352
FXUS63 KLOT 200216
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
916 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).

&&

.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 /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 629 PM CDT Thu Sep 19 2024

Key Messages:

- Period of SHRA possible overnight (30% chance of TS at RFD,
  15% for Chicago area terminals).

- Low chance (<30%) of MVFR stratus Friday morning.

- Winds shift from SE this eve to SW Friday morning and to WNW
  Friday afternoon.

Lake breeze has moved through KMDW/KGYY this afternoon, and was
in the process of moving across KORD at 23Z. Between the lake
breeze and a gradual synoptic shift, winds should settle to
light southeast across the area early this evening.

Farther to the west, a cold front was currently located from
west of KMSP to near KOMA. This front will move east toward the
terminals later tonight, initially resulting in light SE winds
veering more SSW after midnight. In addition, the potential for
scattered SHRA/TSRA will increase within a narrow axis of
moisture and instability ahead of the front. This is expected to
affect KRFD after 06-07Z, and after 08-09Z for the Chicago
terminals. Have included a mention of TSRA at KRFD (roughly
30-40% chance), though not for other airports as guidance is in
good agreement with a steady decrease in lightning potential
farther east into the Chicago area (~15%). SHRA will
exit/dissipate the Chicago area around sunrise.

Some guidance hints at some patchy 1000-1500 foot MVFR stratus
lingering along the frontal zone Friday morning, though any
actual MVFR ceilings are expected to be short lived. The cold
front is expected to move through KRFD by midday and the Chicago
terminals around mid-afternoon, shifting winds to the WNW
around 10-12 knots and clearing out any lingering lower-level
clouds. Can`t completely rule out a scattered SHRA/TSRA ahead of
the front Friday afternoon south/east of Chicago, though the
better chances of that currently look to be off to the east and
southeast of KGYY.

Ratzer

&&

.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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