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000
FXUS63 KLOT 170546
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1246 AM CDT Wed Apr 17 2024

.KEY MESSAGES...

- Conditional thunderstorm threat early to mid Wednesday morning
  mainly I-355 and east, some severe potential.

- Windy again on Wednesday with peak gusts of 40+ mph.

- Next round of rain/showers on Thursday-Thursday evening.

- Much cooler over the weekend with frost/freeze concerns,
  particularly on Saturday night.

&&

.DISCUSSION...
Issued at 241 PM CDT Tue Apr 16 2024

Through Wednesday:

This afternoon, we find the surface low pressure centered over
eastern NE and its warm front extending eastward across the
heart of the CWA. A long line of showers and thunderstorms
extends from east-central IA down through eastern MO and is just
beginning to move across the Mississippi into portions of
western IL. This line will be the first feature to watch for
severe weather this evening. A second are of showers and storms
is located further the west from central IA through north-
central MO.

Precip with the first band will make it into our far west near
the I-39 corridor around mid-afternoon. The activity along the
first band is expected to gradually dampen on its way to our CWA
this afternoon as it outruns the better instability. In fact,
it looks like this line may already be outrunning the
instability quicker than guidance suggests. Regardless, severe
hail is the primary concern with the first band of convection
early on. If storms go surface-based along this line though, if
not sooner, the scope broadens to an all-hazards potential
including a noteworthy concern for a few to several tornadoes.
The tornado threat stems from an impressive low level shear
profile with as much as 40 kt of veering 0-1 km shear expected
this evening resulting in upwards of 300+ m2s-2 of 0-1 km SRH.

That aforementioned secondary band presents a second
opportunity for severe storms tonight for parts of the CWA. This
second push looks to move across the area between mid-evening
through the early overnight hours. This second round will have
more instability aloft to work with, as much as about 1,500
J/kg. This will keep the severe potential alive along this line
during the evening, though the tornado potential may be lower
with the lesser low level instability (e.g. increasing static
stability) and slightly veered surface winds behind the first
line.

While most of the overnight should be quiet, one last chance for
severe storms exists early tomorrow morning. The storm`s cold
front will be working across the area through the morning and a
line of storms may attempt to go up ahead of the boundary.
There`s still plenty of uncertainty on when precip will get
going ahead of the front and if it will even wait until later in
the morning when it`s east of our area. However, the signal for
precip to develop over the CWA has been on a big upward trend
over the past day. If storm`s do fire up, a brief tornado or two
will again be possible with the low level shear still cause for
concern, even if instability has come down quite a bit. We
could see storms develop as early as the predawn hours east of
I-39. Chances build with eastward extent with the seemingly best
potential for storms tomorrow morning being over northwest
Indiana.

Doom

Wednesday Night through Tuesday:

Quiet, precipitation free conditions will be short-lived as our
next rain producing system arrives Thursday morning. An
unseasonably deep/cold upper low over south central Canada will
tighten the baroclinic zone/thermal gradient across the region.
Modest large scale forcing but fairly robust progged low-mid
level frontogenetical circulation will likely bring an area of
rain/showers shifting eastward across the area through the
afternoon, evening, and early overnight period. Chance for any
embedded thunderstorms appears minimal and confined to south of
the Kankakee River. Highs on Thursday will be in the upper 50s
to 60s inland, and cooler near the lake.

Initial surge of dry and cold advection on Friday will result in
a partly cloudy, breezy and dry, seasonably cool day with highs
in the mid to upper 50s. There has been appreciable run to run
variance regarding the magnitude of the colder air mass aloft
arriving this weekend. In the most recent model cycle (00z
4/16), the pendulum swung back to colder, particularly in the
foreign (ECMWF and CMC) suites, which indicate 850 mb temps
plunging to near -10C Saturday morning.

If the colder solutions pan out, breezy CAA driven lows may
flirt with the freezing mark across parts of interior northern
Illinois Saturday morning, with mid 30s to around 40F elsewhere.
Saturday`s highs are currently forecast to reach the lower to
mid 50s, but the colder side of the spectrum points toward
downside potential being 40s for many locales. High pressure
building overhead amidst forecast sub-freezing dew points
Saturday night looks synoptically favorable for elevated
frost/freeze concerns outside of Chicago if skies clear out
enough. Following Sunday`s chilly start, afternoon temps should
recover some vs. Saturday but still likely below normal. Near
normal temps (lower to perhaps mid 60s highs) will then return
to start next work week, with the next precip. chances probably
holding off until just beyond the current day 7 (Monday).

Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1246 AM CDT Wed Apr 17 2024

Main Concern:

- Potential for scattered TSRA to develop over the Chicago metro
  in the early morning hours.

- Wind shift from gusty SSE to SW/WSW behind a cold front in the
  mid morning today

SHRA and TSRA over far northeast Illinois approaching 06z will
remain to the north of DPA and ORD. A strong cold front will
approach the area early this morning. Additional scattered SHRA
and TSRA may develop along/ahead of the front and opted to
include VCTS mention in the Chicago metro TAFs. It`s quite
possible that the convection gets going just east of the metro
terminals.

MVFR CIGs that have blossomed over the Chicago metro and points
south should persist through the night and may linger into the
morning if not later. At RFD, CIGs will likely remain VFR until
behind the cold front passage early this morning when MVFR CIGs
are expected to develop and possibly linger through the day.

Southerly winds gusting to 20-25 kt will shift to
southwest/west-southwest behind a strong cold front, with peak
gusts in the 30-35 kt range. Direction will trend to westerly
in the afternoon and evening and then speeds and gusts will ease
late in the evening and overnight.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 11 PM CDT this evening for the IL
     and IN nearshore waters.

&&

$$

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