textproduct: Chicago

KEY MESSAGES

- An unseasonably warm and humid pattern will arrive on Sunday and persist through at least the middle of the workweek, accompanied by periodic showers and thunderstorms, and a threat for severe weather Monday night through Wednesday.

DISCUSSION

Issued at 337 AM CDT Sat Apr 11 2026

Through Tonight:

Patchy shallow fog outside of Chicago will quickly erode after sunrise this morning. The lingering influence of seasonably strong and expansive surface high pressure over the Great Lakes region will result in an overall quiet daytime period. Following a mostly sunny to partly cloudy start to the day, mid and high level cloud cover will overspread the area and likely limit further temperature rises. Expect highs in the mid 50s to lower 60s (locally mid 60s far southern sections) for inland areas and only in the upper 40s to around 50F near the lake in northeast Illinois and in far northwest Indiana due to onshore flow and afternoon lake breeze reinforcement.

The aforementioned advancing mid and high clouds today are associated with a series of mid-level impulses overtopping broad mid and upper level ridging over the region. Some previously more aggressive guidance has begun to back off with respect to shower coverage this evening due to antecedent dry low levels. Due to fighting dry air and a lack of robust large scale forcing, only isolated to widely scattered light to perhaps moderate showers may occur, primarily north of I-80. Have focused the highest PoPs (40-50%) across far northern Illinois. Mostly, if not entirely dry conditions are favored overnight and as such, we may be able to scale back on low PoPs near and north of I-80. Forecast lows will range from the mid 40s near the lake in northeast Illinois to the mid 50s well inland. As a stout low-level jet moves overhead in the pre-dawn hours of Sunday, breezy southerly winds will likely result in slowly rising temps.

Sunday through Friday:

Western and High Plains troughing will result in fairly amplified downstream ridging through most of the upcoming week, resulting in late spring to early summer-like temperatures. While plenty of dry time can be expected, there will be multiple opportunities for convection.

On Sunday, a strong warm front will surge northward as mid 990s mb surface low pressure tracks towards the Arrowhead region of Minnesota. If mixing is deep enough (ie. cloud cover not too thick), robust southwesterly gradient flow may yield gusts in excess of 40 mph, though will message peak gusts of 35-40 mph due to uncertainty.

A subtropical impulse emanating from near the Baja California will interact with the warm (temps in the 70s, locally ~80F southern CWA) and humid (dew points well into the 50s to ~60F) airmass over the region to result in increasing shower coverage. This will particularly be the case in the afternoon near and northwest of I-55 and area-wide Sunday evening. The synoptic pattern would be more concerning from a severe weather threat perspective if steeper mid-level lapse rates were anticipated. As is, the subtropical nature of the primary forcing mechanism will likely result in modest mid-level lapse rates, limiting instability and equilibrium levels.

For the above reason, confidence is higher in showers than in any thunderstorms materializing. The best chance for embedded thunderstorms is expected to focus over the northwest 1/3 to 1/4 of the CWA from the late morning through the early to mid afternoon until lapse rates further decline. There may then be another window for potential widely scattered embedded storms after midnight into early Monday. Well above normal precipitable water values and deep moist profiles with unusually high freezing levels could very well result in periodic downpours, though limited thunderstorm coverage should tend to keep rates and amounts in check.

During the upcoming workweek, the prevailing mid and upper level pattern will tend to focus ejecting short-wave energy and corresponding stronger large scale forcing to the north and northwest of our area. This lowers confidence in specific details, with more subtle impulses and/or MCVs playing a larger role. On Monday, any lingering convection from overnight should focus southeast and end in the morning. In the afternoon, height rises and dry mid-level air will likely suppress renewed convective development, with the best chance (30-40%) generally well south of I-80. Monday should have similar to slightly warmer highs than on Sunday.

Monday night is the first period of interest for organized convection, and it appears rather conditional, being tied to an MCS that *may* propagate across the upper MS Valley towards the western Lakes. If this impulse doesn't materialize or ends up farther north, chances for storms and any threat for severe weather (wind and hail) will commensurately decrease. On the other hand, in the presence of sufficient forcing, very steep modeled mid-level lapse rates support the level 1-2/5 severe threat.

Tuesday and Wednesday continue to look synoptically supportive for the SPC outlooked severe threats. However, as alluded to earlier, unknown (at this time) mesoscale details can be expected to modulate the relative threat into our area. Neutral height tendencies on Tuesday afternoon and evening, but the presence of the warm frontal boundary near or just north of the area lend to concern for supercellular storm mode and a threat for all severe hazards somewhere in the region. On Wednesday, large scale forcing will uptick some, but there's plenty of uncertainty regarding timing of an approaching cold front. Plus the parent surface low tracking well north and not quite as steep mid-level lapse rates may relegate the primary severe threat (if one materializes locally) to damaging winds. There eventually should be a quieter period later in the workweek, though confidence is low in specifics.

Castro

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 600 AM CDT Sat Apr 11 2026

Key aviation forecast messages:

- Lingering patchy fog early this morning in outlying areas.

- Spotty showers are possible this evening (30% chance).

- Increasing SE winds overnight becoming SW Sunday morning, with gusts to 30 kt likely by midday.

Shallow ground fog continues in low-lying areas outside of Chicago, particularly in river valleys. This is expected to erode and lift after daybreak.

Light and variable winds early this morning will return to a prevailing east to southeast direction after daybreak increasing to around 10 kt. High clouds increase in coverage through the day today ahead of a disturbance approaching the region. Associated decaying showers may then drift into the area into this evening, though coverage may be rather limited as they encounter a drier low-level airmass here locally and have accordingly maintained PROB30s for -SHRA with only minor timing refinement.

Winds then trend SSE late this evening and overnight. Speeds also increase steadily through the night with sporadic gusts into the lower 20 kt range possible by daybreak Sunday. There is a narrow window for low-level wind shear concerns during the ~9-14Z timeframe as a southwesterly low-level jet strengthens overhead (45-50 kt) prior to more efficient mixing setting up after daybreak. Have held off on a formal LLWS mention for now but may need to consider it with later updates. Winds then turn SSW Sunday morning with gusts increasing to near 30 kt by mid to late morning. Scattered showers may approach ORD and MDW by late Sunday morning, though it appears any isolated TS should focus northwest of the Chicago terminals.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

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