368
FXUS63 KLOT 061143
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
643 AM CDT Mon Oct 6 2025

.KEY MESSAGES...

- One last day of unseasonable warmth today before a return to
  more typical early October-like temps for the remainder of the
  week.

- A couple/few waves of showers possibly a few thunderstorms
  this afternoon through early Tuesday morning.

&&

.DISCUSSION...
Issued at 319 AM CDT Mon Oct 6 2025

Water vapor imagery early this morning shows upper trough slowly
moving east across the northern Plains and into the upper
Mississippi Valley and upper Great Lakes. Primary associated
surface low is over Ontario with a cold front trailing south
across eastern MN into central through southwest IA. As the
upper trough and associated sfc low move eastward today, they
will drag the cold front into northwest IL this afternoon and
slowly across our CWA late this afternoon into tonight.

Our CWA will have another day of unseasonable warmth ahead of
the front, as highs once again climb into the 80s with a good
deal of sunshine through the morning before cloud cover begins
increasing this afternoon. Will once again see dewpoints mix out
across most of the warm sector, limiting instability and
convective potential ahead of the front.

Guidance does suggest that there will be a narrow ribbon of
higher dewpoints (60s) pooled along and just ahead of the front,
which is already noted upstream over IA. This narrow axis of
higher boundary layer moisture should allow for an axis of
modest sfc based destabilization along and just ahead of the
boundary with MLCAPE values peaking in the 500-750 J/kg range.
Given the anafrontal nature of the cold front, frontal forcing
along the leading edge of the cold front looks to be pretty
shallow. The shallow depth of forcing and frontal convergence
does raise questions about whether storms will indeed form along
the front and if so, how much coverage.

Having said that, forecast soundings show little or no
inhibition in the narrow instability axis near the front, so it
is possible that modest forcing could be enough for scattered
thunderstorms this afternoon. While instability looks limited,
30-40kt of effective bulk shear could lead to a stronger
thunderstorm or two if storms do indeed develop near the sfc
front.

An additional wave or two of showers (and perhaps isolated
thunderstorm or two) is expected behind the front tonight into
early Tuesday morning in response to strengthening 800-700 mb
frontogenesis associated with the thermally direct circulation
in the right entrance region of the 100kt+ upper level jet. Some
enhancement to the large scale ascent is possible later this
evening and overnight as we potentially get some jet coupling
in the left exit region of a weaker southern stream jet streak.
Guidance varies some on the evolution of precip overnight, the
NAM most aggressive, with strong upper level divergence and
the coupled jet/subsequent strong low-mid level f-gen response.
Confidence in exactly how this plays out tonight is on the
lower end of the spectrum, but considering the high PWATs in
place, if the stronger NAM solution pans out, a few areas could
see some heavier rainfall totals. If the stronger forcing in the
NAM doesn`t verify, then some light showers/drizzle would
remain possible overnight.

Skies will eventually clear out from north to south Tuesday
afternoon, with much cooler temps expected in the wake of the
front. A brisk north wind off the lake will keep temps near the
lake in the low to mid 60s all day. The remainder of the week
and likely next weekend look mostly dry with seasonable temps.
There does look to be an Alberta Clipper-like shortwave progged
to move across the Great Lakes region in the Friday/Friday
night time frame. However, at this point moisture looks limited
and the track north of our area, so based on current model
guidance, the precip chances look minimal with that feature.
Temperatures should return to above average levels over the
weekend in advance of the next trough, which could bring some
rain chances back into the area early next week.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 643 AM CDT Mon Oct 6 2025

Main Concerns:

- Periods of rain/showers mid-late afternoon onward, along with
  a chance of thunderstorms into or through the evening

- Timing of wind shift to northerly late this afternoon-early
  evening

- Deteriorating CIGs late tonight-early Tuesday, with reduced
  VSBY also possible in BR and any drizzle

The first real weather of note in over two weeks will cause
some impacts to flying later today through tonight. VFR, rain-
free conditions should persist until just ahead of, and
especially behind a cold front passage later today. Expect
breezy southwest winds by mid morning until the anticipated
frontal wind shift. Maintained the same timing as in the
previous TAF issuance for the wind shift to north/north-
northeast. GYY may end up seeing lighter and VRB winds for a
time as the frontal push slows, however. Right behind the wind
shift should bring showers and perhaps embedded TS. The chance
for embedded TS should end earliest at RFD, while likely
lingering through this evening over the Chicago metro.

CIGs are expected to deteriorate late tonight, with prevailing
IFR expected at the Chicago metro terminals, and LIFR possible,
especially closer to the lake. Drizzle and light fog may
accompany continued on and off rain, potentially knocking
visibility down into the 3-5 mile range. Some improvement is
expected by mid to late Tuesday morning as any lingering rain
ends, along with 15-20 kt wind gusts from the north-northeast.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 1 AM Tuesday to 3 PM CDT Wednesday
     for the IL nearshore waters.

     Small Craft Advisory from 4 AM Tuesday to 10 AM CDT Wednesday
     for the IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago