Mendota, Illinois Weather Forecast Discussion

453
FXUS63 KLOT 061726
AFDLOT
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1226 PM 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 /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1226 PM CDT Mon Oct 6 2025
Key Aviation Messages for the current TAF period:
- Periods of rain/showers mid-late afternoon onward, along with
  a chance of thunderstorms into or through the evening
- Winds shift to northerly early evening
- Deteriorating CIGs late tonight-early Tuesday, with reduced
  VSBY also possible in BR and any drizzle
Pre-frontal winds out of the southwest will continue through the
afternoon with gusts around 20 knots. As the cold moves through,
winds will switch to the north. Some recent models are
suggesting that the wind shift at Chicago terminals could be at
01Z or even 02Z. With lower confidence in the exact timing, the
TAFs pushed the wind shift at ORD back to 00Z to at least trend
toward a later front arrival.
There is a chance for isolated storms to develop out ahead of
the frontal movement. With lower confidence in thunder at an
individual terminal, VCTS was added for the earliest chances at
22Z. Better coverage of showers is expected after 00Z with the
fronts arrival and maintained the PROB30 for -TSRA during this
period. No changes were made on timing with showers and thunder
chances at KRFD.
Cigs will are expected to deteriorate tonight. MVFR cigs will
become IFR after midnight, with the possibility of LIFR cigs
closer to the lake as showers and drizzle linger into Tuesday
morning. Rain will taper off from west to east after 12Z. MVFR
cigs may linger through the middle of the morning, but
eventually return to VFR conditions. Winds will remain out of
the northeast with gusts around 20 knots for terminals closer to
the lake.
DK
&&
.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.
&&
$$
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NWS LOT Office Area Forecast Discussion