textproduct: Chicago

KEY MESSAGES

- Areas of frost tonight away from the urban Chicago core, with localized sub-freezing temperatures possible.

- While the majority of the time will be dry, there will be a few opportunities for showers Thursday through the upcoming weekend.

- Other than a possible brief warm up Saturday, temperatures will remain mostly near to below average through early next week.

DISCUSSION

Issued at 126 PM CDT Wed May 6 2026

The temperature/frost/freeze forecast tonight remains tricky. An expansive ribbon of upper-level cloud cover continues to stream northeast out of the Colorado Front Range, and this looks to pivot across roughly the southeast third of the forecast area late tonight. Farther to the northwest, cloud cover across Iowa and Wisconsin should generally diminish this evening with the loss of heating/mixing. At this time, it looks like this will open up a window for generally clearer skies this evening/early overnight before additional cloud cover associated with the next disturbance approaches around or perhaps just prior to daybreak Thursday. Any thicker cloud cover that does develop in the region has the potential to significantly alter overnight temperature trends.

With how dewpoints have trended today, even if temperatures dip solidly into the mid 30s, widespread frost development is uncertain. The main concern would be for temperatures to tank much below blended guidance in the dry airmass, with a threat for sub-freezing temperatures to become more widespread than anticipated. Confidence in widespread sub-freezing temperatures is too low to justify a Freeze Warning, however. With this forecast package, elected to hoist Frost Advisories for our Illinois counties north of about I-80 (and outside of the Chicago) where prolonged clearing looks more likely, with a potential for temps to undercut the forecast. Interestingly, raw model guidance remains several degrees colder than statistical (MOS) output. Farther to the south and east, the potential for increased mid cloud cover, and generally warmer guidance precludes the issuance of any frost/freeze headlines at this time, but won't rule out the need for expansions to the headlines this evening. Within the frost advisory area, have limited frost wording to "areas" at this time given the current dry surface layer.

On Thursday, a fast-moving and fairly robust shortwave (currently shifting out of southern Saskatchewan) will push across the region through midday and into the afternoon. Guidance has trended a bit more robust with the associated forcing with this feature, and 500 mb temperatures even look a bit colder than they did yesterday, with pockets of -30 C temperatures forecast to drift across northern Illinois. While the boundary layer will be quite dry (surface dewpoints could fall into the mid/upper 20s), attendant steep low and mid-level lapse rates may make up for the lack of more appreciable moisture, resulting in slivers of weak instability. Additionally, 700-500 mb moisture is forecast to increase markedly near/south of I-80 in the vicinity of enhanced mid- level f-gen.

Based on this, there may be two "corridors" favored for shower activity on Thursday: one across far northern/northeast Illinois tied to colder mid-level temperatures, and a second near/south of I-80 in the vicinity of increased f-gen forcing. Have continued to slightly boost PoPs to account for these regimes on Thursday. The limiting factor for higher PoPs (likelies) right now is the progged dry boundary layer. Given the cold temperatures aloft, can't rule out the potential for a few lightning strikes, but at this time, this signal is too low to justify including a mention in the gridded forecast.

Broad warm advection is forecast to continue through Thursday evening south of about I-80. It remains unclear how much saturation will remain through this time period, but will continue to hold onto some precip chances in this area to account for the persistent isentropic upglide.

Another robust impulse arrives on Friday. Strong low/mid-level convergence will yield and increase in 850-700 mb frontogenesis once again, facilitating an increase in shower (and potentially isolated thunderstorm) development through the late morning and midday. Noting some north-south spread in the track of the shortwave trough, as well as overall trough amplitude which casts some uncertainty in the coverage and location of precipitation. While the latest NBM output has shifted the corridor of highest PoPs a bit farther south, given the current ensemble envelop, elected to continue to hold onto likelies a bit farther to the north.

Saturday looks like it'll be warm and breezy. A cold front, accompanied by another shortwave will bring the next chances for showers to the region Saturday evening into Sunday. This will be followed by another period of near/below average temperatures to start next week.

Carlaw

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/

Issued at 1200 PM CDT Wed May 6 2026

There are no significant aviation weather concerns. BKN VFR cumulus with bases around 7-8 kft AGL will diminish this evening, as will WNW gusts in the 20-25 kt range. WSW/W breezes will re-develop on Thursday, and VFR cloud cover with thicken again with the arrival of the next disturbance. Have introduced PROB30 groups for showers during the extended ORD/MDW TAFs. If more robust activity manages to develop, can't completely rule out the potential for a few lightning strikes given the presence of cold air aloft, but this potential remains too low for a TS mention.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Frost Advisory from 1 AM to 8 AM CDT Thursday for ILZ003- ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013- ILZ019-ILZ020-ILZ021-ILZ103-ILZ105-ILZ106-ILZ107-ILZ108.

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