Timing and strength differences with a cold front will determine where severe storms form Friday

Timing is everything when dealing with the potential for strong storms - especially when it occurs late in the season.  It's only Wednesday, but as early as Monday morning we talked about the potential for severe storms Friday and Friday evening across the Midwest.  However, there are some timing issues we need to discuss before we get too excited.

First, the storm system in question is still off the coast of British Columbia.  Unfortunately, with a storm system over water instead of land there is very little information of the storm being ingested into our numerical weather computers until the system moves on land.  Once on land it is then sampled by numerous surface and upper air observations.  It's those observations that are needed to really get an idea of the strength and timing of developing storm systems.  The second issue has been the timing of the surface cold front and upper level low.  A handful of models want to bring the cold front through late Friday night into early Saturday with the upper level low passing through on Saturday.  Other weather models, however, have significantly delayed the passage of the upper level low until Sunday meaning a later timing of the cold front and possibly a warmer Saturday afternoon. 

Let's talk a little about those timing issues.  The American run GFS and NAM models develop a surface low pressure system in the central Plains late Thursday and lift it northeast into the upper Midwest by Friday afternoon and evening and through Wisconsin by Saturday afternoon.  With this scenario severe storms would be likely just to the west of the Stateline from central Wisconsin through Minnesota, Iowa and Missouri (and possibly into northwest Illinois) ahead of the cold front.  Storms out west would then likely form into a line as they moved east towards the Stateline Friday night before moving into Indiana by Saturday afternoon.  This would keep the chance for showers and a few thunderstorms along with cloud cover around here for Saturday with clearing taking place Sunday.

The European and Canadian models, however, are a lot slower with the passage of the system and don't have the cold front through until Saturday evening.  This would mean temperatures on Saturday would be warmer and the potential for storms on Saturday, rather than Friday, would increase for us. Sunday would be cloudy and rainy and the moderation in temperatures next week would be delayed.  With such a large system expected to move through the middle of the country it's natural for it to slow down a little but I don't think it will be as slow as the European and Canadian models are indicating.  Over the next day and a half the differences between these models should become smaller and smaller.  In the meantime, expect the end of the week to be a little stormy (although not a complete washout) and the end of the weekend to be much cooler than what we've felt all this week. -CK