418
FXUS63 KLOT 250512
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1212 AM CDT Sat May 25 2024

.KEY MESSAGES...

- Periods of showers and thunderstorms are still possible this
  evening before convection moves off to the east after
  midnight.

- Dry and pleasant Saturday-Saturday evening.

- Numerous showers and thunderstorms on parts of Sunday, with an
  attendant afternoon-early evening severe weather threat that
  includes the potential for large hail, damaging winds, and
  tornadoes.

- For Memorial Day and Tuesday, other than a few PM showers and
  maybe isolated thunderstorms, many dry hours, breezy and cool.

&&

.DISCUSSION...
Issued at 325 PM CDT Fri May 24 2024

Through Tonight:

A cold front that will be moving out of eastern Iowa and across
Illinois this evening has supported additional convective
development in the wake of the main cluster that moved through
earlier today.  A few of the new storms have even prompted warnings,
though coverage is much less organized than with the first line. SPC
has hoisted a watch for areas to our southwest, and the southern and
western portions of our forecast area appear to be most favored for
stronger storms during the later afternoon and evening period given
the better shear and instability expected for this area.

The extent of additional development will depend in part on how far
the outflow boundary from earlier activity can work back north as a
warm front and return a less stable airmass into the area. Given the
widespread cloudiness from convective debris, it would appear that
warm advection would be the primary factor supporting renewed
surface destabilization, more so than additional diurnal heating.
Even without significant boundary layer destabilization, the cold
front itself appears to be providing enough convergence to support
renewed development, some of which may be slightly elevated.

Given the cold front progression depicted in various deterministic
models, activity should be dissipating from west to east as we go
later into the evening, with most of the convection exiting to the
east after midnight. A less humid airmass then moves in by early
morning.

Lenning

Saturday through Saturday Evening:

The best day weather wise of the holiday weekend remains in
store, with dry high pressure bringing seasonable temperatures
and low humidity levels. Dew point temperatures are forecast to
mix out into the upper 30s-low 40s in spots during the afternoon!
A lake breeze will quickly develop around mid day and then push
inland through the afternoon. The lake breeze associated
cooling will keep highs near the Lake Michigan shore in the
upper 60s to lower 70s, while inland we`re forecasting mid to
upper 70s, and potentially some localized 80F readings. Prior to
high clouds moving in Saturday evening, light winds, clear
skies, and low dew points will result in temperatures dropping
back into the 50s for most of the area.

Sunday-Sunday Evening:

A rather conditional but potentially ominous severe weather
scenario may unfold on Sunday afternoon. The ejecting short-wave
trough and associated surface low expected to yield a dangerous
severe weather outbreak across portions of the southern and
central Plains on Saturday will quickly move into our area on
Sunday. Sunday will dawn quiet and fairly cool as the system`s
warm front will reside well south of our area.

Rapidly strengthening large scale ascent and strong warm air and
moisture advection will build MUCAPE near and north of the warm
front. Widespread showers and embedded at least scattered
thunderstorms are expected to steadily overspread the area in
the morning through mid day, probably at least in part the
remnants of extensive convection over the Plains Saturday
evening. Thus, Sunday will quickly become inclement for planned
holiday weekend outdoor activities.

The looming question mark is how quickly the air mass near and
south of the lifting warm front will be able to destabilize with
only a short window of time to work with after the earlier day
convection (barring a large change in this thinking). Most
concerning and possibly supportive of rapid recovery is the
strength of the system dynamics quickly overcoming post-lead
convection related stability.

Notably among the 12z models and ensembles, the ECMWF (EPS)
suite had a majority depicting the low deepening to the upper
980s to lower 990s mb as it tracks across northwest Illinois
and into southern/central Wisconsin. This is similar to the
scenario portrayed by recent runs of the 12km NAM, and
importantly, fits pattern recognition wise with past surface
low paths that have produced significant severe weather in our
area.

Because of the likelihood of morning-mid day convection,
confidence is low in how the afternoon will evolve. Big picture
wise, it appears areas with southward extent in the CWA have the
highest chance to more meaningfully destabilize as the warm
front lifts north, which generally aligns with the contours of
SPC`s Day 3 outlook. The above being said, the unusually deep
cyclone for this time of year paired with the late May day
length and sun angle does introduce the plausible concern for
more rapid destabilization. Temps rising through the 70s and dew
points through the 60s near and south of the front along with
very steep mid-level lapse rates of 7.5-8.0C/km may quickly
yield 1000-2000 J/kg of MLCAPE only an hour or two after the
lead convection ends.

Very strong low-level and deep layer bulk shear from a system of
this magnitude entails the potential for areas that destabilize
sufficiently to have a 2-4 hour window all severe hazards
threat. Orthogonal orientation of the deep layer shear vector
(west-southwest) with the cold front suggests that a
supercellular mode is certainly possible ahead of the front.
With winds backed to south-southeast ahead of the cold frontal
trough and strong veering with height up to 850-700 mb, low-
level hodographs will likely be curved and favorable for low-
level mesocyclone development with right moving (deviant)
supercells near the warm frontal zone and in the warm sector.

Where enough 0-3 km CAPE can be realized, 0-1 km SRH up to or
even upwards of 300 m2/s2 and low LCL heights below 700-800 m
introduces a real concern for tornadoes in addition to large to
very large hail and damaging winds. How far north the favorable
ingredients for supercells and tornadoes will reach is huge
question mark, though we`d be remiss to not note that the type
of wind profiles in play may well support the occurrence of
strong tornadoes within the CWA. It just appears that the window
of time where these ingredients may come together appears
relatively short until the winds veer with the approach and
passage of the cold front and brings the threat to an end by the
evening.

Our PoPs are broad brushed and high through Sunday afternoon,
though in reality this detail will need refinement as an entire
afternoon washout appears unlikely. Considering the likelihood
of a couple rounds of storms and the dangers of lightning, let
alone the looming severe weather potential though, our main
message continues to be to pay close attention to subsequent
updates. It does appear that the system has trended progressive
enough to quickly end any evening showers and thunderstorms by
if not before sunset Sunday evening.

Castro

Monday through Friday:

Following Sunday`s storm system, a building upper-level ridge
across the Rockies is expected to setup a cooler, but still
active, northwesterly upper-level flow pattern across the Great
Lakes region early next week. Ensemble guidance suggests that
this pattern will act to steer a few smaller scale impulses
southeastward into our area for Memorial Day and Tuesday.
Accordingly, there are chances (~30%) for some widely scattered
mid-late afternoon and early showers (and possibly a few non-
severe storms) both days. Coverage may be low enough that many
areas remain dry, however. Until the possible PM showers push in
from the northwest, Memorial Day will likely be conducive to
outdoor plans, though cool with highs only in the lower 70s
amidst northwest winds gusting up to 30 mph.

A period of dry (and rather pleasant) weather is expected mid to
late week (especially Wednesday and Thursday of next week) as
surface high pressure sets up shop over the Great Lakes region.
Daytime temperatures are forecast to be in the low to middle 70s
both days under mainly sunny skies.

KJB/Castro

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1212 AM CDT Sat May 25 2024

A surface cold front will shift across the Chicago area
terminals by 08z. In its wake, winds will turn northwesterly and
clouds will clear. Area observations do suggest that a brief
period (up to an hour) of MVFR CIGs may precede the frontal
boundary before conditions quickly clear out behind the front. I
opted to advertise this potential with a couple hour tempo
through 08z. Otherwise, VFR conditions are anticipated through
the remainder of TAF period.

While northwesterly winds will persist in the wake of the cold
front this morning, an afternoon lake breeze will turn the
winds easterly at the main Chicago terminals after 19z this
afternoon. East to southeasterly winds are then expected to
persist tonight as mid and high level cloud cover increases in
advance of the next approaching weather system slatted to impact
the area on Sunday.

KJB

&&

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

&&

$$

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