591
FXUS63 KLOT 221730
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1230 PM CDT Wed May 22 2024

.KEY MESSAGES...

- Westerly winds will gust to around 30 mph today

- A line of showers and storms is expected on Friday

- Another chance for storms Sunday into Monday next week

&&

.DISCUSSION...
Issued at 241 AM CDT Wed May 22 2024

Through Thursday:

A cold front, seen as a well defined fine line on the terminal
doppler radars, is in the process of moving across the Chicago
metro area as of this writing (0730 UTC). Passage of the cold
front should put an end the very impressive synoptic winds that
have occasionally gusted to near 60 mph since early last
evening. It will remain breezy behind the front through the day
today, but not nearly as windy as what we are seeing/saw ahead
of the cold front. Will trim away at the remaining wind advisory
as the front clear the area.

Other than westerly winds gusting to around 30 mph today,
really not expecting any impactful weather through Thursday.
After some morning cloudiness, expect skies to gradually clear
out this afternoon. While much cooler than yesterday, temps
should be seasonably mild today. Thursday will be sunny and
seasonably warm, though a lake breeze will develop in the
afternoon knocking temps back into the 60s at the lakeshore.

- Izzi

Thursday Night through Tuesday:

As the next upper level wave trough ejects out of the Rocky
Mountains early on Thursday, a new surface cyclone will develop
over the Great Plains and travel northeastward. The good news
is a.) all model indications are that this low will be weaker
than Tuesday and b.) it will take a more northerly track, closer
to Minnesota. Regardless, a trailing cold front is schedule to
arrive Friday morning and pass across the forecast area through
Friday. Being so far removed from the low, models are
developing a more marginal set up. There is a good consensus
among models for mid level lapse rates around 7.5C/km, over 2000
J/kg of CAPE above the inversion, as well as a nice veering
wind field through the column. Two main inhibiting factors will
be tied to wind shear and forcing. Models have consistently over
the last several days been projecting fairly weak shear, only
30 knots up to 6km and less than 20 knots below 3km. The jet
streak wrapping around the upper level trough appears to be too
far north to assist with any synoptic lift, so most of the
forcing will have to come from the front itself. However, with
afternoon temperatures expected in the 80s, there should be
enough to tap into the instability aloft and scattered
thunderstorms. With model soundings projecting around 800-1100
J/kg of DCAPE, if there was a severe threat, it could come with
gusty winds as the line passes through.

Better height rises and cold air advection behind the front
will lead to a pleasant Saturday with temperatures in the 70s
under clearer skies.

As the next upper level trough moves over the Plains, another
surface low will move northeastward from Missouri on Sunday into
Monday. However, models are all over with the exact track, but
it seems like Chicago continues to be the proverbial "fork in
the road" with the low avoiding the city to the north or south
depending on which model and which model run you look at. Since
a majority of models are taking the southerly track toward
northern Indiana, its no surprise that the NBM has higher Pops
through Sunday into Monday for areas south of I-80. But given
that it is five days away, confidence will remain in that
showers and thunderstorms are expected, but the details can be
ironed out closer to Sunday.

Beyond Monday, ensemble upper level patterns develop more
northwest flow over the area and the previous trough moves to
the east. As it exits, it could send some weak lobes of
vorticity over the area mid week for another chance for showers
over the area, but confidence being low, PoPs in the extended
were capped at just a chance. But with cooler air filtering in
from the northwest, high temperatures next week should be
around seasonal norms.

DK

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1230 PM CDT Wed May 22 2024

Main Concern:

- Timing of likely lake breeze wind shift on Thursday afternoon

Quiet VFR conditions are in store through this TAF cycle. Gusty
west winds will ease with sunset and become light southwest to
variable overnight. Weak winds aloft on Thursday are expected to
allow a lake breeze to push inland across ORD and MDW (and GYY
beyond its current TAF period), shifting winds to easterly near
10 kt. Indicated a 20z timing of the wind shift at ORD and MDW,
which will be refined as needed in subsequent updates.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 6 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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