Wed Feb 21 Meteorology for Chicago

[Brett Crapser]

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1151 AM CST Wed Feb 21 2024

KEY MESSAGES...

- Chance (30-40%) for thunderstorms late tonight, some stronger
storms with low areal coverage south of I-80

- Much colder weather Friday/Saturday but still seasonal

- Near record warmth early next week

- There is a favorable pattern for strong to severe
thunderstorms in the region early to middle next week, but
still much uncertainty in timing and location

&&

DISCUSSION...
Issued at 313 AM CST Wed Feb 21 2024

Through Thursday Night:

Things are fairly quiet in the near term, but the weather will
be a bit variable over the next 5-7 days, with everything from
rain, thunder and maybe even some snow on the docket along with
a bit of a temperature roller coaster.

Warm advection ahead of a weak upper wave across the plains is
driving an area of mid level clouds. Mid level lapse rates are
decent - around 6-7 deg/km, but 0z soundings and observations
suggest the low levels are quite dry. Therefore, the radar
echoes should remain just clouds, with nary an observation
reporting anything reaching the ground. This wave will exit east
this morning and provide a brief period of clearing. Surface
winds will remain up into daybreak, and thus the warm overnight
readings will provide a springboard for an even milder day into
the low 60s for many, possibly a mid 60 degree reading across
central IL. Records remain in the upper 60s to near 70.
Additional shortwave energy in the plains will bring the return
of clouds this afternoon.

The upper jet to our southeast will maintain its intensity,
while a secondary jet to our north will strengthen a bit.
Increasing southwest flow will bring dewpoints up ahead of a
deepening surface low. Some hi-res guidance tries to break out
some lighter precip farther north this evening with a
strengthening mid-level front, but the focus will be south of
I-80 tonight as a near surface front will tighten. Lift atop
this front, accompanied by fairly steep mid level lapse rates
will promote shower and thunderstorm development. While the
forcing for widespread coverage is not there, expect an uptick
during the later evening into the early overnight hours. SPC has
placed a Marginal risk for severe hail in the Day 1 06z
outlook. Storms will be elevated above the surface, and with low
freezing levels and an elongated and largely unidirectional
hodograph in the 1-6 or 2-6 km shear layer and shear values in
excess of 30-35 kt, the marginal risk seems appropriate. The
limiting factor will be somewhat low MUCAPE and narrow window of
forcing while CAPE is present.

Frontogenetical forcing and some higher rooted isentropic lift
will continue atop the southward sinking front, which will keep
rain showers in the forecast into Thursday morning, but with the
front sinking south, thunder chances will diminish.
Interestingly, there is some disagreement in how far north the
surface low/trough axis will get (6z NAM/0z GFS farther north,
EC/CAMS/Oz NAM farther south). This will determine how long this
band of rain showers lasts and how far north it gets. If the
farther north solutions (which are not currently the favored
solutions in the overall ensemble suite) pan out, this would get
closer to the Chicago metro area and would warrant higher
precip chances. This forcing will ease in the afternoon along
with arrival of drier and cooler air.

KMD

Friday through Tuesday:

After the surface low across the Ohio valley exits to the east,
a pocket of colder air will dislodge from a cold upper trough
over Hudson Bay and dive into the Great Lakes from central
Canada along a decent vorticity maximum on the back side of the
trough. While the forcing mechanisms are decent, and the air
mass will be on the colder side for snow, it will be a narrow
window with limited deeper moisture. This would suggest there
could be some flurries, maybe a brief snow shower given decent
low level lapse rates. Lake effect parameters are modest, given
the fact the airmass is respectably cold and the lake is not
frozen over.

We will be dry and a bit more seasonal in the temperature
department, nothing too unusual for the time of year for Friday
and Saturday. This is in spite of a fairly dramatic drop in
temperatures Friday morning (10-12C drop at 850 mb). Highs will
sneak back to the mid 30s to lower 40s before a rebound to well
above normal temperatures is slated to return to round out the
weekend into early next week. High pressure should keep more sun
around than clouds, though a weak wave on Sunday could bring a
bit more cloud cover, and some ensemble members across the
EC/GEFS/GEPS spectrum (10-15%) bring some rain showers across NW
Indiana.

There is an interesting signal for next Tuesday-Wednesday both
for well above normal warmth, but also for the potential for
stronger thunderstorms in the region in a lower CAPE but high
shear environment. A large scale upper trough will translate
across the country with deep southerly flow ahead of it, strong
upper jet and a deepening surface low all suggest an early
season strong to severe thunderstorm event could materialize
somewhere in the region. ECMWF Extreme Forecast Index values
suggest an unusual to extreme event somewhere from the lower
Great Lakes to the Ohio Valley with over 80-90% on the 50 member
ensemble above the 5 week running climate for maximum and
minimum temperatures, along with CAPE and Shear for late
February. Additionally, the tails of the membership (the higher
end values) approach 700-900 J/KG of CAPE. Colorado State
machine learning probabilities (yesterdays run) also have a 15%
contour for Day 8 nearby. There is significant disagreement in
the timing of any lead shortwaves, and most notably of the
associated cold front and the upper wave - but this is
certainly a pattern to keep an eye on.

KMD

&&

AVIATION /18Z TAFS THROUGH 00Z FRIDAY/...
Issued at 1150 AM CST Wed Feb 21 2024

- PROB30 for showers early Thursday morning, TS possible

Southwest winds will continue to gust in the 20-25 kt range for
the next few hours then gradually ease mid-late afternoon as
they trend westerly. Winds continue to ease after sunset,
becoming light and variable for a time this evening before
prevailing northeast overnight into Thursday morning.

There remains a chance (30-40%) for showers at the Chicago area
terminals early Thursday morning (10-16Z period), currently
covered with PRO30 groups. The better potential exist just
south of the terminals, though it is possible these get
converted to TEMPO groups (including a consideration for TSRA)
if confidence increases in a northward shift in where
showers/storms develop.

Any precipitation will decrease in coverage and shift south by
Thursday afternoon with winds eventually flopping northwest
ahead of an approaching cold front.

Petr

&&

LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$