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of August 13-15, 2005 Answer to Question 12 |
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12. Examine the one-hour radar precipitation accumulations from KTLX ending at about 3:00 a.m. on the 15th.
- How much rainfall occurred in Stephens County between about 9:00 p.m. and 3:00 a.m.? How about Carter County?
- Do these accumulations exceed the 6-hour FFG product?
- How much rainfall occurred in Stephens County between midnight and 3:00 a.m.? How about Carter County?
- Do these accumulations exceed the 3-hour FFG product?
- Look at the one-hour precipitation accumulations for northwest Carter County. Which of the one-hour accumulations exceed the one-hour FFG guidance?
Answer.
- Between 9:00 p.m. and 3:00 a.m, Stephens county received at least 1.6 to 3.5 inches (0 + 0 + 1.0 + 0.3 + 0.3) to (0 + 0 + 1.0 + 1.5 + 1.0). The accumulation between 11:00 p.m. and midnight was missing, so more rainfall could have accumulated. For NW Carter County, 4.6 inches fell (2.0 + 0.3 + 0.3 + 1.0 + 1.0). For SW Carter County, 2.6 inches fell.
- The 6 hour FFG for Stephens county was 2.1 to 2.5 inches, so portions of Stephens county exceeded the FFG value. The 6 hour FFG for Carter county was 2.6 to 3.0 inches. Both NW and SW Carter county exceeded the 6-hour FFG values. (SW Carter marginally exceeded the FFG.)
- Between midnight and 3:00 a.m., Stephens county received 1.6 to 3.5 inches. In other words, all of the rainfall in the 6-hour period we just analyzed fell in the last three hours. SW Carter county received 0.3", and NW Carter county received 3 inches.
- The 3-hour FFG value for Stephens county was 1.6 to 2.0 inches. The 3-hour FFG value for Carter County was 2.1 to 2.5 inches. For this 3-hour period, Stephens county exceeded the 3-hour FFG, and NW Carter county exceeded the 3-hour FFG, but not SW Carter county.
- The 1-hour FFG value for Carter county was 1.1 to 1.5 inches. Only the 9:00 to 10:00 p.m. accumulation (2 inches) exceeded the 1-hour FFG. However, the FFG maps used in this exercise were issued at 1:00 a.m. The 1-hour FFG value that actually in effect for Carter county at 10:00 p.m. was 1.6 to 2.0 inches. Thus, the 1-hour FFG values were not actually exceeded by these radar rainfall estimates. However, the Norman NWS office had previously issued a statement that said that the radar was actually underestimating the amount of rainfall.
The Bottom Line.
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A combination of radar and Mesonet data are useful in monitoring accumulations of rainfall in flooding situations. It is necessary to look at accumulations over 3-hour and 6-hour periods as well as the standard 1-hour product. Subject to its tendency to sometimes underestimate the amounts, the radar is good at estimating precipitation between raingage locations.
