000
FXUS63 KLOT 241719
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1219 PM CDT Wed Apr 24 2024

.KEY MESSAGES...

- Widespread frost and near to sub-freezing temperatures outside
  of Chicago tonight.

- Periods of showers and thunderstorms Friday through Monday
  with heavy rain and the potential for severe weather,
  especially on Sunday.

&&

.DISCUSSION...
Issued at 243 AM CDT Wed Apr 24 2024

Through Thursday:

A cold front has passed through the region and notably cooler air
is filtering southward. The main forecast uncertainty today
revolves around the staying power of post-frontal stratus which
currently isn`t terribly expansive upstream. With this in mind,
continue to advertise a generally cloudy/mostly cloudy start to
the day for most areas, with quick scattering north and west of
I-55, with a slower clearing progression closer to the lake and
across northwest Indiana. Persistent onshore winds will hold
lakeside highs in the low to mid 40s today with mid 50s farther
inland. The airmass aloft will be very dry, with guidance
indicating dewpoints around 800 mb near -40 C. While we certainly
won`t be mixing that deeply today under a building subsidence
inversion, this did look like yet another candidate for
undercutting NBM dewpoints today, mainly where cloud cover is
expected to thin the fastest. The gustiest winds will ease
through the morning, curtailing the fire weather threat,
however.

A cool night is in store tonight, although there are a few
lingering questions about an incoming batch of cirrus and
potential impacts on OLR, as well as potential overnight pockets
of late-night stratus backing into parts of NW Indiana. Based on
upstream satellite imagery, this plume of high-level cloud cover
isn`t particularly opaque and currently doesn`t look like it`ll be
thick enough to notably curtail the temperature fall as winds
weaken and decouple. With this in mind, elected to issue a Freeze
Watch roughly for locales roughly north of I-88, except DuPage and
Central Cook for the potential for more than a few hours of sub-
freezing temperatures. Frost advisories appear probable for the
rest of our area, save perhaps for Central Cook/Chicago.

Temperatures will warm a bit on Thursday, although southeasterly
winds near the lake, followed by a midday lake breeze, will still
suppress highs near the lake to the upper 40s/near 50 degrees.

Carlaw


Thursday Night through Tuesday:

An active pattern is expected for the end of the week, weekend
and possibly into Monday next week. The overall consensus has
not changed too much. One area of low pressure will move from
the Plains Friday to the upper Great Lakes Saturday. Precip
looks to move into the area midday Friday and continue through
Friday evening. Convection during this time period looks to be
mainly elevated with the warm front lifting north across the
area late Friday evening or Friday night.

With the local area expected to be well in the warm sector
Saturday, temps in the upper 70s seem on track, could be some
lower 80s. Plenty of instability on Saturday but not much of a
trigger for storms which may stay west and north of the area,
possibly moving into northwest IL Saturday evening/night. If
storms were to develop Saturday, there would be a severe
potential.

A second area of low pressure will move from the Plains Sunday
to the upper Great Lakes on Monday. The models have slowed the
movement of this low. Depending on any decaying precip Sunday
morning, its possible Sunday may stay dry through mid
afternoon, with the potential for thunderstorms later Sunday
afternoon into Sunday night. These would also have the potential
to become severe. A bit more uncertainty for temps Sunday which
will be dependent on precip trends, but highs could easily
reach the upper 70s to 80 again Sunday.

There`s been an overall lowering in QPF amounts among all the
models and their ensembles, which are still in the 1-2 inch
range by Sunday night. Still quite a bit of uncertainty for
where the heaviest rains may set up and periods of heavy rain
are still possible.

The slower movement of the second low pressure could allow the
cold front to still be moving across the cwa on Monday with a
chance for thunderstorms through early afternoon in the eastern
half of the cwa. Dry weather is then expected Monday night and
Tuesday. cms

&&

.AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1219 PM CDT Wed Apr 24 2024

No major forecast concerns for the terminals through Thursday
afternoon. The lake effect MVFR ceilings will continue to
gradually scatter at the Chicago and northwest Indiana terminals
over the next few hours. Otherwise, VFR conditions are expected
through the conclusion of the forecast period.

As for winds, northeasterly breezes gusting between 15 to 20
kts will subside early this evening with light speeds (around 5
kts) expected overnight. Directions will become southeasterly
around daybreak Thursday morning, but will become easterly
behind a lake breeze Thursday afternoon at the Chicago terminals
and GYY. Though speeds will remain light with values between 6
and 8 kts.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Freeze Watch from midnight CDT tonight through Thursday
     morning for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
     ILZ011-ILZ012-ILZ103.

IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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