179
FXUS63 KLOT 192008
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
308 PM CDT Thu Sep 19 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).

&&

.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 /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1255 PM CDT Thu Sep 19 2024

Key Messages:

- Slow inland push of lake breeze today at ORD/MDW (22-23Z)
- Period of SHRA possible overnight (30% chance of TS at RFD,
  15% at ORD/MDW/DPA)
- Low chance of a period of MVFR stratus Friday morning
- Northwest wind shift behind a cold front Friday afternoon

A lake breeze is evident in surface observations and on radar
imagery early this afternoon and has been slower to push inland
compared to the past few days. Have trended the east wind shift
at MDW a bit later to 22Z based on latest guidance and
maintained 23Z at ORD. Otherwise expect winds to remain SSW
through the afternoon. As we head toward sunset all sites will
gradually trend SSE to SE to at times variable overnight,
remaining light.

Focus then turns to the potential for a period of showers (and
possibly an embedded thunderstorm or two) likely in a decaying
phase with eastward extent. Accordingly, maintained a PROB30
group for TSRA at RFD but left TS out of the Chicago area
terminals with this update. Can`t rule out a few lightning
strikes persisting into the metro, though confidence remains too
low for a formal TAF mention and left the PROB30s as SHRA.

Depending on the coverage of showers, wind directions could be
quite variable overnight, but generally expect them to maintain
a southerly component. In the wake of the showers there is a
signal for a period of stratus development Friday morning. For
now have introduced a scattered 1000-1500 ft layer due to
lingering model variability in cloud coverage.

A cold front is then progged to move through during the late
morning into the afternoon hours turning winds WNW.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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