Area Forecast Discussion

KEY MESSAGES

- Gusty and seasonably cool today.

- Potential (20-40%) for some gusty showers late this morning and afternoon.

- Frost/freeze conditions are expected outside of Chicago and central Cook County late tonight into Monday morning.

DISCUSSION

Issued at 232 AM CDT Sun Apr 19 2026

It's a chilly start to our Sunday with temperatures sitting in the middle and upper 30s early this morning. Mostly clear skies currently will cloud up through the morning with the arrival of a shortwave disturbance traversing the windward side of the broader upper trough. This wave will work across northern IL and drag a back door cold frontal feature southward across the CWA this morning into afternoon. The wave is sharpening up over the northern Plains this morning and is bringing pockets of light precip to the eastern Dakotas and Minnesota. Steep lapse rates between 8 and 9 K/km from the surface to 700-650 mb and low level moisture advection ahead of the front will generate a layer of surface-based instability with the frontal passage. Some decent low level forcing also looks to accompany the front. Put together, we have a good signal for at least some light precip, but some not-so-light precip is on the table as well. It's unclear exactly how much we'll destabilize with some camps suggesting that dewpoints will mix down for the afternoon and the dry near-surface air could help relegate precip to largely sprinkles. Other camps, such as the more recent RAP runs, are more optimistic with the thermodynamics such that a few embedded lightning strikes may even be possible. Guidance resolves largely trace amounts to a couple of hundredths of QPF with this, but a number of high res camps depict streaks of higher amounts closer to or in excess of a tenth, suggesting some pockets of more moderate rainfall are certainly possible. Precip will move into our north and northwest CWA during the mid-late morning and across the I-55 corridor by early afternoon. The primary target area for precip coverage will be near and north of a roughly LaSalle-to-Rensselaer line along the track of the wave while areas farther south should stay dry. Precip should end in our CWA by early evening as the wave pulls away to the east.

Another question to consider is will we see any snow or wintry mix with precip today? While surface temps will be in the 40s to near 50 during the event, wet bulb zero heights as low as 1,000 feet off the ground may allow some snow flakes to reach the surface. Given the convective nature of the precip and instability extending through the DGZ, a bonafide graupel shower could even be in the cards. Better snow/mix chances may exist across our north/northwest this morning given the earlier timing before the BL can heat up. Additionally, while the stronger signal for precip exists immediately ahead of the front, there is a potential for some light post-frontal precip into early evening which also has a seemingly better shot at featuring some snow. However, the wintry precip potential appears to be a low one and in the forecast, went with practically all liquid PoPs.

Deep mixing will into an elevated low level wind field will also promote gusty winds today. Again, uncertainty exists in the depth and efficiency of mixing, but the potential is there for occasional gusts over 40 mph, especially closer to central IL. More frequent wind gusts look to be in the 25 to 35 mph range with the strongest winds generally expected south of I-80. Winds will start out westerly today before veering to northwest for the afternoon with the frontal passage. Afternoon temperatures will range from the middle and upper 40s in our north to middle 50s in our south.

Surface high pressure will center over the area late tonight into early Monday. There won't be much noteworthy cold advection behind the front, but the resultant clear skies and calm winds will bring slightly cooler conditions to tonight/Monday morning. With lows still looking like upper 20s and lower 30s, decided to upgrade our Freeze Watch to a Freeze Warning for the entire CWA outside of central Cook County where conditions will likely stay a few degrees warmer.

Lower and middle 50s are forecast for Monday with mostly sunny skies. Behind the departing trough, upper ridging scooting east into the Plains will advect a much milder airmass and the eastern edge of an EML into the region for Tuesday. Highs are currently forecast to reach the 70s around the CWA. A shortwave feature embedded in the upper flow may attempt to produce some showers Tuesday evening and night, but lots of capping and little forcing favor largely dry conditions. The upper ridge will center over the area on Wednesday and middle to upper 70s are forecast away from the lake. Winds turning onshore will keep areas near the lake notably cooler on Wednesday; lakeside highs are forecast in the 50s. Lower 80s are forecast on Thursday ahead of a large synoptic system that will move across the area and bring additional rain and storm chances to late Thursday and Friday.

Doom

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 622 AM CDT Sun Apr 19 2026

- Scattered showers develop later this morning and afternoon. Chance for a little snow mixing in at RFD, but no accumulations would occur.

- Winds turn WNW/NW and will become gusty into the afternoon. Potential for a few 30+ kt gusts during a brief window towards midday.

- Wind shift to northeasterly this evening and overnight. Small chance for MVFR stratus near the lake overnight.

Light southwesterly winds will increase and become gusty WNW/NW through the morning and afternoon. Deep mixing may support a brief period of stronger wind gusts (30+ kts), but this potential appears too brief and uncertainty too high to justify adding to the TAFs.

A fast-moving disturbance currently racing across southeast Minnesota will move into the region later this morning and afternoon. Based on upstream radar returns and observations, combined with the strong nature of forcing associated with this feature, have converted the previous PROB30s to TEMPOs or showers. At RFD: have included a mention of -SHRASN due to the slightly earlier arrival (towards 15z) and cooler conditions. At the Chicago-area terminals: can't entirely rule out a snowflake or some graupel if more robust convective showers develop, but these chances appear too low to include in the TEMPO groups. Air temperatures at all sites will be above freezing during any precipitation. There is a chance some spotty shower activity lingers near ORD, MDW, and GYY beyond the current TEMPO groups, but this signal has diminished some from previous guidance.

A northeast wind shift will occur at ORD, MDW, and GYY this evening, although some uncertainty regarding the precise timing remains. There is a small potential (30 percent or less) of some MVFR cigs developing off the lake this evening. Eventually, winds will trend light/variable overnight into Monday morning before gradually becoming very light southwesterly towards midday Monday. A lake breeze will push inland through Monday afternoon.

Carlaw

LOT WATCHES / WARNINGS / ADVISORIES

IL...Frost Advisory until 9 AM CDT this morning for ILZ003-ILZ004- ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019- ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ106-ILZ107- ILZ108.

Freeze Warning from midnight tonight to 9 AM CDT Monday for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012- ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039- ILZ103-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Frost Advisory until 9 AM CDT /10 AM EDT/ this morning for INZ001-INZ002-INZ010-INZ011-INZ019.

Freeze Warning from midnight CDT /1 AM EDT/ tonight to 9 AM CDT /10 AM EDT/ Monday for INZ001-INZ002-INZ010-INZ011- INZ019.

LM...Small Craft Advisory from 11 AM this morning to 7 PM CDT this evening for the IL and IN nearshore waters.