007 will be valid from
1200 UTC June 6 - 1200 UTC June 7, 2007.
A Day 3 (99e) issued at 2030 UTC June 5, 2007 will be valid from
0000 UTC June 8 - 0000 UTC June 9, 2007.