Timing out Thursday's severe risk: Breakdown of what you need to know

 The risk for widespread severe thunderstorms has increased across northern Illinois and southern Wisconsin Thursday afternoon and evening, but it's not a guarantee as there are a few things that could limit the severe risk locally. So, let's look at how things will develop through the night Wednesday, into Thursday morning:

REST OF EVENING/OVERNIGHT:

Skies will remain mostly cloudy through the night as a chilly east/northeast breeze keeps temperatures in the 30s. A warm front/stationary boundary currently sits in central and southern Illinois where showers have been ongoing for much of the afternoon. As the front begins to pull back north a surge of moisture will arrive with it. This will lead to an increase in rain showers beginning around 8pm, becoming more widespread through Midnight. Severe weather is not expected but periods of heavy rain will be likely with a few embedded thunderstorms. The steady and heavy rain will continue through the night and into Thursday morning. Rainfall totals by the morning are likely to range between half an inch, up to an inch. Temperatures will remain in the 30s before climbing near 40 degrees by daybreak.

THURSDAY MORNING: 

Ongoing showers and thunderstorms can be expected during the morning Thursday, likely lasting through at least mid-morning. The warm front will still be to our south, which will serve as the focus for the showers during the morning. It is possible that some of the precipitation could last a little longer through the morning and early afternoon. If that occurs it could help keep the warm front to the south a little longer, delaying the return of higher instability until late in the day. Temperatures through the morning will remain in the mid and upper 40s, to low 50s around I-88 by late morning.

THURSDAY AFTERNOON: 

Low pressure near Kansas City will move into Iowa during the afternoon, pulling the warm front north as it does. If the rain from the morning ends around mid-morning, we could see a second wave of showers and thunderstorms develop around Noon, lasting through 3pm or 4pm. The severe threat with this round would be low with the strongest producing heavy rainfall and strong winds.

It looks like our main window for severe weather would occur between 3pm/4pm to 8pm as the main cold front comes through from west to east. This would be when all hazards of severe weather would be possible, including the threat for tornadoes. Instability is expected to increase behind the warm front, spreading north and northeast into Illinois by mid to late afternoon. At the same time, winds within the mid and upper levels of the atmosphere will also be increasing.

As the cold front moves east from Iowa thunderstorms are expected to develop ahead of it before shifting into northwest Illinois. By the afternoon the warm front should be in southern Wisconsin with temperatures locally warming into the mid-60s. Dew points are likely to surge as well, nearing 60 degrees as far north as the state line. This would be enough to help fuel the storms as they shift east into Illinois.

Storms will be more discrete with supercells at the onset which would pose a risk for tornadoes initially, especially over eastern Iowa and areas west of I-39 in Illinois. This risk appears highest for areas that are near the warm front and low-pressure system. As the storms move east, they will form into linear clusters with more of a wind threat through the evening east of I-39.

But as I mentioned above the severe risk (including tornadoes) is not a guarantee. If any of the morning or early afternoon rain and thunder activity last longer into the afternoon, it would decrease the amount of time our atmosphere would have to recover, keeping the highest instability focused further southwest and west of the Stateline. It's also possible that the storms that form out west in Iowa fall apart, losing their strength by the time they reach the Stateline.

Unfortunately, some of these failure points may not be realized until tomorrow, and perhaps even until early in the afternoon. However, if everything were to come together for these storms to develop then there is the risk for tornadoes to occur in northern Illinois. And that is something we will need to keep a close eye on as we go throughout Thursday morning and afternoon.