textproduct: Chicago

KEY MESSAGES

- Waves of showers and thunderstorms will occur today. Showers and storms may produce damaging winds between 4 and 9 PM this afternoon and evening.

- Strong southerly winds are expected today, with peak gusts of 45 to 50 mph southeast of a line from Mendota to Waukegan, and between 55 and 65 mph along and east of the I-57 corridor.

- Wind-whipped snow may occur northwest of a line from Mendota to Crystal Lake tonight into early monday morning. Confidence remains low in how much snow will fall, so a Winter Storm Watch remains in effect for Lee, Ogle, Winnebago, and Boone counties.

- Wind-whipped snow showers may develop across the area Monday afternoon, leading to locally hazardous travel conditions.

- A period of cold temperatures is expected Monday and Tuesday, with overnight wind chills ranging from 5 to 10 below, especially Tuesday morning.

- A clipper system will likely bring a period of snow to the area Tuesday night into Wednesday. Thereafter, a warming trend will commence through the end of the week.

DISCUSSION

Issued at 311 AM CDT Sun Mar 15 2026

Recent surface observations, regional radar data, and GOES-19 water vapor imagery depict the center of a developing low pressure system in central Kansas with expansive radar returns extending northeastward into the Midwest within a tightening low-level baroclinic zone. The expectation is for the surface low to lift northeastward over Kansas City, Missouri this morning, over Davenport, Iowa this afternoon, and over southern Lake Michigan/Lower Michigan this evening into tomorrow. Such a path will squash all four seasons in one day in our local area, with hazards ranging morning thunderstorms, strong to locally damaging non- thunderstorm winds from mid-morning through the afternoon, a threat for damaging-wind producing band of showers and storms this evening, and wind-whipped snow overnight and tomorrow, and wind chills falling below zero Monday night. Good grief!

High Winds Today:

Per RAP mesoanalysis data and regional ASOS data, surface pressures across the Great Lakes are already falling at a rate of 1 to 2 mb/hr in advance of the surface low well to our southwest. A consensus of deterministic model output suggests that surface pressure falls will intensify after daybreak, reaching peak rates on the order of 4mb/hr, 8mb/3hr, and 15mb/6hr centered in Lower Michigan by late morning. Such intense pressure falls will demand a similarly intense low-level mass response, with 925 to 850mb flow progged to increase to 50 to 60 kt, respectively, across eastern Illinois and northwestern Indiana by mid-morning. Even with an increasingly saturated strong low-level inversion beneath a northeastward- advecting EML plume over our area, forecast mixing profiles from the HRRR/RAP/GFS ubiquitously depict the boundary layer tapping into the intense low-level flow primarily along and east of Interstate 57. Indeed, explicit model wind gust output from the HRRR/RAP exceeds 55 mph across far eastern Illinois and northwestern Indiana today, with ensemble means not far behind. As a result, felt there is justification to issue a High Wind Warning for Ford, Iroquois, and Kankakee counties in Illinois and Lake, Porter, Newton, Jasper, and Benton counties in northwestern Indiana with the expectation for southerly wind gusts of 55 to 60 mph. Gusts should begin abruptly between 9-11 AM and continue through the afternoon and evening hours.

Meanwhile, for locations northwest of the new High Wind Warning area, the initial onset of strong winds will be offset by an initial batch of showers and thunderstorms this morning (more on this in the next section below). With that said, the expectation remains for surface gusts to rapidly increase this afternoon behind the morning convection, albeit to a slightly lower degree than points further east (where low-level wind fields will be stronger). So, will go ahead and expand the Wind Advisory to now include LaSalle, Kendall, Kane, DuPage, Cook, Lake, Livingston, Grundy, and Will counties in Illinois with the expectation for frequent southerly wind gusts of 45 to 50 mph mainly this afternoon.

Thunderstorm / Severe Weather Threat Today:

Outside the strong synoptic winds, being in the warm sector of deepening cyclones this time of year often brings consequences in the form of severe thunderstorms. The first opportunities for thunderstorms today will be between 8 AM and 12 PM primarily across Illinois as an apparent large-scale mesoscale gravity wave activates the EML plume into a northeastward-band of convection. While steep mid-level lapse rates (already sampled near 8.5 K/km per the 00Z ILX RAOB) intersecting the minus 10 to minus 30 layer will promote the development of hail, convective-layer shear looks relatively meager during the morning (only about 20 to 25 kt). Accordingly, the main threats with any morning convection should be downpours, lightning strikes, and spits of pea to dime size hail.

This evening, the surface low will move across far northern Illinois and drive a cold front through the area. Ahead of the cold front, the aforementioned strong southerly winds will rapidly advect a plume of lower 50-degree dew points currently across Arkansas and Oklahoma northeastward into Illinois and northwestern Indiana, leading to the generation of very modest but sufficient low-centric MLCAPE on the order of 250-400 J/kt. With low-level strong convergence along and DCVA moving atop the cold front, the expectation remains for a narrow (10 to 15-mile wide) band of shallow (maybe 10-15kft deep) convection to develop along the front and sweep across our area.

While the convective band may not look like much on radar once it develops, it will have access to the aforementioned strong low-level flow (50 to 60kt from 925 to 850mb, some 0-3km shear approaching 50kt). As a result, the band will be capable of producing damaging winds of 60 to 75 mph as it races across our area. If instability were to verify on the upper-end of guidance (some CAMs depict 150 J/kg of 0-3km CAPE), would have to watch for any line breaks or kinks in the convective band or just more discrete convective cells in general to support tornadic mesovortices capable of EF-0 to EF-1 caliber damage. Confidence in the threat for tornadoes in these kinds of set-ups is relatively low and often comes down to surface conditions right as the line of storms is developing, so we should have a better idea of what we're dealing with later this afternoon. For what it is worth, the November 5, 2022 severe weather event appears to be a fair analog in which a narrow band of showers produced swaths of 60 to 75 mph winds and an EF-0 tornado across northern Illinois. Albeit, the threat area today is areawide and later in the day (primarily between 4 and 9 PM), and not just confined to Illinois.

Finally, while perhaps not the most likely our or current expected evolution of convection this evening, would be remiss to ignore the possibility for localized flooding owing to intense cool-season-like synoptic-scale forcing overlapping warm-season like PWATS over 1". Should a southwest-to-northeast oriented band of convection manage to develop this afternoon/evening (perhaps along a pseudo warm- frontal boundary somewhere between I-88 and southern Wisconsin), the aforementioned strong forcing and high moisture values would support localized swaths of rainfall totals in the 1 to 3 inches. Interestingly, successive runes of the deterministic ECMWF and HRDPS have depicted such a threat with convection this afternoon and evening, though other CAM guidance such as the HRRR depict far less impressive QPF footprints. Regardless, should such a swath of rain manage to materialize today, instances of flooding in poor drainage or urban areas cannot be ruled out.

Winter Weather Threat Tonight/Early Monday:

As the surface low lifts into Michigan this evening, the dry slot of the cyclone will race into northern Illinois and northwestern Indiana leading to a transition of anafrontal showers to mist and drizzle. With time through the overnight hours, increasing upper- level diffluence in the right entrance region of 125kt 500mb jet max racing into the Great Lakes will support the development of an expansive "comma head" deformation band of snow along the departing system, more or less centered along the Mississippi River and into Wisconsin. As this occurs, rapid pressure rises in the wake of the surface low (pushing 12mb/6hr) will lead to yet another period of strong winds gusting 45 to 55 mph, this time out of the northwest. The overlap of the strong northwesterly winds and falling snow will lead to an area of hazardous to downright travel conditions with localized blizzard conditions.

Curiously, we continue to see quite a bit of variance in the size, location, and ultimate strength of the deformation band of snow, seemingly tied to subtle differences in the evolution of the surface low. Global guidance, which is probably more representative of synoptic forcing, depicts the deformation shield being quite expansive and encompassing much of northwestern Illinois. In such scenarios, increasingly fluffy and blow-able snow would lead to blizzard-like conditions northwest of a line from Mendota to Crystal Lake with total snow accumulations north of 6 inches. Meanwhile, high-res CAM guidance which is more representative of convective processes, is far less expansive and generally weaker delivering little to no snow accumulations to our area (but still with strong winds). Interestingly, CAM guidance also almost ubiquitously advertises the northeastward-propegation of mesoscale gravity waves within the deformation shield this evening, which would lead to a leading enhancement (with sleet?) and trailing erosion of precipitation associated with localized lift and subsidence of each wave. Taken together, the combined HREF/LREF 10th to 90th percentile snowfall at Rockford remains unusually large, spanning 1 to 8 inches.

All things considered, we simply do not have the confidence to hoist/change winter headlines right right now. So, will have to take the unsavory route of a low-lead time situation by giving the day shift the final say after seeing a couple more suite of model guidance (06, 12Z) to build confidence on the strength and placement of the deformation band. Sometimes, it's easier to decidedly be on the cold side instead of in the path of the center of the low!

Monday and Monday Night:

Behind the departing low pressure system, unseasonably cold air will rush into the area on Monday with 850mb temperatures poised to fall toward -15C. With northwesterly winds blowing 40 to 45 mph, Monday afternoon high temperatures in the lower 20s will feel more like the single digits. Such cold low-level temperatures will also lead to the development of low-level instability by early afternoon, and support the generation of horizontal convective roll-like snow showers from late morning through the afternoon hours. In these kinds of regimes, conditions can vary from perfectly fine to near blizzard-like across short distances around and within the snow showers. So, this time period may require a Winter Weather Advisory for parts or all of the area.

Monday night, 850mb temperatures will tumble toward -20C leading to a downright chilly night. Overnight lows will range from the mid single-digits northwest to the lower teens southeast, with wind chills falling to the minus 5 to minus 10 range. While uncomfortably chilly, record lows at Chicago and Rockford should not be threatened.

Borchardt

Tuesday through Saturday:

The unseasonably cold start to the day on Tuesday will lower the ceiling for the day's high temperatures, with temperatures presently favored to peak only in the 20s. With expansive surface high pressure shuffling its way eastward across the region, no precipitation is expected during the daytime hours.

A fast-moving clipper-type shortwave diving southeastward out of Alberta will arrive at our doorstep Tuesday night. Nearly all members of the 00Z EPS and GEFS output a modest amount of QPF across our forecast area Tuesday night into Wednesday morning, which would translate to measurable snowfall with air temperatures not expected to rise above freezing until sometime during the mid to late morning on Wednesday. Some ensemble spread remains with the strength of the inbound wave and how much moisture it will have to work with, but overall, this doesn't have the look of a blockbuster snowfall event.

A strong upper-level ridge initially centered off the California coast will reposition itself eastward over the Desert Southwest for the latter half of the week. This will kick off a warming trend here locally through the end of the workweek as the associated thermal ridge expands eastward, with highs in the 50s and 60s expected to make a return.

The jet stream draped around the ridge's northern periphery will also get nudged eastward, which will steer any subsequent clipper waves/systems emanating out of Canada more to our east. That said, one additional clipper shortwave may still get close enough to our area Wednesday night into Thursday for another round of precipitation to occur here. This outcome is reflected only in a minority of Grand Ensemble members at this time, so made no changes to the dry NBM forecast at this juncture.

Ogorek

AVIATION /06Z TAFS THROUGH 12Z MONDAY/

Issued at 1233 AM CDT Sun Mar 15 2026

A strong spring storm system is expected to move across the Great Lakes through the TAF period leading to periods of inclement weather.

A recent surface analysis revealed the center of a low pressure system in the central Plains with pressure falls on the order of 1 to 2mb/hr downwind in the Great Lakes. As the low matures and lifts toward eastern Iowa later this morning, surface winds will gradually increase in magnitude while turning clockwise toward the southeast. LLWS criteria (both speed and direction) will continue to be met through the overnight hours as a southerly low-level jet (sampled near 50kt at 2kft per regional VWPs) continues to stream overhead.

After daybreak, winds are expected to turn south to southwesterly (wind direction 180-190) and increase markedly with frequent gusts above 30kt by 15Z. At GYY, gusts may exceed 40 or even 45kt this morning. A band of showers and storms is expected to sweep across the terminals generally between 15-19Z associated with a large-scale mesoscale gravity wave interacting with a plume of steep mid- level lapse rates. The inherited TEMPO groups cover the threat of morning convection well. Behind the initial batch of showers and storms, increasing low-level warm air advection and the approach of DCVA as the surface low lifts toward the Wisconsin state line will encourage the development of scattered showers by early afternoon. Cigs will also trend lower, with BKN025 favored by mid afternoon.

This evening, surface winds will increase even further ahead of an approaching cold front. With pressure falls nearing 10mb/3hr this afternoon into Lower Michigan, am concerned that southerly gusts could exceed 40-45kt from 18-22Z or so, with a play for even gusts exceeding 50kt at GYY. So, will nudge winds upward in the outgoing TAF. A band of strongly forced convection (with or without lightning) is then expected to sweep across the terminals along the cold front (looking at 21-00Z window). The band will be accompanied by a brief (sub-hour) period of visibility less than 3 miles as well as an abrupt southwesterly (wind direction 230 or so) wind shift and gusts of 40-50 kt. Behind the convective band, showers and drizzle is expected to prevail as cigs build down toward IFR as the dry slow of the synoptic- scale cyclone noses overhead. The inherited TAFs handle such trends excellently.

Toward the end of the TAF period, tumbling temperatures will facilitate a transition of any ongoing precipitation from rain to snow. Am still seeing quite a bit of variance in the coverage of what will be an eventual broad shield of snow and timing of a northwesterly wind shift, owing to subtle differences in the structure of the low as it moves into Lower Michigan. For now, will maintain a mention of 3SM SN at ORD/MDW and a northwesterly wind shift as early as 06-07Z though adjustments are all but likely in later forecasts. Gusts will continue to tag 30 to 35kt through the overnight hours after the northwesterly wind shift.

Borchardt

LOT WATCHES/WARNINGS/ADVISORIES

IL...Winter Storm Watch from this evening through Monday evening for ILZ003-ILZ004-ILZ008-ILZ010.

Wind Advisory from 10 AM this morning to 9 PM CDT this evening for ILZ023-ILZ032-ILZ033-ILZ039-ILZ108.

IN...Wind Advisory from 10 AM CDT /11 AM EDT/ this morning to 9 PM CDT /10 PM EDT/ this evening for INZ001-INZ002-INZ010- INZ011-INZ019.

LM...Gale Warning from 7 AM this morning to 9 PM CDT this evening for the IL and IN nearshore waters.

Gale Watch from late tonight through Monday afternoon for the IL and IN nearshore waters.