textproduct: Chicago

KEY MESSAGES

- Warm, dry and breezy conditions this afternoon will result in an elevated risk for brush fires across northern Illinois.

- Cooler conditions Sunday into Monday, with the next chance for higher rain chances on Tuesday.

DISCUSSION

Issued at 237 AM CDT Sat May 9 2026

In the very near term, the combination of light rain on Friday, clearing skies early this morning, and ongoing weak moisture advection has allowed for the development of shallow dense fog across central Illinois. Southwest winds in excess of 20 knots as low as 500 feet AGL will advect the fog and/or very low stratus over the far southern CWA through daybreak. Any fog/stratus should quickly erode after sunrise.

As a longwave trough centered over Hudson Bay remains nearly stationary through the weekend, a small Pacific-based wave will phase with a lobe from the longwave trough. The phased system will rotate across the Great Lakes region, pushing a cold front across the forecast area this afternoon.

The trough axis extending from the longwave trough is currently evident on IR satellite as a band of mid-level cloud cover over Wisconsin and far southeast Minnesota. Meanwhile, the faster Pacific wave will overtake the lead trough late this afternoon over or just east of the area. Expectations are for mid-level showers to yield virga late morning into early afternoon before potentially lowering and producing sporadic sprinkles or showers at the surface. That said, nearly the entire area will remain dry today.

Stout mixing, especially post-frontal, is expected this afternoon amid decent dry-air advection. With the upstream airmass characterized by dew points in the 20s, afternoon RH values will likely fall to 30 percent or lower. At the same time, low to mid-level flow will maximize over the area. While there remains some question as to whether the stronger gradient flow will be reflected all the way down to the surface, the deep mixing should allow the deepest eddies to tap into the strong flow (50 knots at 700 hPa) and produce a period of strong WNW/NW gusts mid afternoon. Would not be surprised to see a high sustained/gust ratio, with sustained winds under 20 mph and a few of the highest gusts 35mph or higher. With the combination of high winds, lower RH, and marginally low fine fuel moisture, elevated brush fire conditions are possible across northern Illinois for several hours this afternoon. Given the mild weekend day and the expectation for a higher prevalence of outdoor activities, will highlight messaging a bit more.

Yet another wave currently over Wyoming will quickly track across central Illinois late this evening into the overnight hours, yielding an axis of light rain across the far southern CWA. Cooler and drier conditions are expected Sunday and Monday. With high pressure nearing from the northwest and dry conditions noted by PWATs under 0.3 inches, another unseasonably chilly night with some patchy/areas of frost for outlying areas is in store Sunday night. A Frost Advisory may be warranted for interior portions of northern Illinois.

Broad ridging over the western CONUS will begin to edge eastward next week. However, decent agreement with a stronger trough (and rain/thunderstorm chances) digging southeastward across the Great Lakes Tuesday into early Wednesday will likely re-inforce, or at least slow the departure of, the long-standing trough over eastern Canada. This will also likely dampen the ridge and keep the warmest air to the south and west of the area.

Kluber

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/

Issued at 551 AM CDT Sat May 9 2026

Key messages:

- Increasingly gusty west-northwest winds this afternoon, possibly with some occasional gusts in excess of 30 kt after 19Z.

- Winds abate by sunset early evening and turn northerly as a cold front moves across the area. There is also a small potential (~15% chance) that winds briefly shift northeast at the Chicago terminals with a lake breeze early this evening.

The gusty west-northwesterly winds expected in advance of an approaching cold front this afternoon will be the primary weather story/concern across the area terminals. While gustiness up to 20 kt will begin to develop mid to late this morning as the surface gradient begins to strengthen, it appears even stronger wind gusts of at least 25-30 kt will materialize this afternoon as deep diurnal boundary layer mixing begins to top 10,000 feet AGL. Interestingly, westerly winds at this level are expected reside in the 40-45 kt range for a period mid to late this afternoon. Accordingly, the potential for mixing down this higher momentum aloft does add some concern for at least some sporadic higher gusts (perhaps around, or a bit in excess of, 35 kt) at area terminals this afternoon.

Winds will quickly abate early this evening and begin to turn northerly around sunset as the surface cold frontal passage occurs. While the winds may briefly shift more northeasterly off the lake with the front early this evening, directions should settle more northerly during the late evening and overnight hours.

Outside of the winds today, VFR conditions are expected at the main terminals, with only some higher Level CIGs around 10,000 feet possible this afternoon. A low (~20%) chance also exists for a few isolated light showers or sprinkles this afternoon.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from noon today to 7 PM CDT this evening for Winthrop Harbor IL to Gary IN.