286
FXUS63 KLOT 122323
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 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 /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 623 PM CDT Mon Aug 12 2024

Forecast concerns for the terminals...

- Patchy fog possible late tonight outside of Chicago

- Lake breeze generating easterly winds at ORD, MDW, and GYY
  Tuesday afternoon

A broad surface high continues to reside over the Great Lakes
this evening with a modest shortwave disturbance pivoting
through aloft. While the disturbance has kicked off some showers
across eastern IA and central IL, northeast IL and northwest IN
remains dry with just BKN VFR clouds overhead. These VFR clouds
are expected to gradually scatter out this evening which may
set the stage for some patchy fog to develop outside of the
urban core of Chicago tonight. Guidance continues to be rather
bullish with the coverage and intensity of the fog, but the
very shallow saturated layer noted in forecast soundings leads
me to believe that guidance is over done. Nevertheless, the
signal for patchy fog remains sufficient to maintain the
prevailing mention in the TAFs at DPA, RFD, and GYY. Any fog
that does materialize tonight should be shallow and result in
MVFR visibilities (4-6 SM range) at worst. The fog is expected
to erode shortly after daybreak on Tuesday leaving FEW to SCT
VFR skies for the rest of the period.

Otherwise, expect light and variable winds to prevail through
the end of the forecast period. Though areas near Lake Michigan,
namely ORD, MDW, and GYY; will see a lake breeze develop late
Tuesday morning and result in light (5-7 kt) easterly winds for
the afternoon.

Yack

&&

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

&&

$$

Visit us at weather.gov/chicago