This Thread should say Season is DEAD for the southern plains. It looks like this system coming in late next week is a yawner for the southern plains and we know that typicaly starting June 1st everything goes way north. The last time i remember a season like this was I think was 2009 when the crazy death ridge took over in May and nothing happened. I would like for others to weigh in on why this year for the Plains has been so quiet and before someone says the stupid drought caused it how do you explain last years outbreaks? Just wondering...
Jacob Ferden brought up a point a few pages back that I overlooked when discussing my thoughts on the lack of good chase opportunities this season (I suggest going back a few pages and reading those posts) - repeated frontal intrusions.
Certainly it's a combination of factors including a building drought and repeated frontal intrusions (lots of cold fronts) sweeping the Gulf of moisture and leaving tornado alley high and dry for much longer than usual. Drought obviously is an indicator of the amount of rain the region has been getting, and you need rain to get severe weather. Also, drought causes dry soil which leads to higher sensible heat flux (and lower latent heat flux), which begets hotter, drier days. This can both amplify the cap and increase LFCs and decrease PBL RH due to the lack of low-level moisture.
The repeated frontal intrusions are reminders of the extended and very cold winter the central and eastern CONUS experienced. Apparently this cold pattern is taking its sweet time letting go, too. Throughout the winter there was a persistent eastern US trough and western US ridge which drove cold air down from Canada a lot. Since this continued into the early spring, it kept temperatures much colder than average and that delayed green-up significantly.
This map of departure from average relative greenness shows how most of tornado alley remains starved of vegetation. The drought also delayed green-up and has resulted in reduced vegetation cover than usual for this time of year. Vegetation plays a big role in getting more moisture into the atmosphere through evapotranspiration. With less vegetation there is less evapotranspiration.
Finally, I think the large scale height pattern has not been the most conducive for severe weather activity so far. While troughs, some strong, have made their way across the SGP, many were plagued with issues like timing (the wave not ejecting until late evening or ejecting early in the morning before instability could build). Also, this week in particular serves as a good example: when you have a high-amplitude pattern that settles a trough axis over the Mississippi River valley, wave dynamics says not to expect rapid changes. We've had two or three of these patterns set up this spring so far.
The drought was similar to last year, although last year the drought was either holding steady or starting to wane compared to this year where it is building quickly. Also, I wouldn't really say last year had a bunch of outbreaks. The vast majority of last year's spring season was concentrated in a two-week period that featured many significant, but generally single-tornado or single-tornadic-supercell days. These were the result of very rich moisture present with a few troughs that maintained strong flow at their bases (which is somewhat unusual given you should usually find sub-geostrophic flow at the base of troughs) and that didn't blow through the central US and up into Canada like many of this year's troughs have been doing. Also, while last year featured a cold spring, the winter wasn't really cold, and the cold air stopped intruding by mid-May when things got going. I think that's why this year has been so lackluster even compared to a similar year like last year.
