688
FXUS63 KLOT 140458
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1158 PM CDT Mon May 13 2024

.KEY MESSAGES...

- Scattered showers and isolated thunderstorms will persist
  overnight into Tuesday morning.

- Renewed localized flooding may develop overnight in areas that
  already received heavy rain, particularly along/south of I-80.

- Showers and storms may return by the end of the week.

&&

.UPDATE...
Issued at 907 PM CDT Mon May 13 2024

A surface low centered in northern Missouri will continue to
crawl eastward. Ahead of this feature southerly mid level flow
continues to advect over an inch of precipitable water
northward, as seen on the 00Z DVN sounding. As such, scattered
showers are expected to continue through the overnight and into
Tuesday morning. The flooding threat will be monitored closely
through the night with the additional rainfall over soggy soils,
but rain rates have notably diminished from what was observed
this afternoon. The main update to the forecast was paring
thunder chances back for most of the forecast area. There
remains better instability to the west near the Quad Cities and
to the south in Central Illinois and Indiana so thunder chances
remain there, but other than an isolated rumble or two, the
remainder of northeastern Illinois and northwest Indiana,
including Chicago, are expected to be mainly thunder free. The
rain will likely remain through Tuesday morning. For more
information on the rest of the forecast, see the discussion
below.

DK

&&

.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 /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1158 PM CDT Mon May 13 2024

A low pressure system continues to slowly move across the middle
Mississippi River Valley. As a result, waves of showers will
meander through the airspace overnight and tomorrow morning. By
early afternoon, coverage of showers should become highest south
and east of the terminals as the low begins to pull away.

The ceiling forecast remains tricky, especially through the
first 12 hours of the TAF. Largely VFR cigs at press time
should become dotted with pockets stratus based between 800 and
1500ft through the overnight hours. Experimental probabilistic
cloud guidance only paints around a 40 to 50% chance for such
cigs to prevail for more than a few hours at any given
terminal, mainly at some point between midnight and 8 AM. For
now, felt the course of least regret was to maintain the
inherited timing of MVFR and IFR developing at each terminal
keeping in mind observations will likely bounce back and forth
between categories throughout the night. Should stratus fail to
develop at all, adjustments will be made with the 09Z AMD or 12Z
TAF. At least pockets of MVFR appear poised to continue into
the afternoon before dry air slowly filtered into the region and
causes cloud bases to rise.

Finally, light and variable winds at press time should become
northeasterly by daybreak and remain as such through the
entirety of the TAF period.

Borchardt

&&

.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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