April vs. May: Why such a difference in severe weather

The month of May, on average, is the most active month in terms of tornadoes, followed then by June. So far, there have only been 45 reported (remember this number is preliminary) tornadoes and we are over half way through the month. On average we should see numbers nearing three hundred (this is the three year average). Now I'm sure no one is complaining about this after such an active and devastating April. But it has us wondering, what's the difference? Why such a lack in severe weather, so far, this month?

Well, we've just been backed up. The 11th of May held the greatest number of tornadoes where 16 were reported. Before and after that very few severe weather episodes occurred. The month of April was very unusual for many reasons. A constant cool pocket of air resided over much of the northern tier of the U.S. while heat and humidity were found in the south. Widespread and devastating tornado outbreaks tend to occur when you have a very strong jet stream, coupled with cool/dry air, move over top very warm and humid air. This is exactly what played out last month. For the most part, the jet stream remained positioned over the southern states and just didn't move. This is very typical of what you would see during La Nina.

The month of May has been quite different. The jet stream hasn't been as strong and the heat and humidity has remained confined further south. In fact, this past week and a half the main jet stream has been shifted further to the north and two cut-off lows, one in the east and one in the west, have remained blocked or stationary. The low in the east has pulled down cool and dry Canadian air keeping a lot of the moisture from the gulf from moving northward. Now because the winds within the jet stream haven't been as strong and the moisture has been in short supply we just haven't seen much severe weather this month. Which, believe me, I'm not complaining about!

According to the National Weather Service in Wichita, KS , only eight tornadoes have been reported across the state of Kansas so far; two in February, six in April and zero in May! Climatologically, May and June are the most active period tornado wise for the central and southern Plains. The last time this happened where no tornadoes were reported for this long of a stretch was 1984!

So what does the rest of the month look like? Well, the low in the east will finally begin to move into the Atlantic by this weekend as the blocking high across the North Atlantic breaks down. This will allow the low in the west to slowly move east, pulling a little more moisture with it as it does. Thunderstorm chances will be on the increase through the end of this week across the Plains while our shot for storms will come this weekend.