409
FXUS63 KLOT 132318
AFDLOT

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

.KEY MESSAGES...

- Showers and storms likely late Wed night into Thursday AM,
  locally heavy rainfall possible.

- Second round of showers and storms likely late Thursday,
  mainly during the evening and night, some could be strong to
  severe with locally heavy rainfall.

- Widely scattered showers and a few storms Friday-Sunday
  afternoons/evenings.

&&

.DISCUSSION...
Issued at 320 PM CDT Tue Aug 13 2024

Through Wednesday:

No impactful weather is expected through the daytime tomorrow
as surface high pressure continues to slowly drift eastward
through the region. Still can`t rule out some stray showers
popping along the lake breeze or any other subtle convergence
zones farther inland this afternoon as has been seen in
southeastern Wisconsin, but otherwise, favor a dry forecast
through tomorrow afternoon. High temperatures tomorrow should be
just a touch warmer than they were today at most locations,
with cirrus debris from convection to our southwest likely to
blunt solar insolation just enough to prevent temperature
readings from making a run at 90 degrees.

Ogorek


Wednesday Night through Tuesday:

Focus in the extended forecast period remains the potential for
a couple rounds of showers and thunderstorms late Wednesday
night through Thursday night. At the beginning of the period a
mid-upper shortwave ridge and surface high pressure will be in
the process of shifting east of the region in response to an
approaching upper level trough/developing low. An associated
surface low is forecast to lift northeast through Minnesota with
a lead warm front lifting across the region bringing a return
to warm and humid conditions (low 70s dewpoints) to the local
area and set the stage for possible strong to severe
thunderstorm development and heavy rainfall.

There continues to be an array of potential outcomes for the
first round of storms as an upstream MCS tracks into the area.
This includes anything from the MCS decaying as it moves into
the area (particularly on the northern extent) to a more
persistent cluster of showers and thunderstorms moving across
the entire forecast area.

There remains a signal for a lull in activity during the
afternoon hours before a likely second round of showers/storms
during the evening hours. The strength of these storms and
overall coverage ultimately depends upon what transpires in the
morning hours. A slower exit and greater coverage could limit
our destabilization and reduce the evening storm severity and
coverage. Meanwhile, less coverage and quicker ending AM storms
could allow for stronger destabilization aided by steeper lapse
rate plume advecting into the area. This would support the
higher-end instability values depicted by some model guidance.
This unstable airmass coincident with increasing deep layer
shear with the approaching wave as well as some low-level shear
(increasing LLJ), would supportive of all severe hazards.
Overall, will begin to message the potential for strong to
severe thunderstorms, though this threat remains conditional.

Looking more specifically at the heavy rain potential, a
higher-end thunderstorm training event does not appear to be
the most likely outcome (based on the latest guidance) when
comparing this set-up to our typical Maddox patterns for flash
flooding. Modeled PWATs will be toward the upper echelon for
this time of year, however, and suggests any storms that develop
will be capable of high rainfall rates and any training of
storms in this environment would support at least a localized
flash flooding threat.

Friday through the weekend the closed mid-upper low will
meander across the Great Lakes region which likely will allow
for diurnal scattered showers and isolated thunderstorms each
day. Beyond Friday temperatures will be in the 70s to low 80s
through the end of the period.

Petr

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 618 PM CDT Tue Aug 13 2024

Forecast concerns for the terminals...

- Lake breeze turning winds easterly at ORD, MDW, and GYY
  Wednesday afternoon

High pressure overhead will continue to promote dry and VFR
conditions through Wednesday afternoon. Light and variable winds
will prevail across northern IL and northwest IN tonight with
the exception of the Chicago terminals which will see a light
southeast wind. Winds will become southeasterly areawide
Wednesday morning with speeds remaining light in the 5-7 kt
range. Though a lake breeze is expected to develop and move
through the Chicago area terminals (ORD, MDW, and GYY) between
17z and 19z Wednesday resulting in a more easterly wind
direction and slightly higher speeds around 8-10 kts.

Skies will remain mostly clear through Wednesday afternoon, but
VFR cloud cover is expected to increase Wednesday evening ahead
of an approaching storm complex. However, any notable showers
and/or storm chances are expected to hold off until after the
conclusion of this TAF period.

Yack

&&

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

&&

$$

Visit us at weather.gov/chicago