636
FXUS63 KLOT 072023
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
323 PM CDT Mon Apr 7 2025

.KEY MESSAGES...

- On and off scattered snow showers can be expected
  into/through this evening.

- Periods of cold rain are expected on Wednesday, following
  lingering showers Wednesday night into Thursday.

- Temperatures will warm to above normal early next week.

&&

.DISCUSSION...
Issued at 323 PM CDT Mon Apr 7 2025

Through Tuesday Night:

Isolated to scattered snow showers and blustery, unseasonably
chilly conditions will persist into this evening, followed by a
clearing and rather cold overnight into Tuesday. A few more 700
mb shortwaves will interact with lingering steepened low-level
lapse rates due to the strong cold air advection. With the
activity toward and just beyond sunset, it appears a bit greater
coverage should focus over northeast Illinois, just inland of
the lake, with little/no snow showers in much of northwest
Indiana. Accumulations and road impacts remain unlikely with the
snow showers, though any heavier snow showers will continue to
knock visibility down to 1-2 miles at times.

As 1025 mb high pressure builds overhead overnight, clearing
skies and much lighter winds will result in a rather cold start
to Tuesday morning, with low-mid 20s common (mid 20s in Chicago)
and a few upper teens readings probable in typical colder/sheltered
spots of interior northern Illinois. Highs on Tuesday will be
about 10 degrees below normal with highs in the 40s, but likely
only upper 30s along the immediate lakeshore. The strong April
sun and lighter winds will (thankfully) make for a less
unpleasant afternoon than today. While the surface high is just
off to the east Tuesday evening, the very dry air mass in place
and only slowly increasing high clouds will result in quickly
falling temperatures (upper 20s to lower-mid 30s), that will
then slowly rise overnight.

Castro/Carlaw

Wednesday through Monday:

A fast westerly upper jet streak will emerge across Kansas and
Nebraska by Wednesday morning. Surface cyclogenesis is expected
to take place across southern Nebraska, and an associated
uptick in southwesterly 925-850 mb flow will yield a steady
increase in warm advection across much of our region. The
initial batch of warm advection-driven precipitation is expected
to blossom across our area into Wednesday morning (primarily
after daybreak) as a result.

While there`s still some remaining uncertainty regarding how
the low-level thermal profiles will look as precipitation
initially breaks out, wet bulb temperatures may remain just cold
enough to support a period of wet snow into Wednesday morning
(or wet snow mixing with rain), focused mainly across northeast
Illinois and parts of northwest Indiana. If the p-type can be
all snow for an hour or two after onset with heavy enough rates,
can`t rule out patchy slushy coatings on elevated/colder
surfaces, though temperatures above freezing should otherwise
generally limit impacts.

Thereafter (by late Wednesday morning), intensifying warm
advection should transition any lingering wet snow or mix over
to a cold rain through the day on Wednesday and into Wednesday
evening. In earlier model cycles, there was a plausible
scenario for precip to start to transition back to wet snow
Wednesday night into Thursday morning as the primary surface low
pulls away to our east. Most of the model guidance, however,
has backed off on this potential, though can`t completely rule
it out yet.

Rain shower chances will continue through Thursday as a series
of additional vort maxes pivot nearby, but activity should shift
and/or focus southward Thursday afternoon, and then come to an
end by Thursday evening. Broad troughing will linger overhead
through the end of the week, although the introduction of drier
air largely looks to suppress precipitation chances late in the
week and into the weekend. Temperatures will remain below normal
on Friday, coolest lakeside, then return to seasonable inland
(upper 50s-low 60s) on Saturday. There`s a signal for above
normal temperatures Sunday-Monday with highs reaching into the
70s, and this warmth possibly making it all the way to the
lakeshore for a change.

Carlaw/Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1221 PM CDT Mon Apr 7 2025

Aviation forecast concerns for the 18Z TAFs:

- Strong north-northeast winds through this evening, with gusts
  near 30 kts early this afternoon gradually easing. Winds turn
  northwest winds around 10 kt Tuesday, with a lake breeze shift
  to east for KORD/MDW mid-late afternoon.

- Scattered snow showers this afternoon, with occasional IFR vis
  and lower MVFR ceilings. Snow showers should diminish early
  this evening. No accumulations expected, only vis impacts.

Sharp cold front pushed south across the terminals earlier this
morning, turning winds north-northeast with gusts around 30 kt.
These winds will persist through this evening, with a very
gradual easing from peak speeds early-mid afternoon. Eventually,
winds are expected to turn north-northwest and eventually
northwest after midnight tonight, persisting around 10 kts into
Tuesday. The surface pressure gradient does look to relax enough
tomorrow afternoon to allow an easterly lake breeze to move
across KORD/KMDW in the 20-22Z time frame.

Withing the colder air behind the cold front, increasing
surface-based instability has allowed for the development of
convective snow showers to develop into far northern and
northeast IL/northwest IN. Prevailing conditions are expected to
be VFR visibility and high-based MVFR ceilings, though brief
deterioration into IFR vis is possible as scattered snow showers
track over individual terminals. No accumulation is expected.
Snow showers will persist through the afternoon and early
evening, but should quickly diminish after sunset. VFR
conditions are then expected overnight through Tuesday.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning until 4 PM CDT this afternoon for the IL
     nearshore waters.

     Small Craft Advisory until 7 AM CDT Tuesday for the IL
     nearshore waters.

     Gale Warning until 7 PM CDT this evening for the IN nearshore
     waters.

     Small Craft Advisory from 7 PM this evening to 4 PM CDT
     Tuesday for the IN nearshore waters.

&&

$$

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