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000
FXUS63 KLOT 181737
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1237 PM CDT Thu Apr 18 2024

.KEY MESSAGES...

- A couple waves of showers expected today, though main period of
  more persistent rain (and maybe a few storms south of I-80)
  comes later this afternoon into the evening.

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

- Next period of showers (50-70%+ PoPs) Monday night into
  Tuesday, followed by another cool-down mid next week.

&&

.DISCUSSION...
Issued at 329 AM CDT Thu Apr 18 2024

Through Friday:

A compact surface low will track east-northeastward from the
southern Plains out across the Mid-Mississippi Valley today as a
cold front advances east-southeastward into the Midwest. Along
and south of the track of this low, a very moist and unstable
warm sector (amidst a well sheared environment) will support an
episode of severe thunderstorms this afternoon and evening. The
primary severe threat will largely be south of our area today
(roughly from MO eastward across central and southern IL) as the
surface low tracks into central parts of IL into this evening.
While this is the case, there is a low (20-30%) chance that the
severe weather threat (mainly a hail and locally strong wind
gust threat) could extend as far north as some of my far
southern counties in central IL and IN (areas roughly to near
the U.S. 24 corridor). Ultimately, however, this will be
dependent on the precise track the surface takes across central
IL into this evening.

While the main severe weather threat looks to remain south of
the area today, all of northern IL and northwestern IN will be
getting rain from this system (and maybe a few embedded non-
severe thunderstorms as far north as the I-80 corridor) today
through this evening. The rain looks to come in two waves, with
the second wave bearing a majority of the rainfall expected. The
first wave of showers will be associated with the remnants of
the of the shower and thunderstorm activity currently tracking
across northern MO and far southern IA. This activity will be on
a weakening trend as it shifts east-northeastward into north
central and parts of northeastern IL later this morning (after
9am). For this reason, it`s plausible that not everyone gets
much rainfall with this first wave, which could persist in some
areas through early afternoon (around 1 pm) before weakening and
moving out of the area. Much of the area may then experience a
few hour period of dry time this afternoon in advance of our
next wave of rain.

Our second (and most substantial) wave of rain is expected to
develop northeastward into the area later this afternoon (likely
after 3 or 4 pm this afternoon) and persist into the evening
(through 10 or 11 pm). This period of showers (and even a few
embedded non-severe storms) will develop in response to the
rapidly strengthening frontogenetic circulation along and north
of the surface low track. Accordingly, we are expecting a
roughly a 3 to 5 hour period of good rainfall as this enhanced
frontogenetic circulation interacts with seasonably high
precipitable water values up to 1.2". The rain (and any embedded
storms) will come to an end from northwest to southeast through
the evening as the surface low, and the associated forcing,
quickly peels off to our east. Total rainfall amounts should
generally be under an inch for most, but would not be surprised
to see a few locations get some more health amounts of an inch
to an inch and a half.

A cooler airmass will begin to work into the area in the wake
of this evenings exiting system as a large upper low settles in
across the Upper Midwest and Great Lakes into Friday. In this
pattern, breezy and dry westerly winds will develop during the
day Friday as deep diurnal mixing ensues. This will support wind
gusts up to around 35 mph through the afternoon as temperatures
peak in the upper 50s.

KJB

Friday Night through Wednesday:

The chilliest air mass since early April will be with us through
the weekend, bringing frost/freeze concerns during the overnight
and early mornings. Robust cold air advection (CAA) Friday night
will drop 850 mb temps down to -5 to -10C from south to north by
early Saturday. Northwesterly winds will ease from the blustery
early evening conditions, but likely still remain elevated enough
to preclude any frost development. The strength of the CAA will
still be sufficient for favored interior northern Illinois
locations to dip into the lower 30s, with some spots right around
32F. Not expecting to need any frost/freeze headlines given the
winds and only isolated 32F readings, but will message a limited
freeze risk in the HWO for Friday night.

Sprawling surface high pressure of ~1030 mb centered over the
central and northern Plains on Saturday will gradually weaken and
slide southeast through Sunday night. The position of the high
pressure, CAA, and steep low-level lapse rates will make for a
breezy and chilly (by recent standards) Saturday with highs only
in the upper 40s to lower 50s, locally mid 50s southern CWA. While
the surface ridging won`t be ideally positioned for completely
calm winds on Saturday night, given the very dry air mass (dew
points in the 20s), the boundary layer should still effectively
decouple for light enough northwest winds for strong radiational
cooling. In addition, any daytime strato-Cu will likely erode
quickly in the evening. The stage appears set for widespread lower
30s lows outside of Chicago and a decent coverage of sub-freezing
readings, with even some localized upper 20s plausible. While
conditions for frost development may not be quite ideal, we
continued to advertise areas of frost outside of Chicago. Frost
and/or freeze headlines will likely be needed.

Sunday won`t be as cool as Saturday after the frosty start with
plenty of strong late April sunshine boosting high temps into the
mid to upper 50s. The dry surface high pressure does look to be
close enough to our south to bring in Sunday night as another
target for areas of frost development outside of Chicago as lows
dip to the mid to upper 30s. Monday`s high temps will further
warm up to seasonably pleasant levels in the lower to mid 60s from
warm advection on the back side of the surface high. High level
and then mid clouds will arrive in the afternoon in advance of our
next system.

Medium range guidance is in solid agreement (for this lead time)
in a robust Pacific hybrid/clipper type short-wave affecting the
area Monday night into Tuesday. Warm advection wing showers will
quickly blossom in the late evening and overnight. At least
scattered showers may then persist into Tuesday morning ahead of
the system cold front, though breezy southwest winds should allow
for seasonable high temps. Temperatures should then however steadily
fall in the late afternoon in the robust CAA behind the cold
front, accompanied by some steepening lapse rate driven shower
activity. Below normal temperatures, potentially cooler than in
the official forecast, are probable Tuesday night into Wednesday,
coolest near the lake during the daytime hours.

Castro

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1236 PM CDT Thu Apr 18 2024

Key Aviation Impacts for the current TAF period:

- Showers and VFR clouds through mid afternoon at area
  terminals

- A line of storms moves in this evening with lower cigs and
  vis, gustier northeast winds, and steadier rain rates.
  Isolated lightning is possible, but confidence was too low to
  add to the TAFs

- VFR and westerly winds after midnight

The first round of rain is moving over northern Illinois and
Northwest Indiana at the time this discussion was posted. Cigs
have remained above 6000 feet and all the lightning remains
closer to St. Louis. There may be breaks in the rain through
22Z, so switched the TEMPO to prevailing -SHRA with the rain in
eastern Iowa moving west.

By 22Z, a cold front will through the area. This front will
provide an opportunity for steadier rain rates, gustier
northeasterly winds and cigs/vis down to MVFR levels. There is a
chance for IFR conditions to develop but confidence was not
there to add it to the TAF currently. As the system transpires,
if stronger cells develop, amends will be needed to reduce
conditions. Lastly, there will be instability present such that
isolated lightning strikes are possible as the front moves
through. Strong consideration was given to adding thunder to
KGYY`s TAF, but kept it out for now. The stronger confidence in
thunder remains south of area terminals where stronger
instability will be located in Central Illinois. But if there is
more consistent strikes developing, it will be added to TAFs
tactically.

Recent model runs have sped up the front so that the strongest
cells might be east of terminals by 03Z. -SHRA was added for the
chance for lingering precipitation behind the front.
Additionally, as the front passes, winds will slowly switch from
the northeast to the north west and weaken. Clouds will drift
east. There is a chance clouds between 2000 to 3000 feet linger
a through midnight, but there was moderate confidence in skies
clearing out so prevailed VFR through the end of the TAF.
Quieter conditions are expected on Friday with stronger wind
gusts around 25 knots in the afternoon.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory from 10 AM to 7 PM CDT Friday for the IL
     nearshore waters.

     Small Craft Advisory from 4 AM to 7 PM CDT Friday for the IN
     nearshore waters.

&&

$$

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