890
FXUS63 KLOT 111135
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
635 AM CDT Tue Mar 11 2025

.KEY MESSAGES...

- Lake enhanced cold front will send temperatures tumbling
  today, particularly near the lake.

- Powerful spring cyclone will impact the area Friday into
  Saturday and bring with it:
 - Windy and unseasonably warm conditions Friday and Friday
       night
 - Potential for a brief period of gusty showers and/or
       thunderstorms Friday night
     - Very strong winds Saturday with gusts over 40 mph likely

&&

.DISCUSSION...
Issued at 310 AM CDT Tue Mar 11 2025

Today:

A cold front will move southward across the area this morning,
but the bigger story will be the secondary lake enhanced front
that should lag a couple of hours behind the initial synoptic
cold front. Push of colder air will be much more formidable
behind the lake enhanced front with the magnitude of the
temperature drop decreasing the farther inland you get from the
lake. Right near the lake temperatures are expected to fall into
the upper 30s, then into the 40s within about 50 miles of the
lake and into the 50s farther west and south from there. Strong
and gusty north to northeast winds are expected behind the lake
enhanced front, which should begin to taper off gradually this
afternoon, then more abruptly at sunset. The gusty winds will
send wind chills dropping into the 20s near the lake this
afternoon, a far cry from Monday`s near 70 degree temperatures!

Wednesday-Thursday:

The front is expected to become quasi-stationary over central
portions of Illinois and Indiana with our CWA on the cool side
of the boundary. A 20+ degree temperature gradient is expected
across our CWA Wednesday, with cool easterly winds off the lake
keeping areas near the lake struggling to get into the lower
40s. While lake chilled air will modify as it moves inland, some
footprint of the lake cooling is likely to keep temperatures
roughly north of I-80 only in the 50s. Farther south, away from
the chilling influences of the lake, temperatures south of I-80
should rebound back into the lower 60s, particularly well south
of I-80.

Thursday looks to largely be a repeat of Wednesday. Thurday`s
high temps from the NBM are nearly 10F warmer across the CWA,
however with continued cool easterly flow off the lake, there
is concern that highs may not rebound that much north of I-80
and particularly near Lake Michigan.

Friday:

A vigorous mid-latitude cyclone is still progged to develop
over the central Plains Friday as strong negatively tilted mid-
upper level trough emerges east of the Rockies and closes off a
mid-upper level low. Strong surface pressure falls well ahead
of the strong surface cyclone should drive the front that had
been stalled over downstate IL quickly northward. The last few
runs of the GFS and ECMWF both favor some sunshine Friday, which
should help send temps soaring well into the 70s across the
area. NBM has highs in the mid 70s across most of the CWA and
those highs look reasonable, however, if there is a good deal of
sunshine there is certainly room for temps to overperform and
some areas get near 80.

Pressure gradient will be strengthening through the day with
south-southeasterly winds likely growing quite strong in the
afternoon. Bufkit soundings from the GFS and forecast soundings
from the ECMWF would both support the potential for gusts to
near 40 mph Friday afternoon.

The synoptic gradient is expected to be backed slightly in
response to the rapidly deepening cyclone west-southwest of the
CWA. The slight easterly component could cause the northward
surging boundary to get hung up near the IL shore north of
Chicago. While the warmth should eventually make it to the
immediate north shore suburbs, it might not be until late in
the afternoon or early in the evening.

One last note, assuming there is a good deal of sunshine Friday
afternoon, deeper mixing of the boundary layer could result in
the somewhat shallower surface moisture mixing out and
dewpoints dropping lower than forecast. While this isn`t
explicitly depicted in any of the global models, if it were to
happen that could complicate instability and convection
forecasts a bit Friday night.

Friday night:

A fairly sharp synoptic dryline arcing north-northwestward
across Missouri and Iowa should be the focal point for
convection developing Friday afternoon. Some moisture should
pool just ahead of the dryline, though increasingly marginal
with northward extent. The more marginal moisture north will
probably be compensated for by colder mid-level temps and strong
synoptic forcing. Really too far out to have a good handle on
widespread convection will be Friday afternoon as drylines are
not typically known for QLCSs and the shear vector orientation
is currently progged to be fairly perpendicular to the dryline
over Iowa and northern Missouri, which would tend to favor more
cellular type convection. Obviously large scale synoptic forcing
will be strong, so could that be enough to result in a more
organized QLCS developing? Time will tell...

The convective mode of storms to our west will probably play a
role in the fate of this convection as it moves east into
Illinois well after dark Friday night. A well developed QLCS
would stand a better chance of surviving east across Illinois
after dark, despite what looks to be a fairly precipitous
increase CINH and decrease CAPE with eastward extent across IL.
Obviously given the kinematic fields, any QLCS type feature
could pose a severe convective wind threat in spite of entering
a potentially more hostile environment.

Conversely, if convection is more cellular and scattered in
nature Friday afternoon, then the chances of it surviving well
into the night across Illinois would be greatly diminished.
Since last night, ECMWF and GFS have shown dryline/prefrontal
trough feature weakening Friday night as it moves east into
Illinois. It is also still possible that this cyclone slows some
in guidance between now and Friday with Friday afternoon
convective initiation being farther west, which would also
lessen our convective and severe threat locally.

Overall, there are a lot of uncertainties with regards to our
convective/severe potential Friday night. At this point, the
potential mitigating factors certainly seem to strongly
outweigh the positives. However, given the very strong winds
and shear with this system, Friday night will bear close
watching over the next couple/few days. This time period will
get within the range of the NAM later today and tonight; it will
be interesting to see how widespread depiction of the
convection will be along the dry line in IA/MO with the NAM.

Saturday:

Dryline/prefrontal trough should clear our CWA by sunrise
Saturday. NBM carries some rather high chance pops Saturday, but
with dryslot surging into our CWA those pops seem pretty
questionable. The vast majority of EPS and GEFS members would
support no rain Saturday and if this trend continues, a later
shift will likely be able to remove precip chances from
Saturday.

The bigger story Saturday looks to be the synoptic high wind
potential. The dry slot of a vigorous mid-latitude cyclone like
the one models have been consistently depicting, are notorious
for producing strong to potentially damaging winds. Strong
subsidence, deepening into the powerful low/mid level winds can
result in damaging winds over a corridor within the dry slot.
Still a bit far out to lock into exactly where the strongest
winds will be and how strong they will get, but certainly it
looks like our CWA could be close to or directly impacted by
the highest winds. In collaboration with neighboring offices,
NBM winds were bumped up and the official forecast now has
gusts peaking near wind advisory criteria. This is still 5 days
out and a lot can change, but the threat for a synoptic high
wind event certainly exists in the region Saturday.

Sunday-Monday:

Looks like a brief cool down to near normal temps Sunday,
followed by a potential warm up and a return to above average
temps by Monday. Given the volatility of the forecast prior to
this period, forecast confidence is on the lower end of the
spectrum for this time frame.

- Izzi

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 635 AM CDT Tue Mar 11 2025

Key Aviation Messages for the current TAF period:

- Gusty northeast winds today

A cold front will cause winds to shift to northwest around the
start of this 12Z TAF cycle. This initial front will then be
followed by a secondary, stronger front off the lake that will
flip winds sharply to the northeast with its passage. Wind gusts
will increase along and behind the secondary front to around 25
knots. Guidance has continually been projecting 14Z with that
secondary lake front`s passage. But with the secondary lake
boundary just passing Green Bay (at the time this discussion was
posted), obs will be watched closely in case it picks up or
slows down in speed. Lastly, obs up north have been showing SCT
to BKN 2500-3000. Most models agree with that, but if the lake
enhances the front, there is a slight chance for cigs down
between 1500-2000 feet. VFR conditions are expected through most
of the TAF period, but there is a short window along and behind
the front that could provide MVFR cigs. For now, there is low
confidence for inland extent and so there is no formal mention
in the TAFs, simply left as a SCT015. However, satellite trends
for clouds over the lake will be monitored and an AMD may be
needed.

Winds will remain out of the northeast through the end of the
TAF period with diminishing gusts in the late afternoon.

DK

&&

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

     Small Craft Advisory until 4 AM CDT Wednesday for Northerly
     Is. IL to Michigan City IN.

&&

$$

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