NWS Chicago Decision Support Briefing - Forecast

108
FXUS63 KLOT 120717
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
217 AM CDT Thu Mar 12 2026

.KEY MESSAGES...

- Strong to very strong winds develop overnight tonight and
especially Friday morning/early afternoon with a potential for
60 mph wind gusts during this time. Wind Advisories and High
Wind Watches issued for the area.

- Small potential for snow/graupel squalls with a cold front
late tonight, mainly across far northern Illinois.

- Strong storm system may bring a threat for strong storms to
the region on Sunday, followed by accumulating snow and
another round of very strong winds Sunday night into Monday.

- Turning much colder early next week with Monday morning and
Tuesday morning wind chills below zero.

&&

.DISCUSSION...
Issued at 216 AM CDT Thu Mar 12 2026

Through Friday Night:

A roughly 1030 mb surface high across southern Oklahoma will
continue to build eastward across the ArkLaTex today and this
afternoon. Lingering breezy westerly winds have continued to
subside early this morning, but some additional west to
northwesterly breezes will develop later this morning and
afternoon. Regional water vapor loops reveal a small but fairly
robust vort max pushing across Iowa and into Wisconsin at this
hour, with some corresponding scattered mid-level cloud cover
and radar echoes. Other than tossing perhaps a little more mid
deck across our north this morning, this shouldn`t amount to
much in our area. Cloud cover will thicken up late this
afternoon and evening in advance of the next disturbance.

Model guidance is in good agreement depicting a surface low
deepening towards the mid 980s mb range at is quickly scoots
east across central Minnesota and eventually into northern lower
Michigan late tonight and on Friday. An very impressive low-
level mass response is progged to materialize across the region
late this evening and overnight as a 70-80 knot southwesterly
LLJ spins up. As is typical during the overnight hours, some
degree of near-surface static stability will keep the vast
majority of this flow out of reach, but the presence of a
tightening pressure gradient and persistent pressure falls will
likely yield a gradual uptick in the frequency and intensity of
gusts through the night.

Additionally, an area of showers will develop along the nose of
an incoming upper jet streak. Lapse rates above 600 mb become
fairly steep after about midnight tonight as forcing
intensifies. Have painted high-end chance to likely PoPs across
the northern half of the area, and some guidance trends would
suggest we may need to continue boosting PoPs a bit even across
our far south. Can`t rule out the potential for a few lightning
strikes given the steep lapse rates, but chances at this time
appear a bit too low to warrant adding a formal mention to the
grids.

The intense surface low will eventually drag a cold front
through the region sometime around or just prior to daybreak
Friday morning. Forecast soundings reveal a modest uptick in
low-level instability ahead of the front, although whether or
not there`s any true surface-based instability remains in
doubt. Nonetheless, given the magnitude of low-level forcing in
place, there`s a small potential that a few shallow/low-topped
showers develop along/ahead of the front. While air temperatures
are forecast to be at or above freezing, wetbulb zero heights
are forecast to fall to under a few hundred feet. If robust
convective elements can develop, couldn`t entirely rule out a
threat for some snow and/or graupel squalls as the front comes
through. Regardless, it will be windy, and the front itself will
likely be accompanied by a fairly sharp uptick in wind gusts.

Through the rest of Friday morning, low-level lapse rates are
forecast to steepen as cold advection spreads across the region.
This will begin to allow for more efficient and persistent
momentum transfer out of a 50-60 knot 850 mb jet, leading to
another likely surge in wind speeds and gusts through midday.

Regarding wind headlines: somewhat of a tricky scenario in this
case. At least sporadic wind advisory-level gusts (around 45
mph) will be possible after about midnight tonight, with more
of a notable increase likely not occurring until well after
midnight and especially along and behind the cold front with the
strongest gusts focusing primarily in the 7 AM to 1 PM
timeframe. Forecast soundings are fairly concerning during this
time window, revealing mean boundary layer winds easily over 40
knots and at times nearing 45 knots. With strong pressure rises
developing in the cold advection regime, it certainly seems
plausible that at least sporadic 60+ mph wind gusts materialize
through the morning across parts of the area. One fly in the
ointment is the potential for expansive lower stratus to
develop, particularly across northern Illinois, through Friday
morning. This can limit the degree of mixing from time to time
(not always), but regardless is a source of some uncertainty in
how strong winds will get.

That said, concern and confidence was high enough for a period
of very strong winds to justify the issuance of a High Wind
Watch for the northeastern half of the forecast area from late
tonight through mid afternoon Friday. In this area, the longest
overlap of steepening BL lapse rates and lingering strong
925-850 mb flow will lead to the highest chances for peak gusts
to exceed 55 mph at times. Periodic wind gusts this high are
possible elsewhere, but may not be as frequent. Have confined
the time window of the High Wind Watch to encompass the period
of worst conditions (rolled a little earlier to capture the
cold frontal passage), while the Wind Advisory runs a bit
longer to encompass the potential overnight, LLJ-induced gusty
winds and the late-afternoon ramp down. Winds will ease sharply
Friday evening as the pressure gradient relaxes.


Carlaw


Saturday through Wednesday:

The active stretch of weather looks likely to continue over the
weekend and into the start of next week, potentially including a
low to non-negligible threat for strong to severe storms,
accumulating snow, and strong to very strong gradient/non-
thunderstorm winds.

Model guidance is coming into better alignment regarding the
evolution of a diving/digging shortwave trough and subsequent
cyclogenesis across the central plains Saturday night into Sunday.
As this occurs, intensifying warm advection/southerly low-level
flow will impinge on a remnant 850 mb baroclinic zone which
should result in blossoming frontogenetically-enhanced
precipitation into early Sunday morning. At this time,
deterministic and ensemble guidance suggests that the majority
of this precipitation may focus north of the Wisconsin state
line, but there will be some threat for snow or a mix across
northern Illinois Saturday night.

Thereafter, a break in the action is possible as the fgen zone
rolls northward and a deepening surface low tracks somewhere
near or just north of Kansas City. Guidance is reasonably well
clustered with the low track immediately to our west on Sunday.
Unfortunately, this track would slingshot the system warm front
northward into our region, potentially as far north as I-80 on
Sunday afternoon/early evening. With less of a EML in place and
generally lower dewpoints compared to this Tuesday`s severe
weather episode, instability is not nearly as significant. Most
soundings generally hold onto some level of surface convective
inhibition, but with such robust forcing and a rapidly-
intensifying wind field, this will be a period to monitor
closely with a potential for low-topped strong convection.

Eventually, the surface low will push north and east of the
region allowing a much colder thermal profile to spill in from
the north and west. The trend in the guidance lately is towards
more of an accumulating snow potential in our region Sunday
night into Monday as a result (particularly west of the Fox
Valley and across northwest Illinois), with surface temperatures
potentially falling through the 20s with increasingly strong
north to northwesterly winds. Will start to beef up the
accumulating snow mention a bit in our graphical messaging
today. If trends continue, this has the potential to be an
impactful episode of winter weather somewhere in the region.

A much colder airmass will descend on the region through Tuesday
as this storm system pulls away. 850 mb temperatures falling to
around -20C may support some lake effect snow into Tuesday
morning, but likely to focus mainly north and east of our region
given WNW/NW boundary layer flow.

Thereafter, active and very dynamic synoptic flow will continue
through the rest of the week with notable ridging developing
across the western CONUS. Associated downstream troughing may
guide continued disturbances through the region, although
generally quieter conditions look to develop locally with
moderating temperatures through the end of next week.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1236 AM CDT Thu Mar 12 2026

Primary forecast concern this period is very strong winds
tonight into Friday.

Westerly winds under 10kts are expected early this morning.
Speeds may increase into the 10-12kt range later this morning
with some higher gusts at times through early afternoon. Winds
will turn southerly late this afternoon and may briefly become
south/southeast early this evening, before turning back to
south/southwest during the evening. Speeds will steadily
increase this evening and will be in the mid 20kt range by late
evening into early Friday morning, as gusts increase into the
mid 30kt range. Higher gusts, possibly into the mid 40kt range
are possible after daybreak Friday morning behind a strong cold
front that will turn wind directions westerly.

There will also be a chance of showers early Friday morning,
with the highest coverage near the IL/WI state line, but a few
of these showers may spread across all the terminals in the
early morning hours. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory from 1 AM to 6 PM CDT Friday for ILZ003-ILZ004-
ILZ008-ILZ010-ILZ011-ILZ019-ILZ021-ILZ032-ILZ033-ILZ039.

High Wind Watch from late tonight through Friday afternoon for
ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ023-ILZ103-ILZ104-
ILZ105-ILZ106-ILZ107-ILZ108.

IN...Wind Advisory from 2 AM to 7 PM EDT Friday for INZ019.

High Wind Watch from late tonight through Friday afternoon for
INZ001-INZ002-INZ010-INZ011.

LM...Gale Warning from 10 PM this evening to 10 PM CDT Friday for
the IL and IN nearshore waters.

Small Craft Advisory until 4 AM CDT early this morning for
Northerly Is. to Gary IN.

Small Craft Advisory until 10 AM CDT this morning for Gary to
Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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