If you were using the NWS’s estimate of Kansas rainfall Friday morning, after two lines of strong thunderstorms moved through, you would be tempted to issue strongly-worded flash flood warnings as the estimate was that as much as ten inches of rain had fallen over a very large area about 40 mi. WSW of Wichita. Streams would have been rising rapidly and roads closed.
Only one problem: The actual rainfall was 2 to 3.3″, substantial yes, but not enough to cause flooding given the recent very dry conditions.
The erroneous rainfall estimates were caused by “hail contamination” — where the radar senses the very high reflectivity from wet hail and mistakenly computes and totals it as rain resulting in a wild overestimate.
On this blog and elsewhere the past year, weather fans and others have heard a lot about dual-polarization (DP) radar. DP, when perfected, will solve this problem and many others.
As you know, the NWS is installing dual-polarization radar across the U.S. An easy to understand explanation of dual polarization radar is below, featuring the lovely and talented Cat Taylor.
When perfected, D-P will allow better warnings of tornadoes, flash floods, and heavy snow. Below is an example from 9:50pm Thursday evening showing large hail (red) inside a northern Oklahoma severe thunderstorm warning polygon as it moved northeast. The storm produced hail at least 1.5″ in diameter. DP nailed it! The hail is clearly differentiated from the heavy rain (dark green).
You can see that the actual area of hail in red (this is the same storm that moved into Kansas and caused the hail contamination shown in the uppermost image) versus the areas of rain. Instead of estimating what is rain and what is hail, this radar measures them.
This is still a powerful tool that we don’t yet fully understand. The same was true of Doppler radar in the ’90′s. It took us time and trial and error to sort out everything it can do. Now, Doppler allows is to issue much more accurate and timely warnings of tornadoes and damaging winds.
The next few years of working with DP promise to be very fruitful and represent yet another step forward in the improvement of warnings of extreme weather.