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of August 13-15, 2005 Answer to Question 1 |
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1. 10:00 pm CDT on August 13th. A second straight day of scattered marginal severe weather has just ended. Some impressive rainfall totals (see map below) caused minor flas h-flooding in parts of western Oklahoma. Examine the Flash Flood Guidance from the Arkansas-Red River Basin Forecast Center.
- About how much rainfall in one hour would it take to cause flooding along I-35 south of OKC? In three hours? In six hours?
- What about Beckham and Greer Counties in far western Oklahoma?
- Why do you think these values are different?
Answer.
- The RFC's flash-flood guidance (FFG) is a "rule of thumb" that predicts how much rainfall is needed during certain periods to produce flash-flooding.
- The product indicates that counties in south-central Oklahoma will require 2.5"-3.5" of rainfall in one hour to produce flash flooding. The values increase to 3"-4" for three-hour totals and 4"-5" for six-hour totals.
- One-hour FFG for Beckham and Greer Counties is less. (1.6"-2.0" for one hour; 2.1"-2.5" for three hour; 3.1"-3.5" for six hour).
- The FFG is less for the western counties because they have received heavy rains in recent hours. Because of the rains, their soils are more saturated, and unable to accommodate as much new rainfall.
The Bottom Line.
FFG is useful for estimating rainfall needed to produce flooding. It has some limitations, such as:
