548
FXUS63 KLOT 121935
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
235 PM CDT Mon Aug 12 2024

.KEY MESSAGES...

- Mainly dry conditions and warming temperatures through
  Wednesday.

- Periods of showers and thunderstorms Wednesday night into
  Thursday, with additional shower chances continuing, at least
  through Friday.

&&

.DISCUSSION...
Issued at 235 PM CDT Mon Aug 12 2024

Through Tuesday:

Broad mid and upper level troughing overhead along with an
embedded shortwave will continue to yield increased cloud cover
and at least a low chance for light showers, mainly south of
I-80 this afternoon. That said, a fairly parched sub-cloud layer
may continue to inhibit much more than sprinkles out of this.
An additional area of diurnally-enhanced cumulus is building in
the vicinity of a Dubuque to Joliet line, and also north of the
Wisconsin state line, with more of a congested/towering look to
things near and west of the Mississippi River. Instability
remains in short supply across the local area with lower surface
dewpoints and slightly warmer air in the 700-500 mb layer, so
not expecting anything more than a rogue shower/sprinkle out of
any of this.

Skies largely look to clear this evening and overnight, except
perhaps across our far southern locales where we`ll hang onto some
enhanced mid-level moisture overnight. With pretty much calm
winds and surface dewpoints in the 50s, temperatures will probably
dip into the mid (maybe even low) 50s in our typical cool
spots/near river valleys. Can`t rule out some spotty shallow fog,
but this doesn`t look to be enough to warrant a mention in the
gridded forecast.

Temperatures will be a few degrees warmer on Tuesday (with cooler
conditions near the lake). Expecting some thickening cumulus in
the vicinity of the lake breeze with a small potential for a
sprinkle, although the better potential for showers will remain
relegated to our north across Wisconsin.

Carlaw


Tuesday Night through Monday:

It currently looks like we`ll manage to squeeze out one more dry
day on Wednesday, but an unsettled weather pattern will develop
thereafter as high pressure slinks off to the east. Some
guidance suggests precip chances may increase as early as late
Wednesday afternoon near our I-39 corridor locales, although
given the depth of dry air under 10 kft in forecast soundings,
suspect things may take a little while longer to fully saturate.

Better rain and thunderstorm chances look to arrive overnight
Wednesday into Thursday morning as broad but fairly intense warm
advection materializes as an intensifying southwesterly low-
level jet pivots out of northern Missouri. While there still
appear to be some timing, magnitude, and placement differences
across the guidance suite, from a conceptual model standpoint,
this will be a period to monitor for a locally heavy rainfall
threat as intensifying WAA interacts with a reservoir of modest
elevated instability and a notably moistening profile with PWATs
nearing or exceeding 2 inches. The placement of a SW-NE
oriented LLJ, with northwest 500-300 mb flow aloft spells some
threat for training convective elements during this time frame.

How any lingering morning activity shakes out Thursday will have
a significant impact on how things evolve during the afternoon
and evening. There`s some potential for airmass recovery and
destabilization as low to mid 70s dewpoints attempt to inch
their way in across our southwest, but this is not universally
agreed upon by today`s guidance. If sufficient airmass recovery
takes place, an additional round of afternoon and evening
thunderstorms would likely result with a synoptic wave expected
to be traversing the Upper Midwest. There might even be a low
severe threat in our west and south given expected very low LCLs
and some increased clockwise curvature noted in hodographs, but
this is all contingent on morning activity/timing.

Confidence in the forecast evolution Friday and beyond begins to
degrade, with differences in the handling of the aforementioned
broad upper wave and when/where/if it closes off. At this range,
the blended chance PoP offering on Friday looks reasonable, with
scattered showers and perhaps a few storms as mid-level
temperatures cool a bit. If the upper wave remains a bit more
progressive, we`ll manage a dry weekend, but quite a few ensemble
members maintain a slow enough evolution that at least lower-end
precipitation chances seem warranted at this point.

Carlaw

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1211 PM CDT Mon Aug 12 2024

Main Aviation Concerns:

- Ceilings lower and increase in coverage this afternoon, with
  VFR conditions prevailing.

- Lake breeze expected to develop this afternoon, however exact
  timing remains unclear but expected 19-21z.

- Overnight could see clearing skies, along with potential for
  patchy fog development outside of the metro.

A weak wave disturbance will continue sliding through central
Illinois this afternoon, with shower chances expected to remain
south of the LOT terminals. Some increased mid and upper level
clouds increase this afternoon, with developing low level cu
field anticipated over the next couple of hours...seeing
developing cu over nw IL and portions of north central IN. A
lake breeze is expected to develop this afternoon, however exact
timing remains in limbo...anticipated between 19-21z from GYY
to MDW, and ORD. Conditions turn calm and clear overnight,
potentially bringing patchy fog to the region into early
morning.

Baker

&&

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

&&

$$

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