839
FXUS63 KLOT 200833
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
333 AM CDT Fri Sep 20 2024

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms ending early this
  morning

- Widely scattered thunderstorms expected to redevelop this
  afternoon, mainly near/east of I-57

- Unseasonable warmth will continue through Saturday, with one
  last 90 degree day of 2024 possible

- Periods of showers and some thunderstorms late Saturday night
  possibly continuing into Monday

&&

.DISCUSSION...
Issued at 333 AM CDT Fri Sep 20 2024

Water vapor imagery early this morning shows a vigorous
shortwave trough moving eastward across the upper Great Lakes.
Some showers and scattered thunderstorms across northwest and
far northern IL appear to be on the southern flank of the
stronger ascent associated with this shortwave trough. The
convection across northern IL is generally expected to be on a
weakening trend over the next few hours as it moves east of the
more slowly eastward advecting instability axis and the 30-35kt
low level jet weakens/veers. By sunrise, expecting just some
widely scattered showers and possibly an isolated thunderstorm,
with activity expected to dissipate by completely by 13/14z.

Weak cold front extending from NW WI south across eastern IA and
into northern MO at 08z with move eastward across the CWA today.
Low level moisture pooling along and just ahead of the cold
front (sfc dewpoints mid-upper 60s) will support an axis of
moderate instability (MLCAPE 1000-1500 J/kg) by early afternoon
across eastern portions of the CWA. Mid-upper level trough
associated with the initial shortwave over the upper Great
Lakes should be east of the area this afternoon. However,
another, more subtle shortwave is evident on water vapor
imagery moving into western MN early this morning. This wave
should remain to our north, but perhaps close enough to provide
just enough synoptic ascent to allow some widely scattered
convection to develop along the approaching weak cold front.
Can`t rule out development midday over far eastern IL near/east
of I-57, but the slightly better chances should be over
northwest IN this afternoon. Any afternoon convection should be
out of our CWA by late afternoon. Can`t completely discount a
marginally severe storm, but there are quite a few negatives
working against a meaningful severe threat, including weak
synoptic forcing and weak frontal convergence, substantially dry
mid-upper level air mass, and poor mid-level lapse rates.

There`ll be a break in the precip threat tonight into Saturday
with Saturday looking to feature a good deal of sunshine.
Guidance is in pretty good agreement with afternoon 925mb temps
22-25C, which would support high temps near or just above 90F.
Lower dewpoints in the wake of today`s front should keep heat
indices generally lower than ambient temps, but it will still be
quite warm for this time of year. In fact, highs could end up
just a couple/few degrees shy of the record high of 94F (both
Rockford in 1920 and Chicago in 2017).

Saturday night into Sunday, closed mid-upper level low over the
desert southwest will move eastward and emerge over the central
High Plains. Lead shortwave emanating from this upper low,
possibly with some assistance from strong upper low tracking
east across the U.S. Canadian border, should lead to showers
and thunderstorms developing across the Cornbelt and northern
Missouri later Saturday into Saturday night. This activity is
expected to move/develop eastward into our CWA, possibly as
early as later Saturday night, but more likely Sunday. Moisture
pooling near a stalled boundary certainly would support some
potentially beneficial rainfall, though there are some lingering
questions about the magnitude of forcing this far east of the
main southern stream upper low and south of the northern stream
shortwave. Assuming the boundary stays parked nearby, the
threat of showers and thunderstorms could continue Sunday night
and even into Monday as the weakening upper low over central
Plains moves east into the mid Mississippi Valley Monday.

In the wake of that system, long wave troughing should become
established next week over the Great Lakes which should result
in more seasonable temperatures and dry conditions as Canadian
high pressure ushers in a dry and very pleasant air mass.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1240 AM CDT Fri Sep 20 2024

SHRA/TSRA extending from southern WI southwest into IA will
continue to move gradually eastward into northern IL during the
predawn hours. This activity is likely to weaken as it
progresses eastward across northern IL. Greatest TS chances
remain at RFD (60% chance) with chances dropping off
considerably over the Chicago terminals (less than 30%). In
fact, it is possible that this activity could dissipate
altogether before reaching Chicago toward dawn, but maintained
TEMPO for now and will monitor trends overnight.

Cold front will move across the terminals today with southwest
winds this morning expected to veer to west and then northwest
by later this afternoon. Lake breeze will develop this afternoon
and result in wind shift to northeast at GYY and could threaten
MDW with a wind shift to northeast for a time early this
evening. Confidence wasn`t high enough to include wind shift in
MDW TAF, but this will need to be monitored for later TAF
issuances.

- Izzi

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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