IWX issues Area Forecast Discussion (AFD) at Dec 1, 6:32 PM EST327
FXUS63 KIWX 012332
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
632 PM EST Sun Dec 1 2024
.KEY MESSAGES...
- Lake effect snow showers continue tonight, but primary impacts
are expected late Morning monday into early Tuesday morning.
- Lake effect snow accumulations of 4 to 8 inches possible
across southwest Lower Michigan into portions of northwest
Indiana. Locally higher amounts are possible. Heaviest lake
effect snow showers are expected generally along and north of
Indiana Toll Road.
- Unseasonably cold conditions persist through much of this
period with more lake enhanced snow showers possible Wednesday
night into Thursday.
&&
.UPDATE...
Issued at 615 PM EST Sun Dec 1 2024
Band of light snow showers has been persisting from SW of
Marshall to Coldwater and Angola for the past couple of hours,
but appears to be both shifting somewhat east and also weakening
as the flow shifts more SW. Glance at MI and IN highway and
truck cams shows limited concerns and accumulations at this
point, so will monitor the progress for possible increase in
pops in these areas. Confidence in evolution of renewed pops
late evening into overnight remains low as evolution of the
trough later this evening/overnight remains low with hi res
models not even handling the current setup favorably. Will leave
pops alone late eve into overnight for now and revisit in a few
hours.
&&
.DISCUSSION...
Issued at 350 PM EST Sun Dec 1 2024
The major forecast theme has not changed much, with the
potential of heavy lake effect snow showers and continuation of
unseasonably cold air as the main weather stories. In terms of
headlines, St. Joseph Co Michigan has been added to the Winter
Storm Watch for lake effect snow, otherwise no headline changes
with the afternoon package.
Light lake effect snow showers continue this afternoon as a
sheared vort max continues to dive southeast across the Ohio
Valley. Mid level subsidence should result in some drying of mid
levels/bulk of DGZ later this evening which should cause some
diminishment in lake effect snow shower coverage/intensity.
Otherwise, through early this evening, bulk of more notable lake
effect snow showers should remain roughly from Holland to
Battle Creek where fetch is maximized.
Mid level RH will increase late tonight into Monday morning as
another more amplified mid level trough drops southeast from the
Upper Midwest. This should ramp up lake effect snow showers late
tonight into Monday morning with some aided convergence from
sharpening of aggregate troughing across far eastern Lake
Michigan. At this time, not expecting snowfall rates to be that
high through mid morning, but by afternoon guidance continues to
depict inversion heights and lake induced equilibrium levels
increasing steadily as a -22 deg C 700 mb cold core drops across
the southern Great Lakes. These more favorable lake effect
parameters combined with what should be well defined lake
aggregate troughing should increase potential of more
significant snow showers in the afternoon. This still could be a
multiband setup, but a few more intense bands could become
established with this synoptic setup, especially late afternoon
into early evening as better low level lift becomes more focused
in DGZ. One negative factor for more organized banding during
the day Monday could be a potential of some modest shear in the
convective boundary layer.
Expecting some impacts to travel Monday morning into Monday
afternoon, but potential of greatest impacts may be centered on
the Monday night into early Tuesday period. Over the past
several days, guidance has provided some subtle suggestions of
mesovort potential across southern Lake Michigan, with an
adjoining more dominant single band possibly shifting into
northwest IN/southwest Lower MI during the night. The greatest
snowfall rates of this event could potentially be associated
with a dominant band during this period. The lowest confidence
item in this forecast is the intensity/location of this
potential adjoining single band Monday night, specifically how
far west this band potential will reach which will be somewhat
dependent on mesovort strength over southern Lake Michigan.
Hires guidance does still suggest this potential band would be
more progressive in nature Monday night into early Tuesday which
could limit overall snow accumulations. Taking all of the above
into account and the lingering uncertainties, have maintained
Winter Storm Watch headline for Berrien/Cass Counties MI and
added St. Joseph County IN (greatest concern along and north of
Toll Road) to the Winter Storm Watch due to the potential of
this area possibly experiencing accumulations from both phases
of this event that could lead to some locally higher storm total
amounts. Impacts are certainly possible for La Porte County,
with the greatest concern Monday evening into Monday night. Have
opted to keep La Porte County out of headlines at this time
given lower confidence in this later second portion of the
event.
Some impacts are possible for the Tuesday morning commute with
lingering lake effect snow showers possible, but backing wind
profiles, crashing inversion heights, and subsidence should
result in decaying lake effect snow showers late morning into
the afternoon. A brief period of low level warm advection
commences later Tuesday into Tuesday night in advance of next
amplified upper trough dropping across the Great Lakes. Short
residence time of deeper moisture appears to be a limitation
despite strong forcing, and initial dry nature of airmass should
support snow over rain for Wednesday/Wednesday night. Some lake
effect snow showers are likely Wednesday night into Thursday in
wake of this system with breezy/windy conditions for much of
the Wednesday through Friday period. Additional snow shower
chances are possible for next weekend with another northwest
flow wave. Some moderation in temperatures is possible next
weekend, but medium range guidance suggests local area could be
prone to additional intrusion of colder air just beyond this
forecast valid period.
&&
.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 615 PM EST Sun Dec 1 2024
Abundant TAF challenges through the period, especially at KSBN
with lake effect clouds and snowshowers.
KFWA was on the edge of the MVFR cloud deck and should mix out
within the next several hours. Made some minor adjustments to
their TAF as well, with VFR conditions expected thereafter.
Can't rule out a return to MVFR cigs and maybe some lighter snow
showers after 18Z Mon, but not worth adding at this point.
At KSBN, the main band of lighter snow showers/flurries has
pushed east and given the SW flow at the airport suspect this
will remain through a good portion of the TAF period, resulting
in pushing back of the arrival of snowshowers to closer to 18Z
Mon. MVFR cigs will likely stick around, although there could be
a period of VFR cigs at times as drier air temporarily mixes
into the low levels (as shown by the slowly climbing cigs the
past several hours). Although a more favorable flow
re-establishes near/after 18Z, with more of a multi band setup
for snow showers, many of which will likely drop flight
conditions to IFR or possibly lower in the more persistent
bands. Confidence in impacts to KSBN remain low as some signals
indicate the worst could remain just north of the airport. Have
remained conservative for now.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Storm Watch from Monday morning through Tuesday morning
for INZ104-204.
OH...None.
MI...Winter Storm Watch from Monday morning through Tuesday morning
for MIZ078-177-277.
MARINE...Small Craft Advisory until 7 PM EST this evening for LMZ043-
046.
&&
$$
UPDATE...Fisher
DISCUSSION...Marsili
AVIATION...Fisher
Source:
IWX issues Area Forecast Discussion (AFD) at Dec 1, 6:32 PM EST---------------
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