964
FXUS63 KLOT 130522
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1222 AM CDT Tue Aug 13 2024

.KEY MESSAGES...

- 20% chance for patchy ground fog late tonight into Tuesday
  morning outside of Chicago.

- 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.

&&

.UPDATE...
Issued at 853 PM CDT Mon Aug 12 2024

Main forecast change this evening was to add a patchy fog
mention late tonight into Tuesday morning.

A broad surface high continues to reside over the Great Lakes
with a weak shortwave disturbance aloft that is starting to
pivot into northern IN. With the loss of forcing from this wave
skies have been gradually clearing and that is expected to
continue through the night. The combination of clear skies and
light winds should set up a good night for radiational cooling
with temperatures expected to dip into mid-50s outside of the
urban core of Chicago where lower 60s are forecast.

As temperatures cool, dew points are expected to slowly increase
which looks to generate marginally favorable conditions for
patchy ground fog late tonight into Tuesday morning (3 AM to 8
AM CDT). While guidance remains in great agreement on fog
developing tonight, the very shallow saturated layer noted in
forecast soundings continues to cause uncertainty as to coverage
and density of any fog. Regardless, felt the signal was
sufficient enough to warrant the addition of a 20% chance for
patchy fog to the forecast with the evening update for areas
outside of the urban core of Chicago. Given the aforementioned
shallow nature of the fog I suspect the coverage will be limited
to mainly river valleys and rural areas with visibilities
generally expected to be in the 4 to 6 mile range at worst.

Any fog that does materialize tonight will erode by 9 AM Tuesday
morning leaving us with another dry and seasonable afternoon.
Though temperatures on Tuesday are expected to be a bit warmer
than today with highs in the mid-80s areawide.

Yack

&&

.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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1222 AM CDT Tue Aug 13 2024

VRB winds 5 knots or less are expected for most of the period
as high pressure drifts over the area. However, a lake breeze is
expected to cross ORD/MDW late this morning and produce E winds
of 5 to 10 knots.

VFR conditions are expected at ORD/MDW, with the potential for
periods of sub-VFR visibility from MIFG or BCFG at RFD/DPA/GYY
through mid-morning.

Kluber

&&

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

&&

$$

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