This upcoming week is likely going to bring one of the biggest severe weather/tornado threats that has been seen in quite a while. Over the past two years, the tornado season has been unusually quiet, and this year hasn’t been any different so far. I mentioned on the site and on social media several weeks ago that I believed that a window would open up later in April going into May that could prove to be active and quite dangerous. We are quickly approaching that window, and this week will be the beginning of a several week period of severe weather across parts of the United States.
Before I get into the details on this week, I want to point out that some of the most memorable tornado seasons are years that were not-so-active. One recent example of that would be the 2013 EF-5 Moore tornado, which occurred during an anonymously quiet season. Regardless of the overall numbers when everything is all said and done, I can assure you that the people who are impacted by a tornado care nothing about averages, because to them, it was a bad season. With that said, the period we’re entering could bring an average to above average number of tornadoes, so I do believe we’re about to come out of this “tornado drought.”
Persistent mid and upper-level ridging over the western U.S. and Alaska (due to warmer waters in the Gulf of Alaska and along the western U.S. coast) has kept the overall pattern unfavorable for severe weather over the last couple of years. However this week, there’s going to be a change in the overall pattern, with troughing developing over the West Coast and broad ridging farther east. In fact, medium and long-range model guidance continues to hint at this being a trend the rest of this season. I believe this will be the case at least through parts of April going into May, and we’ll just have to wait and see what happens beyond that. All of this means that a more active severe weather period is probably on the way.
I want to briefly break down this week’s severe weather/tornado threat. Keep in mind that there could be an outbreak of tornadoes on Wednesday and Thursday. I use the term ‘outbreak’ loosely because different people interpret that differently than others. In this article, I’m going to use SPC’s maps, but I’ll be making my own maps and will try to have them posted by sometime Tuesday.
The atmosphere is going to be very unstable across the Southern Plains on Tuesday, but it seems that a strong cap is going to be in place. A cap is a warm-layer of air on top of a cooler-layer of air at the surface. It’s almost like putting a lid on the atmosphere, which hinders storms from developing. If those air particles can’t rise, then they can’t cool (adiabatically), condense, and develop into towering clouds and produce precipitation. However, there are times when this cap can be eroded away with a certain mechanism, such as heating the surface, adding moisture to the atmosphere, or a favorable pattern forcing the air upward vertically or moving that above-the-surface warm-layer out of the area.
This is the exact situation that we have on Tuesday. IF the cap can be taken care of, then supercells capable of producing severe weather could occur and maybe even produce a few tornadoes. SPC has only placed a marginal risk for severe weather across parts of the Southern Plains, not because they don’t think these storms will be severe but because it’s uncertain whether or not they’ll actually develop due to the cap. Hopefully that makes sense, and let’s hope that the cap holds strong.
SPC’s Tuesday Outlook:
Wednesday and Thursday:
All of the ingredients are going to be in place to support the development of supercells across portions of the Southern Plains on Wednesday, and that threat will shift farther east on Thursday, which will include parts of the Southern Plains and extending northeastward into the central and upper Mississippi Valley and approaching parts of the Great Lakes.
Moisture is going to get pumped northward from the Gulf of Mexico, and the heating of the day will make the environment very unstable. Wind shear (the changing of wind direction and speed with height) is going to be favorable for storms to develop, maintain their strength, and rotate. The mid and upper levels will be supportive enough to lift the air up through the atmosphere, causing air particles to cool, condense, and form towering clouds and precipitation. All modes of severe weather including tornadoes will be possible. The low-level winds will also be supportive (another necessary component) for the development of possible tornadoes.
Even though the main focus seems to be on Thursday, which I agree could be a big day, Wednesday needs to be watched just a closely. Storms that develop on Wednesday could carry over into the next day, and that will need to clear out in order for the environment to be as conducive for discrete supercells. All of the necessary ingredients will be available both days, but it only takes one small hindrance to cause a bust. At this time, I don’t expect this to be a bust for either day, so residents that are in the zone outlined on SPC’s maps below need to adequately prepare for this potential outbreak of severe weather.
Again, I’ll post my own maps on Tuesday and will also try to post another article. If I see that the severe weather threat is going to extend farther east beyond Thursday, I’ll detail that later in the week.
SPC’s Wednesday Outlook:
SPC’s Thursday Outlook: