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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 250526
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1226 AM CDT Thu Apr 25 2024

.KEY MESSAGES...

- Widespread frost and sub-freezing temperatures, outside of
  Chicago, tonight

- Periods of showers and thunderstorms expected starting Friday
  and continuing at times through the weekend, with some threat
  for severe weather, particularly later in the weekend. Locally
  heavy rainfall possible as well.

&&

.DISCUSSION...
Issued at 250 PM CDT Wed Apr 24 2024

Through Friday:

High pressure will continue to drift into the Great Lakes
tonight which will aid in eroding the lingering lake effect
cloud cover in northwest IN and diminishing the breezy northeast
winds. Therefore, conditions look favorable for a decent night
of radiational cooling, which should allow temperatures to dip
into the upper 20s and lower 30s outside of Chicago with
readings in the mid-30s in the city. Dew points are also
expected to remain in the upper 20s to lower 30s tonight which
will keep maximum RH values near saturation and promote
widespread frost development, especially outside of downtown
Chicago. While confidence in these frost/freeze conditions
remains high, there is some mid-level cirrus across the central
Plains that is progged to drift overhead tonight. Given that the
cirrus should thin as it moves into the drier airmass overhead
I suspect it will do little to reduce the aforementioned
radiational cooling and resultant frost development. Thus, the
Freeze Watch for areas north of I-88 has been upgraded to a
Freeze Warning and expanded to include DuPage county. A Frost
Advisory was also issued to encompass the rest of our forecast
area with the exception of central Cook County.

After a chilly start to our Thursday temperatures are expected
to rebound into the lower 60s as winds become southeasterly and
warm advection begins. However, a lake breeze is forecast to
develop and surge inland Thursday afternoon resulting in notably
cooler temperatures for areas along the lakeshore. Otherwise,
skies will remain mostly to partly sunny before cloud cover
starts to increase Thursday night ahead of our next weather
system on Friday.

Yack

Friday Night through Wednesday:

Large scale pattern change is expected as a pair of vigorous
shortwave troughs will help carve out a deep western trough.
Amplification of a downstream ridge over the eastern U.S. will
allow for much warmer weather, though the expected proximity of
the jet stream should result in stormy periods starting Friday
and continuing through the upcoming weekend.

First shortwave trough is progged to eject out onto the central
High Plains Friday, with strong model consensus in closing off a
mid-upper level low as trough acquires negative tilt with time.
First slug of warm air advection driven showers and eventually
some thunderstorms should overspread the area Friday afternoon and
continue into Friday evening. Strong moisture transport associated
with a broad 50-60kt low level jet should drive PWATS to 200-250%
of normal by Friday evening. This certainly suggests that some
locally heavy rainfall totals are possible, with probs of precip
totals >1" in around half of the GEFS and EPS members.

By 12z Saturday, this lead trough is expected to lift north to the
Upper Mississippi Valley, which should drive the sfc low and
associated warm front well north of the area. Though NBM maintains
some precip chances on Saturday, it is increasingly looking like
Saturday could be a good candidate for a potentially dry day
this weekend. With warm front well to our north and height rises
in the wake of the departing trough, potential will be there
for temps to get quite warm if stratus doesn`t linger into the
afternoon. With some sunshine, high could make a run at the
lower 80s and with dewpoints at least in the upper 50s, that
would feel "muggy" compared to today!

Another round or two of showers and thunderstorms are possible in
the Saturday night through Sunday night time frame as the next
strong shortwave trough ejects out from the western long wave
trough. Hard to pinpoint timing at this distance as a lot will
depend on upstream convective coverage and eventually evolution.
Synoptically, general set-up looks generally favorable for a
severe threat in the region Sunday afternoon/evening, but where
that threat set-ups will likely hinge on how things evolve
Saturday and Saturday night.

Cold front is expected to move across the region Sunday night,
likely bring an end to the shower/thunderstorm threat. Pacific air
mass behind this front means temps will remain mild early next
week, with highs possibly still in the upper 60s/low 70s behind
the front early next week. A warming trend is expected heading
into the middle of next week back to well above average temps.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
-- Changed Discussion --
Issued at 1226 AM CDT Thu Apr 25 2024

Only forecast concern this period is a lake breeze this
afternoon. Light and variable winds overnight will become
southeast after daybreak and may briefly become southerly before
turning back to the southeast. A lake breeze will move inland
this afternoon shifting winds easterly at ORD/MDW/GYY with
speeds to 10kts. Winds will turn back to southeasterly this
evening with speeds under 10kts, though speeds are expected to
increase to 10-12kt by daybreak Friday with some gusts possible.
cms
-- End Changed Discussion --

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
IL...Freeze Warning until 8 AM CDT this morning for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ103.

     Frost Advisory until 8 AM CDT this morning for ILZ019-ILZ020-
     ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ105-ILZ106-ILZ107-
     ILZ108.

IN...Frost Advisory until 8 AM CDT /9 AM EDT/ this morning for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...None.
-- End Changed Discussion --

&&

$$

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