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000
FXUS63 KLOT 230018
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
718 PM CDT Mon Apr 22 2024

.KEY MESSAGES...

- Low-topped showers Tuesday afternoon into the early evening,
  with a few thunderstorms between about 4pm-9pm. Monitoring a
  low but non-zero severe threat late in the day NW of I-55.

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

- Warmer by the end of the week with periodic showers and storms
  Friday into next weekend.

&&

.DISCUSSION...
Issued at 325 PM CDT Mon Apr 22 2024

Through Tuesday night:

In the wake of elevated fire weather conditions this afternoon,
breezy southwesterly and much milder conditions are in store
tonight, in the warm sector of the fast moving clipper-like
low pressure system that will affect the region through
tomorrow night. A thickening cloud deck along with the
southwesterly winds gusting up to 25-35 mph will keep low temps
propped up in the lower to mid 50s.

A lead wing of warm advection will develop this evening and
overnight in advance of the aforementioned surface low, which
will be pinwheeling across the Minnesota Arrowhead region. With
the sub-cloud layer down through the boundary layer remaining
quite dry/parched, coverage of shower activity should remain
pretty sparse with a good deal of sub-cloud evaporation likely.
That said, there`s sufficient forcing to keep an indication of
low PoPs (primarily 20-30%) in the official forecast for
occasional light showers/sprinkles.

By early Tuesday morning, the combination of better jet dynamics,
mid-level moistening, and enhanced 700 mb convergence/f-gen will
lead to an expansion of steadier precipitation south of I-80, or
near and south of I-80. Coverage continues to look high enough
to maintain a corridor of categorical PoPs, and given the
magnitude of convergence, some isolated rainfall amounts over an
inch still appear possible south of I-80. On the northern
periphery of the main area of rain, a consistent signal for
gusty winds exists in the guidance suite with plentiful sub-
cloud evaporation and steeper near-surface lapse rates. There
may be a few spits of rain as far north as the I-88 corridor or so.

On Tuesday afternoon, particularly late in the afternoon and
perhaps into the early evening, we`re continuing to monitor a
low but non-zero threat for strong to even severe thunderstorms
in advance of an incoming cold front. While surface dewpoints
will only be in the 40s to perhaps near 50 degrees, 500 mb
temperatures will be cooling towards -25 to -30 C as the core of
an impressive vort lobe approaches. These values are more
typical of late January, and are under the 10th percentile for
the date based on ILX`s sounding climatology.

This will provide a boost to available instability which would
otherwise be much lower given the surface dewpoints. In
addition, guidance is in very good agreement suggesting that
low-level lapse rates will steepen markedly immediately ahead of
the incoming front. All of this supports a narrow ribbon of
instability (all below about 400 mb) along/ahead of the cold
front, focusing along and northwest of I-55. Given the intense
forcing associated with the incoming DCVA/height falls, would
expect scattered low-topped showers and a few thunderstorms to
develop given equilibrium levels around -25 C.

Effective deep layer shear, while not terribly high and
modulated by the lower ELs, may be just sufficient to facilitate
a little updraft organization. Given the low freezing levels and
steep mid-level and low-level lapse rates, any stronger/taller
cores could support a threat for some marginally-severe
(quarter-sized) hail and gusty winds. In addition, given the
set-up and enhanced near-surface vorticity near the front, can`t
entirely rule out the potential for a hybrid supercell/landspout
tornado potential with sufficient destabilization, though LCLs
may be a bit too high. The threat remained a bit too low/uncertain
for an SPC day 2 severe delineation in the mid day update, but
is something we`ll be keeping an eye on given recent guidance
trends.

The cold front will be sweep across the area Tuesday evening
with a few lingering showers, possibly a thunderstorm across the
eastern/southeastern half or so of the area during the early
evening, until about 8-9pm. Winds will quickly shift northerly
behind this front and breezy conditions are expected with gusts
into the 20-25 mph range for a time inland, and 25-30 mph if not
briefly higher near Lake Michigan. Despite lows in the lower to
mid 30s for interior northern Illinois by Wednesday morning,
conditions look unfavorable for frost formation due the
stronger winds.

Carlaw/Castro

Wednesday through Monday:

Another period of chilly late April weather is slated for
Wednesday and Wednesday night following a notable cold frontal
passage late Tuesday. Cool northeasterly winds and cold air
advection in the wake of this front will persist on Wednesday in
advance of a surface high moving into the Great Lakes. The net
result will be high temperatures remaining several degrees below
average in spite of a good amount of sunshine through the day.
Currently, we expect high temperatures Wednesday afternoon to top
out in the 40s along the lakeshore, and into the 50s farther
inland. This will then set the stage for areas of frost as light
winds and mostly clear skies Wednesday night allow temperatures to
drop into the lower to middle 30s, coolest outside of Chicago.

Temperatures rebound back into the 60s later in the week and
eventually into the 70s for a period this weekend. However, the
main weather story later in the period will be the shift towards a
much more active and stormy pattern as an active belt of
mid/upper-level southwesterlies steer a couple of notable Pacific
waves northeastward into the the Upper Midwest Friday through the
weekend. The first Pacific wave is expected to induce lee
cyclogenesis across the central High Plains Thursday night, before
tracking into the Upper Midwest on Saturday. While the primary
threat for severe thunderstorms looks to largely remain west-
southwest of our area (in closer proximately to the surface low
and cold front) on Friday, increasing warm/moist advection over an
approaching surface warm front will likely drive a period of
elevated showers and thunderstorms across our area during the day.

The threat of severe storms may begin to increase over parts of
our area late Saturday into Sunday as a second wave, and
associated surface low, quickly track northeastward somewhere
along the lingering cold frontal boundary just to our west-
northwest. Unfortunately, the timing and exact evolution of this
second storm system into Sunday is of lower confidence at this
time. This is noted by some rather significant spread amongst the
ensemble guidance at this range. Nevertheless, with the larger
scale pattern supportive, we will have to keep an eye on the
potential for another period or two of stormy weather (some
possibly even severe with locally heavy rainfall) across parts of
our area Saturday afternoon through Sunday.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 718 PM CDT Mon Apr 22 2024

Key Messages:

- Scattered sprinkles and light showers around at times tonight.

- Scattered showers likely at the Chicago metro terminals
  tomorrow afternoon with a few thunderstorms possible.

- Blustery southwesterly winds throughout most of the TAF
  period, with a wind shift to northwesterly winds likely
  sometime tomorrow afternoon or early evening.

Scattered sprinkles and light showers currently in Iowa and
northwest Illinois will pass near or over the terminals this
evening before departing during the early overnight hours. Not
expecting these to cause any meaningful ceiling lowerings or
visibility reductions.

A more coherent rain band will set up in the area after daybreak
and persist through the early afternoon. This band is favored to
generally remain just south of the terminals with its northern
periphery expected to remain roughly along or near GYY`s
latitude. It is possible that this rain band will position itself
farther to the north than currently expected and potentially
affect the other Chicago metro terminals (particularly MDW), but
thinking that the likelihood of this happening is too low to
warrant the introduction of any formal mention of that in the
MDW, ORD, and DPA TAFs, especially since the ceiling lowerings
and visibility reductions on the northern edge of the rain band
shouldn`t be as pronounced as farther to the south.

Another batch of showers is then expected to blossom along a
surface boundary tomorrow afternoon, and potentially grow deep
enough to support lightning production. The stronger
thunderstorms will likely produce wind gusts that will be a bit
stronger than the ambient wind field across the area, in
addition to lowering ceilings and temporarily causing sub-VFR
visibilities. Given the expected positioning of the surface
boundary tomorrow afternoon, this thunderstorm potential should
remain southeast of RFD.

Lastly, southwesterly winds will likely continue to gust into
the 20-30 kt range through most of the rest of the TAF period.
The gustiness may become a bit more sporadic at times tonight,
which could result in low-level wind shear materializing, but
think that winds should remain elevated enough on the whole to
preclude the need for a LLWS mention in the TAFs. Winds should
then shift to a northwesterly direction behind the aforementioned
surface boundary tomorrow afternoon or early tomorrow evening.
There will likely be another wind shift to a northerly or north-
northeasterly direction behind the passage of the true cold
front towards the end of the current 30-hour ORD and MDW TAF
period, but will let that be addressed in the 06Z TAFs.

Ogorek

&&

.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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