451
FXUS63 KLOT 132335
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
635 PM CDT Mon May 13 2024

.KEY MESSAGES...

- Showers and thunderstorms will continue tonight, with isolated
  to scattered storms persisting into Tuesday. The storms
  through mid-evening may produce locally strong wind gusts in
  excess of 40 mph.

- A localized area of heavy rainfall remains possible roughly
  from the central Chicago metro through the I-80 corridor
  through mid-evening.

- Chances for showers and storms appear poised to return by the
  end of the week.

&&

.DISCUSSION...
Issued at 306 PM CDT Mon May 13 2024

Through Tuesday Night:

An uncapped atmosphere with abundant sunshine this morning
allowed scattered storms to develop roughly with a low-level
moisture discontinuity late this morning. Given generally
outflow dominant storms within a weakly sheared environment,
coverage of upstream storms increased as outflow boundaries
propagated downstream. We have seen a few attempts at stronger
short-lived cores capable of producing near-severe gusts and
large hail, but most cores have so far resulted in small hail
and some gusty winds over 40 mph.

As we progress through this evening, three features and their
subsequent interactions will drive the overall storm evolution.
The main mid-level forcing continues to be driven by a NW to SE
oriented trough lifting across central Illinois. Meanwhile, a
cold front over the northern tier of counties in Illinois will
continue to drift southward. Between these two features, outflow
boundaries with ongoing convection will result in localized
forcing before the front and trough ultimately converge early
this evening. Simply put, a messy radar display of convection is
expected for several hours, with gusty winds and locally sub-
severe hail persisting.

Regarding flooding concerns, we have seen some instantaneous
rainfall rates over 4"/hr, but the small footprint of individual
cells or small clusters have limited flooding concerns to only
localized standing water before waters quickly subside. With
that said, the merging of the several features noted above will
likely support a loose west to east line of higher storm
coverage and locally heavy rain across portions of the central
and southern Chicago metro through early evening. Confidence in
the potential of Flash Flooding remains low enough to preclude
issuance of a targeted Flash Flood Watch at this time, but the
need for a Flash Flood Warning cannot entirely be ruled out.

Beyond this evening, the northern deformation axis of an upper-
level low drifting eastward across central Illinois will allow
showers and some storms to persist overnight and into Tuesday
before slowly departing southeast through Tuesday night.

Kluber


Wednesday through Sunday:

On Wednesday, a surface ridge will move through the Great Lakes
allowing for plenty of sunshine and a rebound in highs back into
the upper 60s to mid 70s. With an onshore component to the
wind, temperatures along the Lake Michigan shoreline will remain
in the upper 50s to lower 60s.

The forecast from Thursday onward carries much lower forecast
confidence than is typical due to the apparent differences in
how a myriad of upper level shortwaves riding through the jet
stream in the northern Pacific Ocean will interact with a soon
to be eastward drifting cut off low emanating from southern
California. Will the cut off lows phase with the northern jet
stream leading to aggregate troughing across the Great Lakes, or
will they have a missed connection leading to pronounced
ridging across the northern US? Timing will be everything, and
right now, the ensemble envelope is as vast as can be. Perhaps
the only item of confidence is that there should be a wave (or
waves?) of precipitation sometime in the Thursday to Sunday
timeframe, warranting more or less continuous low-to mid-range
chance PoPs (20-50%). Anything beyond that is pretty much a
throw of a dart at this point.

Borchardt/Izzi

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 635 PM CDT Mon May 13 2024

Key Aviation Impacts for the current TAF period:

- Continued showers through Tuesday morning

- Widespread MVFR cigs are expected overnight with localized
  IFR conditions possible

- Drier, VFR conditions are expected by Tuesday evening

Scattered showers over the region will continue through the
overnight into Tuesday morning. An isolated lightning strike
through 02Z cannot be completely ruled out, but the chances are
diminishing. Winds have become generally west to northwest as
the front has arrived. However, as it moves east over the lake,
winds are expected to flip to the northeast after 01Z and
remain below 10 knots. Cloud bases will remain aloft through
this evening, but are expected to lower to at least MVFR levels
after midnight. There remains a chance for IFR cigs to develop
overnight, but confidence is lowering as models have been
pulling back on the potential. 09Z to 13Z window looks like the
most likely time with the signal closer to KMDW/KGYY and areas
to the south.

MVFR conditions will continue through Tuesday as the scattered
showers slowly pivot and start to move southeast of the area.
Low confidence on the exact timing, but showers should gradually
taper off in the afternoon. Clouds will remain in the area, but
slowly become VFR in the late afternoon/evening as northeast
winds prevail.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 4 AM Tuesday to 7 AM CDT Wednesday
     for the IL nearshore waters.

     Small Craft Advisory from 7 AM Tuesday to 7 AM CDT Wednesday
     for the IN nearshore waters.

&&

$$

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