Saturday, February 23rd - First Severe Weather Threat Of The Year Across Portions of Illinois (Update #2)

We continue to watch what most certainly will be a very impressive and strong storm system moving into and through portions of the Midwest this weekend, centered on Saturday and Saturday evening. Not only will this system bring a brief period of very warm temperatures to portions of the state, but the first real threat of severe thunderstorms as well, favoring the southern half of Illinois at this time. 

I dig a bit more into the details below...

Taking a look well back to our west and southwest early this morning, we can see our strong upper level low slowly spinning its way across the southwestern U.S. and the Baja region of Mexico. Over the next 24 to 36 hours, this strong upper level low will emerge out into the southern plains and begin its track off to the northeast, moving through the central plains and eventually the Midwest later Saturday. 

By Saturday afternoon, the very potent upper level low will be located off to our west across the central plains with the very strong associated mid-level jet streak beginning to nose into the Midwest and portions of Illinois by the late afternoon hours. This strong jet stream up around 18,000ft will help set the stage for a setup conducive for severe weather, along with a few other ingredients mentioned below. 

The other ingredient that will be aiding in this severe weather threat will be the rapid advection of sufficient low level moisture into portions of the state during the day on Saturday. As you can see from the loop below, dew points well into the 50's quickly advect northward into the southern half of the state during the morning and afternoon hours. This is caused by both strengthening low level winds on the east side of the developing low pressure system and favorable trajectories off the Gulf of Mexico to advect that moisture northward. Usually in the summer, these types of dew points aren't enough for severe weather but given its early in the season and along with the very strong wind shear, it will be sufficient to get strong to severe thunderstorms. The loop below runs from this evening through the late evening hours on Saturday. 

The initial push of warm air and moisture advection on Saturday morning could lead to the first round of thunderstorms, favoring the southern portion of the state as shown below. 

The more serious time frame for severe thunderstorms will come during the mid-afternoon to evening hours across the southern half of the state. The combination of very strong winds aloft, sufficient moisture, and developing instability from both surface heating and colder temperatures aloft will set the stage for thunderstorm development by this time. The main threat at this time looks to be strong damaging straight line winds with any stronger thunderstorms but, given the sufficient low level major, most likely lower cloud bases and strong low level wind shear, we cannot rule out a few tornadoes at this time. The thing holding back a more serious tornado threat is the low amounts of instability, which is to be expected for this time of year. Even so, expect the strong wind shear and moisture to compensate somewhat, owing to a damaging wind threat. Also, with several rounds of thunderstorms possible over a given area downstate, the threat of flooding will also be there as well. 

Below is our latest TAM/severe weather thunderstorm threat outlook and as we've said several times, we are favoring downstate for the most serious threat of severe thunderstorms. 

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