DISCUSSION
Issued at 223 AM CDT Wed Sep 11 2024

Through Thursday Night:

Mid-summer like daytime warmth but otherwise quiet and dry conditions are in store. Western North American wildfire smoke will continue to be present aloft at varying thicknesses.
Expecting some effect on temperatures today, reducing the chance of any locations hitting 90F, though mid to locally upper 80s are forecast away from Lake Michigan, where some modest afternoon lake breeze cooling will occur. As has been the case in this dry pattern, undercut raw model and blended guidance low temps for tonight and trended toward MOS, with low-mid 50s prevalent outside of Chicago.

High level cloud cover will stream northward in advance of Francine's remnants Thursday afternoon and night, which combined with the smoke aloft, will slightly limit the ceiling on inland temps Thursday afternoon. A synoptic easterly component and lake breeze influence will cap shoreline highs in the upper 70s to around 80F. Forecast lows Thursday night are in the upper 50s to lower 60s outside Chicago and mid-upper 60s in and near the city, milder due to the increased high clouds plus smoke aloft.

We'll continue to message a threat for grass and brush fire starts due to the very dry conditions and warm temperatures today and Thursday.

Castro

Friday through Tuesday:

The forecast later this week for our area will largely be dependent upon the evolution of the remnants of Francine following its landfall later this evening on the LA Gulf Coast.
Ensemble guidance continues to favor a northward trajectory up the Lower Mississippi Valley into Thursday. Thereafter, it appears rapidly building upper-level heights and a building ridge across the Great Lakes (Rex block pattern) will largely limit its northward progress, essentially causing it to become quasistationary while gradually weakening across the TN Valley into this weekend.

While a notable increase in tropospheric moisture will exist in the vicinity of Francine's lingering circulation, a wall of exceptionally dry air will exist on its periphery, with modeled 700 mb dewpoints on Friday morning in the -15 to -25 C range across northern Illinois. An initial push of locally-enhanced DCVA is forecast to arrive south of I-80 through Friday morning along with generally modest upper divergence owing to relatively weak flow above 500 mb. Given the tremendously dry airmass across northern Illinois, it seems likely that we'll struggle to moisten the lower-levels appreciably, resulting in a general erosion of the northern terminus of any precipitation shield/bands of showers. Have continued to generally confine slight to chance PoPs well south of I-80 during the day Friday as a result. Certainly can't rule out some light activity develop farther to the north, but chances appear low at this time.

Any activity should tend to refocus inland, with increasing moisture the farther west you go with dry east to northeasterly trajectories off the lake continuing near the lake into Friday night and Saturday. With limited large scale forcing overhead on Saturday, precipitation coverage may remain pretty spotty, and most locations/hours look to remain precip-free. From Sunday and beyond, run-to-run differences in the handling of a secondary moist surge are evident, with some signal this ends up largely missing the region with another plume of very dry mid- level air sneaking in from the east. For now, have continued to trim PoPs to generally advertise on slight chances Sunday into Monday given uncertainties.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/
Issued at 620 AM CDT Wed Sep 11 2024

VFR with no significant concerns.

SSW to SW (200-230 deg) winds will increase to 7-10kt away from the lake later this morning. A lake breeze will gradually push inland from the Illinois shore this afternoon, while hugging the IN shore, which may briefly result in an east-northeast wind shift at GYY in the late afternoon. MDW's TAF favors an east- southeast wind shift (7-8 kt speeds) at around 23z with medium confidence. The boundary will probably wash out near ORD around sunset/00z this evening, and winds will likely turn light southeasterly even if the lake breeze doesn't make it across the airport.

Light/VRB to calm winds will return to outlying terminals with sunset. Initially light easterly winds will gradually increase Thursday morning, to near 10 kt by midday. The varying thickness smoke layer aloft will remain at about 15kft AGL.

Castro

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