May is considered the "peak" month for severe weather in the nation, and on average, sees the most tornadoes annually. Overall, here in Western Wisconsin, we've been quite fortunate this severe weather season with no major storm outbreaks thus far. This last week of the month however, could bring one, or possibly more severe weather threats to the region. The makings of a severe weather event are on the table for Thursday afternoon (May 24), and if things come together as needed, some of us could be dealing with damaging winds and isolated tornadoes, along with heavy rainfall. The rain will be welcome, as we've been mostly dry since the first week of the month.
The severe threat will come with the arrival of a cold front during Thursday afternoon. A wave of low pressure will be developing to the south, riding northward along the front, and tracking just to our west. The image below shows the location of the low pressure and associated warm and cold fronts, as progged or forecast, by the Global Forecast System model at 7pm CDT Thursday.
The front will generate the lift needed to develop the storms, while the strong jet stream winds pushing through will help sustain them. Below is a look at the upper level, 250mb winds, or jet stream level (roughly 35k feet) for the same time frame. Notice how the stream pushes towards Wisconsin, then takes a jog to the northeast. This is a short wave, or piece of additional energy within the upper trough carved out by the jet. Winds at this level are progged to be 70-90kts at this time, which translates to plenty of wind energy at this level to enhance storm updrafts. The short wave takes on what we call a negative tilt, shown by the red line. This feature will aid development while adding more lift to the atmosphere as storms develop and move through.
One important ingredient for severe weather that has been lacking much of this month here in the Upper Midwest is abundant low level moisture. Dew points have been very low for the time of year, with several high pressure systems sliding through with Canadian origins. This has kept much of the moist air well to the south. The strong southerly flow out ahead of this front does reach fairly far to the south, and over the last day or so has managed to transport higher levels of moisture northward. Below is a map of surface observations from Wednesday night. I have outlined in green the area where dew point temperatures are well into the 50's and lower 60's. It is a fairly narrow corridor, but as it shifts eastward with the front, should be enough to help destabilize the atmosphere and help any storms that form, turn severe.
One question that will remain until we get within a few hours of this event will be how much instability will be present to fire these storms as the front nears. This will be determined by how much sunshine we see prior to the storms arrival, but at this point I expect any sun to be fairly limited. With the other ingredients in play however, I believe even lower end instability will be offset by the favorable jet stream winds. With surface winds blowing from the south and mid and upper level winds more from the southwest, some directional shear will help storms rotate, should supercells form. As of this writing the Storm Prediction Center has Western Wisconsin under a "slight risk" of severe weather, with a 30% probability of severe weather within 25 miles of a given point. It's also of note they have the same area hatched for a 10% probability of "significant" severe weather within the same area.
Despite the set up favoring severe storms, you just really never know until the event begins to play out. The best advice is to stay informed and be ready to take action should severe weather threaten.