Area Forecast Discussion

KEY MESSAGES

- Threat for scattered thunderstorms this afternoon and evening. Severe threat is low, but a few strong/gusty cores possible, particularly south of I-80 and east of I-55.

- Warming trend through Thursday (except cooler near Lake Michigan on Wednesday).

- More widespread showers and thunderstorms with another cold front Thursday night into Friday.

DISCUSSION

Issued at 218 AM CDT Tue Apr 21 2026

Water vapor loops reveal a low-amplitude shortwave embedded within broad northwest flow, shifting steadily eastward into northwest Iowa and southwest Minnesota early this morning. Ahead of this feature, a southwesterly LLJ is near its nocturnal peak, but will remain robust as it drifts across parts of the region later this morning. Expectation is for surface gusts to increase quickly after sunrise as mixing into the base of this arriving speed max begins. Based on the progression and orientation of the strongest flow, there will likely be a notable north- south wind gust gradient across the area today, with the strongest occurring roughly east of I-55 and south of I-80. Given fairly deep mixing (at least to 850-800 mb), have boosted wind gusts towards the NBM 90th percentile (35+ mph), which aligns better with mean boundary layer flow near 30 knots during the afternoon. In addition, have dropped dewpoints across the southeast third of the forecast area, in-line with the deeper-mixing guidance as dewpoints mixed out pretty aggressively yesterday. With the overlap of stronger winds and afternoon RH values near 30 percent, parts of the area may approach an elevated grass fire danger as fine fuels have dried (Midewin RAWS site at 7% yesterday afternoon), and this part of the forecast area hasn't received nearly as much rain as farther north. Will highlight this in the fire weather planning forecast.

The other focus today is on what appears to be an increasing potential for thunderstorms this afternoon and into the evening. The aforementioned shortwave will transport a renewed elevated mixed layer into the area later this morning and afternoon. The upper air sounding at North Platte, NE sampled the core of this EML plume as it advected off the high terrain, with surface-5 km lapse rates near dry adiabatic. As this plume of steep lapse rates arrives in our northwest through the morning, forecast guidance nearly ubiquitously shows increasing saturation taking place at the top of the EML near 600-500 mb along with the eventual initiation of very high-based convection rooted above 6-7 kft. Convection initially may remain fairly sparse, but it's now looking like at least widely scattered coverage is in the cards across parts of northeast Illinois and into northwest Indiana during the afternoon hours.

As this activity continues southeastward, there may be a potential for some stronger wind gusts as cores work into an area of deeper mixing. Overall, instability will not be that significant (likely a few hundred J/kg), but the dry sub-cloud layer may promote some strong gusts to the surface, particularly south of I-80 and east of about I-55. If cores manage to sustain and grow (which also is not entirely clear at this point), couldn't rule out the potential for a few 60+ mph gusts and perhaps some small hail. Additional convection may develop this evening as a southwesterly LLJ impinges upon some increased 925-850 mb moisture and overtops a remnant outflow boundary. The orientation of the mid and upper level wind field (strong veering to the northwest with near-surface southerlies) can result in localized training issues. So while the multi-model CAM suite really doesn't show a strong signal for this now, will need to keep an eye on this in the vicinity of the IN/IL state line this evening.

A backdoor front/boundary will push into the region through late tonight. This will turn winds onshore at the lakefront tonight. Can't rule out that areas of fog bleed in off the lake late tonight into Wednesday morning, but neutral to mixy modified Richardson number values increase uncertainty as to how far inland this threat will extend. For now, have confined fog wording to the first mile or two of the lakefront.

Onshore flow will persist through the day on Wednesday, and the combination of stratus and fog over the lake will likely hold high temperatures in the mid 50s lakeside while inland locales will warm into the 70s. This boundary looks like it'll stall somewhere in our southwest through the day, and with dewpoints forecast to rise to near 60 to its south, additional thunderstorm development will be possible through midday and afternoon hours Wednesday. Forcing appears very nebulous, and likely limited to general near-surface convergence, so made no alterations to the NBM-delivered slight chances in our far southeast at this time.

Warmer and breezy conditions will develop on Thursday, with lingering lake cooling likely confined to portions of Lake County (IL) and perhaps far northeastern Cook. The next synoptic trough will roll overhead through Thursday night delivering another round of more widespread showers and some thunderstorms into Friday morning. Limited instability, the climatologically unfavorable time of night, and overall weak flow suggests the severe threat should remain pretty low with this activity.

A cold front will send temperatures back to near-normal levels over the weekend. Another system will bring the next chances for showers and storms back to the area early next week.

Carlaw

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 1250 AM CDT Tue Apr 21 2026

Key messages for the 06Z TAF period:

* SSW winds occasionally gusting to near 20 kt overnight.

* Marginal LLWS setup overnight.

* A few hour period of showers and thunderstorms Tuesday afternoon in Chicagoland. Associated gusty winds and vsby reductions possible.

* A signal for dense fog/low stratus late Tuesday night into early Wednesday. Low confidence on impacts.

SSW winds overnight will increase a bit after 06-07Z as a low level jet builds overhead with occasional gusts into the teens to near 20 kt. A marginal LLWS environment will set up overnight with as many as 40 to 45 kt of SW flow at 2kft AGL. SW winds will build quickly during the morning to feature 20 to 25 kt gusts for much of the daytime.

A broken line of showers and possible thunderstorms is anticipated to develop over northern IL and drop south across the Chicago metro this afternoon. Periods of moderate rainfall with MVFR vsbys will be possible. Localized gusty winds to near or over 30 kt are also on the table. Favored window for ORD/DPA is 19-22Z, and 20-23Z at MDW/GYY. While not out of the question, precip looks to remain east of RFD this afternoon.

Winds will ease and veer to NW behind the showers, then onto NE by late Tuesday night, likely with some variable behavior at times during the evening. Dense fog is favored to develop over southern Lake Michigan and protrude inland late Tuesday night into early Wednesday. There is a lot of spread in model guidance as to how exactly this will play out at the terminals; whether more of a fog issue vs stratus, how low conditions may get, and how far inland impacts will spread. At least a period of MVFR cigs and vsbys is strongly favored, although a noteworthy signal exists for IFR to LIFR conditions after around 08Z, mainly around Chicago and Gary. With the uncertainties, introduced MVFR for the last few hours of the ORD/MDW 30-hr TAF, but refinements will be needed as confidence in impacts grows.

Doom

LOT WATCHES / WARNINGS / ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 9 AM this morning to 4 PM CDT this afternoon for Wilmette Harbor to Calumet Harbor IL.

Small Craft Advisory from 9 AM this morning to 7 PM CDT this evening for the IN nearshore waters.