000
FXUS63 KLOT 230736
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
236 AM CDT Tue Apr 23 2024

.KEY MESSAGES...

- Low-topped showers and thunderstorms this afternoon into the
  early evening. Level 1/5 severe weather threat from about 3 to
  8 pm NW of I-55 and north of I-80.

- Areas of frost likely (60%+ chance) late Wednesday night.

- Periods of showers and thunderstorms Friday through Sunday
  with locally heavy rain.

&&

.DISCUSSION...
Issued at 236 AM CDT Tue Apr 23 2024

Through Wednesday:

Recent analyses reveal frontogenesis is intensifying across
southwest Iowa and southeast Nebraska early this morning as 35 to
40 kt 700 mb southwesterlies are intercepting northwesterly
flow at the leading edge of an advancing disturbance. Showers
and thunderstorms have been gradually expanding in the vicinity
of this region of enhanced warm advection, and we`ll continue to
see an expansion as this disturbance translates eastward
through the morning. Seems like there`s been a modest northward
wobble with the track/location of the primary f-gen circulation,
which will be overlapped by strengthening upper jet divergence
later this morning and towards midday. Latest indications are
the main shield of rain will set up perhaps just north of I-80
and points south. A pretty formidable dry layer across northern
Illinois will likely chew away at precipitation chances. We
continue to see a signal for a brief period of gustier winds
developing this morning (8 am to 11 am) on the northern terminus
of the rain shield as sub-cloud evaporation and deeper mixing
works to transport 35 to 40 knot flow associated with a
diurnally weakening low-level jet towards the surface. Don`t
have gusts as high as some of the hires guidance reflected in
the gridded forecast, although did nudge gusts up a bit in this
corridor across the Kankakee River and up towards I-80.

This main slug of stratiform rain will push south of the region
through the early afternoon. Attention will then turn towards the
main upper vort and an initial pre-frontal trough feature.
Guidance continues to depict a rapid steepening of 0-3 km lapse
rates north and west of I-55, as well as surface dewpoints
increasing through the upper 40s to near 50 degrees. While
these values typically wouldn`t be associated with surface based
instability, 500 mb temperatures will be falling towards
about -25 C during the afternoon across northern Illinois as
the parent trough presses southward across Wisconsin. This
combination is expected to yield a pretty quick increase in
instability in a narrow ribbon along and just ahead of a
northwest wind shift. While MLCAPE values won`t be particularly
high--only about 500 J/kg within this narrow axis--most of it
will occur within the 0-3 km layer where a fair amount of
veering and strengthening of the wind field will be taking
place.

The current expectations are for scattered low-topped
convection to develop, possibly as early as 2 PM, but most
likely during the 3 to 8 PM time frame as the more notable
cooling at 500 mb occurs. With equilibrium levels progged to
only be about 20 kft or so, effective deep layer shear will be
reduced a bit as it`ll be occurring within the roughly 0-3 or
0-4 km layer. Still, guidance depicts deep shear values nearing
40 knots as an impressive 90 kt 500 mb jet streak intensifies
overhead. This will certainly be sufficient to facilitate some
degree of storm and updraft organization and mid-level rotation.

Given the steep lapse rates and low freezing levels, hail (mainly
small, but a few instances up to the size of quarters) along with
strong/gusty winds will be the main threats with any more
developed cores. With the cold temperatures aloft, LCLs aren`t
forecast to be all that high today even amidst 50 degree
dewpoints, and sufficient looping/clockwise turning hodographs
below 2 km suggest that the (brief) tornado threat, while low,
isn`t entirely zero today. Most hires guidance is suggesting
that cells won`t necessarily be tied to the region of enhanced
near- surface vorticity on the wind shift (instead forming a bit
ahead of it), but if any updrafts do manage to sustain in that
narrow ribbon, a hybrid supercell/landspout tornado threat could
also emerge on the wind shift boundary itself. The main area of
interest for the strong-severe threat is roughly NW of I-55 and
north of I-80. Convection will probably continue for an hour or
three after sunset but should gradually diminish with the loss
of heating.

The true cold front will arrive late this evening. Cold advection
will send temperatures back into 30s tonight. Across interior
northern Illinois, overnight lows will probably dip towards
freezing in spots. This doesn`t look like much of a frost set up
given the expected winds, and with such a limited area of near-
freezing temperatures, don`t anticipate needing freeze warnings
for the region tonight. Wednesday will feature highs in the low to
mid 40s near the lake, with low to mid 50s farther inland. There`s
some uncertainty regarding how expansive post-frontal stratus
will remain through the morning, but did end up boosting cloud
cover through early afternoon across the eastern half of the
forecast area.

Carlaw

Wednesday Night through Monday:

High pressure will be moving across the area Wednesday night
with light winds expected. There does appear to be a chance for
some high clouds overnight into Thursday morning but these may
not affect low temps too much, which are expected to be in the
lower 30s for most areas outside of Chicago. Areas of frost
still look on track for now for most of the area.

The Friday through Sunday time period is looking quite active
with periods of showers and thunderstorms. One area of low
pressure is expected to move from the central Plains to the
upper Great Lakes on Saturday and then a second area of low
pressure is expected to take a similar track Saturday night into
Sunday night. Precipitable water values approach 1.5 inches
Friday into Friday night and again on Sunday and these two time
periods appear to be the most likely for precipitation as
trends currently show. Ensemble QPF amounts by Sunday night are
generally in the 2-3 inch range for the local area, with some
isolated higher amounts. If these materialize, trends will need
to be monitored for flooding concerns, especially over the
weekend after the first expected wave of precipitation Friday.

The potential for severe storms will also need to be monitored
but given the proximity to the surface low, its possible that
more widespread or organized severe weather may remain west and
northwest of the area Friday with perhaps a little better chance
for a severe potential Sunday with the second area of low
pressure and possible cold front moving across the area. Though
there is still quite a bit of uncertainty with timing/location
differences among the models. If the front moves through the
area Sunday evening/night, then Monday may end up quiet. A
slower evolution/fropa, would introduce the potential for some
thunderstorms Monday. cms

&&

.AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/...
Issued at 1229 AM CDT Tue Apr 23 2024

Forecast concerns include...

Chance of thunderstorms this afternoon.
Strong/gusty southwest winds through this evening.
Wind shift to northeast late this evening.
Scattered showers overnight/Light rain mid/late morning.

Scattered showers will continue to move across the terminals
over the next few hours. There may be a brief vis reduction in
the heaviest showers, but otherwise vfr cigs/vis expected and
these will end by 08z-09z. Another area of rain is expected to
develop across west central IL after sunrise and spread east/
northeast through the mid/late morning, likely reaching MDW/GYY
and perhaps as far north as DPA/ORD. Cigs/vis also expected to
remain vfr with this rain. However, there will be mvfr cigs/vis
just south of the terminals with this rain. If the entire rain
area shifts further north than currently expected, than some
mvfr cigs/vis may get as far north as GYY. This rain will shift
east and end during the early afternoon. There will then be a
chance of thunderstorms from mid afternoon through early
evening. These are expected to develop just northwest of the
Chicago area terminals and then move southeast across the
terminals. Current prob mention has this potential handled for
now and expect to transition to tempo mention with later
forecasts if trends continue.

Southwest winds will gust into the mid 20kt range through mid
morning. Winds are likely to diminish some where the rain
develops later this morning but gusts will still be possible.
As winds turn more west/southwest this afternoon, gusts into the
30kt range will be possible. Winds will continue turning
northwest this evening then are expected to shift northeast late
this evening. High confidence for the wind shifts, but only
medium confidence for timing, which may need refinement with
later forecasts. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT Wednesday for the IL and
     IN nearshore waters.

&&

$$

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