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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS63 KLOT 201550
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1050 AM CDT Sat Apr 20 2024

.KEY MESSAGES...

- Areas of frost late tonight/early Sunday morning away from
  Chicago.

- Next rain/shower chances Monday night through Tuesday.

- Another period of inclement weather with periods of showers and
  storms possible towards the end of next week into the weekend.

&&

.UPDATE...
-- Changed Discussion --
Issued at 1050 AM CDT Sat Apr 20 2024

Minor adjustments to going forecast at mid-morning, primarily to
increase cloud cover a little quicker across the north and
northwest parts of the forecast area, as well as to lower max
temps a degree or in these same areas where cloud cover will be
thickest this afternoon. Clouds will linger into this evening,
before eventually clearing from the northwest overnight. Will
need to contemplate potential frost/freeze headline especially
west and northwest of Chicago for later tonight with the full
afternoon forecast package.

Ratzer
-- End Changed Discussion --

&&

.DISCUSSION...
Issued at 201 AM CDT Sat Apr 20 2024

Through Sunday:

Temperatures are slowly dropping into the mid 30s across
northwest IL early this morning and with dewpoints in the 20s,
its possible some locations will see lows near freezing across
these areas, along/west of the Fox Valley and north of I-80.
Winds are expected to remain in the 10 mph range which should
limit any frost formation early this morning.

While this morning will start sunny, clouds will be on the
increase through mid/late morning with mostly cloudy skies
expected this afternoon into this evening. This cloud cover may
keep temps down a few degrees than previously expected and now
have highs generally in the upper 40s north to lower 50s south.
If the cloud cover arrives faster, limiting the amount of sun
and warming, these temps may still be a few degrees too warm.
Northwest winds will also be gusting into the mid 20 mph range.

There have been some flurries and sprinkles with this cloud
cover across the upper midwest early this morning and its
possible there could be a few sprinkles here later this
afternoon into the early evening but confidence is too low for
any mention at this time.

This cloud cover is expected to move out of the area tonight,
which will allow for clearing skies and light winds, setting up
good radiational cooling with low temps likely in the lower 30s
across the northwest cwa and mid 30s elsewhere. Frost may become
widespread across the northwest cwa by Sunday morning, while
temps remain near or perhaps a degree or two below freezing. Its
possible that if the clouds do clear, a frost advisory may be
needed vs. a freeze warning. And because of this potential, have
decided not to go with a freeze watch for tonight/early Sunday
morning. Temps look to rebound back to near 60 for highs Sunday
under mostly sunny skies. cms

Sunday Night through Friday:

High pressure will shift across the ArkLaTex on Sunday night
which will start the process of turning winds out of the
southwest. It looks like winds will remain sufficiently decoupled
to allow temperatures to fall into the mid 30s with another round
of frost development for areas outside of Chicago.

The pressure gradient will tighten up on Monday with increasing
high-level cloud cover through the afternoon in advance of the
next approaching disturbance. With the increased surface
gradient, southwesterly winds look to gust towards 30 mph during
the afternoon. Given the state of fine fuels (both the Indiana
Dunes and Midewin RAWS sites indicated 9 percent fine fuel
moisture on Friday), with afternoon RH values expected to fall
towards 30 to 35 percent, Monday looks like it may be another
day with an elevated fire danger.

Precipitation chances will increase Monday evening as low-level
warm advection ramps up in advance of a notably divergent and
compact upper jet streak. It doesn`t look like there will be much
in the way of instability with this initial batch of forcing, with
this initial batch of precip in the form of scattered showers. The
core of the main vort max and attendant mid-level height falls (80
to 100 m/12 hours) will then spread overhead through Tuesday
afternoon ahead of an incoming cold front. The GFS continues to
depict deeply-mixed soundings during a brief window during the
early afternoon with sufficiently deep/cold equilibrium levels to
support showers and even a few thunderstorms. While notably more
subdued, the latest ECMWF also generates just enough surface-based
instability to justify a mention of isolated thunder into the
evening.

Precipitation chances will diminish Tuesday evening as the main
trough axis shifts to the east. Breezy northerly winds will push
another cooler airmass into the region, with highs on Wednesday
likely not getting out of the mid to upper 40s near the lake.
Winds may also slacken sufficiently Wednesday night for another
round of frost development outside of Chicago. Temperatures will
then warm back into the 60s through the end of the week, although
lakeside locales across NE Illinois will likely remain in the
upper 40s and 50s with persistent southeasterly winds. Intermittent
shower and thunderstorm chances will return towards Friday and
through the weekend.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z SUNDAY/...
Issued at 610 AM CDT Sat Apr 20 2024

There are no significant aviation weather concerns. West-
northwesterly breezes will continue this morning and afternoon,
with occasional gusts towards 20-25 knots. VFR cloud cover will
develop later this morning and afternoon with bases around 4-5
kft agl. While there`s a small potential for a few sprinkles
later today, the chances are much too low to warrant a mention
in the TAFs. Winds will gradually ease tonight. Additional
northwesterly breezes will develop on Sunday afternoon.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 1 PM CDT this afternoon for Gary to
     Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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