Spring has certainly arrived in the midwest, and we'll enter the second week of April will a chance at our first organized severe weather of the season.
Monday has started the week under fairly benign cloudy skies - but we get unsettled from here on out in Champaign-Urbana, central Illinois, and the entire midwest broadly speaking. I'll address the threats chronologically as we head through the week.
Monday night will see our first potentially unsettled night as we see a chance for thunderstorms develop during the overnight hours. The severe weather threat is fairly low with any storms on Monday night, and any severe threat should be limited to marginally severe hail. It looks like the best chance for thunderstorms in CU will be as approach Midnight, into the early morning hours.
Tuesday, we see our first legitimate severe weather threat, albeit an isolated threat. A surface boundary will stall out across central Illinois during the day and serve as a focal point for thunderstorm development during the afternoon and evening as an upper level wave moves over the area. As of right now, I believe storm coverage should be relatively isolated in our area on Tuesday. We'll have a cap (warm layer of air above the surface that acts to supress thunderstorm development) overhead that should keep a lid on things for much of the day. That said, with the boundary in our area, given the right chain of events during the day we could see scattered storms develop locally, and any storms will have the potential to reach severe levels.
Wednesday things again look to increase in both intensity and coverage. The surface boundary/warm front will again be draped across our backyard with afternoon/evening thunderstorms possible. Instability will be quite strong by mid-afternoon on Wednesday, and with a much weaker cap storms could be considerably more widespread than on Tuesday. The biggest threats on Wednesday will be damaging wind and large hail. While instabilty will be quite intense providing the fuel for thunderstorms to reach severe levels in our area - the turning of winds with height, or wind shear that is necessary to cause storm rotation and the potential for tornadoes is quite low on Wednesday. Given the presence of the warm front in our area which can at time compensate for lower wind shear, the potential for an isolated tornado is worth mentioning. But again, while I think we could see the potential for widespread thunderstorms on Wednesday, the tornado threat will be fairly low, though it certainly won't be non-existant.
Thursday. Thursday is the potential big one. There is still some disagreement between computer forecast models *and* we are still 72 hours out which lends plenty of time for change so I'll hold off on sounding all of the alarms now. That said, the signals are there for a potential significant severe weather outbreak across Illinois, Iowa, and Missouri on Thursday, with the threat for widespread damaging winds, large hail, and perhaps tornadoes. I'll save the details on this one for upcoming posts (and if the threat remains this high there will be plenty of updates), but the storm system is drawing some early comparisons to past events that did produce tornadoes in central Illinois.
During the day on Thursday, the boundary that will have spent the previous several days stalled out across central Illinois will quickly lift north as a warm front leave central Illinois under a warm and humid airmass that will rapidly become unstable during the afternoon. As an upper level wave digs into the region a surface low pressure center will intensify over Iowa and move into northern Illinois. A cold front will swing across the Mississippi River during the afternoon with severe thunderstorms likely developing ahead of it - perhaps in the form of widespread damaging winds and large hail. The big wild card in this one is again going to be wind shear. There are some signals that adequate wind shear will be present in the lower levels of the atmosphere for rotating supercell thunderstorms that will carry with them an attendant tornado threat - but forecast model agreement is not 100%. While most forecast models do suggest widespread severe weather, there are still some suggestions that the wind shear required for a more robust tornado threat may not materialize.