-- Discussion containing changed information from previous version are highlighted. --
905
FXUS63 KLOT 201541
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1041 AM CDT Fri Sep 20 2024

.KEY MESSAGES...

-- Changed Discussion --
- Widely scattered showers and thunderstorms expected to
  redevelop this afternoon, mainly near/east of I-55

- 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
-- End Changed Discussion --

&&

.UPDATE...
-- Changed Discussion --
Issued at 1041 AM CDT Fri Sep 20 2024

A cold front continues to make progress across northwest IL this
morning which will aid in developing additional showers and
thunderstorms this afternoon. Ahead of the front, a plume of mid
to upper 60s dew points has started to destabilize the
atmosphere as noted by the developing accas over the Chicago
metro. While coverage of showers has been rather limited thus
far, this accas plume will need to be monitored as it tracks
into northwest IN towards midday as it may allow showers and
thunderstorms to develop a tad earlier than originally
anticipated. That said, have gone ahead and introduced some
20-30% chances for showers and thunderstorms to the forecast
from now (1040 AM) through this afternoon to account for the
potential for earlier development, especially for areas along
and east of I-55.

Furthermore, a subtle 500 mb speed max is also being analyzed
over northern IL and southern WI this morning which is providing
around 30 kts of effective shear. While this magnitude of shear
may lead to some stronger storm cores this afternoon, the threat
for widespread severe weather remains rather low (<10% chance).
Though, a localized thunderstorm wind gust of 50 to 60 mph
cannot be completely ruled out just yet. Thus the Storm
Prediction Center has maintained a level 1 of 5 risk for areas
east of I-55 for this potential. Regardless, showers and storms
are expected to taper early this evening as the aforementioned
front exits to the east.

Aside from the showers and storms, another unseasonably warm
afternoon is expected areawide with otherwise mostly to partly
sunny skies. High temperatures will once again top out in the
mid to upper 80s, but a weak lake breeze will keep readings
slightly cooler (in the upper 70s to lower 80s) at the
lakeshore.


Yack
-- End Changed Discussion --

&&

.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 /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 625 AM CDT Fri Sep 20 2024

VFR conditions are expected at the terminals today as some
widely scattered thunderstorms develop east of the terminals
early-mid afternoon. Winds will become southwesterly this
morning before shifting to northwest this afternoon. A lake
breeze is expected to develop this afternoon, surging into NW
Indiana first, resulting in a wind shift to northeast at GYY.
The lake breeze will probably move across MDW late this
afternoon. The most likely scenario is the lake breeze will stay
east of ORD with northwest wind prevailing or will barely reach
ORD as it dissipates around sunset. In either of those
scenarios, a sustained easterly wind wouldn`t develop.

- Izzi

&&

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

&&

$$

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