676
FXUS63 KLOT 131734
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1234 PM CDT Tue Aug 13 2024

.KEY MESSAGES...

- Showers and t-storms likely Wed night into Thursday morning,
  locally heavy rainfall possible

- Second round of showers and t-storms possible late Thursday,
  mainly during the evening and night

- Widely scattered showers and a few storms possible Friday
  afternoon and evening

&&

.DISCUSSION...
Issued at 311 AM CDT Tue Aug 13 2024

Through Wednesday:

A NW to SE oriented surface ridge will drift northeastward
across the forecast area through tonight, promoting generally
light and variable winds with the exception of a lake breeze
along Lake Michigan in the afternoon. Added low-level
convergence along the lake breeze should allow for a more
prominent cumulus field late morning into early evening across
northeast Illinois. Though higher low-level moisture will be
shallow, would not be surprised to see some localized clusters
of cumulus grow sufficiently deep to generate a few sprinkles
around and west of the I-294 corridor. Otherwise, expect near
normal temps in the low 80s under mostly sunny skies today.

Dry conditions should persist tonight through Wednesday as a
couple convective complexes well to the southwest bring periods
of upper-level debris clouds over the area. Barring any higher
mid to upper-level cloud cover on Wednesday, temps should push
solidly into the mid 80s during the afternoon.

Kluber


Wednesday Night through Monday:

The well advertised wetter, stormier pattern still looks poised
to begin Wednesday night and continue at times into at least
Friday, with long breaks in the rain likely during this time
frame.

Synoptically, a seasonably strong shortwave trough is progged
to move east into the northern Plains later Wednesday into
Wednesday night. After a couple of days of return flow from the
Gulf of Mexico, a quality, seasonably moist air mass is expected
to have spread north across the eastern Plains into the
Cornbelt by Wed evening. Height falls in advance of this trough
will help support the development of 40-50kt nocturnal low level
jet Wednesday evening. There is surprisingly good agreement
that initially, the strongest moisture convergence associated
with this low level jet will focus over northern IA and southern
MN where initial MCS development is expected.

Overnight Wednesday night into Thursday morning, this low level
jet will begin to veer, leading to stronger moisture
convergence developing southeastward toward eastern IA and
western IL. This should result in MCS propagating (moving +
developing) southeastward. Conditions look quite favorable for
heavy rainfall and potential flash flooding west of our CWA
Wednesday night into early Thursday morning with a pretty
textbook set-up training convection being advertised. Will need
to keep an eye on this threat sneaking into our western CWA
given the inherent uncertainties associated with forecasting
placement of nocturnal convection 48 hours out, but certainly at
this point, all signs point toward the meaningful flash flood
threat remaining west of our CWA Wednesday night.

By Thursday morning, continued veering and eventual weakening
of the low level jet should push the more favorable area for
convection squarely into our CWA. However, by this time would
anticipate the convection will likely be in a weakening phase
with a decreasing threat of heavy rainfall and flash flooding.

The decaying MCS moving across the area Thursday morning raises
some pretty substantial questions about the degree of air mass
recovery that will be able to be achieved Thursday afternoon.
The composite warm front/outflow boundary will likely be south
and west of our CWA Thursday morning, but as the upper trough
and associated sfc low track east Thursday afternoon, the front
and more unstable air mass should gradually be pulled northeast
into our CWA. Westerly flow in the mid levels should advect a
plume of steeper lapse rates eastward into the area Thursday
afternoon, aiding in the destabilization and air mass recovery.
Best chances meaningful destabilization will be over western
CWA, with it getting increasingly iffy with eastward extent as
isentropic ascent/warm air advection could keep some elevated
convection going into the afternoon, limiting chances for air
mass recovery.

Still seeing some sizable spread in the guidance with respect
to the depth and location of what looks to be a closed upper
mid- upper level low within the broader shortwave trough
Thursday afternoon and evening. This has pretty big implications
of shear profiles and magnitude/timing of synoptic forcing that
could drive a second round of sfc based convection later in the
afternoon or more likely in the evening into our CWA. If
everything were to fall into place just right, then there could
be an appreciable severe weather threat into our CWA or nearby
late Thurs afternoon into Thursday evening. However, with
several models suggesting more modest kinematic fields and
resultant shear profiles, and big uncertainties regarding the
quality of air mass recovery following morning storms, it is way
too early to message a severe threat. Something that will need
to be watched though...

It is looking increasingly likely that this upper low/trough
and associated cooler mid level air will be close enough to the
area Friday afternoon that there could be scattered showers and
a few storms. How progressive this trough will be this weekend
will be key in determining whether (mainly diurnal) rain chances
shift east of our CWA or potentially linger into the weekend.
The size, track, and strength of Tropical Cyclone Ernesto as it
lifts north into the mid-latitudes will probably play a role in
how quickly this trough moves eastward. NBM pops in the "slight
chance" range for the weekend look reasonable at this point with
no changes made.

- Izzi

&&

.AVIATION /18Z TAFS THROUGH 00Z THURSDAY/...
Issued at 1234 PM CDT Tue Aug 13 2024

There are no significant aviation forecast concerns for the
current TAF period. Winds will largely remain under 10 kts,
peaking in magnitude in the immediate wake of lake breeze
passages both this afternoon and tomorrow afternoon. VFR
conditions are also expected to prevail with cloud cover
expected to consist of diurnal VFR cumulus and high-level
cirrus at times. Some hi-res guidance is suggesting that fog may
develop at or near some of our TAF sites late overnight tonight,
but confidence in this occurring is too low to warrant
advertising any visibility reductions in the 18Z set of TAFs.

Ogorek

&&

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

&&

$$

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