842
FXUS63 KLOT 092057
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
357 PM CDT Sun Mar 9 2025

.KEY MESSAGES...

- Spring-like warmth is expected through most of this week
  (with a few exceptions for areas near the lakefront and on
  Tuesday/Tuesday night behind a lake-enhanced cold front).

- A period of windy and stormy weather is expected late Friday
  into Saturday. There is a chance that storms could be strong
  to severe during this time frame.

&&

.DISCUSSION...
Issued at 357 PM CDT Sun Mar 9 2025

While today was an unseasonably mild day, tomorrow looks to be
even milder as abundant sunshine has another day to further warm
up the dry air mass that is in place over the region. The HRRR
and RAP had a pretty good handle on today`s temperatures, which
overperformed most other guidance by a few degrees, so have
leaned on those models again for tomorrow`s highs, which range
from the mid 60s to low 70s in our forecast grids. Tomorrow`s
daily high temperature records for Chicago and Rockford (both 70
degrees set in 1955) could be threatened if temperatures
tomorrow end up overperforming guidance a little more, but for
now, our forecast grids explicitly have highs falling just one
to two degrees shy of the 70 degree record for both cities.

Southwesterly winds will turn breezier tomorrow afternoon as a
low pressure system tracks along the Minnesota/Ontario border
and Lake Superior, yielding a tighter surface pressure gradient
across the Midwest. This system`s trailing cold front will drop
southeastward across the Midwest late tomorrow into Tuesday,
likely reaching our forecast area Tuesday morning not
accompanied by any precipitation. With Tuesday morning`s lows
likely only falling into the 40s and still a good bit of
sunshine expected to be observed, a good chunk of our forecast
area should still see high temperatures climb into the 50s and
even low 60s (most likely in our southern counties). However,
lake enhancement to the cold air advection behind the front will
cause much colder air to charge inland from Lake Michigan with
temperatures quickly falling into the 30s during the late
morning and afternoon (quite possibly quicker/earlier than
presently depicted in our forecast grids).

After a seasonably cool Tuesday night, temperatures will start
to moderate again over the latter half of the week as the
surface pressure configuration/evolution generally favors winds
retaining a southerly component, promoting an extended period of
warm air advection. Above to well above normal temperatures are
thus expected Wednesday through at least Friday, when the
persistent warm air advection looks to culminate in widespread
70+ degree readings being observed during the daytime. Though
again, the influence of the still-cold Lake Michigan may still
be felt in locations closer to the lakeshore. This particularly
looks to be the case for the Illinois portion of the lakeshore
on Wednesday and again on Thursday, when onshore flow or the
development of a lake breeze will keep locations closer to the
lake several degrees cooler than locations farther inland.
There is also still some ensemble support for rain occurring
across mainly southeastern portions of our forecast area on
Thursday/Thursday night as a weakening shortwave trough passes
by to our south. The latest NBM came in with PoPs below 15%,
which is probably a bit too low based on the latest ensemble
runs, but with the strongest forcing likely to remain to our
south, opted to leave these be for now.

Aside from the unseasonable warmth, the other main forecast
item of note is the windy and likely stormy weather that is
expected to close out the week. For several consecutive days
now, ensemble and deterministic guidance has remained in
remarkable agreement on a deep, potent upper-level trough
ejecting into the Great Plains on Friday and quickly acquiring a
negative tilt as it pivots into the Midwest. There is likewise
strong ensemble agreement on the general idea of this trough
ejection spurring the development of a rapidly deepening surface
low that would track from the central High Plains into the
Upper Midwest Friday into Saturday. An anomalously deep surface
low like this developing east of the Rockies this time of year
almost always spells trouble in the form of severe weather for
somebody, and this case looks to be no different with both CSU
and NSSL experimental machine learning probabilities putting a
broad swath of the central and eastern CONUS under the gun for
potential severe weather Friday into Saturday. With the surface
low likely tracking to our northwest and with an anomalously
warm and moist air mass in place over the region, northern
Illinois and northwest Indiana would be among the areas at risk
for this potential severe weather, mainly late Friday, but also
potentially on Saturday as well depending on how quickly the
system evolves.

However, with this system still being 5-6 days out, there are
still unsurprisingly a lot of finer scale uncertainties that
need to be resolved over the coming days, particularly with
respect to timing and how far north the better quality Gulf
moisture will be able to protrude. The latest multi-model/multi-
ensemble consensus favors convection developing well to our
west/southwest and not arriving here until after sunset, when
any surface-based instability would likely begin to trend
downward, which could be one limiting factor the potential for
severe weather occurring in our area. The better quality
moisture and associated greater surface-based instability could
also get pinched off to our south, which would also tend to limit
the overall threat for severe weather here locally. That being
said, the overall forcing for ascent looks strong, and it looks
like there will be no shortage of low-level and deep-layer
shear given the strong kinematic fields in the vicinity of the
low, so it won`t take much instability for us to potentially
have a problem on our hands. For now, have just boosted our
thunder probabilities from the NBM initialization for late
Friday/early Saturday, but we`ll need to continue to closely
monitor trends in forecast guidance over the coming days to
better gauge the extent to which severe weather may end up being
a possibility here locally.

Otherwise, with the deep surface low compacting pressure
gradients across the region, Friday and Saturday both look like
they could be pretty windy days. If some of the deeper/more
aggressive model solutions verify, then we may eventually need
to hoist a wind headline for at least one of these days, but
that`s something that is still several days down the road.

Ogorek

&&

.AVIATION /18Z TAFS THROUGH 00Z TUESDAY/...
Issued at 1224 PM CDT Sun Mar 9 2025

Only forecast concern this period are wind speeds/gusts.
Southwest winds will continue through the period with speeds
increasing this afternoon and gusts into the lower/mid 20kt
range mid/late this afternoon. Speeds are expected to diminish
under 10kts shortly after sunset this evening and speeds may
continue to diminish to 5kts or less overnight. Speeds will
increase into the 10-15kt range early Monday afternoon with
gusts to 20kts possible. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 PM CDT this evening for Winthrop
     Harbor to Wilmette Harbor IL.

&&

$$

Visit us at weather.gov/chicago