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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 171144
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
644 AM CDT Wed Apr 17 2024

.KEY MESSAGES...

- Conditional severe weather threat exists through 10-11 am this
  morning, primarily near and east of I-55.

- Gusty westerly winds developing today, with peak gusts 40+
  mph.

- Next round of rain/showers on Thursday-Thursday evening

- Much cooler over the weekend with frost/freeze concerns,
  particularly on Saturday night

&&

.DISCUSSION...
Issued at 332 AM CDT Wed Apr 17 2024

Through Tonight:

A conditional threat for isolated to widely scattered
thunderstorms continues through around 10 am this morning,
especially for areas near and east of I-55. Aside from a few
festering elevated thunderstorms near the surface warm front in
Lake County IL, much of the local area remains quiet at the moment
thanks to a weak capping inversion based around 850 mb. While
this is the case, a very moist (surface dewpoints into the low
60s) and unstable airmass remains in place this morning as our
area remains positioned in the warm sector of the main storm
system centered across extreme southeastern MN. The concern for
addition thunderstorm development this morning looks to be tied to
the approaching cold front, which currently is sparking off some
renewed weak convection across far eastern IA out near the
Mississippi. This developing convection also appears to be aided
by a mid-level impulse sliding east-northeastward across the
southern periphery of the main Upper Midwest mid-level cyclone.

While confidence is not the highest with the amount of renewed
storm development that will occur with the frontal boundary early
this morning, it is concerning that any storms that develop could
become severe with an attendant damaging wind and tornado threat
given the amply sheared prefrontal airmass. At this time we think
there is roughly a 20 to 30% chance that a broken line of storms
develops across eastern IL (near and east of I-55) around, or
just prior to daybreak this morning. These storms, should they
develop, will then progress east-northeastward with the front
through mid-morning. The threat of severe storms will then
quickly end after 10-11 am this morning as the front finally
clears my northwestern IN counties. We will continue to monitor
near term trends over the next couple hours.

Conditions will dry out this afternoon, but skies are likely to
remain mainly cloudy. It will also be a rather windy day as winds
quickly increase from the west following the frontal passage.
Accordingly, westerly winds gusts of 35 to 40 mph are expected
through the afternoon into early evening before they abate
overnight. Cooler weather will also filter into the area today in
the wake of the front. Consequently, temperatures will hold in
the 60s through the day before dropping off into the 40s tonight.

Thursday Through early next Week:

Our precipitation maker will shift eastward across the Midwest
on Thursday. The surface low track with this system looks to
progress eastward across central parts of IL and IN through Friday
evening. With our area expected to be on cool northern side, it
appears the main threat for thunderstorms and severe weather will
remain south of the area. However, our area will be in line for a
period of rain Thursday into Thursday evening.

With the eastward depart of Thursday`s weather maker, a much
cooler airmass will work into the area for the end of the week as
a large upper low settles in across the Upper Midwest and Great
Lakes into Friday. In this pattern, breezy (but dry) westerly
winds (gusting 35 mph) are likely Friday, with temperatures
peaking in the mid to upper 50s. This much cooler period of
weather will persist through the weekend, as daytime highs only
peak in the 50s. Overnight low temperatures each night Friday
night through the Sunday night are expected to drop into the
30s. This may foster early morning frost development outside of
Chicago, particularly on Sunday morning when light winds and
clear skies near a surface high may even support some
subfreezing lows in some areas.

KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
-- Changed Discussion --
Issued at 643 AM CDT Wed Apr 17 2024

Main Concerns:

- Widely scattered SHRA and TSRA may develop over the Chicago
  metro terminals and then shift east by 14-15z

- Wind shift to gusty southwest to west with a cold front
  passage this morning

A cold front is sweeping across northern Illinois this morning.
Highest SHRA/TSRA coverage ahead of the front as of this writing
is from far northern Illinois into Wisconsin. If convection can
develop farther south and intensify, it will likely produce
lightning and at least air space impacts. Given the current
minimal coverage on radar at the latitude of the terminals, held
onto VCTS mention for the next 1-3 hours, latest at GYY.

South-southwest winds gusting to 25-30 kt will quickly shift to
southwest behind the front and then west this afternoon. Peak
gusts through early evening will be in the 30-35 kt range. MVFR
CIGs that developed overnight will temporarily scatter this
morning in the immediate wake of the front and then likely fill
back in during the mid-late morning.

On Thursday morning, light northwest winds will "flop" over to
northeast as our next weather system approaches. Some VFR -RA
may arrive after 15z, with prevailing rain/showers expected
after the current ORD and MDW 30-hour TAF.

Castro
-- End Changed Discussion --

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 3 AM CDT Thursday for the IL and IN
     nearshore waters.

&&

$$

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