IWX issues Area Forecast Discussion (AFD) at Dec 1, 12:11 PM EST023
FXUS63 KIWX 011711
AFDIWX
Area Forecast Discussion
National Weather Service Northern Indiana
1211 PM EST Sun Dec 1 2024
.KEY MESSAGES...
-Lake effect snow will continue through early Tuesday, mainly
along and north of the Toll Road. 1-2" is possible in Michigan
through early Monday.
-Heavy lake effect snow is possible late Monday into Monday
night. 5-8" with locally higher amounts is possible across
southwest Michigan.
-Unseasonably cold temperatures will persist through much of this
week.
&&
.DISCUSSION...
Issued at 337 AM EST Sun Dec 1 2024
Stretched midlevel shortwave/vort max is currently passing through
the area as clearly seen on latest water vapor imagery. Primary
impact from this wave will be a subtle veering of low level flow
that will allow nuisance lake effect snow to drift further south
into IN/OH. Anticipate LES to set up along/north of US-6 today.
Inversion heights remain low (around 4 kft) and 300 deg wind
direction is not ideal with a lot of dry air entrainment to overcome
(single digit surface dewpoints over WI). Primary ascent is just
below the DGZ so flake size will remain on the small side.
Visibilities may drop down to 1-2 SM at times later today but any
accumulation will remain light. Anticipate 1-2" in isolated areas
across our MI counties with most locations under an inch. Highs will
remain in the 20s with lake effect cloud shield setting up generally
along/north of US-30. Lows tonight will be similar to present values
with low teens south of the lake effect clouds and low 20s in the
north.
Lake effect parameters improve substantially on Mon. Another
shortwave dives just SW of our area with low level winds (at least
briefly) becoming northerly during the day. Much more ideal fetch
anticipated with Lake Superior connection. Lake-induced pressure
trough likely to support the development of a single, dominant band
along the western MI shoreline with some hints of a possible
mesolow/"type-VI" event. Inversion heights soar to around 10 kft
with strong ascent focused in the DGZ. 0-2km theta-e lapse rates
also drop below zero. The biggest negative factor will be a
potentially limited time window for dominant band organization. The
00Z NAM is the most aggressive here given a longer duration of
favorable low level flow extending well into Mon night. However,
other available models show a more limited window with a slightly
more NW fetch. In this scenario, our NW CWA likely still sees a
quick 2-4" late Monday. If the NAM verifies and a dominant band sets
up for all of Monday night then local amounts exceeding 8" are quite
possible. While uncertainty remains on the high side given
inconsistencies in low level flow from latest NWP, still felt it was
prudent to issue a watch at this point given the potential for
significant snowfall. Kept the watch confined to Berrien/Cass where
a conservative first-guess QPF still came out to 5-8." If the band
materializes then the South Bend area will need at least an advisory
and a warning is not out of the question but confidence is much
lower for the IN counties. Forecast confidence should increase
with the next model cycle as more hi-res guidance becomes
available.
With active weather in the short term did not deviate much from
blended initialization in the long term. A warm front lifts through
the region early Wed with some low potential for precip. Much better
chances expected Wed night into Thu as cold front passes and lake
effect snow lights up once again. Still a lot of details to work out
but NBM low chance PoP's will likely need significant increases if
extended period of cold, cyclonic, NW flow materializes as shown in
deterministic GFS and ECMWF. Forecast confidence drops significantly
in days 6-9 given significant differences in model guidance
concerning evolution of next longwave trough.
&&
.AVIATION /18Z TAFS THROUGH 18Z MONDAY/...
Issued at 1208 PM EST Sun Dec 1 2024
South Bend is right on the southern edge of a lake effect
snow/flurries band moving over northern Indiana. It is possible
that KSBN gets a few more hours of light snow showers before the
winds shift to be westerly, or they could end up completely dry
early this afternoon. Dry overnight with prevailing MVFR
ceilings at KSBN and low end VFR at KFWA. Better chances for
lake effect snow, especially at KSBN, arrive at the end of the
TAF period by 15-18Z Monday as a trough pivots through the Great
Lakes region.
&&
.IWX WATCHES/WARNINGS/ADVISORIES...
IN...None.
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.
&&
$$
DISCUSSION...AGD
AVIATION...Johnson
Source:
IWX issues Area Forecast Discussion (AFD) at Dec 1, 12:11 PM EST---------------
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