textproduct: Chicago

KEY MESSAGES

- Increased grass/brush fire threat today due to unseasonably warm, dry, and breezy conditions.

- Periodic shower and thunderstorm chances return Friday and into next week.

- Low (level 1 out of 5) severe weather threat as early as Friday afternoon, but mainly Friday evening/night along with a threat for locally heavy rainfall.

DISCUSSION

Issued at 208 AM CDT Thu Jun 4 2026

Today will feature warm, dry, and breezy conditions as high pressure shifts eastward and broad/deep southwesterly flow takes its place. Southwesterly winds, gusting occasionally as high as 25 mph, will push mid/upper 80 degree temperatures all the way to the lake today. The one small exception may be the extreme NE parts of Lake County, IL where some onshore component could linger through the afternoon. The combination of southwesterly breezes, RH values falling towards 25 percent, and the dry state of fuels will lead to a modestly elevated fire danger this afternoon.

The airmass will undergo a substantial character change late tonight into Friday as a plume of deep tropospheric moisture to our west finally sloshes eastward. As this moist plume advances, precipitation chances will start to increase across our far NW Illinois locales towards the pre-dawn hours on Friday.

Somewhat of a tricky forecast evolution is apparent into later Friday morning and afternoon. Some (but not all) guidance has trended a bit more robust with an initial lead shortwave trough advancing overhead during this timeframe (potentially along with a convectively-augmented MCV), with a notable increase in southwesterly 700 mb flow occurring through midday. The ECMWF and NAMNest have consistently shown this signal over the past few model cycles, and now seeing more of this flow enhancement appearing on guidance like the CMC and GFS to some degree. Mid and upper-level temperatures are not forecast to be that cold-- actually quite "mild" for this time of year with H5 temps progged near -8 C. This will result in very muted mid-level lapse rates and associated overall instability even though surface dewpoints will be in the upper 60s to near 70 degrees.

At this time, it looks like limited instability (ascent arriving too early in the day) and a lack of stronger deep-layer wind shear should tend to limit the potential for stronger storm development. That said, with a tropical-like atmosphere advancing into the region, it won't take much insolation to erode surface-based convective inhibition. A slower progression (shortwave arriving more into the mid afternoon), or more instability could result in an increased threat for a few low- topped supercells and an attendant brief tornado threat given enlarged hodographs in the 0-4 km layer. This does not appear to be the most likely outcome at this time, however, with general scattered shower and thunderstorm activity anticipated into the afternoon hours.

A nocturnally-intensifying LLJ interacting with a remnant quasistationary boundary in the region looks to drive an additional increase in shower and thunderstorm development Friday night into early Saturday. Somewhat steeper mid-level lapse rates will yield a bit more MUCAPE during this period, and the effective deep layer shear is also forecast to increase somewhat. This may support some degree of organization with a threat for a few stronger storms on the nose of the incoming LLJ. Lots of uncertainty though regarding both the orientation of the LLJ (west-east vs southwest-northeast) and location of the low-level theta-e/moist axis, so there remain quite a bit of uncertainty regarding the coverage and placement of nocturnal storms. If robust convection develops in the region, upwind Corfidi vectors weakening under 10 kts elucidate the potential for some training as strong veering takes place through 500 mb. Given all of the mesoscale uncertainties at this range, the general level 1 out of 4 ERO looks appropriate at this range.

Some convection may linger into Saturday morning, but with the main shortwave departing to the east, synoptic forcing is expected to diminish, suggesting precipitation coverage should as well. If a mature/widespread MCS doesn't plow through the region through early Saturday morning, a moist and unstable airmass is forecast to develop with daytime heating. Widely scattered thunderstorms would then be possible, focusing on any remnant boundaries and/or a developing lake breeze during the afternoon. Activity will probably fester into the evening. On Sunday, the main axis of deeper moisture and associated shower and storm chances looks to focus mainly across the southwest half of the CWA.

The moist axis then looks like it'll attempt to slosh back across the area early next week, with additional chances for showers and isolated storms at times.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/

Issued at 636 AM CDT Thu Jun 4 2026

Key concerns for the 12Z TAF period:

* Showers and possible thunderstorms Friday morning with associated MVFR

Light SSW winds will open up after 14Z and gust to around 20 kt during the day, especially the afternoon and early evening. They'll subside for the evening but occasionally continue to tag 10+ kt. SW winds will ramp up early Friday in advance of a system of precip.

Showers and potential embedded thunderstorms will approach RFD early Friday, just beyond the scope of the current 24-hour TAF period. They look to work into Chicagoland mid to late morning. Fair amount of uncertainty exists in coverage/occurrence of precip during the morning with generally higher coverage expected during the afternoon. A period of MVFR cigs is favored with these showers during the morning and into afternoon.

Doom

LOT WATCHES/WARNINGS/ADVISORIES

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