textproduct: Chicago

KEY MESSAGES

- The warmest temperatures since May 15th are anticipated today, as high temperatures flirt with the 90 degree mark.

- Shower and thunderstorm activity tonight and on Thursday should focus near and/or north of the IL/WI state line, though may then sag southward into parts of our area Friday-Saturday.

- There is a signal for hot and humid conditions to develop across the general region next week with periodic shower/storm chances.

DISCUSSION

Issued at 327 AM CDT Wed Jun 11 2025

Through Thursday:

A quasi-zonal upper-level pattern characterized by broad but low amplitude ridging over the northern CONUS will continue to expand eastward into the Great Lakes today as surface high pressure settles over the southeastern CONUS. This will set the stage for a much warmer day today as breezy west-southwesterly winds drive a lower-level thermal ridge (featuring 850 mb temps of +18 to +19C) overhead. This very warm low-level airmass in combination with mostly sunny skies through the day is expected to push temperatures well into the 80s this afternoon. Some 90 degree readings are also in play for much of northern IL.

A weak west-to-east oriented cold frontal boundary is expected to sag southward across southern WI later this afternoon, then into far northern IL tonight before stalling near the I-88 corridor into Thursday. Another very warm day (highs well into the 80s) will be in store again for most on Thursday, though persistent onshore easterly flow north of the surface frontal boundary will result in much cooler conditions (70s) near the northern IL Lake Michigan shore Thursday afternoon.

The local threat for thunderstorms in this pattern tonight continue to trend lower for IL and IN. Instead, the primary threat for showers and thunderstorms tonight will be tied to the path of a couple of mid-level impulses currently shifting eastward into the western Dakota's early this morning. With these expected to track eastward across southern MN and into WI later this evening into tonight, the current expectation favors most, if not all of the storms tonight to remain north of the WI stateline. Moisture profiles south of the stateline look to remain rather poor, thus supporting increased capping. Accordingly, we continued the trend of cutting back POPs tonight, with only a slight chance (20%) still in the forecast for areas near the WI state line.

Our local threat for showers and thunderstorms on Thursday has also trended lower, owing to continued poor moisture and a split mid-level flow regime. Instead, the main threat for storms on Thursday looks to be focused to our north in closer proximity to the lower-level baroclinic zone, and also to our southwest in association with a southern stream mid-level low shifting into the Ozarks. Consequently, POPs have been lowered into the slight chance (20%) category for our area on Thursday.

KJB

Thursday Night through Tuesday:

Overall the forecast for a soggy end to the work week remains on track. While guidance has started to converge on a solution, there remains a fair amount of uncertainty particularly with timing and precipitation coverage Friday and Saturday. Therefore, expect the details discussed here to undergo further refinement with future forecast updates.

A stalled frontal boundary is expected to be draped across portions of northern IL and northwest IN (likely near I-80) Thursday night. While it does appear that subtle mid-level ridging should be in place overhead, still cannot completely rule out a lingering isolated shower and or thunderstorm (especially Thursday evening north of I-80) if something is to develop Thursday afternoon along the front. Otherwise, a mostly dry night should be seen across the area with mostly cloudy skies and temperatures in the lower to mid- 60s though some locations near the IL-WI line could see some upper 50s.

Heading into Friday, an upper low is expected to be lifting into the Mississippi Valley Friday morning and then eject into the Ohio River Valley Friday night. As this occurs, warm advection is expected to increase across northern IL and northwest IN (characterized by southerly winds) which should force the aforementioned front back north through the day. The increasing forcing, due to the low and warm advection, within the presence of 1.5 to 1.6 inch PWATs should lead to a fair coverage of showers and embedded thunderstorms Friday afternoon. Note that while guidance has come into better agreement on this evolution, there remains some uncertainty in the arrival time of the low and how far north the front will be when the low gets here. Therefore, there is a scenario in which the northern half or so of the area could remain mostly dry with those to the south (namely south of I-80) seeing a greater coverage of showers and storms. For now have maintained the NBM's broad 40-50% POPs areawide but suspect future forecasts should be able to trim these back somewhat. Regardless, temperatures on Friday look to warm into the lower to mid-80s for most though lingering onshore winds will keep cooler conditions along the northeast IL lakeshore.

Thunderstorm coverage is expected to diminish Friday night as we lose diurnal heating, but scattered showers are expected to persist as the upper low meanders its way across the region broadening into an baggy trough as it does so. At the same time, a secondary shortwave trough is forecast to pivot across the Upper Midwest Friday night through the day on Saturday which should push the front back through the area. The combination of frontal convergence and the broad forcing from the decaying upper low will redevelop showers and thunderstorms on Saturday particularly during the afternoon hours. That said, with the aforementioned uncertainty as to how far north the front will get on Friday will ultimately dictate the northward extent of the rain coverage for Saturday. Though, the frontal orientation should favor the greatest coverage of showers and storms along and east of I-55. As for temperatures, highs on Saturday should still reach into the lower 80s for areas south of the front while those to the north and near Lake Michigan see readings in the mid-70s and mid-60s respectively.

The front will continue to move through the area Saturday night which should finally allow showers and storms to exit our area by Sunday morning. However, lingering northwest flow aloft in combination with northeast surface winds will keep temperatures several degrees cooler Sunday afternoon with highs in the mid to upper 70s inland and in the 60s near the lake. It should be noted that our official forecast does have 20% POPs due to a handful of guidance members being slower with the front's exit, but the expectation is for dry conditions areawide.

As previously advertised, a broad upper ridge is still forecast to develop across the central CONUS late Sunday into early next week. While our area will be cooler on Sunday as mentioned above, temperatures are forecast to begin to warm on Monday with highs back into the 80s. However, lingering east-southeast winds on Monday will keep temperatures in the 70s near the lake. Temperatures will continue to warm through the rest of next week (including near the lake) as the ridge axis meanders eastward. That said, there remains a notable signal in guidance for periodic disturbances to round the ridge next week as well which could result in chances for showers and thunderstorms at times especially towards the later half of the week.

Yack

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 622 AM CDT Wed Jun 11 2025

Forecast concerns for the terminals...

- Increasing southwest winds this morning with gusts in the 20-25 kt range expected.

- Frontal boundary to move through late tonight resulting in a northeast wind shift.

A surface high will continue to drift across the region today resulting in mostly clear skies and warm temperatures. However, a plume of SCT to BKN mid-level clouds will swing through the area this morning before eroding by midday. Winds will increase this morning as diurnal mixing allows the atmosphere to tap into the low-level jet overhead resulting in 20-25 kt gusts. The strongest gusts are expected through 18-19z with occasional 18-20 kt gusts lingering through the afternoon. Gusts will fully subside this evening with speeds remaining around 5-7 kts overnight.

A weak disturbance will pivot across portions of the Midwest this evening and overnight leading to the development of scattered showers and thunderstorms. These showers/storms are expected to remain in WI, but an isolated shower or two may drift into far northern IL (including RFD) after 03z. Due to lower confidence (<20% chance) have opted to maintain a dry forecast for now. That said, a frontal boundary will pivot through northern IL and eventually northwest IN late tonight (after 06z) resulting in a northeast wind shift at the terminals. Otherwise, expect VFR conditions to prevail tonight through Thursday.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

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