WINTER, SPRING, AND TORNADO SEASON...
The weather has improved around the Midwest the past few days but the extreme conditions of the previous week have increased the Winter Severity Index produced by the Midwest Climate Center. In places like Dubuque, the Quad Cities, Des Moines, and Omaha the cumulative effects of the winter are now considered extreme.
Below are the current ratings for some cities in my area. You'll notice in all of these summaries things were going along fine until the first of the year and then we spiked, especially in the period January 10th through February 20th.
The Quad Cities
Cedar Rapids is technically considered severe but sets on the border of extreme.
From a meteorological perspective, spring starts March 1st and lasts through May. The reason we do this is for book keeping purposes. It's far easier to compose statistics for a 3 month period than to mess with the official start of winter and spring which varies from year to year and encompasses portions of 5 months.
My purpose for bringing this up is Monday is March 1st and the start of meteorological spring. The spring outlook has been issued by the fine folks at NOAA. For your viewing pleasure here's what the three month temperature and precipitation outlooks portray. About what you would expect in a moderate La Nina.
For me, one of the more compelling aspects of spring is the advent of the severe weather season. There is always lots of conjecture regarding tornado season and how bad it may (or may not) be. To be honest, our skill at forecasting the intensity of a severe weather season is quite limited. It only takes one major outbreak or a single deadly event such as Joplin in 2011 (160 deaths), to skew the numbers. It's even harder to predict a geographic region such as the central Midwest.
However, one factor which has been known to play a role in upping the risk of severe weather is the La Nina. It tends to increase the strength of the upper level jet. It also has a tendency to create a zonal (west to east flow) creating a pronounced thermal gradient for storms to form along. When they do the shear necessary for supercells is enhanced. Supercells are rotating thunderstorms and the ones most likely to produce significant severe weather including tornadoes. We will have a La Nina to contend with this year and studies have shown that nationally the severe risk is higher with La Nina than its counterpart El Nino. See the difference below.
Again, I really can't say with any assurance how things will play out but my belief is the threat is higher than average in my area for severe weather events. Additionally, its been some time since we have had an outbreak (15-20 tornadoes in a given day). That tells me the law of averages will eventually come into play and our day will come. When? That's anybody's guess.
One thing's for sure, the season is set to kick in over the south where March and April are big months. Typically, my area doesn't get much activity until April but I've seen some pretty good tornadoes as early as mid-March and I've seen the season hold off as late as the tail end of May. Lot's of factors will go into how soon things pop in the Midwest but if the above normal temperatures come early so too will the threat. Here's some severe weather probabilities based on time of year from the Storm Prediction Center. Notice the surge north from the end of March to mid-May as warmth and humidity builds into the Midwest.
One thing that's apparent in the short term is that the pattern is not conducive to storms or significant precipitation over the central Midwest. A minor disturbance Friday night may bring a light mix of rain or snow showers with minimal amounts. The next one Saturday travels to the northwest and its significance will be the warming it brings for Saturday with highs in the 40s. This is what the EURO ad GFS are showing for total precipitation into next Wednesday.
More important, neither model indicates any snow the next 6 days and well beyond. Here are the snowfall forecasts from the EURO and GFS into Wednesday of next week.