446
FXUS63 KIWX 050122
AFDIWX

Area Forecast Discussion
National Weather Service Northern Indiana
822 PM EST Wed Dec 4 2024

.KEY MESSAGES...

- Lake effect snow, gusty winds, and reduced visibility are
  expected this evening overnight into Thursday. Wind gusts up
  to 50 mph are possible, especially near the lake.

- A winter storm warning and winter weather advisory are in
  effect for southwest Michigan and northern Indiana. Hazardous
  travel is possible in these areas.

- Arctic air arrives Wednesday night with wind chills in the
  single digits to below zero. The cold will last through
  Friday.

- Friday and Saturday will be dry, then rain is expected Sunday
  night and Monday.

&&

.UPDATE...
Issued at 822 PM EST Wed Dec 4 2024

Strong arctic cold front now just reaching BEH and SBN with
impressive 50-60 mph gusts will continue east through the
remainder of the forecast area this evening. The stronger gusts
have been on the southern fringe of an organized line(s) of
convection tracking through southern MI. Ptype has quickly
changed over to a burst of moderate/heavy wet snow given intense
dynamic/evaporative cooling with visby`s briefly down to a
quarter/half mile within the line as it tracks east-southeast
through southern MI, possibly clipping far northeast IN and nw
OH later this evening. Have opted to handle with a SPS for now
as a marginal temp profile and lagging influx of arctic air
likely keeps this initial activity from being a true snow
squall/flash freeze situation. Warmer and less organized
convective activity to the south likely limits impacts, removed
from the more robust ascent plume associated with a vigorous
vort max tracking through lower Michigan. Forecast is on track
otherwise with no changes to headlines overnight into Thursday
as we transition over to a wind driven lake effect event in our
wnw flow LES belts.

&&

.DISCUSSION...
Issued at 228 PM EST Wed Dec 4 2024

A deepening clipper like system traverses the northern Great Lakes
today into Thursday. Strong gradient winds across the area will have
some likihood of being brought to the surface today as the morning
clouds move eastward and mixing slowly gets started. The majority of
the winds are forecast to stay under 40 mph today. It will be when
the strong shortwave dives across Lake MI and its attendant arctic
front moves through the area that we get our higher impact outcomes.
In terms of wind, the greatest chance for 40+ mph winds appears to
come at night when the cold air arrives on top of the warmth of the
mid to upper 30 degree highs to cooling into the teens tonight. This
is going to have to rely on the cold air doing the majority of the
mixing in this case to allow for more than an hour or two of cold
front forced gusts. This is outside of normal timing climatology for
when gusts really surpass advisory criteria, which is during the
morning hours. Although this could be influenced by the amount of
warm advection events we have that often create an inversion to
limit coupling by the afternoon. Still think this is strong
enough to go with an SPS for areas south of US-30 this evening
into the overnight. Should there be any wet roadways from
precip across the area today, there is a chance for some flash
freezing, although wind gusts and the normally dry nature of
these systems may cut down on those chances.

The other longer-lasting impact will be the snow. Trajectories
finally swivel around to northwest around or just after 00z this
evening. Initially, winds are nearly unidirectional and with minimal
shear to shread dominant banding. However, by 12z, stronger shear
around 15 kts through the cloud bearing layer may be enough to break
up any remaining single banding and allow for more of a multibanded
set up for Thursday. The trend will also be to have the lake-induced
inversions drop from a good 700 mb/9500 ft to below that 7500 ft
threshold for better lift early Thursday morning. The trend will
also be to lose moisture content through the lowest 1 km of the
profile and the inversion heights drop during the morning as well.
By the morning, HREF guidance seems to indicate that the Michigan
counties of Cass and St Joseph and portions of Berrien county will
be on their way to reaching warning criteria with 6 to 8 inches by
12z. It may take a little bit more effort for the southern row of
counties in the warning to attain their snow threshold especially if
it is going to be less efficient and if there`s a trend towards a
multiband orientation. HREF snowfall brings the 3 to 6 totals over
24 hrs by 12z Thursday morning to 6 to 8 inches by Thursday evening
in St Joseph and parts of Elkhart. Still, it appears the best area
for lowered visibility both from fractured snow flakes and blowing
and drifting snow appears to be the closer you are to Lake MI where
the strongest winds are expected to be and some snow pack still
remains from our last event, but some of the visibility issues may
extend farther downstream of Lake MI as well and this could help
verify those warnings at an impact-based level. 30 kt gusty winds
are expected to last through the morning, but dissipate during the
afternoon. Lake effect snow continues through the day Thursday, but
may even sputter and start into Thursday night as that`s when
trajectories finally turn more southerly. Mentioned the teens for
lows tonight, but highs Thursday and Friday struggle reach freezing
with a majority in the mid 20s to around 30 both days. The worst
wind chills are below zero south of US-30 tonight with the strong
gusty winds helping out there.

Finally, the north and western side of the approaching surface high
pressure begins influencing the area with warm advection around
midday Friday. Expect dry weather, then, Friday as result. Saturday,
a sprawling warm front from a low pressure system in Canada scrapes
our north. Am skeptical anything comes out of it based on moisture
profiles, but the NAM solution indicates it might. As a result of
the warm front and the high pressure system stalling in the Gulf of
Mexico, warm advection sets up and allows temperatures to arrive
back in the 40s on Sunday. This is all in the nick of time because
the upper low in the southwestern US through all this time finally
meanders northward toward the area Sunday night into Monday with
chances for rain. We`ll have to see how much snowpack exists at that
point and heavy it rains because all that rain/meltwater goes
somewhere and it may be into the rivers. Additionally, we`ll have to
see what happens to road temperatures because there could be some
onset freezing rain.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 534 PM EST Wed Dec 4 2024

Arctic front on track to plow through KSBN by 01z and KFWA by
2-3z with ~40 kt gusts and convective rain/snow showers
immediately along and behind. Brief reductions in visby`s to
less than a mile possible with snow showers/squalls, with some
rain potentially mixing in initially. Attention thereafter will
turn to the lake effect response as cold/strong west-northwest
flow trajectories overspread the relatively warmer lake waters
overnight into Thursday. This lake plume to primarily remain
north of KFWA with times of light snow showers and cigs in the
high MVFR to low VFR category. Lake effect snow multibands will
be in and out of KSBN with variable flight conditions (LIFR to
MVFR) overnight into tomorrow, with some improvement possible
later Thursday as drier air starts to lower inversion heights
and the overall intensity of the LES. West winds will continue
to gust up to near 30 knots otherwise into Thursday.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...Winter Storm Warning until 1 AM EST Friday for INZ005-006-104.
     Winter Weather Advisory until 1 AM EST /midnight CST/ Friday
     for INZ007>009-014-103-116-204.
     Wind Advisory until 4 AM EST /3 AM CST/ Thursday for INZ012-
     013-015-017-018-020-022>027-032>034-203-216.
OH...None.
MI...Winter Storm Warning until 1 AM EST Friday for MIZ078-079-177-
     277.
     Winter Weather Advisory until 1 AM EST Friday for MIZ080.
MARINE...Gale Warning until 1 PM EST Thursday for LMZ043-046.

&&

$$

UPDATE...Steinwedel
DISCUSSION...Roller
AVIATION...Steinwedel