552
FXUS63 KLOT 062316
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
516 PM CST Thu Mar 6 2025

.KEY MESSAGES...

- A narrow swath of accumulating wet snow is likely on Friday,
  especially areas north of I-80, where some localized travel
  impacts could materialize.

- Spring-like warmth is expected early next week.

&&

.DISCUSSION...
Issued at 311 PM CST Thu Mar 6 2025

Through Friday Night:

All is quiet across the area early this afternoon as sunny skies
prevail through the remainder of the day. However, attention is
quickly turning to a notable mid-level impulse currently noted in
the Water Vapor imagery ejecting out of a larger scale trough
centered across southern California. This feature will quickly
induce lee cyclogenesis east of the front range of the Colorado
Rockies this evening, before rapidly shifting eastward across
the lower Missouri Valley on Friday.

As cyclogenesis ensues into this evening, an increasing southerly
lower-level wind response will interact with, and tighten, a
west-to-east oriented lower-level baroclinic zone draped north
of the surface low from NE eastward into IA. This results in
the rapid development of banded precipitation (mainly in the
form of snow) through the evening in close proximity to this
baroclinic zone. While this developing band of snow will largely
remain west of our area through much of the night, it is
expected to develop eastward through the overnight hours.
Accordingly, snow is expected to onset in parts of western IL
after 3 am, then across the remainder of the area by around
daybreak Friday morning. Periods of wet snow are thus favored in
most areas through the mid to late morning hours, before the
continued focus into the afternoon begins to set up across far
northern IL (north of I-80).

While the strength of this system is expected to begin a
weakening trend as it encounters more confluent mid-level flow
across the western Great Lakes on Friday, there is concern that
the snow could still fall at moderate to heavy rates at times
(up to an inch per hour) during the day. This occurs as a band
rather potent 850 to 700 mb frontogenesis looks to set up right
across northern IL during the late morning and afternoon. Model
cross sections across this tightening baroclinic zone depict a
rather classic frontogenetical response, with forced ascent
right into a layer characterized by negative Theta-E lapse rates
(convectively unstable) through the DGZ. This suggests that
some narrow heaver bands of snow will certainly be possible,
with the most favored location for this being areas from I-80
northward late in the morning through the afternoon.

Surface temperatures are expected to remain near freezing across
far northern IL during the snowfall into Friday afternoon.
Accordingly, this raises questions as to the extent of road
snow accumulations and hence if any real travel impacts other
than reduced visibilities will materialize. Current thinking is
that most areas will only see some slushy accumulations on area
roads, but in an areas that happen to fall under one of the
more intense narrow bands, the snow may fall at a high enough
rate to result in some snow covered roads and possible travel
impacts. Unfortunately, however, the exact locations in which
these narrow intense snow bands setup are very difficult to
pinpoint with any degree of skill this far out. At this time,
the most favored area for these bands of heavy snow resides
north of I-80 in northern IL.

The precipitation will come to an end by early Friday evening.
Total snow accumulations of 2 to 3 inches (locally higher) look
favorable north of I-80. Farther south, (south of I-80) warmer
temperatures will limit accumulations, and may even result in
any lingering precipitation falling as rain in the afternoon.

KJB


Saturday through Thursday:

Over the weekend, an upper-level ridge will become established
over the western CONUS, resulting in northwest flow aloft over
the Midwest. One upper-level shortwave embedded in this
northwest flow will pass close by late Saturday/early Sunday.
However, with most of the latest ensemble guidance keeping any
measurable precipitation with this wave well to our north and
northeast, and with forecast soundings showing that there will
be a good bit of dry low-to mid-level air to work through here,
have maintained a dry forecast for this time period as output
by the NBM.

Early next week, the western CONUS ridge will slide eastward,
while the surface pressure configuration will generally promote
southerly winds and warm air advection across the central
CONUS. While there are still some finer-scale uncertainties that
will need to be ironed out over the coming the days (including
the influence of the still-cold Lake Michigan and how far north
any warm fronts will surge), on the whole, the first half of
next week looks very much spring-like with widespread 60+ degree
temperatures likely to make an appearance in the area. However,
we will have to keep an eye out for a potential back door cold
front to sneak into northern IL late Tuesday into Wednesday.
Overall, have low confidence on this occurring at this time,
but if it does, expect locations across northern IL (especially
near Lake Michigan) to turn much cooler. Otherwise, with a
longwave trough replacing the ridge out west, there is a fairly
strong signal in ensemble guidance for lee cyclogenesis
occurring in the Plains next week. Whatever comes of that will
likely lead to our next precipitation chances sometime later in
the week.

Ogorek/KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 517 PM CST Thu Mar 6 2025

The main aviation weather concerns for the 00Z TAF period
include:

* Snow Friday morning and afternoon with associated IFR cigs and
  vsbys expected

Conditions will be quiet through the night with westerly winds
at under 10 kt. A system of snow will work into the area early
tomorrow and is expected to reach the TAF sites near daybreak,
or not long thereafter. A quick reduction down to IFR is
anticipated after the onset of snow. LIFR conditions will be
possible at times if a heavier band of snow can materialize,
although confidence in LIFR is currently low. It`s also possible
that some rain will mix in with the snow at times, especially
toward the beginning and end of the event. The snow should move
away from the airfields early in the evening and conditions are
expected to quickly recover to VFR. Accumulations could push and
inch or two by the end of Friday.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 6 PM CST this evening for the IN
     nearshore waters.

&&

$$

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