textproduct: Chicago

KEY MESSAGES

- Warm and primarily dry conditions through the day on Friday.

- Periodic chances for showers and some thunderstorms will return Friday night into next week, though there will continue to be plenty of dry hours.

DISCUSSION

Issued at 257 PM CDT Wed Sep 17 2025

Through Friday:

Expect very similar weather conditions to the past 24 hours over the next 24-30 hours, followed by early signs of the gradual breakdown of the entrenched blocking regime. Lows will be in the mid to upper 50s outside of Chicago (a bit milder than this morning) and in the lower to mid 60s in the city. While we don't have any patchy fog explicitly mentioned in the gridded forecast late tonight into early Thursday, wouldn't be surprised at very patchy ground fog in typical favored spots.

On Thursday, skies should trend to partly cloudy into the northwest 1/3 or so of the CWA (vs. nearly full sun everywhere today). Otherwise, 925 mb temps will nudge up 1-2C, supporting highs a bit warmer than today, primarily in the mid-upper 80s away from the lake breeze cooling footprint inland of Lake Michigan. A few 90F readings are possible near and south of a Sterling to Watseka line. Thursday night will be tranquil with lows in the upper 50s to mid 60s, mildest in and near Chicago.

A slow moving upper low will drift southeast across the Dakotas on Friday. This will sharpen up the mid-upper ridge axis over the northwestern Great Lakes, helping maintain the dry daytime conditions. As the moist axis finally begins to slosh east from near the MS River on Friday afternoon, can't completely rule out a few showers near/west of I-39 (20-25% chance). Friday's high temps will be similar to those of Thursday. The minor exception will be near Lake Michigan, where the lake breeze will surge farther inland earlier than on Thursday as it becomes enhanced by a backdoor cold front.

Castro

Friday Night through Wednesday:

The aforementioned upper low will move across Minnesota Friday night and then become an open wave embedded in a larger trough over the north central US and Canadian Prairies on Saturday. Associated weak surface low pressure will lift north and drag a warm front slow across the region. A fairly sharp instability west-east instability gradient will nudge eastward and open up the potential for scattered showers and isolated to widely scattered storms Friday night, aided by the closest pass of the trough to our northwest. The evening should be dry over the eastern half of the area, including the Chicago metro, with low rain chances shifting east overnight.

Large scale forcing will shift farther north on Saturday- Saturday night, with any isolated to widely scattered showers and isolated storms confined to north of I-80 (20-30% PoPs) but particularly up into Wisconsin (closer to the better forcing). Expect another warm day on Saturday, also paired with a bit higher humidity (dew points in the 60s), followed by a mild night Saturday night by late September standards.

A more amplified trough then looks to eject out of the northern Plains Sunday-Sunday night and move across the Great Lakes Sunday night through Monday night. Associated with this feature should be a more formidable cold front, potentially accompanied by a bit higher coverage of showers and thunderstorms to the area during this timeframe. Wind shear values and PWATs Sunday through Monday look to be rather modest, so the threat for severe weather and/or torrential rainfall (and flooding) still looks low at this time. Additionally, southerly flow will keep high temperatures in the 80s through the early part of next week. To note, the Autumn Equinox is on Monday September 22nd at 1:19 PM CDT this year, so temps will be well above normal to close out astronomical summer.

Looking towards Tuesday-Wednesday next week, the forecast becomes much more uncertain as guidance is struggling with the evolution of the Monday/Tuesday trough. Some guidance suggests the trough will morph into an upper low and remain parked over the Great Lakes through a good portion of next week while others suggest a return of upper ridging as the upper low becomes "buried" in the southern Plains. If the upper low solutions materialize then cooler and wetter conditions would be favored. However, if renewed upper ridging comes to fruition then a return to dry conditions with generally above normal temps would be favored, albeit with persistent northerly flow keeping it cooler near the lake. After our official PoPs peak in the 40-50% range Sunday evening through Monday evening, they do trend down to only 20-30% on Tuesday and <15% on Wednesday, so a majority of the ensemble members seem to favor the 2nd outlined scenario.

Castro/Yack

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1159 AM CDT Wed Sep 17 2025

There are no significant aviation weather concerns anticipated through the period.

Surface high pressure will remain dominant across the Great Lakes region through the period. This will foster light winds, and favor inland shifting lake breezes with easterly winds both today and again on Thursday. Otherwise, aside from some patchy ground fog potential outside of the main Chicago terminals towards daybreak Thursday morning, VFR conditions are expected through the period.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.