UPDATE
Issued at 916 PM CDT Thu Sep 19 2024

No significant changes made to the overnight forecast this evening, with only modifications being to slow arrival of rain chances a bit across our far west/northwest counties until closer to and after midnight.

Latest surface analysis depicts a cold front stretching from the arrowhead of Minnesota, southwest to a weak surface wave the the IA/NE/MO border area south of Omaha. Scattered showers and thunderstorms have been occurring ahead of the front from central IA into the northern Lakes, within a narrow axis of low- level moisture (upper 60s/near 70 surface dew points). The greatest coverage and intensity of storms was across northeast IA into western WI, on the nose of a 30 kt low level jet and in association with a mid-level short wave lifting northeast across the upper Mississippi Valley. Scattered showers and thunderstorms are expected to move slowly east into northern IL later tonight as the front slowly pushes east, though with better forcing lifting to our north, gradual weakening and veering of the LLJ and unfavorable diurnal timing should result in decreasing intensity and lightning potential from west to east across the forecast area.

Other than trimming pops across our northwest cwa in the mid- late evening period, going forecast appears on track trends through early Friday and no other significant changes appear warranted.

Ratzer

DISCUSSION
Issued at 308 PM CDT Thu Sep 19 2024

Through Friday Night:

The main forecast concern for the next 24 to 36 hours will be the potential for isolated to widely scattered showers and thunderstorms late tonight through Friday afternoon. The driving force for the showers and storms is the broad trough across the northern Plains that will gradually drift into the upper Midwest and Great Lakes overnight into the day on Friday. At the same time, showers and thunderstorms are expected to develop along a cold front (currently stretching from central MN to eastern IA and southern NE) and move into northern IL and eventually northwest IN after midnight tonight. However, the current airmass over northern IL and northwest IN remains rather dry (dew points currently in the upper 40s to lower 50s) which should weaken the showers and storms as they approach the I-39 corridor. Thus the coverage of showers and storms overnight will likely be in a diminishing state meaning some areas may miss out on needed rainfall depending on how far into northern IL showers and storms can survive.

Regardless of storm coverage tonight, the cold front is expected to still be over northeastern IL and northwest IN on Friday which may allow for additional showers and storms to develop Friday afternoon. The uncertainty, however; is how much the atmosphere will be able to destabilize ahead of the front given the limited moisture and potential for scattered cloud cover. If the atmosphere is able to destabilize prior to the front exiting, then another period of scattered showers and isolated thunderstorms could materialize mainly for areas along and east of I-55. Additionally, the aforementioned trough will generate a modest upper jet across the area which is forecast to generate around 30 kts of effective shear Friday afternoon.
This shear could support a more robust shower or storm core which in turn may result in a threat, albeit low <10% chance, for localized gusty winds and maybe some small hail. Therefore, SPC has maintained their level 1 of 5 severe risk for areas east of I-55 Friday afternoon for this potential but I suspect most storms will remain sub-severe.

Any showers and storms that develop Friday afternoon will move out of the area by Friday evening resulting in dry conditions for Friday night. Otherwise, expect temperatures to remain unseasonably warm with highs in the mid-80s and overnight lows in the upper 50s to lower 60s.

Yack

Saturday through Thursday:

Following another unseasonably warm day in advance of an approaching cold front Saturday (inland highs well into the mid to upper 80), a significant weather pattern shift is slatted for the second half of the weekend. Most notable, will be the transition towards a wetter and cooler pattern Sunday and Monday as the upper trough over southern CA ejects east-northeastward into the Plains and Midwest. While there continues to be the usual uncertainties at this time range, much of this revolves around the finer scale details of the troughs evolution as it moves across the Midwest into Monday. Accordingly, there continues to be a strong signal in the model and ensemble guidance for a couple periods of beneficial rainfall across our area Sunday through Monday. In fact, some potential also exists for some areas of moderate to heavy rainfall for a period Sunday night into Monday as deep rich moisture (PWATs 1.5"+) feeds into the area just in advance of the approaching upper trough and the associated surface low. There may also end up being a favored period of strong thunderstorms across parts of the area into Monday.
However, the favored location for these storms is still largely unclear, and will ultimately be dependent upon the exact location and track of the surface low, as well as the location of the associated frontal features.

Following the departure of this early week storm system, mean upper troughing and northerly lower-level winds across the Great Lakes will foster a much cooler, more seasonable type pattern across our area. Accordingly, expect daily high temperatures to primarily be in the middle 70s following early morning lows in the 50s.

KJB

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/
Issued at 1240 AM CDT Fri Sep 20 2024

SHRA/TSRA extending from southern WI southwest into IA will continue to move gradually eastward into northern IL during the predawn hours. This activity is likely to weaken as it progresses eastward across northern IL. Greatest TS chances remain at RFD (60% chance) with chances dropping off considerably over the Chicago terminals (less than 30%). In fact, it is possible that this activity could dissipate altogether before reaching Chicago toward dawn, but maintained TEMPO for now and will monitor trends overnight.

Cold front will move across the terminals today with southwest winds this morning expected to veer to west and then northwest by later this afternoon. Lake breeze will develop this afternoon and result in wind shift to northeast at GYY and could threaten MDW with a wind shift to northeast for a time early this evening. Confidence wasn't high enough to include wind shift in MDW TAF, but this will need to be monitored for later TAF issuances.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...None.