textproduct: Chicago

KEY MESSAGES

- We'll be monitoring for fog development this evening into early Friday, with the highest chance for dense fog near the lake.

- Following an unseasonably warm and breezy Friday afternoon, yet another severe weather threat is anticipated late day Friday into Friday evening, especially near and west of I-39. There will also be an areawide threat for flash flooding.

- Cooler air arrives into early next week and may bring frost/freeze concerns early Sunday and Monday mornings.

DISCUSSION

Issued at 340 AM CDT Thu Apr 16 2026

Through Friday Night:

The pattern regionally will be much quieter today through Friday morning, giving a brief break to dry out ahead of the final storm system in the very active/stormy pattern over the past week. As the main mid-upper trough axis over the mid and upper MS Valley early this morning shifts over the central Great Lakes by early to mid afternoon, expect widely scattered showers and perhaps a few thunderstorms to focus over the northeast 1/3 to 1/2 or so of the CWA. A lake breeze slowly pushing inland may provide additional focus to modestly enhance coverage near the lake. Precipitable waters merely a bit above average for mid April (vs. prior summer-like values this week) will still be sufficient for localized downpours with any deeper cores, but limited coverage will keep flooding potential below 5%. Highs will range from the upper 60s to mid 70s (warmest south of I-80) for most today, though note that temps will fall into the 50s near the lake this afternoon.

Attention for late today through tonight will turn to pre- existing fog and very low stratus over southern Lake Michigan. Onshore winds (and cooling temps/decreasing T/Td spreads) in the wake of the lake breeze passage this afternoon could enable patchy fog to bleed inland near the shoreline prior to sunset this evening. Weak surface high pressure building over the lake will be conducive for a period of good radiational cooling and associated fog development and/or lakeshore fog slowly oozing inland. While confidence isn't all that high in the fog potential with inland extent, there's been enough of a signal for patchy fog mention over the northeast half of the CWA. Where confidence is higher, near the lake in northeast Illinois and far northwest Indiana, indicated areas of fog. Strengthening southerly flow aloft in the pre-dawn hours of Friday may help erode some of the fog, with the possible exception of near the Illinois shore.

Friday will be the last summer-like warm and breezy day of this stretch, out ahead of an elongated surface low lifting towards western Lake Superior. Expect highs in the low to locally mid 80s away from the far northeast IL shore (which will reach the 70s towards evening), and southerly winds gusting up to 30-35 mph. Lingering lower dew points from underneath the departing surface high, south-southeast winds to offset surface moisture return with eastward extent, and parched mid-levels will yield a fairly sharp instability gradient from the MS River to the longitude of Lake Michigan by mid Friday afternoon.

The big question marks for the severe setup are with respect to timing of convective initiation near the MS River and storm mode upon approach into northwest and north central Illinois late Friday afternoon or early Friday evening. Deep layer shear will once again be supportive of severe weather. If CI occurs earlier (by ~3-5pm) and sufficiently close to our far western CWA counties, there may be a play for initial discrete/semi- discrete supercells with an attendant damaging hail threat. Even in northwest IL, fairly large T/Td spreads may serve to limit an appreciable supercellular tornado threat.

After possible initial discrete mode, there should be a quick transition to linear/QLCS as cold frontal forcing takes hold. The CI timing question is thus also important in terms of far east a damaging wind (and embedded QLCS tornado threat) will reach Friday evening until the threat wanes overnight Friday night. The drier profiles into the Chicago metro and points south will be deeply mixed with inverted V characteristics at the low levels. So even if storms get going early and transition to a weakening squall line outracing the instability gradient, an earlier arrival would still entail a severe wind threat with outflow dominant segments. All in all, SPC's initial day 2 outlook appears reasonable for the setup, with a 3/5 threat level (enhanced risk) near/west of I-39 tapering to a 1/5 threat level (marginal risk) near/east of I-57.

A very progressive MCS/QLCS Friday night would limit the residence time of torrential rainfall and the associated flooding threat. However, with the very rainy stretch of late for much of the area and already elevated river levels, it won't take much for renewed rises and localized flash flooding. Backbuilding of convection behind initial outflow boundary, on the other hand, would correspondingly increase the flood threat, especially for hardest hit areas this week and/or urban areas.

Saturday through Wednesday:

The most recent model guidance for Saturday has pretty unanimously sped up the timing of the cold frontal passage to yield a mostly dry day for much of if not the entire area. Best chance (30-50%) for any lingering showers past sunrise will be for areas southeast of I-55. Saturday afternoon will trend to partly cloudy to mostly sunny, albeit notably cooler (50s) with blustery west-northwest winds. Cold air advection and dry advection through Saturday night-early Sunday will set the stage for lows in the 30s, possibly flirting with freezing across parts of interior northern IL. Following a cool and breezy Sunday, ~1030 mb high pressure over the region will likely be conducive to at least areas of frost as temps dip to the low-mid 30s outside of Chicago, if not colder in spots. Agricultural interests are advised to monitor forecasts through early next week.

Cooler conditions into Monday morning will be short-lived however, with extended guidance favoring a return to high amplitude ridging across the central CONUS and unseasonably warm temperatures returning to the region by midweek next week. Despite the returning warmth next week away from Lake Michigan, a blocky mid-upper pattern will favor much less stormy conditions over the local area, giving a longer period to dry out from this week's rain.

Castro

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1233 PM CDT Thu Apr 16 2026

Key Messages:

- A few VCTS expected early this afternoon, particularly across the main Chicago area terminals.

- Easterly wind shift with lake breeze either late this afternoon or early this evening.

- Fog/low CIGs likely to shift in off the lake this evening and tonight. Potential exists for LIFR to VLIFR conditions are the Chicago area terminals overnight.

A weather disturbance, currently shifting overhead across the northeastern IL terminals, is fostering a few lines of showers and thunderstorms early this afternoon. While these will be around the terminals through around 19Z, the main focus for these is expected to expected to gradually slide east- southeastward through the afternoon. However, a few additional isolated showers and storms could develop overhead in the wake of this activity. Should any additional showers develop, their coverage should remain low and largely hit or miss.

Otherwise, breezy westerly winds this afternoon will shift east- northeasterly by early this evening as a lake breeze pushes inland across the Chicago area terminals. Thereafter, concerns will reside around the increasing potential for fog and very low CIGs to move in off of Lake Michigan this evening and tonight. Accordingly, we have opted to hit the VSBYs and CIGs harder with the morning update, particularly for the main Chicago area terminals. Admittedly, however, conditions could be even be lower than what is currently highlighted in the TAFs for a period tonight. Conditions should then improve by mid- morning on Friday as the surface flow begins to increase and become gusty from the south-southeast.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

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