Over the past several days we have been keeping close tabs on the evolving conditions that could transpire for Wednesday afternoon and evening in North Central Wisconsin. Let's first establish what is headed our way that could lead to a severe weather outbreak. A potent cold front on Tuesday was slowly moving east across the Dakotas, spurring showers and strong to severe storms. In advance of this boundary, very warm and humid air continued to surge north, with dew point temperatures reaching oppressive/almost tropical ranges in the lower to mid 70s. In addition, a notable area of vorticity was following in lock step along the front, and a wave of very strong jet stream winds of 110 kts just behind the front extending across Montana into the western half of the Dakotas. Meanwhile the surface features spelled out an occluded wave of low pressure centered over southern Manitoba, Canada, with a warm front located in the central Dakotas and the cold front trailing not far behind to the northwest. The storms had taken a bit of a break during the late morning and early afternoon Tuesday, but as daytime heating continued into the late afternoon, a batch of storms developed and will work their way east into western Minnesota Tuesday night into Wednesday morning.
So how does this all relate to us downstream in the Wisconsin River Valley? For starters, that very warm and humid air is likely to be in place for Wednesday across the area, as highs soar into the mid to upper 80s, and dew points are anticipated to climb well into the 60s if not a few spots near 70. More so, the aforementioned surface features to the west on Tuesday are expected to be draped across our region from Wednesday afternoon into the evening. Before I get too far, let me make use of a graphic I made up earlier this year explaining surface fronts and where the threat of severe weather can be the greatest.
The main differences from how this is depicted and what the surface fronts will look like on Wednesday is that the low will be much farther north in southern Canada, while the Triple Point (meeting of the warm front, cold front and occluded front) will be located across northwestern Wisconsin during the afternoon hours on Wednesday. This area is expected to continue eastward for Wednesday night. One other minor difference is that the above featured map was typical of a spring scenario in our area. During the summer, that Better Risk of Severe Weather does expand about 50 miles north of the warm front, considering the overrunning effect of the higher dew points, along with the strong energy from the sunshine this time of year. No less, the ingredients are there for a good chunk of our area Wednesday afternoon into the evening to have storms that can be severe, with damaging winds, large hail, heavy downpours of rain and yes a threat for isolated tornadoes. Based on past studies, locales closest to the triple point as it is heading through have the best risk for tornadoes to spin up, primarily because this is where the most twisting is going on in the atmosphere. Winds in this area are converging from the south & southwest just in advance of the Triple Point, swinging around to the west & northwest just behind it. As I mentioned, peak time on Wednesday afternoon into the early evening for this to take shape could be in western and northwestern WI, but it could extend into North Central Wisconsin.
So the as I alluded to, the ingredients are there, but will conditions pan out for a severe weather outbreak? If you recall back to June 7, 2007, a wave of low pressure made a bee line straight through Central Wisconsin, meaning when storms broke out, they had plenty of twisting in the atmosphere to work with as the rumbled by. That morning ended up being mostly cloudy due to storms that had rolled through to our south and west earlier in the morning, but died out just after daybreak. So interestingly, that day the daytime heating feature never really got into full effect. No less, the trigger was there to get the ball rolling on an active afternoon of severe weather. On to Wednesday's set up. Will there be sunshine prior to the arrival of the fronts from the west, which would help to destabilize the atmosphere? It looks like there should be. Will the humid and very warm air set up shop in our region? That seems to be a good bet. However, the key to all of this is, where the storms fire and how will they track once into our viewing area? That will be answered as the storms come to fruition on Wednesday.
There are a bunch of different parameters we look at here in the weather lab that point to instability in the atmosphere. The lifted index value (LI), CAPE value, Bulk Richardson Index, the Theta E, etc. Based on weather computer model output, they are all in the possible to probable range for severe thunderstorms. In addition the convective temp for the storms to break out on Wednesday is in the mid 80s. That's the first step, will the air temperature make it that high to initialize the storms locally? Another feature we look at is the change in the wind direction from the surface up to about 500 MB (up to about 20,000 ft). There is a notable change in direction of roughly 60 degrees from the southwest to west during the early to mid afternoon on Wednesday, before becoming mainly uni-directional (winds blowing from the same direction) toward evening. The spread in wind during the afternoon isn't perfect for the development of tornadoes, but there is at least a possibility. Beyond lets say 7 or 8pm, the storms would transition from featuring rotation potential to that of causing straight line winds, which can be just as damaging.
Overall, we likely will have an active afternoon and evening of storms on our hands Wednesday...it is just a matter of the severity of the storms as they go through our area. Add to that, whether the atmospheric conditions will pan out in relation to what the computer models are depicting or if a few wrenches could get thrown into the works to cause just scattered strong storms and nothing more. We shall see. In the meantime, you can be sure we will have live coverage on 24/7 Weather Wednesday afternoon and evening. If conditions warrant, there could be a few live updates outside of news time on Newschannel 7. We'll we watching the storms and work to help you stay safe!
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