First let’s concentrate on what is going on right now and into tonight. Below is the current Day 1 Outlook from the SPC showing a large area highlighted under a slight risk of severe thunderstorms across the upper Midwest and into parts of western New York. The main threat will be damaging winds from these storms but some large hail and a couple of isolated tornadoes cannot be ruled out.

Here is the Day 1 SPC Outlook showing the wind damage probabilities for this evening and tonight. A 30% chance of severe wind within 25 miles of a point is expected to occur from areas in southeastern Wisconsin east toward central Michigan. In fact, most of this area is also under a 10% or greater probability of seeing significant wind gusts in excess of 65 knots.
The threat for damaging winds will mainly occur due to straight-line winds associated with lines of thunderstorms. This is the scenario that the current 18Z NAM model run depicts as well. This line or lines of storms should fire up this evening and continue into the overnight hours. Below is the 18Z NAM run showing what the reflectivity may look like this evening around sunset.
A line of storms may be most intense right across and around that 30% area the SPC highlighted for wind damage this evening and tonight. Taking a look at the modeled CAPE and EHI values, we also see that the storms will have plenty of instability to work with.
Storms are expected to continue through the overnight hours and into tomorrow morning, weakening some in the early morning hours. However, a line or lines of storms will refire up tomorrow most likely around midday and continue to track off to the east. Here is a look at what the 18Z NAM model is thinking right now as far as what the reflectivity may look like. The top image shows the reflectivity around midday into the early afternoon whereas the bottom image shows how it may appear in the late afternoon.
Once again, these storms will have plenty of instability with CAPE values approaching and exceeding 3000 J/kg according to the 18Z NAM model.
EHI values will also be good in some areas meaning that if any supercells can form out in front of the main line of storms, there could be a chance for a couple tornadoes.
For these reasons as well as the great upper air support that these storms will be dealing with, the SPC has issued a moderate risk for some areas in their Day 2 Outlook. These areas are the areas highlighted in purple, indicating a 45% chance of severe weather within 25 miles of a point, as well as a 10% or greater probability of significant severe weather within 25 miles of a point.
A large area is looking at a slight chance of severe weather tomorrow but the areas at the greatest risk will be from southern Illinois into Indiana, Ohio, Pennsylvania, New Jersey, New York, southern Vermont and New Hampshire, Massachusetts, Rhode Island, and Connecticut. Residents of these areas should prepare for the potential of straight-line wind damage, as this will be the main threat tomorrow and tomorrow night. If the line of storms becomes strong enough, and this is the current thinking, we could be looking at a long lived derecho which could produce damage similar to the one that affected areas of the Midwest and Mid-Atlantic on June 29th of this year.
The threat could be very extreme in some areas tomorrow so stay alert to the latest weather conditions and remember to seek shelter in advance of these storms.