660
FXUS63 KLOT 121130
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
630 AM CDT Wed Mar 12 2025

.KEY MESSAGES...

- Powerful spring cyclone will impact the area Friday into
  Saturday and bring with it:
 - Very windy and unseasonably warm Friday afternoon and Friday
       night
 - Fast moving line of showers and thunderstorms Friday night,
       some threat for severe thunderstorms
     - Very strong, possibly damaging winds, Saturday morning/early
       afternoon with some gusts over 50 mph likely

&&

.DISCUSSION...
Issued at 329 AM CDT Wed Mar 12 2025

Synopsis:

Not much change in guidance with respect to the very powerful
extratropical cyclone that will develop over the central High
Plains Thursday night, then taking a sub-980mb surface low
from eastern Colorado early Friday morning to Minnesota by
daybreak Saturday. Aloft, a vigorous mid-upper level low will
close off in the base of the negatively tilted upper trough with
>100kt 500mb speed max nosing around the base of the trough.
This classic, very intense cyclone will bring high winds and
severe thunderstorms to a large part of the country through
Saturday.

Today through Thursday:

Tuesday`s cold front has stalled out over southern Illinois and
southern Indiana early this morning. This front should remain
quasistationary today and then drift north tonight into
Thursday, but our CWA should remain to the north of the boundary
through Thursday. Easterly winds off the lake will keep
temperatures fairly cold near the lake with a footprint of lake
cooling extending pretty far inland given the eastern synoptic
gradient. This will set up a large temp gradient across our CWA
this afternoon and again Thursday afternoon. Highs near the lake
will be in the 40s both days. Elsewhere, highs today should be
in the upper 50s to lower 60s well inland from the lake and
north of I-80, and in the 60s south of I-80 today. Temperatures
away from the footprint of the lake cooling should warm about 5F
Thursday with 60s away from the lake north of I-80 and lower/mid
70s well south of I-80 Thursday.

Friday:

Pressure falls in advance of the aforementioned deep cyclone
will allow the warm front to surge northward across our CWA late
Thursday night/early Friday morning. Significant mid-upper level
height rises in advance of this cyclone actually look to keep
the brunt of the high cloudiness north of our CWA Friday,
leaving skies fairly sunny most of the day with the exception
of some possible afternoon fair weather cumulus. The sunshine
will promote a fairly deeply mixed boundary layer which has a
few implications:
- Should be able to mix into the increasingly strong southerly
  flow off the deck, probably resulting in gusts over 40 mph
  during the afternoon, with advisory level gusts over 45 mph
  possible over western CWA.
- Temperatures seem likely to end up on the higher end of the
  guidance spectrum, if not a bit above guidance (well into the
  70s with some low 80s possible).
- Potentially mix out the somewhat shallow boundary layer
  moisture in the afternoon, which could have implications for
  both available instability in our area as well as possible
  fire weather concerns.

One exception to the warmth Friday will be Illinois north shore
area. Synoptic winds will be backed slightly from due south and
should cause the warm front to get hung up just inland from the
Lake Co IL and perhaps far northern Cook county lakeshore. These
area will likely miss out on the near record warmth during the
daytime hours Friday.

Friday night:

Very strong winds are expected to continue into the evening
hours with advisory level gusts over 45 mph probably continuing
well into the evening, particularly in the urban heat island
of Chicago where temperatures through the evening time should
only fall from the 70s into mid-upper 60s by late evening.

An east and northeastward surging dryline is likely to result in
explosive severe thunderstorm development Friday afternoon from
southwest Iowa arcing southeast and south into and across
Missouri. This convection is expected to move rapidly
northeastward into western IL early Friday evening and then
across northern IL and extreme northwest Indiana during mid-late
evening hours, possibly lingering just past midnight in
northwest Indiana.

There are a number of areas of uncertainty regarding the
convective and severe threat in our CWA Friday night:

First, guidance is pretty gung-ho about convection becoming
linear quickly after initiation. Given the magnitude of forcing
with this powerhouse cyclone, it is hard to disagree with that
line (pardon the pun) of thinking despite the propensity for
drylines to not be big QLCS producers.

Next, it isn`t clear how much (if any) instability will be
available for the forecasted QLCS to feed off of as it rips across
our CWA. As already mentioned, it is possible that more
aggressive mixing out of the boundary layer moisture could
result in weaker than expected instability in our area. Certainly
seems likely that it will encounter an increasingly hostile
environment with respect to instability the farther east the
line gets. Could the line dissipate before making it through our
CWA? It`s possible, but most available guidance suggests it
will survive east across most of our CWA and given the magnitude
of forcing, that is hard to dispute with any degree of
confidence.

Finally, even with meager instability, shear and wind fields
with this cyclone will be extreme. It wouldn`t be hard for even
a weakening line of showers to mix down some 50kt+ gusts. If a
more vigorous QLCS were to make it into the CWA, then potential
for significant gusts (65kt+) and even a low end QLCS tornado
threat would exist.

Given the uncertainties, but fairly high ceiling with the very
strong wind fields, the new SWODY3 from SPC looks very
reasonable, including the higher threat over our southwestern
CWA where there is a better chance for more favorable
instability to overlap the extreme wind profiles.

Saturday:

A pronounced dry slot is progged to nose northeastward into the
region in the wake of the dry line late Friday night into
Saturday morning. This is a classic region for very strong and
potentially damaging synoptic winds to occur in powerful
cyclones like this one. GFS Bufkit soundings show max mixed
layer winds >50kt Saturday morning over much of our CWA. GFS and
ECMWF both would have the core of the strongest winds over our
CWA from between about 09-18z Saturday. Have increased winds in
our official forecast above NBM guidance, but well below
potential magnitude of how strong winds could get. It is common
for the magnitude of the winds to be stronger than modeled in
these type of set-ups. A swath of damaging gusts >50kt is quite
possible either in or near our CWA very late Friday night into
Saturday morning.

NBM pops continue to be likely far too high with our CWA progged
to be in the heart of the dry slot with skies likely clearing
out Saturday morning. The one exception may be some showers far
northwest CWA Saturday morning if the dry slot ends up just a
bit to the southwest. Given everything else going on, opted to
make no big changes to precip chances from NBM for now for
Saturday, but given current model guidance, we`re likely
eventually headed for a dry forecast most of the CWA.

Depending on how much dewpoints mix out in the wake of the
dryline and how much rain falls with the earlier convection, we
could also be dealing with a potent fire weather situation
Saturday given the very powerful winds.

Sunday onward:

Some lingering wrap around showers are possible Saturday night
into early Sunday, especially northern CWA as the big closed
cyclone wobbles away from the region. Colder, near normal
temperatures are expected Sunday, followed by a warming trend
heading into early next week.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 630 AM CDT Wed Mar 12 2025

VFR conditions are expected through the TAF period.

The easterly winds over performed overnight, with occasional
gusts to 20 knots. There are still stronger winds around 30
knots about 2000 AGL. As the sun rises, there is a chance an
isolated gust up to 20 knots mixes down to the surface, but
there is low confidence and impacts are not expected.

Winds will gradually weaken to below 10 knots and return to the
northeast late this morning. As winds become light overnight
into Thursday morning, there is a chance for some low level
saturation allowing some mist and lowering vis to develop. Model
soundings show just enough dry air to lower confidence and keep
reduced vis out of the TAF at present.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 10 AM CDT this morning for Winthrop
     Harbor to Northerly Is. IL.

&&

$$

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