255
FXUS63 KLOT 121634
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1034 AM CST Wed Feb 12 2025

.KEY MESSAGES...

- Snow, possibly heavy at times, will result in hazardous travel
  conditions from later this morning into this evening. Snow may
  mix with or change to freezing drizzle this evening,
  especially southeast of I-57.

- A burst of heavy snow is possible late Friday afternoon into
  Friday evening, especially north of I-80.

- Another winter storm may bring accumulating snow, possibly
  heavy snow, to the region Saturday into Saturday evening.

- Cold weather, with periods of subzero wind chills, is likely
  Sunday through the middle of next week.

&&

.MESOSCALE...
Issued at 1030 AM CST Wed Feb 12 2025

Deep layer ascent is continuing to increase across the area this
morning. There are some transient bands of heavier snow that
will expand in coverage across the remainder of northeast
Illinois and eventually northwest Indiana. The more significant
concern for half mile visibility in moderate to heavy snow will
continue to be across northern Illinois. AMDAR soundings and
mesoanalysis shows deep dendritic growth layers extending from
northwest IL and into northern IL this morning. Therefore snow
ratios will be on the higher side along and north of I-88 at
least for a time this morning into early afternoon. These are
the areas that will rapidly accumulate snowfall today and will
likely have 1 inch per hour rates at times through the
afternoon. There is another band of snow back across IA that
will pivot across northern IL late this afternoon.

Farther south the thermal profile is on the edge of what would
be conducive to the fluffier snowfall rates, and the main
banding will remain NW of the city. That said, visibilities
across NW MO and NE IA have fallen to 1/2SM to 3/4SM at times,
albeit shorter lasting in the synoptic snowfall. Satellite
imagery suggests the deeper forcing from warm air advection will
last about 3 hours or so. There could be a lull to lighter snow
late afternoon. Forecast cross sections suggest a redevelopment
of higher rooted frontogenesis forcing this evening as the 850
mb low moves overhead, and there are some better lapse rates
aloft would then bring an uptick in snowfall over the Chicago
area this evening, but we will need to see how widespread this
coverage would be.

The threat for freezing drizzle looks a little bit less than
earlier. Forecast soundings look to remain deep enough to keep
snow going (albeit lighter) tonight, with the exception of areas
well southeast of I-57.

Snowfall totals have come down slightly over the Chicago area
and south, we did slightly nudge them up with higher snowfall
ratios toward the Rockford area and along the Wisconsin border
where deeper dendritic snow may keep ratios up a bit higher.
There is no real change to the message. Updated WSWs will be
sent shortly.

KMD

&&

.DISCUSSION...
Issued at 318 AM CST Wed Feb 12 2025

Through Thursday:

Forecast remains on track with our winter storm today into
tonight. Winter Weather Advisory remains intact, with snowfall
amounts expected to range from 4-6" from I-80 north (highest
amounts likely near the lake in the northern suburbs) and 2-4"
farther to the south with a period of freezing drizzle possible
this evening.

Early morning GOES vapor imagery depicts a broad upper trough
over the western CONUS, with flatter west-southwest flow
downstream from the Plains to the Great Lakes and northeastern
U.S. A significant short wave, which will be the main driver of
our winter storm today, was propagating from the Four Corners
region into the central and southern Rockies. Less obvious were
a series of low-amplitude ripples within the flatter flow from
the Plains into the Midwest. One of these was transiting from IA
into northern IL, evident in a modest increase in south-
southwesterly mid-level flow and associated warm advection
isentropic ascent. This area of lift was also juxtaposed beneath
a region of upper level divergence in the right entrance of a
120 kt jet streak over Ontario. An area of light snow has
developed overnight from southeast IA into northern IL, and this
will continue through the remainder of the predawn hours as the
wave tracks east. A dusting to perhaps a few tenths of an inch
of accumulation are possible mainly north of the I-80 corridor
before intensity diminishes to flurries and more spotty very
light snow by sunrise.

Snow is then expected to redevelop from the west and increase
in intensity by mid-morning, as WAA/isentropic ascent ramps up
downstream of the approaching main short wave. Steady heavier
snow will overspread northern IL through midday and into
northwest IN by afternoon. Highest snowfall rates are expected
to occur from west to east in roughly the 10 AM through 6 PM
window, as a combination of strengthening upper level jet
dynamics, strong mid-level warm advection and associated
transient frontogenetic banding develop ahead of the mid-level
circulation during the midday and afternoon hours. While surface
cyclogenesis occurs well south of the area across the lower
Mississippi Valley, a rather shallow slope of a broad low-mid
level baroclinic zone to the north does look to support decent
this combination of stronger forcing into the western Great
Lakes. In fact, the 00Z HREF suggests some potential for brief
snowfall rates of an inch per hour this afternoon roughly
between the I80 and I-88 corridors where the better f-gen signal
resides, though with a limited duration.

Forecast soundings and time/height sections then indicate some
loss of saturation across the southern half of the forecast area
by early evening, as the 850 mb circulation passes. While this
will allow precipitation intensity to diminish, loss of
saturation at temps colder that -8C or so would also support a
period of freezing drizzle or a mix of light snow/freezing
drizzle especially along and south of I-80 during the evening
hours. Better saturation, a colder column and weak deformation
forcing would likely maintain light snow farther to the north.
Cooling of the column and backing north winds (as the now
deepening surface low lifts toward the eastern Lakes) also looks
to support a period of lake- enhancement/effect snow shower
potential into northeast IL this evening. Lake parameters
increase in favorability toward and after midnight, though
continued backing of the low- level winds should make lake
effect fairly transient as the band propagates southeast across
the Chicago area and into northwest IN by 1-2 am, and then
quickly east of the cwa around or just after sunrise Thursday.

Colder and blustery conditions are then expected Thursday.
Though sunshine will return, temperatures are only expected to
reach the teens and lower 20s as wind chills struggle to get
much above zero.

Ratzer


Thursday Night through Tuesday:

Fast moving high pressure will move across the Ohio Valley and
southern Great Lakes region Thursday night into Friday morning.
Light/calm winds in the evening will turn light southerly
overnight. With clear skies expected, temps will drop quickly to
near zero for most areas, a few degrees warmer in the metro
area, a few degrees cooler elsewhere. Even with the light winds,
wind chills may still dip to -10 below for some areas across
northern IL Friday morning. High temps look to rebound in the
upper 20s for most areas, perhaps lower 30s across the south.

The models continue to be in good agreement with a quick moving
burst of warm air advection snow late Friday afternoon into
Friday evening. Blended pops are now in the 90-100% range along
with QPF in the 0.2 to 0.3 inch range. With a DGZ depth possibly
in the 50-100mb, snow/water ratios may be higher than the
current 10:1 or so. This would yield the potential for several
inches of snow accumulation. Biggest uncertainty with this time
period, is the southern extent of the snow, which may remain
along and north of I-80. Did not make any changes to the pops
but they may need to be lowered across the southern cwa if the
current trends continue. Once this snow potential shifts east of
the area, there looks to be a lull, possibly several hours
overnight into Saturday morning.

Low pressure is then expected to move mainly along or just north
of the Ohio River Saturday afternoon into Sunday morning.
Despite fair agreement among the ensembles, small changes to the
track of this low, in addition to when it begins to deepen,
will have large impacts on the forecast for the local area.
While a mix with rain is possible across the southern cwa, the
current expected track would likely favor mainly snow for most
of the area. A low deepening further to the southeast of the
area, would like favor a light snow versus a stronger/deepening
low closer to our area, leading to a period of moderate and
perhaps heavy snow. With all this said, still too much
uncertainty for any specifics for this time period. Depending on
the track and the prevailing wind direction, lake effect snow
may continue across far northeast IL Saturday night and then
move into northwest IN Sunday morning.

Confidence for early next week is decreasing. The models are now
showing Sunday possibly leaning more cloudy with some support
for flurries and then a possible clipper system for Monday
afternoon into Monday night, though only limited support among
the models. While temps will be much colder going into early
next week, these precip/cloud uncertainties lower confidence
for temps. Blended guidance has low chance pops from Monday
afternoon onward and can`t improve on that from this distance.
blended guidance low temps are generally in the single digits
below zero across most of the area, single digits above zero for
the metro. Also can`t improve on these at this time either.
Wind chills will possibly be in the -20 range Monday morning.
cms

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 525 AM CST Wed Feb 12 2025

Forecast concerns include...

Snow late this morning through early Thursday morning.
Gusty northeast winds today.
Wind shift to northwest late this evening.

There is an area of light snow that is currently moving
northeast and away from the terminals with only flurries
expected through mid morning. Prevailing snow will spread over
the terminals late this morning and continue into the early
overnight hours Thursday morning. The heaviest of the snow is
expected during the early to mid afternoon hours today for the
Chicago terminals, a few hours earlier at RFD. Its during this
time that prevailing visibilities are expected to be under 1sm
and may be in the 1/2sm range for a few hours. The snow is
expected to diminish in intensity during the late afternoon and
its possible visibilities may improve above what is currently
expected, possibly into the 3-5sm range. Then another period of
light snow is possible this evening, with snow eventually
tapering off to flurries early Thursday morning and ending by
daybreak Thursday morning. There is the potential for some light
freezing drizzle to mix with the snow early this evening but
confidence is too low to include at this time.

Mvfr cigs will likely lower into ifr this afternoon and then
prevailing ifr cigs are expected to continue into this evening
then lifting to mvfr overnight and scattering out Thursday
morning.

Northeast winds will continue into this evening, with gusts into
the 20kt range expected. Speeds will diminish this evening as
winds turn northerly in the mid evening and then will shift to
the northwest late this evening and early Thursday morning.
Gusts will increase into the lower 20kt range overnight and
continue into Thursday morning. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Weather Advisory until 3 AM CST Thursday for ILZ003-
     ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-
     ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-
     ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Winter Weather Advisory until 3 AM CST /4 AM EST/ Thursday for
     INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 6 PM CST Thursday for Winthrop
     Harbor IL to Gary IN.

     Small Craft Advisory from 3 AM to 6 PM CST Thursday for Gary
     to Burns Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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