314
FXUS63 KLOT 200540
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1240 AM CDT Fri Sep 20 2024

.KEY MESSAGES...

- A few showers or thunderstorms may sneak into northern
  Illinois toward daybreak Friday. Additional showers and storms
  may develop during the afternoon along and east of I-55.

- A pattern change is expected this weekend into early next week
  with the arrival of several waves of showers and storms
  (favoring Sunday and Monday).

&&

.UPDATE...
Issued at 916 PM CDT Thu Sep 19 2024

No significant changes made to the overnight forecast this
evening, with only modifications being to slow arrival of rain
chances a bit across our far west/northwest counties until
closer to and after midnight.

Latest surface analysis depicts a cold front stretching from
the arrowhead of Minnesota, southwest to a weak surface wave
the the IA/NE/MO border area south of Omaha. Scattered showers
and thunderstorms have been occurring ahead of the front from
central IA into the northern Lakes, within a narrow axis of low-
level moisture (upper 60s/near 70 surface dew points). The
greatest coverage and intensity of storms was across northeast
IA into western WI, on the nose of a 30 kt low level jet and in
association with a mid-level short wave lifting northeast across
the upper Mississippi Valley. Scattered showers and
thunderstorms are expected to move slowly east into northern IL
later tonight as the front slowly pushes east, though with
better forcing lifting to our north, gradual weakening and
veering of the LLJ and unfavorable diurnal timing should result
in decreasing intensity and lightning potential from west to
east across the forecast area.

Other than trimming pops across our northwest cwa in the mid-
late evening period, going forecast appears on track trends
through early Friday and no other significant changes appear
warranted.

Ratzer

&&

.DISCUSSION...
Issued at 308 PM CDT Thu Sep 19 2024

Through Friday Night:

The main forecast concern for the next 24 to 36 hours will be
the potential for isolated to widely scattered showers and
thunderstorms late tonight through Friday afternoon. The driving
force for the showers and storms is the broad trough across the
northern Plains that will gradually drift into the upper
Midwest and Great Lakes overnight into the day on Friday. At the
same time, showers and thunderstorms are expected to develop
along a cold front (currently stretching from central MN to
eastern IA and southern NE) and move into northern IL and
eventually northwest IN after midnight tonight. However, the
current airmass over northern IL and northwest IN remains rather
dry (dew points currently in the upper 40s to lower 50s) which
should weaken the showers and storms as they approach the I-39
corridor. Thus the coverage of showers and storms overnight will
likely be in a diminishing state meaning some areas may miss
out on needed rainfall depending on how far into northern IL
showers and storms can survive.

Regardless of storm coverage tonight, the cold front is
expected to still be over northeastern IL and northwest IN on
Friday which may allow for additional showers and storms to
develop Friday afternoon. The uncertainty, however; is how much
the atmosphere will be able to destabilize ahead of the front
given the limited moisture and potential for scattered cloud
cover. If the atmosphere is able to destabilize prior to the
front exiting, then another period of scattered showers and
isolated thunderstorms could materialize mainly for areas along
and east of I-55. Additionally, the aforementioned trough will
generate a modest upper jet across the area which is forecast to
generate around 30 kts of effective shear Friday afternoon.
This shear could support a more robust shower or storm core
which in turn may result in a threat, albeit low <10% chance,
for localized gusty winds and maybe some small hail. Therefore,
SPC has maintained their level 1 of 5 severe risk for areas east
of I-55 Friday afternoon for this potential but I suspect most
storms will remain sub-severe.

Any showers and storms that develop Friday afternoon will move
out of the area by Friday evening resulting in dry conditions
for Friday night. Otherwise, expect temperatures to remain
unseasonably warm with highs in the mid-80s and overnight lows
in the upper 50s to lower 60s.

Yack


Saturday through Thursday:

Following another unseasonably warm day in advance of an
approaching cold front Saturday (inland highs well into the mid
to upper 80), a significant weather pattern shift is slatted for
the second half of the weekend. Most notable, will be the
transition towards a wetter and cooler pattern Sunday and Monday
as the upper trough over southern CA ejects east-northeastward
into the Plains and Midwest. While there continues to be the usual
uncertainties at this time range, much of this revolves around
the finer scale details of the troughs evolution as it moves
across the Midwest into Monday. Accordingly, there continues to be
a strong signal in the model and ensemble guidance for a couple
periods of beneficial rainfall across our area Sunday through
Monday. In fact, some potential also exists for some areas of
moderate to heavy rainfall for a period Sunday night into Monday
as deep rich moisture (PWATs 1.5"+) feeds into the area just in
advance of the approaching upper trough and the associated
surface low. There may also end up being a favored period of
strong thunderstorms across parts of the area into Monday.
However, the favored location for these storms is still largely
unclear, and will ultimately be dependent upon the exact location
and track of the surface low, as well as the location of the
associated frontal features.

Following the departure of this early week storm system, mean
upper troughing and northerly lower-level winds across the Great
Lakes will foster a much cooler, more seasonable type pattern
across our area. Accordingly, expect daily high temperatures to
primarily be in the middle 70s following early morning lows in
the 50s.

KJB

&&

.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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