Last Update: 5/19/2025 6:50 PM CDT
Relatively breezy and cool conditions will persist throughout the rest of this evening across the Chicago area with a mid 1030's surface high over the Hudson Bay, and an increasing surface pressure gradient in our area in response to deepening low pressure across the Great Plains leading to a cool northeasterly lake wind of 10-15 mph, gusting to 20 mph. This has limited high temperatures to the upper 40's, while areas further inland have reached the mid-upper 50's. Skies have remained mostly cloudy throughout the day due to a mid latitude cyclone and its parent trough slowly pushing east from the high plains, with the leading band of cloud cover driven by broad synoptic ascent arcing from central Indiana north into Lake Michigan, then WNW into the upper peninsula and northern Minnesota. Shallow plumes of mid-level moisture and marginal MUCAPE resulted in weak bands of showers and storms popping up across west central Illinois earlier this afternoon, though they gradually fizzled out as they moved east due to the drier low-mid level air mass in place in northern and eastern Illinois.
Aided by a mid level shortwave pushing NNE out of Missouri later this evening, the warm front currently stretching ESE from the surface low in northeastern Kansas through southern Illinois into eastern Tennessee will gradually work its way northwards into north central Illinois throughout the night. Increasing southwesterly synoptic ascent ahead of the front associated with this wave, along with increasing positive vorticity advection and diffluence ahead of the front will result in widespread coverage of showers throughout northeastern Illinois through early tomorrow afternoon. Thunderstorm chances look to be out of the question with this system, with lapse rates generally below 7 °C/km, and negligible CAPE, which can likely be attributed to the relatively cool air mass in place over our area, in addition to evaporative cooling that will come with any rainfall. Despite meager instability, the depth of moisture being advected into our area ahead of the warm front will likely bring periods of soaking rainfall, with PWATS greater than 1.5" in the southwestern regions of the Chicago metro. This combination of marginal instability and heavy amounts of moisture will result in rainfall rates potentially exceeding 0.2" inches/hour in the hardest hit areas. Most model guidance depicts total QPF between 1 to 2 inches by Thursday afternoon after rain chances have ceased.
As the main swath of widespread rainfall exits to the north of our area early tomorrow afternoon, the warm front should stall out somewhere south of I-80 and north of the Kankakee River, due to the colder air mass in place over our area, along with strong easterly surface winds in response to the surface low tracking into northwest Illinois near Davenport. This will allow for continued coverage of scattered showers throughout northern Illinois, with potentially a few elevated thunderstorms as mid level lapse rates increase tomorrow afternoon. Best chances for strong thunderstorms remain farther south of our area near the warm frontal boundary, where sunnier skies and consequential solar insolation may increase low level instability enough to allow for surfaced based storms. This will be limited by meager low level shear, as well as persistent low level capping.
Heading into tomorrow night, expect continued coverage of light scattered showers through Wednesday morning as a mid level shortwave pushes east through central Illinois. With the surface low approaching northwestern Illinois tomorrow morning, gradually increasing the surface pressure gradient, expect breezy easterly sustained winds of 15-25 mph, and gusts up to 40 mph to persist through tomorrow evening. As a result, high temperatures tomorrow will be limited in the northwest suburbs near the lake to the upper 40's and low 50's, while areas of the south of the city will reach the mid-upper 50's, and possibly 60's south of I-80. Persistent cloud cover should keep lows to the upper 40's and low 50's areawide for the next several nights.
Winds will shift to the WNW overnight tomorrow night, and along with persistent cloud cover and shower coverage, highs on Wednesday will be limited to the low-mid 50's areawide. The closed upper low will elongate as it gradually meanders eastward from the upper Midwest into the Great Lakes throughout the afternoon on Wednesday, sending another weak mid-level shortwave southeastward across northwestern Illinois Wednesday evening. This may result in periods of light scattered showers primarily in the northern half of the Chicago metro Wednesday night through Thursday morning, and will also maintain the relatively cloudy and cool conditions.
Guidance starts to get very inconsistent with the progression of the Great Lakes upper low system after Thursday, although EPS and GEFS ensembles generally indicated that upper level flow should remain approximately geostrophic through the first part of the weekend, diminishing chances for any precipitation. NBM also indicates generally dry conditions, although still on the cool side, with highs topping out in the low 60's and lows in the mid-upper 40's, with northerly winds and partly cloudy/mostly sunny skies.