For Amateur and Professional Meteorologists - Tornado Forecasting Tip

There was a lot of controversy in the Midwest yesterday morning as to whether the forecast tornadoes in the Midwest would develop because of heavy morning rains in the region.

Tornadoes require "instability," defined as air rising -- perhaps violently ( > 100 mph) in extreme cases -- if it is warm and humid near the ground and cold aloft. Conventional wisdom is that extensive morning rain diminishes tornado potential. 

But that's often not true. Take look at the radar of the same region eight hours later. 

While the morning rain may keep the air less unstable, I've documented that some of the very biggest tornadoes and tornado outbreaks are preceded by after midnight on the day of the event. My speculation is the evaporating rain lowers the "lifting condensation level" (LCL). Lower LCL's = tornadoes more likely, other factors equal. 

Here are just some of the events I have examined:

• April 27, 2011 (see radar image nearby from 4:49am). 
• April 3, 1974 (not only was there morning rain in the region, there was a tornado SE of IND)
• April 11, 1965, (Palm Sunday Outbreak)
• April 26, 1991 (Wichita-Andover F-5 and several other ≥ F-4 tornadoes)
• June 8, 1974 (Central and Northeast Oklahoma, OKC and TUL both hit)
• April 14, 2012 (regional outbreak in the central Great Plains)
• April 9, 1947 (Woodward Tornado, it was rainy along the 150+ mile path most of the day)
• June 8, 1968 (Topeka Tornado, rained most of the day)

And, of course, yesterday. 

So, if you are a forecaster, do not assume that rain in the morning on a dynamically-driven tornado day will lessen the chances. 

One other item: Significant tornadoes are uncommon in regions of rising pressure. For example, at 6pm CDT (when the diurnal pressure curve favors rising pressure), the pressure was falling rapidly in the Midwest (below).

I will provide more "non-model" forecasting methods from time to time going forward.