931
FXUS63 KLOT 272030
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
330 PM CDT Thu Mar 27 2025

.KEY MESSAGES...

- Scattered showers and storms are expected to develop across
  the region overnight. A few storms will produce pea to nickel
  size hail.

- Friday will be warm and breezy with highs in the 70s and
  southwesterly winds gusting 30 to 40 mph.

- Rain and/or thunderstorms possible at times over the weekend
  with a substantial temperature gradient stalled across
  northern Illinois. Some thunderstorms Sunday afternoon could
  be severe and produce damaging winds, especially toward
  central Illinois and Indiana.

&&

.DISCUSSION...
Issued at 208 PM CDT Thu Mar 27 2025

Through Friday night:

RAP-mesoanalysis data augmented by observations depict a broad
region of low surface pressure along the lee side of the Rocky
Mountains well to the west of a surface high pressure system
centered over the Carolinas. A broad warm frontal zone extends
eastward from the region of low pressure from the Dakotas
through the middle Mississippi River Valley. Showers and a few
thunderstorms continue to fester along the terminus of a remnant
low-level jet and within a region of persistent mid-level
frontogenesis acting upon the broad warm frontal zone, mainly
from northwestern MO into central IL. Our area hence remains
clouded over, leading to muted temperatures and light winds.

Tonight, the broad region of low pressure well to our west will
consolidate and shift eastward into the northern Plains. As this
occurs, low- to mid-level southwesterly flow will increase across
the mid-Mississippi River Valley, facilitating the northeastward
advection of a reservoir of steep mid-level lapse rates (an
elevated mixed layer, or EML) northeastward into the Great
Lakes. As the warm frontal zone sharpens and low-level jet
reintensifies overnight in our general region, isentropic
surfaces will become increasingly sloped allowing for efficient
upglide. While confidence in showers and storms developing is
hence high (>90% chance in our general region), exactly where
carries lower confidence (40-60% at any given point) owing to
differences amongst model suites and the rate at which the base
of the EML moistens. Global guidance such as the EPS/GEFS favors
the axis of showers and storms extending from southeastern Iowa
to central Indiana, while high-res guidance summarized by the
HREF favors the axis further to the north from eastern Iowa to
Lake Michigan. This is a fairly typical source of forecast
uncertainty with convection along a northeastward-advancing EML
plume and intensifying low-level jet, and can be prone to
sizable forecast errors in observed locations of storms.
Regardless of where storms do develop, modest convective-layer
shear of 20-25kt and relatively low freezing levels near 10kft
will support periodic instances of pea to nickel size hail, in
addition to healthy downpours and frequent lightning strikes.
(Note that HREF 48-hour PMM QPF supports a few swaths of >1"
rain somewhere in our general region overnight).

Showers and storms may linger beyond daybreak across
northeastern or eastern Illinois tomorrow morning until the
continued eastward- advancement of the surface low to our
northwest pulls the warm front into Wisconsin. Decreasing
cloudiness will allow for mixing heights to quickly build upward
into the remnant low-level jet, though a strengthening low-
level inversion may tend to limit the efficiency with which
winds can mix to the ground (e.g. WAA-driven winds). With that
said, forecast mixing profiles among HRRR/RAP/NAM3 data support
frequent gusts of 30 to 40 mph. Temperatures will also rocket
upward tomorrow, with 850mb temperatures north of +10C and at
least partial sunshine supporting highs well into the 70s. In
all, tomorrow will look and feel quite a bit different compared
to today.

Winds will remain breezy after sunset tomorrow night as
temperatures only gradually fall (offsetting immediate
decoupling). Overnight lows will fall toward the upper 50s.

Borchardt

Issued at 327 PM CDT Thu Mar 27 2025
Saturday through Thursday:

A surface low will quickly track eastward across central
Wisconsin Friday night. An associated cold front is expected to
shift southward toward northern Illinois late Friday night and
Saturday, with continued notable model spread in the southward
extent of the front. The GEFS and related deterministic GFS have
trended much farther north and more inline with the EPS/CMCE,
which does increase confidence that the front will not surge
through the entire area during the day Saturday. But both
climatology and pattern recognition would suggest guidance may
be a bit farther north than what ultimately conspires. The
latest official forecast shows the front pushing over far
northeast Illinois during the morning, shifting southward to
near the I-80 corridor in the afternoon, then stalling during
the night. Max temps in the mid 70s are likely south of I-80 on
Saturday while temps in the 50s and 60s fall through the
afternoon. Behind the front, drizzle or fog is possible.
Meanwhile, a convectively-enhanced wave over eastern Texas will
shift across Indiana and result in a pocket of showers and
perhaps a few storms east of I-57 Saturday afternoon into the
evening.

Focus then turns to the potential for severe weather over at
least part of the forecast area on Sunday. Veering low-level
flow with only spotty upstream precip Saturday night should
allow the front to retreat northward as a warm front. A larger
surface low will then traverse along or near the warm front
somewhere across northern Illinois on Sunday. Like on Saturday,
the GEFS has trended much farther north with the warm front,
with much of the guidance now putting at least the southern half
of the CWA in the warm sector. Impressive mid to upper-level
dynamics, specifically with an incoming 70 knot mid-level jet
streak, over modest low-level moisture advection will support a
severe storm risk south of I-80, and potentially farther north
and west depending on if/where the warm front stalls due to the
influence of Lake Michigan. Shear vectors and somewhat
unidirectional low-level flow favor linear/bowing convective
segments with wind damage as the primary threat (large hail with
any more discrete cells). But again climatologically speaking,
both the storm mode (broken/bowing segments) in this area and
the potential for a stalled warm front does support a low end
tornado risk (particularly QLCS). Still a few days out to refine
these details, with frontal location being a significant factor
with how the forecast evolves this weekend.

As the main trough axis crosses the forecast area Sunday night,
a residual band of rain should mix with or even transition to
snow. There is a low (10%) chance of some minor slushy
accumulations late Sunday night into Monday morning, but overall
expectations are for little impact at this time. Drier and
colder conditions will follow through Tuesday with highs
generally in the 40s. Long-term ensemble guidance continues to
highlight another strong mid-latitude cyclone across the central
CONUS Wednesday into Thursday morning. Forecast uncertainty is
quite high locally as the associated warm front is favored to
settle across the western Great Lakes region. Any push into the
warm sector of this system will bring an increase in severe
storm potential.

Kluber

&&

.AVIATION /18Z TAFS THROUGH 00Z SATURDAY/...
Issued at 1220 PM CDT Thu Mar 27 2025

Key messages for the 18Z TAFs:
- VFR cloud cover this afternoon with a few isolated light
  showers and sprinkles.
- A period of more substantial showers and thunderstorms
  overnight, with IFR CIGs and VSBYs.
- Low clouds clear by mid Friday morning, then southwest winds
  become increasingly gusty (up to around 35 kt) through the
  afternoon.

An area of VFR cloud cover based in the 6-8,000 ft range
continue to stream across the area early this afternoon. There
are also some isolated showers, though most of these are
expected to remain southwest of the main Chicago terminals.
Otherwise, expect light (10 kt or less) south to southwest
winds to back more southeasterly into this evening as a warm
front begins to approach the area from the south.

The warm front will lift northward across the terminal airspace
around daybreak Friday morning. As it begins to move into the
area after midnight, a period of showers and thunderstorms are
likely (60%+ chance) to develop and move across the terminals.
Current best timing for these storms look to be in the 08-12z
timeframe. The only change for the new set of TAFs was to
convert the PROB30 to a Tempo group for these storms.
Otherwise, should see some lower CIGs and VSBYs with these
showers and storms, with IFR conditions favored.

Once the warm front lifts north of the area after daybreak
Friday, expect the low clouds to scatter out for the day, with
VFR conditions returning. The only other note for Friday will be
the increasing southwesterly winds through the day. Expect winds
to gust up around (or even a bit above) 35 kt, especially during
the mid and late afternoon hours.

KJB

&&

.MARINE...
Issued at 208 PM CDT Thu Mar 27 2025

Thursday night into Friday, a seasonably strong low pressure
system will move from the northern Plains toward the Great Lakes
causing a surface warm front to lift northward through the
region. Behind the front, southwesterly winds will increase in
magnitude with gale force gusts in excess of 35 kt possible,
starting Friday afternoon and lasting through early Saturday
morning. Since water temperatures remain cold (leading to
stability over the lake), the threat for gale force winds will
be within the first 2 to 3 miles of the lakeshore. A Gale Watch
remains in effect for the Illinois and a portion of the
northwestern Indiana nearshore Friday afternoon through early
Saturday morning.

NWS Chicago

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Watch from Friday afternoon through late Friday night for
     the IL and IN nearshore waters.

&&

$$

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