471
FXUS63 KLOT 121743
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1243 PM CDT Mon Aug 12 2024

.KEY MESSAGES...

- Mainly dry conditions and warming temperatures through the
  first half of the week.

- Periods of showers and thunderstorms starting Wed night and
  continuing into Thur night.

&&

.DISCUSSION...
Issued at 308 AM CDT Mon Aug 12 2024

Through Tuesday:

A mid-level wave tracking ESE across Iowa early this morning
has continued to generate scattered high-based showers and a few
thunderstorms as it shifts away from modest mid-level lapse
rates and into a drier antecedent airmass east of the
Mississippi River. Overall expectations are for the mid-level
showers to track toward central Illinois late this morning
through this afternoon. It is possible that the entire forecast
area will remain dry today, but have opted to maintain a slight
chance of showers and sprinkles roughly southwest of a line from
Dixon to Morris to Rensselaer. Otherwise, today will feature
mostly cloudy skies, light winds, and high temps in the mid 70s
to around 80F.

The continued dry airmass will persist over the area through
Tuesday. A seasonably strong upper-level jet will remain
generally inactive in terms of precip generation, especially
given rising mid-level heights and little flow (<10 knots)
below 700 hPa. There is less than a 10% chance that some high-
based cumulus along an afternoon lake breeze in northeast
Illinois grow sufficiently deep to squeeze out a few sprinkles.
Otherwise, expect mostly sunny skies with high in the low 80s
(normal for mid-August).

Kluber


Tuesday Night through Sunday:

Seasonably warm temperatures and comfortable humidity values (by
mid August standards) expected Wednesday as transient shortwave
ridging traverses the Midwest.

After several days of return flow from the Gulf of Mexico, a
very warm and moist air mass is expected to spread north into
the central Plains by Wednesday. A formidable trough is progged
to move east into the Cornbelt and upper Midwest Wednesday night
with height falls in advance of this feature likely to lead to
strengthening nocturnal low level jet Wed night. Strong theta-e
advection is likely as this low level jet drags the very moist
air mass over the Plains east toward the MS Valley, likely
resulting in an MCS or two Wednesday night. Guidance would tend
to favor areas to our west Wed night, but as low level jet
veers and trough continues eastward, this activity will probably
spill into our western CWA after midnight.

Low confidence in how Thursday will play out with morning
showers and thunderstorms expected across the CWA. Convection
should generally be in a weakening phase, so greater threat for
for heavy rainfall and possibly even some severe wx looks to
stay west of our CWA. However, cold pool/outflow from the Thurs
morning convection could stifle the synoptic warm front`s
northward progress Thursday. This will play a big role in temps
Thursday afternoon as well as potential recovery needed for a
second round of storms Thursday evening as cold front and
associated upper trough move across the region.

In the wake of Thursday night`s cold front, look for somewhat
less humid air mass to eventually filter into the region Friday
and through the weekend. Confidence in the evolution of synoptic
features heading into the weekend is lower than average. For
several days now, some ensemble members of the GFS and ECMWF and
even occasionally an operational run or two have been showing a
mid and upper level low closing off somewhere in the region. The
00z ECMWF has come in with a closed mid-upper level low sitting
and meandering across the Great Lakes region this weekend, but
this time it has much stronger ensemble support. Interestingly,
potential tropical cyclone #5 (PTC5) being monitored over the
Tropical Atlantic may end up playing a role, as it is progged to
develop into a powerful tropical cyclone and re-curse north
across the western Atlantic this weekend into early next week.
Often times when strong, large hurricanes (like what the ECMWF
is showing) move north into the mid-latitudes, they frequently
result in a slowing down of the progression of systems and some
blocking developing. Obviously confidence in how this plays out
is low with so many moving pieces, but should an upper low cut
off over the region, there could be bouts of scattered showers
and storms over the weekend, favoring the afternoon and evening
hours with peak heating. NBM does carry chance pops Friday and
into the first part of the weekend, which seems more than
reasonable at this distance as there is still a lot to work out
in how the pattern will evolve. It`s still possible that PTC5
isn`t as prominent as shown by the ECMWF and a more progressive
trough moves east of the area and leaves us with dry and
seasonable weekend. Stay tuned...

- Izzi

&&

.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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