NWS Chicago Decision Support Briefing - Forecast

780
FXUS63 KLOT 141123
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 AM CDT Fri Mar 14 2025

.KEY MESSAGES...

- A powerful spring cyclone will impact the area this afternoon
into Saturday and bring with it:
- Very windy and unseasonably warm this afternoon into
tonight, wind gusts to 40-50 mph expected.
- Near critical threat of grass/brush fire spread this
afternoon and evening.
- Fast moving line of showers and thunderstorms tonight,
with a threat for severe thunderstorms.
- Very strong, possibly damaging, winds Saturday morning
through early afternoon with gusts over 50 mph likely.
- Elevated threat of grass/brush fire spread Saturday.

- It will be much cooler late Saturday night into Sunday with
widely scattered rain and snow showers possible, followed by
above normal temperatures quickly returning for Monday-
Tuesday.

- Precip chances will return by the middle of next week.

&&

.DISCUSSION...
Issued at 340 AM CDT Fri Mar 14 2025

Conditions For Today:

Early this morning, we find a stationary frontal boundary draped
across central IL. A reservoir of upwards of 500 J/kg of MUCAPE
resides near and north of the front according to latest SPC
mesoanalysis. Moist, southerly isentropic ascent atop the front
has managed to stir up a batch cumulus clouds that`s since moved
over the area. But, just out to our west near Davenport, one
lone elevated cell found a way to go up and produce maybe a
couple of dozen lightning strikes and briefly heavy rain. Given
the drier ambient environment and general lack of forcing, we`re
not expecting much in the way of rain or storms this morning in
our area, but a few isolated showers/storms could conceivably
fall on our northwest CWA up until around daybreak.

This front will work its way northward through the morning.
Quick moisture advection behind the front beneath a steep
near-surface inversion may result in some fog developing
along/near the IL lakeshore during the morning. No models
explicitly resolve any really low visibilities, but forecast
soundings suggest that some lower visibilities near the lake are
possible. Any fog should dissipate by the afternoon as
temperatures pull away from dewpoints.

Today should be abnormally warm thanks to sunny skies and
persistent warm advection. 925mb temps look to push 20C which,
especially on a day like today, should favor highs in the upper
70s to around 80 degrees. Winds out of the southeast will keep
the Lake (IL) and northern Cook County shores several degrees
cooler than inland areas. For details on the windy conditions
today, see the separately-labeled section of the short term
discussion below.

Thunderstorms This Evening and Tonight:

The focus for this forecast is an expected line of strong to
severe thunderstorms slated for this evening and tonight.
Regional water vapor imagery this morning showcases a high-
amplitude upper trough with base moving over the Four Corners
region. As the trough ejects from the Rockies, the system will
see rapid lee cyclogenesis with the help of a strengthening
500mb jet max around the base of the trough which will obtain
around 120 kt of flow by this afternoon. A dry airmass following
the low out of the Desert Southwest will clash with a northward
flux of Gulf moisture causing a dry-line feature to set up
along the Mississippi Valley later today. As the aforementioned
jet max swings around to the leeward side of the trough and
interacts with this dryline feature, rapid convective
development is expected across KS, MO, and IA. Initial storm
mode looks to be discrete/semi-discrete cells before a strong
meridional kinematic field quickly transitions mode to linear
convection. This QLCS will then move across northern IL and
northwestern IN during the latter part of evening and stretching
into the early overnight hours of tonight.

Out ahead of this storm, between 1,000 and 1,500 Joules of
MUCAPE are expected to overspread the area in the warm
advection wing during the afternoon today. Luckily, this number
is expected to wane significantly by the time storms move in
closer to mid-late evening. Some models were previously
resolving some scattered convection out ahead of the line
earlier in the evening, which would have had access to the
better instability. However, models have really backed off on
this signal and now it looks as though this one discrete line
will be the whole show. It`s unclear how much instability we`ll
hang onto by the time the storm`s arrive, but models generally
resolve less than 1,000 Joules of MUCAPE coincident with the
storms. The NAM, in typical NAM fashion, maintains closer to
1,200 J/kg. A huge majority of that instability will be housed
above the lowest few hundred millibars. Most models resolve
little to no 0-3 km CAPE and just about all agree that there
will be some MLCIN in the 0-3 km layer. The RAP and HRRR are the
most aggressive with low level CAPE and resolve a very narrow
corridor along the QLCS with more-than- sufficient instability
for a QLCS tornado threat. This low level instability will play
a pivotal role in evolution of the overall severe threat, but
especially the QLCS tornado potential. There is also a worry
that long, curved low level hodographs and impressive dynamic
forcing could promote further lift/stretching and compensate for
a lack of low level instability.

The QLCS is expected to be on a general weakening trend as it
moves across our area and encounters a more marginal
environment with eastward extent. All hazards will be possible,
but damaging winds are the biggest concern for tonight with such
strong flow right off the deck (70 kt at 850mb and 55kt at
925mb) that shouldn`t be difficult to drag down to the surface
in this line of storms. A QLCS tornado potential exists
seemingly area-wide, although the much better chances appear to
be across our south and west where the storm should have more
low level instability and somewhat better shear to work with.
Steep mid level lapse rates with several hundred Joules of
MUCAPE will also promote a severe hail threat, although
significant hail does not look likely.

The storms will move away to the east overnight. Despite being a
rather short-lived event over any given area, models generally
paint between half an inch and three quarters of an inch of QPF
around the CWA with localized swaths closer to one inch. In
addition to the severe hazards mentioned, brief periods of heavy
or torrential rainfall are likely given the deep layer moisture
and forcing. Flooding does not look overly concerning, but some
localized ponding or pooling will be possible following any
heavier rain cores.

Strong Non-Thunderstorm Winds Later Today Through Saturday:

Little has changed with the wind forecast through Saturday.
As a result, we did not make any changes to the going wind
headlines for later today through this evening. However, we did
opt to include the remainder of our southeastern counties across
east central IL into northwestern IN in the High Wind Watch for
Saturday.

A rapidly tightening pressure gradient amidst a deeply mixed
boundary layer will foster increasingly gusty (40-45 mph) south-
southeasterly winds this afternoon and evening. Southerly winds
will remain strong and gusty through the evening, and may actually
end up peaking in intensity in excess of 50 mph during the mid to
late evening hours in response to an intensifying low-level jet
overhead (increasing to 50-60 kt around 1,500 ft AGL). We should
then see the advisory level winds tail off overnight following the
passage of a line of severe convection.

This lull in the winds will be short lived, however, as south-
southwest winds are slatted to quickly ramp-up again across the
area around (or shortly after) daybreak Saturday morning as the
systems dry slot punches northeastward into the area. Signs
continue to point towards the possibility of a period of damaging
winds, which could top 60 mph, particularly from mid morning
through early afternoon (roughly 9 am to 2 pm Saturday). A High
Wind Watch remains in effect for this potential, with the addition
of my southeastern counties to this watch. Winds speeds are
expected to gradually ease into Saturday evening.

Doom/KJB


Sunday through Thursday...

Forecast thinking during this period has not changed, please
reference the previous discussion below for more on this.

Conditions will be substantially colder and blustery on Sunday
as the synoptic cold front associated with the Friday/Saturday
storm system sweeps through the area. There is a potential for
some lingering precipitation (mostly snow) Sunday morning
towards midday as a sharp mid-level trough axis pushes overhead.
There`s plenty of variability regarding the amount of moisture
availability, so for now haven`t made any changes from the NBM-
delivered slight chance PoPs. Any light snow doesn`t look overly
impactful with temperatures expected to rise through the 30s
through the morning. Blustery W/NW winds will occur through the
day, with some gusts to around 30 mph appearing likely. While
temperatures will be notably colder, very dry air is slated to
arrive, and this may end up dropping RH values under 30 percent
at times during the afternoon. Fine fuels will likely still be
recovering from Friday night - Saturday`s precipitation, so it`s
unclear what impact this will have on any fire weather
potential, but will continue to monitor this for a potential
slightly elevated fire danger.

A warming trend will commence on Monday and Tuesday as deep
south to southwesterly flow re-establishes itself across the
region as another sprawling area of low pressure begins to take
shape across the central plains. Depending on how warm things
get on Monday, this potentially could be another elevated fire
weather day with moisture transport still lagging and gusty
southerly winds expected to develop with increasing mixing.

For Tuesday night and beyond, models and ensemble guidance have
consistently been showing significant lee cyclogenesis taking
place to our west as another intense/positively-tilted trough
axis swings through the Great Basin and across the Continental
Divide. While there`s still a fair deal of spread in the
associated surface low tracks, the signal for a robust storm
system in the region during the Tuesday night-Thursday timeframe
continues to grow. Showers and thunderstorms, followed by a
potential changeover to a little snow on the backside appear
possible.

Carlaw/KJB

&&

.AVIATION /12Z TAFS THROUGH 18Z SATURDAY/...
Issued at 623 AM CDT Fri Mar 14 2025

Forecast concerns through the period include...
- Strong gusty south-southeast winds (gusts up around 40 kt at
times), particularly mid to late afternoon through this
evening.
- Quick hitting line of severe thunderstorms with strong wind
gusts (possibly in excess of 55 kt) late this evening (likely
in the 03-06Z timeframe).
- Strong south-southwest wind gusts 45 to 50 kt redevelop after
daybreak Saturday morning.

An unusually strong early spring storm system developing across
the central High Plains early this morning, will rapidly shift
northeastward across the Upper Midwest into Saturday morning.
The rapidly strengthening surface gradient with this strong low
will foster increasingly south-southeasterly winds through the
day. Expect south-southeasterly winds (~160 direction) to gust
into the 30-35 kt range by early afternoon, then increase up
around 40 kt later this afternoon into the evening.

A line of severe thunderstorms are also expected to develop
across southeastern IA southward across central and eastern MO
late this afternoon. This activity will then rapidly shift
eastward across all of northern IL and northwestern IN mid to
late this evening. Even stronger wind gusts (perhaps 50-60 kt)
may accompany this line of storms. Best favored timing for these
looks to be in the 03-05Z timeframe at KRFD and more in the
04-06Z period at the Chicago area terminals.

The winds may ease for a period following the passage of the
storms overnight tonight. However, another period of strong
gusty south-southwest winds (up to 50 kt) is expected to
develop early Saturday morning. Expect these winds to peak in
intensity during the mid morning through the early afternoon
hours.

KJB

&&

.MARINE...
Issued at 340 AM CDT Fri Mar 14 2025

A powerful area of low pressure will move from the central
Plains this morning to near or over western Lake Superior on
Saturday. South-southeasterly winds will become increasingly
strong this afternoon and especially during the even. We`re
expecting a period of gale force (35-40 kt) winds through the
pre-dawn hours of Saturday, perhaps followed by a brief lull.
After daybreak Saturday, even stronger winds from the south-
southwest are likely, with a distinct potential for 50 kt storm
force speeds/gusts through the morning. For the impending
periods of very strong winds, an unseasonably warm air mass will
likely confine the strongest winds to the first few/couple
miles of the Illinois and far northwest Indiana nearshore. A
Gale Warning remains in effect from 4 PM Friday to 9 AM
Saturday and a Storm Watch also remains in effect from 9 AM to
3 PM Saturday.

Castro/KJB

&&

.FIRE WEATHER...
Issued at 340 AM CDT Fri Mar 14 2025

The Fire Weather Watch has been upgraded to a Red Flag Warning
for all of interior northern IL. A Fire Weather Watch remains
in effect for the Lake and Cook counties in northeast IL and for
northwestern IN. Both are in effect from 1 PM to 9 PM CDT today.
High temperatures this afternoon will climb to near 80F,
particularly away from Lake Michigan. The very dry fuels,
unseasonable warmth, and strong winds (gusting 40-50 mph later
this afternoon and evening) will lead to an high fire danger.
In spite of minimum relative humidity (RH) values possibly
remaining above the 25% criteria in much of the area, the
extenuating factors (very strong winds, summer-like temps, and
very dry fuels) may still enable fires to quickly grow out of
control.

Strong southerly winds will continue through Friday evening out
ahead of a fast moving line of severe thunderstorms that will
cross the area through the pre-dawn hours of Saturday. Rain is
expected areawide, though rainfall totals may only reach one
half inch. That said, the rain should moisten fuels going into
Saturday. Particularly if the rain under-performs though, even
stronger south-southwesterly winds gusting up to 50 to 60 mph
and RH in the 40-50% range may dry out some fuels enough to
cause an elevated fire danger again on Saturday.

Castro/KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory from noon today to 1 AM CDT Saturday for ILZ003-
ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012-ILZ019-ILZ020-
ILZ021-ILZ032.

Red Flag Warning from 1 PM this afternoon to 9 PM CDT this
evening for ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-ILZ032-
ILZ033-ILZ039.

High Wind Watch from Saturday morning through Saturday
afternoon for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-
ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

Fire Weather Watch from 1 PM CDT this afternoon through this
evening for ILZ006-ILZ014.

Wind Advisory from 4 PM this afternoon to 4 AM CDT Saturday
for ILZ006-ILZ013-ILZ023-ILZ033-ILZ039-ILZ103-ILZ104-
ILZ105-ILZ106-ILZ107-ILZ108.

IN...High Wind Watch from Saturday morning through Saturday
afternoon for INZ001-INZ002-INZ010-INZ011-INZ019.

Fire Weather Watch from 1 PM CDT /2 PM EDT/ this afternoon
through this evening for INZ001-INZ002-INZ010-INZ011-
INZ019.

Wind Advisory from 4 PM CDT /5 PM EDT/ this afternoon to 4 AM
CDT /5 AM EDT/ Saturday for INZ001-INZ002-INZ010-INZ011-
INZ019.

LM...Small Craft Advisory from 10 AM this morning to 4 PM CDT this
afternoon for the IL and IN nearshore waters.

Gale Warning from 4 PM this afternoon to 9 AM CDT Saturday for
the IL and IN nearshore waters.

Storm Watch from Saturday morning through Saturday afternoon
for the IL and IN nearshore waters.

&&

$$

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