textproduct: Chicago

KEY MESSAGES

- Areas of dense fog this morning, mainly near/north of I-80.

- Severe thunderstorms this afternoon and evening, with the potential for very large hail, damaging winds and tornadoes.

- Potential for heavy rain and localized flash flooding this afternoon into Wednesday morning.

- Small potential for damaging winds continuing into Wednesday morning across our far south and east.

- Possible very strong winds (45+ mph) Thursday night/Friday.

DISCUSSION

Issued at 218 AM CDT Tue Mar 10 2026

Through Wednesday:

Moisture is surging northward early this morning, with dewpoints pushing into the upper 50s and lower 60s across the region. Pockets of low stratus and shallow mist have recently developed, and this will likely continue to expand north of I-80 and the Illinois river through daybreak. Surface flow is forecast to remain elevated, but given the recent expansion of sub 1/2 mile visibilities, may need to shortly consider a dense fog advisory for parts of the area.

Somewhat of a complex severe weather setup is evident later this afternoon and overnight with quite a few moving parts and generally nebulous/subtle large scale forcing mechanisms. As a result, uncertainty in the precise evolution of thunderstorms remains, but the ceiling on the severe weather potential in the region is anomalously high for this time of year.

The first key player is an extremely shallow cold front that's presently rolling down the lake and into southern Wisconsin. This front is so shallow that it's not super evident even on the MKX radar, which certainly corroborates forecast soundings from the NAM/NAMNest which depict the frontal inversion sitting around 700 feet. These types of shallow early-spring boundaries, enhanced by the cold Lake Michigan waters, often have an impressive ability to plow southward and undercut even deep moist boundary layers. This backdoor front will likely be no exception, with the latest guidance suggesting this will surge south to near a Rockford - St. Charles - Crown Point, IN line by midday before settling in the vicinity of to just south of the I-80 corridor by mid afternoon before becoming quasistationary. Behind the front, temperatures will fall to either side of 40 at the lake front and into the 40s/maybe low 50s inland as low stratus expands atop the frontal inversion (perhaps with some scattering in the afternoon, mainly along the southern flanks).

While low stratus may develop south of I-80 through this morning, forecast soundings indicate this would be quite thin and likely to mix out readily. As a result, temperatures are expected to surge through the 70s, if not tagging 80 on a localized basis towards central Illinois this afternoon. As dewpoints surge into the mid and upper 60s, an expansive elevated mixed layer characterized by 500-700 mb lapse rates of 7.5 to 8.5 C/km will overspread the region leading to a rapid increase in conditional instability with MLCAPE values progged to rise through 2000-3000 J/kg during the afternoon.

Given the relatively warm base to the EML, things look to remain decidedly capped to surface-based convection through at least early afternoon in the region until additional modest ascent approaches the region. Key player number 2 appears to be a mid-level perturbation that's currently pushing out of the Texas/Oklahoma Panhandles and into Kansas. While global guidance has been fairly steady with this feature, have noted some more significant variability with the CAMs which in turn seems to be playing a role in the run-to-run variability in convective initiation (or not) by early-mid afternoon near and southeast of the Quad Cities as the associated DCVA with this feature is not particularly robust, and the main jet core may only provide a glancing blow across the sharpening quasistationary/warm frontal zone before continuing northeastward. Whether explosive CI occurs in the vicinity of a Moline to Ottawa line in the roughly 2 - 4 PM timeframe remains a key detail to iron out. If CI occurs here, this would be very concerning as semi-discrete supercellular convection would be favored within a strongly sheared environment supportive of all severe hazards including tornadoes, very large hail, and damaging winds.

While recent runs of the HRRR have mostly lost this convection, the fact that the globals (GFS, ECMWF, UKMET) have been fairly steadfast in convective development in this zone remains concerning. Have continued to paint generally chance PoPs in this corridor during the afternoon, and we'll need to carefully monitor this zone. *IF* semi-discrete cells can develop and sustain in this corridor south of the front, there would certainly be a threat for strong tornadoes.

Isentropic upglide atop the sharp frontal inversion and generally increasing large scale forcing as the main surface low approaches will eventually lead to an expansion of thunderstorms through Tuesday evening. Surface based convection south of the front will continue to pose an all-hazards severe threat deep into the evening. While a tornado threat can occasionally occur many miles north of warm fronts, this appears unlikely in this scenario given the sharp nature of this feature. As a result, any storms that track northeast and atop the frontal zone will quickly become elevated. Additional convection will likely develop through the evening across northern Illinois in response to intensifying upglide. Storms north of I-80 will pose a threat for large--possibly very large--hail as forecast soundings reveal strong inflow-layer storm-relative winds and significant 1-7 km shear. If very robust supercells can sustain themselves, isolated damaging wind gusts may still materialize north of the front as well.

Regarding the flood potential: the generally southwesterly 500-mb flow does not exactly match some of the more significant Maddox type flash flood setups in the region. That said, corridors of southwest to northeast oriented thunderstorms will pose a threat for at least localized training and very heavy rainfall. Some increased concern for flooding exists for areas like Chicago and parts of the south and southeast burbs where heavy rain fell a few days ago. Will refresh the Hydrologic Outlook. Can't rule out needing a targeted flash flood watch down the line if corridors of heavier training convection can be nailed down a bit better.

The threat for strong-severe storms will possibly continue through the night. Noting another early window Wednesday morning as a possibly convectively-augmented MCV and vort lobe tracks across our southeast around or just after daybreak. Guidance suggests that surface-based convection would still be possible during this period south and east of a Pontiac to Rensselaer line, with a threat for damaging wind gusts and perhaps some embedded QLCS tornado threat. This is somewhat lower confidence at this time, but something we'll need to keep an eye on.

Fairly widespread showers will then occur through the balance of Wednesday as a cold front eventually works its way through the area. Some potential for precip to turn over to a little snow across far northern Illinois, but temperatures look to remain largely at or above freezing. Could even be a potential for a little drizzle as things come to an end, but again not concerned about wintry impacts at this point due to forecasted temperatures. Have boosted wind gusts Wednesday afternoon as steepening boundary layer lapse rates should result in at least periodic 30-40 mph gusts.

Carlaw

Wednesday night through Monday:

It looks like we'll get a bit of a break Wednesday night into Thursday as high pressure shifts off to our south. Noting some potential for fairly deep mixing Thursday afternoon and for RH values to fall around or under 30 percent with breezy westerly winds. Given the rain forecast over the next 24 hours, this doesn't currently look like an elevated fire threat.

Thursday night into Friday, a robust surface low (mid 980s mb) is forecast to track to our north, roughly across the Door Peninsula and across northern lower Michigan. Noting a huge low- level mass response in the guidance, with 850 mb winds near 70 knots advertised across the guidance suite. While some low cloud cover and at least spotty light precipitation (rain) will be possible, deeper-than-typical mixing during the late overnight hours into Friday morning may yield a significant increase in winds and gusts. Concerningly, latest GFS soundings show roughly 50 knot boundary layer mean winds during this time frame, with even steeper lapse rates developing after daybreak Friday in the cold advection regime. Have increased winds and gusts a bit with this forecast package above the NBM-delivered values, but suspect things remain notably underdone at this time. At this time, can't rule out high wind warning type winds and gusts into Friday morning given the latest model trends.

Winds will ease sharply Friday evening as high pressure again builds across the region. There may be some additional precip chances towards the Saturday night - Sunday morning timeframe as mid-level frontogenesis intensifies in advance of the next disturbance, although the latest guidance suggests most of this activity may end up shifting largely north of the region. Thereafter, global deterministic and ensemble guidance suggest the potential for yet another robust surface low spinning up in the general vicinity on Sunday and into Sunday night. A much colder airmass bottled up to the north looks like it'll get dislodged with this feature and surge across our area during this timeframe. While model variability remains somewhat high at this juncture, can't totally rule out old man winter making another return to the region with some snow potential into early Monday morning.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/

Issued at 642 AM CDT Tue Mar 10 2026

Primary forecast concerns include...

Wind shift to northeast this morning. Ifr/lifr cigs this morning and again tonight. Scattered strong/severe thunderstorms this afternoon/evening. Dense fog with lifr/vlifr vis at times.

The cold front is slowly working its way down the northeast IL shore of Lake Michigan and will move across the Chicago terminals over the next few hours, shifting winds to the north/ northeast. Directions will turn northeast later this morning and more easterly this afternoon. Speeds in the 10-12kt range are expected with higher gusts at times. A warm front will lift northeast this evening and may allow winds to briefly turn back southerly at MDW/GYY but confidence is low. Winds will turn to the north/northwest overnight.

Areas of dense fog continue across parts of northwest IL and at RFD and this fog is expected to slowly lift this morning. Additional fog may move inland from Lake Michigan today and also tonight, but confidence is low. If this fog were develop inland, it may also become dense.

Ifr and possibly lifr cigs are expected to move back across the Chicago terminals behind the cold front noted above. These cigs are then still expected to lift back to low mvfr this afternoon. Cigs may tank back into lifr or vlifr this evening and if they do, they may persist through daybreak Wednesday morning, before slowly lifting later Wednesday morning.

Scattered thunderstorms are expected to develop during the mid to late afternoon and likely right over, or just southwest of the terminals. Confidence for exact location remains medium and maintained prob mention for now, but tempo mention may be needed with later forecasts. The highest coverage of thunderstorms may still be during the early/mid evening which the current tempo handles well. The thunderstorm activity is expected to be moving east and southeast of the terminals by 06z. Scattered showers are expected to continue overnight into Wednesday morning. cms

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 11 AM CDT this morning for ILZ003- ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012.

IN...None. LM...Small Craft Advisory from 10 AM this morning to 4 AM CDT Thursday for Winthrop Harbor to Northerly Is. IL.

Small Craft Advisory from 10 AM Wednesday to 4 AM CDT Thursday for Northerly Is. to Gary IN.

Small Craft Advisory from 10 AM Wednesday to 10 AM CDT Thursday for Gary to Burns Harbor IN-Burns Harbor to Michigan City IN.