Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 122352
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
652 PM CDT Tue Mar 12 2024

.KEY MESSAGES...

- Unseasonably mild and breezy conditions will continue to result
  in an elevated fire weather threat through sunset this evening.

- A few isolated showers and/or storms possible (20% chance)
  overnight tonight, primarily southwest of the area.

- Several rounds of more substantial showers and storms capable of
  producing heavy rain and localized flooding likely (80%+ chance)
  late Wednesday night through Thursday night. Some storms may
  also become severe across parts of the area, particularly on
  Thursday.

- Below average temperatures expected to arrive late in the
  weekend into early next week with a non-zero chance of snow
  showers

&&

.DISCUSSION...
Issued at 308 PM CDT Tue Mar 12 2024

Through Wednesday:

Windy, warm and dry conditions will continue to create an
elevated fire weather threat through the remainder of the
afternoon. However, this threat will wane quickly with sunset
this evening as wind gusts abate and RH levels improve.

We continue to monitor the possibility of some isolated
thunderstorms across portions of the area tonight. Storm
development currently appears most favorable this evening along
the nose of a developing modest low-level jet across parts of MO
eastward into west central IL. The parent mid-level impulse is
expected to track south of much of the area across down state IL
later tonight. This suggests the threat is low for much more than
a couple of isolated showers or storms in my south overnight.
Accordingly, we have reduced pops to around 20% during the
overnight period.

Wednesday will be another unseasonably warm day for the area,
especially for areas inland from Lake Michigan. Temperatures will
once again top out well into the 60s to the low 70s. However,
cooler conditions (temperatures in the 50s) are expected near the
northeast Illinois Lake Michigan shore during the afternoon thanks
to passage of a back door frontal boundary, which will result in a
wind shift to primarily onshore cool east-northeasterly flow.
Cloud cover will also increase across the area in the afternoon,
and there are some signs that some isolated to widely scattered
afternoon showers (and maybe a storm or two) will develop within a
warm advection (isentropic upglide) regime. While not looking to
be a washout, we do continue to carry some 20-30% probabilities
for showers, mainly during the mid to late afternoon hours on
Wednesday.

KJB

Wednesday night through Tuesday:

The primarily focus during this period continues to be late
Wednesday night through Thursday night. A trough currently
moving across the Pacific Northwest will develop into a closed
low as it digs in across the Desert Southwest into Thursday. As
this occurs, model and ensemble guidance remain in strong
agreement that a notable mid-level impulse will then be ejected
east-northeastward from this western trough across the Midwest
and into the western Great Lakes region late Thursday into
Thursday evening. At the surface, a quasi-stationary warm
frontal boundary is expected to set up somewhere near the I-80
corridor on Thursday as surface low pressure develops across
southern IA into west central parts of IL. This frontal boundary
is expected to separate much cooler weather (temps in the 40s)
from much warm weather (temps in the 60s to around 70) south of
the front on Thursday. For this reason, temperatures will vary
considerably from north-to- south on Thursday, and the
temperature gradient will likely be much sharper then what is
currently depicted in the forecast database.

Robust convection is expected to develop in association with
this approaching disturbance, and will likely come in a few
waves. The first of these waves of convection looks to come
sometime late Wednesday night into Thursday morning in response
to increasing low-level moisture transport and isentropic
ascent. The presence of steep mid-level lapse rates (7.5C per
km) suggests that these storms will produce a notable hail
threat into early Thursday morning. Locally heavy rainfall will
also be probable with these storms as precipitation water values
increase over 1". Additional waves of convection then appear
likely through the day and into the evening, especially along
and north of where the low-level baroclinic zone settles. Latest
guidance favors the front stalling near the I-80 corridor. With
little movement on the front owing to reinforcement from
elevated convection to the north, some flooding concerns may
develop, especially if the heavy rain falls over the more urban
areas in and around Chicago.

South of the warm front, prospects of a more formidable severe
thunderstorm set-up will hinge on the amount of low-level cloud
cover trapped under a modest EML through the morning and
potentially into the afternoon. Conditional upon low-level
clouds clearing by mid-day, organized convection with all severe
hazards appear possible, particularly along the warm front and
farther west toward the surface low and triple point into
eastern Iowa in the evening. Latest thinking here aligns well
with SPC`s Day 3 outlook depicting a Slight Risk into the
western CWA.

Thursday`s mid-level low will begin to shear to the east while
being absorbed into the broader westerlies aloft Thursday night
through Friday. Residual light rain can be expected into Friday
morning, with low-level stratus struggling to erode until a
deeper layer of dry air advects in from the northwest Friday
night. Multiple waves embedded within deep troughing across
Ontario this weekend into early next week will then provide some
quick bouts of precip as we head into some of coldest
conditions since mid-February. Temps may struggle to rise much
into the 30s early next week, with sufficient moisture to
support at least minor snow accumulations with any well-timed
system.

KJB/Kluber

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 650 PM CDT Tue Mar 12 2024

Gusty southwest winds will quickly ease with sunset this
evening becoming light and variable overnight. Winds remain
light Wednesday morning though likely turn east to northeast
behind a weak backdoor front. There is a chance they remain
light and variable through much of the morning at the Chicago
area terminals if the front happens to stall out a bit further
north before a reinforcing lake breeze then turns winds
northeast in the afternoon around 10 kt.

There remains a chance for high based showers developing
Wednesday afternoon into early evening, currently accounted for
in PRO30 groups in the 18-02Z timeframe. An isolated
thunderstorm cannot be ruled out though confidence is not high
enough to include TS in the TAFs.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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