textproduct: Chicago

KEY MESSAGES

- Severe thunderstorms are expected this afternoon and evening, with the potential for very large hail, damaging winds and tornadoes (some strong).

- 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 253 PM CDT Tue Mar 10 2026

Through Wednesday...

Early this afternoon, an awfully sharp frontal boundary is draped across the area from a roughly Pontiac to Kankakee to Rensselaer line. North of the front, we find cool northerly winds and temperatures in the 40s. Meanwhile across our south, temperatures are in the middle and upper 70s with dewpoints into the lower 60s. This front is tied to a broad center of low pressure pushing into southwestern IA. On the eastern flank of this wave, a shortwave low- to mid-level impulse is scooting from western into northern IL which will act as the trigger for our initial chances for severe weather today.

18Z RAOB from DVN depicts steep mid level lapse rates within resident EML plume and a rather formidable capping inversion. an 18Z balloon from ILX shows recorded over 2,000 J/kg of weakly- capped surface based CAPE. Latest SPC mesoanalysis depicts a reservoir of MLCAPE over central and extending into northern IL and CIN steadily decreasing. Since late this morning, there's been a wide expansion of stratocumulus over central and north- central IL beneath the capping inversion in the open warm sector. And as of 2 PM, some congestus was noted west of the CWA and regional radar shows showers have recently been pulsing southeast of the Quad Cities.

A very close eye will be kept on the evolution of these initial radar echoes southeast of the Quad Cities in the next few hours, as they may develop into the first storms of the day. Large scale forcing isn't great early on this afternoon, but will be aided by the passing of this shortwave and soundings from SPC mesoanalysis suggest the cap is becoming rather fragile upstream and even weaker with eastward extent into our southern CWA. All severe hazards would be possible with any storms that develop along and south of the front early this afternoon with over 2,000 J/kg of MLCAPE extending into our south, 50 to 60 kt of effective shear, and long, veered low level hodographs favoring supercell updrafts. Damaging winds and large hail, while too favored closer to and south of the front, will also be possible with any storms that develop and track north of the boundary where the low levels will remain stable.

During the late afternoon into early evening, an expansive SSW LLJ will strengthen considerably across IL and nose into northern IL. This will bring a notable uptick in moist upglide and large scale forcing around the area. As a result, convection is anticipated to blossom over the area toward early evening with the highest coverage of storms expected near and south of the front. South of the front, the low-level wind profile will become increasingly curved, which will drive up the strong tornado potential. Modeled SRH is maximized south of the front after 23Z. For these reasons, the late afternoon through early evening period defines the window of highest concern from a significant tornado perspective.

Damaging to destructive straight line winds will be moreso favored with storms along and south of the front where a lack of low level stability will make it far easier for such winds to be drawn to the surface, but damaging winds are certainly a concern with any storms around the area. The large hail threat is active basically area-wide including with any storms that track north of the front with substantial mid level shear and instability overspreading the CWA. The biggest hail concerns are too favored closer to the surface boundary, but extends north across most of the Chicago metro area.

Storms will be capable of producing heavy to torrential downpours and localized flash flooding may occur with any given storm. Flooding concerns are highlighted on either side of the front where we expect the overall highest storm coverage and see the highest potential for training or regularly recurring convection along the boundary. Basically all available deterministic guidance resolves at least a county-wide swath of 1-2" QPF near or on either side of I-80 through the end of the event. Accordingly, made the decision to hoist a Flood Watch for areas near and north of I-80. Will need to keep a close eye on storm evolution into tonight and continued convective chances overnight to possibly consider an expansion into our southern counties, but initial flooding concerns aren't quite as high down in these parts, especially given the largely rural landscape.

Taken altogether, any storm this afternoon an evening may produce destructive hail larger than 2 inches in diameter, damaging winds, flash flooding and, if close to the warm front, strong tornadoes.

The elongated center of the surface low will stretch across the area tonight with stable low level air immediately in its wake. This should shunt the strong storm and heavy rain potential after midnight to areas along and southeast of the low track, which equates to areas south of I-80. However, occasional showers and storms may very well continue across northern IL through much of the overnight. The severe threat could linger into the overnight across our southern counties where greatest forcing and instability will be focused, but stabilizing low level air and decreasing low level shear should have the tornado threat winding down late evening into the overnight. Hail and damaging winds would be the primary concern with continued convection into the overnight down south, but showers and largely sub-severe convection are expected.

A trailing wave will lift into northern IL very late tonight into early Wednesday which will provide additional, likely widespread showers for the better part of Wednesday while the synoptic system and cold front make their way out of the region. There's a signal for some wintry mix or snow showers to develop across our north in the cold advection, but no accumulations or wintry impacts are anticipated. Temperatures will be cooling during the daytime tomorrow around much of the area in the wake of the cold front with afternoon temperatures progged in the 30s and 40s.

Doom

Wednesday night through Tuesday...

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 /18Z TAFS THROUGH 00Z THURSDAY/

Issued at 106 PM CDT Tue Mar 10 2026

Aviation forecast concerns for the 18Z TAFs:

- IFR/LIFR ceilings through Wednesday morning, with occasional IFR/MVFR vis in fog/rain. Improvement to MVFR Wednesday afternoon.

- Period of TSRA mainly this evening. Low confidence in TS as early as ~21Z, higher confidence after 00Z. A few strong to severe storms possible with hail/wind. Greatest thunderstorm potential shifts south/southeast of terminals after 06Z.

- Northeast winds 10-15 kt become north-northwest predawn Wednesday and northwest around daybreak. Gusts 25+ kts by afternoon. Surface cold front has pushed south of the terminals as of midday. Northeast winds and extensive IFR (LIFR at GYY) ceilings behind the front will linger this afternoon, though may try to briefly improve to lower-end MVFR. Some fog continues to reduce vis at RFD and off the lake at GYY and will likely persist in these locations. The frontal zone will become nearly stationary along/just south of the I-80 corridor later this afternoon, and is expected to become the focus for shower and thunderstorm development into tonight. Various model guidance depicts development of SHRA/TSRA as early as 21Z in the vicinity of the terminals this afternoon, though confidence in coverage is fairly low at this time and have elected to maintain prob30 mention in the 21Z-24Z period. Confidence increases in higher coverage from 00Z on however, ahead of an approaching mid- level disturbance and associated surface wave along the front. While the highest probability of severe TS will generally be along/south of the aforementioned stationary front (south of the terminals), storms north of the boundary may still be capable of producing isolated large hail and damaging winds. Current high-res model trends indicate the greatest storm coverage would shift south/southeast of the Chicago area after 05-06Z or so. SHRA will likely continue in some fashion through daybreak Wednesday (with a low by non-zero TS chance through perhaps 08-10Z).

Winds will shift northwest Wednesday morning as the surface low pressure wave moves off to the east of the area and the cold front begins to push off to the southeast. Speeds/gusts increase by midday as low-level colder air spreads in, with gusts in the 25+ kt range during the afternoon. IFR cigs should gradually rise to MVFR during this time, and while dry in the TAFs there is a chance of some spotty lingering SHRA.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...Flood Watch through late tonight for ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107- ILZ108.

IN...Flood Watch through late tonight for INZ001-INZ002.

LM...Small Craft Advisory until 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.

Dense Fog Advisory until 10 PM CDT this evening for the IN nearshore waters.

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