922
FXUS63 KLOT 212004
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
304 PM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Near record heat expected today, fittingly for the last day
  of astronomical summer

- Scattered showers/t-storms tonight likely to build to a
  steadier beneficial rain with some embedded thunderstorms
  Sunday

- Another period of showers and possible storms Monday night
  into Tuesday, highest chances along and east of I-55

&&

.DISCUSSION...
Issued at 304 PM CDT Sat Sep 21 2024

Through Sunday night:

Another unseasonably warm afternoon is underway with high
temperatures solidly in the upper 80s and lower 90s as of 2 PM
CDT. However, the upper ridge that has been supporting the
summer-like warmth as of late is starting to break down as it
gets shoved east by the approaching upper-low over the southwest
CONUS. Ahead of the upper-low, strong warm advection has been
interacting with a plume of moisture across IA and western IL
which has resulted in some showers and thunderstorms taking
shape near the Mississippi River. While these showers and storms
have so far been able to maintain themselves, they should begin
to interact with the much drier airmass over northern IL and
dissipate prior to reaching the I-39 corridor. Though, there is
a small (around 15%) chance that an isolated shower or storm
could briefly impact portions of western Ogle and/or Lee
counties this afternoon before the showers/storms dissipate.

A better coverage showers and thunderstorms is expected to move
into northern IL overnight as the aforementioned upper-low
ejects a couple of vort maxes into the western Great Lakes and
forces a cold through the area. While this setup will allow
showers and storms to persist throughout the day on Sunday,
there still is expected to be some occasional breaks in the rain
on Sunday particularly during the late morning and early
afternoon hours before the front arrives. Furthermore, the
widespread rain will also limit the amount of destabilization
that will occur Sunday afternoon which means that the coverage
of thunderstorms should gradually wane through the day on Sunday
as well. That said, rainfall totals from this system are
expected to be solidly in the 0.5 to 1.0 inch range areawide but
there is the potential for some pockets of higher rainfall
amounts in excess of an inch. Unfortunately, the convective
nature of the showers/storms make it difficult to pinpoint the
exact location of these higher amounts. Regardless, antecedently
dry conditions should allow the ground to soak up the rainfall
without issue so the risk for flooding remains very low.

The aforementioned cold front will begin to push the bulk of
the showers and storms south of I-80 by Sunday evening which may
allow most of northern IL to dry out Sunday night. However, the
presence of an upper vort overhead in combination with
lingering moisture may allow at least isolated showers to
persist Sunday night. Aside from the rain, winds will quickly
build behind the front Sunday evening especially over the open
waters of Lake Michigan with gusts in the 15 to 25 mph possible.
These breezy northerly winds will also build waves on the lake
and create dangerous swimming conditions and hazardous boating
conditions. Though, the front will finally usher in notably
cooler air in its wake which will lead to more typical mid-
September temperatures for Sunday and Sunday night.

Yack

Monday through Saturday:

Cooler, more typical late September, weather is slatted for least
the first half of next week following a period of showers and
storms with a cold frontal passage on Sunday. Shower and storm
chances with this first initial disturbance and cold front should
shift south of the area for Sunday night into Monday. This should
thus result in mainly precipitation free conditions in our area
for daylight hours of Monday. However, expect temperatures to only
top out around 70 as cool northeasterly winds prevail.

While much of the day Monday is now looking dry, there is an
increasing signal for another period of showers and possible
storms Monday night into Tuesday as the central Plains mid/upper
trough finally ejects eastward across the Mid-Mississippi Valley.
As it does, it should also foster the development and
consolidation of a northeastward shifting area of low pressure
across IL and IN Monday night into Tuesday. The track of this wave
largely looks to favor the southeastern half of the area through
Tuesday (roughly areas along and southeast of a line from Waukegan
to Mendota, IL). Some low potential (<20%) also exists for a few
strong storms Tuesday, primarily across IN in closer proximity to
the evolving warm sector.

Following the departure of this wave of low pressure late Tuesday,
the weather pattern looks to become rather "blocky" across the
central CONUS as a building upper ridge overspreads the upper
Midwest and western Great Lakes regions. Models often struggle
with the evolution of blocky patterns like this, so there is not
a lot of confidence in the details. However, it looks like Rex
Block type pattern with northern stream ridging and a southern
stream upper low somewhere in the central Plains or lower Missouri
Valley region. If this Rex Block develops as ECMWF suggests with
pretty good ensemble agreement, then dry and warmer than average
weather could return later in the week into next weekend.

KJB

&&

.AVIATION /18Z TAFS THROUGH 00Z MONDAY/...
Issued at 1245 PM CDT Sat Sep 21 2024

Key messages:

- A lake breeze is expected to to provide northeast winds at
  GYY, east winds at MDW, and southeast winds at ORD - though
  magnitudes should be 10 knots or less

- A front will move through from west to east providing chances
  for showers and thunderstorms around northern Illinois and
  northwestern Indiana Sunday

There is a line of showers and storms over the Mississippi River
gradually moving east, but are starting to fall apart as they
interact with the drier air in Illinois. The line will be
monitored, but given the lower confidence in impacts, TAFs were
kept dry through this evening. The main impact in the afternoon
will be a lake breeze that develops. Winds ware expected to turn
to the northeast at GYY, near east at MDW, and southeast at ORD.
However, wind speeds should be at or less than 10 knots.

A front is expected to move across the airspace early Sunday
morning. Confidence with the onset of rain is low at this time,
it was opted to maintain the going PROB30 mention prior to 12Z
Sunday morning. Confidence then increases with showers at the
terminals after 12Z. While there will be instability present,
latest guidance is keeping the threat of storms more isolated
and embedded in nature. Therefore, the VCTS mention was
maintained in the latest TAF. The chances for thunder are
expected to diminish after 18Z, though showers should remain
around the terminals.

Winds ahead of the boundary will be out of the southeast before
flipping to the southwest. Guidance is suggesting that by
midday, wind directions will slowly become more westerly and
even northwesterly after 21Z. A lower level jet aloft may
develop and with proper mixing, occasional stronger gusts up to
20 knots are possible Sunday afternoon.

DK

&&

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

&&

$$

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