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000
FXUS63 KLOT 221759
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1259 PM CDT Mon Apr 22 2024

.KEY MESSAGES...

- Gusty southwest winds today, along very low RH, leading to an
  elevated fire danger.

- Low-topped showers and a few thunderstorms Tuesday afternoon.
  Monitoring a low but non-zero severe threat late in the day NW
  of I-55.

- Frost may develop again late Wednesday night.

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

&&

.DISCUSSION...
Issued at 253 AM CDT Mon Apr 22 2024

Through Tuesday

The surface pressure gradient will tighten up over the region
today as high pressure continues to build south across the Lower
Mississippi River Valley as an upstream low scoots across
Manitoba. Gusty southwesterly winds will be the result today as
relatively deep boundary layer mixing taps into 30 knot flow
around 800 mb. Expecting gusts up to 35 mph across northwest
Illinois, and in the 25 to 30 mph range across NW Indiana where
the gradient flow will be a bit weaker. Additionally, have dropped
afternoon dewpoints closer to the stronger-mixing guidance such
as the HRRR, RAP, and RDPS, with this suite of guidance handling
Sunday`s dewpoint trends much better than the globals. Mid-level
moisture changes little today, suggesting a similar mixing-out of
dewpoints in the mid 20s, and wouldn`t be surprised if these end
up even a bit lower than currently advertised. The one caveat to
this will be how quickly the thick/opaque cirrus arrives, with
some potential for this to tamp down on mixing a bit, but all
indications at this point are that this won`t do much to curtail
the RH fall. With this in mind, even with the ongoing greenup,
fine fuel moistures are low (8 percent measured at both the
Midewin and Indiana Dunes RAWS sites Sunday afternoon), so we`ll
continue to highlight the elevated threat for fire spread today.

A lead wing of warm advection will develop this evening and
overnight in advance of the aforementioned low which will be
pinwheeling across the Minnesota Arrowhead region. With a still
parched boundary layer, coverage of shower activity may remain
pretty sparse with a good deal of sub-cloud evaporation likely.
Have cut PoPs during the overnight hours as a result.

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.
Coverage looks high enough to justify an introduction of
categorical PoPs, and given the magnitude of convergence, some
isolated rainfall amounts over an inch appear possible. 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.

On Tuesday afternoon, and particularly very late in the afternoon,
we`re monitoring a low but non-zero severe weather potential in
advance of an incoming cold front. While surface dewpoints will
only be in the mid 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. The threat currently
remains a bit too low/uncertain for an SPC day 2 severe
delineation, but is something we`ll be keeping an eye on given
recent guidance trends.

Carlaw

Tuesday Night through Sunday:

A cold front will be moving across the area Tuesday evening with
a few lingering showers, possibly a thunderstorm across the
eastern half or so of the area during the early evening. Winds
will shift northerly behind this front and breezy conditions are
expected with gusts into the mid 20 mph range into Wednesday.
Higher gusts will be possible near Lake Michigan. Despite lows
in the mid 30s for parts of the area by Wednesday morning, not
expecting much frost formation due the stronger winds and at
least partly cloudy skies for the eastern cwa. Highs Wednesday
will be cooler, possibly only in the mid/upper 40s for northeast
IL and northwest IN, 50s further inland.

Areas of frost are expected for most of the area away from
Chicago late Wednesday night into early Thursday morning with
mainly clear skies and light winds. Lows generally in the lower
30s outside Chicago. Temps expected to rebound back into the
lower 60s for highs Thursday with easterly flow keeping temps
cooler near Lake Michigan.

An unsettled and potentially active pattern is setting up across
the region for the end of the week into this weekend. There
remains uncertainty for timing/location and the initial rounds
of potential precipitation may be delayed as drier easterly
flow erodes their progress into the area. Eventually surface low
pressure is expected to track northwest across the upper
midwest on Saturday. Another surface low appears to develop over
the Plains by Sunday, which lifts northwest Sunday night into
Monday with another period of active weather possible for the
local area. The blended pops are generally in the 60-70 percent
range from Friday through Monday. While these may be fine for
certain time periods, they are likely too high for several days
in a row as there will be dry periods. However, from this
distance, there is too much uncertainty to try to add detail to
these higher pops. cms

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1257 PM CDT Mon Apr 22 2024

The primary aviation weather concerns for the 18Z TAF period
include:

* Scattered showers likely Tuesday afternoon with a few
  thunderstorms possible

Gusty SW winds will continue through the rest of today. Gusts
will become less frequent this evening and overnight, but
should continue to gust as high as 25 to near 30 kt at times.
Winds will be similar in direction and magnitude during the day
tomorrow, turning more westerly by late afternoon.

A few pockets of light, non-impactful rain may move over the
terminals late this evening and overnight. Conditions should be
dry during the first half of the day tomorrow. A swath of
widespread showers is expected to pass south of the terminals
during the morning and afternoon, but there is a non-zero chance
that a little bit of light rain could extend as far north as
ORD or MDW. A better chance for rain comes in the late afternoon
and early evening as a cold front swoops across the area. A few
thunderstorms will also be possible during this time.
Conditions are expected to remain VFR through the current
period.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT Wednesday for Winthrop
     Harbor IL to Burns Harbor IN.

     Small Craft Advisory from 10 PM this evening to 10 PM CDT
     Wednesday for Burns Harbor to Michigan City IN.

&&

$$

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