Area Forecast Discussion
Issued by NWS Marquette, MI

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FXUS63 KMQT 101821

Area Forecast Discussion
National Weather Service Marquette MI
121 PM EST Sun Dec 10 2017

.SHORT TERM...(Today and tonight)
Issued at 444 AM EST SUN DEC 10 2017

Main concern in the short-term will be increased lake effect
snowfall potential today. Advisories continue into early this
afternoon for most locations along Lake Superior.

Today: A trough sagging south across Lake Superior this morning,
will be the initial focus for lake effect/enhanced snowfall across
the Keweenaw and eastern Lake Superior early this morning. The
trough will into the rest of the central and eastern U.P. by
daybreak. This will allow snow coverage and intensity to increase
across north central and eastern Upper Michigan as the trough
passes. Then, winds are expected to shift to the north and northwest
into the early afternoon hours. This, along with a quick moving
shortwave moving through, will allow lake effect snow to continue on
the back side of the trough as 850mb temperatures drop to around
-18C to -20C across the area. The Keweenaw will see the better
chances of snow early in the day as the trough sinks southward;
however, the northerly wind direction is less favorable for heavier
lake effect snow in that area and inversion heights drop to around
3kft by late morning. This should allow for only a few additional
inches of snowfall through mid to late morning, when the advisory
expires. Over the north-central and eastern U.P., the increase fetch
across Lake Superior, along with the aforementioned colder
temperatures aloft, will allow for at least moderate lake effect
snow even after the initial trough slides through the area early
this morning. Inversion heights are progged to be around 5kft to
7kft this morning before coming down to around 3kft by early to mid
afternoon when the advisory is set to expire in those locations.
Current thinking is that a widespread 3 to 6 inches of snow could
fall from near Big Bay to Harvey and point east near Lake Superior.

Tonight: A weak ridge will slide across the Upper Great Lakes this
evening, which will allow the lake effect potential to steadily
diminish across all of the U.P. This will especially be true by late
evening as the next low pressure system begins to approach from the
west, effectively shifting winds to the south across the area. This
doesn`t necessarily mean that the snow will completely end; however,
as another shortwave and associated surface low brush past the
western U.P., the chances for light system snow will increase late
tonight for the far western U.P. and along the WI border. At this
point, this looks to be just a quick shot at some light snow for
those areas with an inch or two accumulation expected. Most of the
U.P., away from the WI border will see an inch or less.

.LONG TERM...(Monday through Saturday)
Issued at 401 AM EST SUN DEC 10 2017

Overall, we`re not looking at much of a pattern shift through the up
and coming weekend as longwave troughing remains anchored across the
central and eastern CONUS. As a result, multiple embedded shortwaves
will dig across the region bringing chances for periodic area-wide
snow and then lingering lake effect behind these systems. The main
forecast concern in the near-term is the potential for a heavy lake
effect snow across the north wind snow belts beginning Monday
afternoon and running through at least Tuesday night into Wednesday.
Temperatures will start off well below normal early this week, but
should gradually warm towards normal by the end of the weekend.

Monday: Initially we will split two shortwaves, one well south of
the region and the other rapidly digging south across Lake Superior.
As a result, a clipper system will track across the region, possibly
lingering a bit and/or deepening as height falls associated with the
approaching northerly wave arrive. The warm air advection ahead of
the secondary wave looks like it will be enough to produce
widespread snow across the area on Monday, with lake enhanced snow
possible north of Lake Michigan. This snow looks like it will impact
the west during the morning commute and then gradually transition
eastward through the mid-morning and afternoon across the central
and east, respectively. While QPF is low per the high-res and medium
range models associated with the system snow, forecast soundings
continue to show some depth through the DGZ along the leading edge
of the warm air advection. Therefore, expect the snow to be somewhat
efficient, but probably closer to 15-20:1 as thermal profiles
hugging the -10C line. 1-3``, with localized amounts up to 4`` look
possible Monday morning through the afternoon. As the secondary wave
charges south across the area Monday afternoon, lake effect snow
will pick up in the north wind snow belts across the west and north
central during the afternoon and evening, respectively. This will be
the beginning of the LES event expected to persist through Wednesday.

Monday night through Wednesday: Moderate to heavy LES will
overspread the north wind snow belts through Monday night and is
expected to linger into Wednesday. The medium-range models continue
to consistently show strong unidirectional north-northwest flow
through much of the time period as the main 850mb low associated
with the secondary shortwave slowly pushes over the eastern Great
Lakes. The combination of unidirectional flow and increasing cold
air advection, with 850mb temperatures expected to drop down to
around -20C, will allow the LES machine to kick in full force during
this time period. Forecast soundings show ample lift through the DGZ
as a result, with inversion heights/equilibrium levels reaching 8-
12k feet. This seems reasonable given the lingering weak vort maxes
expected to rotate across the region during this time period.
Therefore, the synoptic setup coupled with the mesoscale and
microphysics details support highly efficient LES, with SLRs of at
least 30:1, across the north wind snow belts. Certainly headlines
will be needed during this event; however, confidence was high
enough given the model-to-model and quasi run-to-run agreement
(mainly the only difference being the onset and longevity of the
event), to issue a winter storm watch for LES for Marquette and
Alger counties as a warning for heavy LES definitely looks possible.
The main concern is for areas east of a line from Big Bay to
McFarland across Marquette county and much of Alger county as a very
strong/highly efficient LES band is expected to develop and linger
across the area for upwards of 24-36 hours. In fact, with this wind
direction the air mass upstream of Lake Superior will already be pre-
conditioned by Lake Nipigon, which will inhibit drier air from
working into the region and prolonging the LES. It is difficult to
pinpoint exactly where this band will develop and linger; however,
areas that do remain under this band for long periods of time may
pick up at least a foot to a foot and a half of snow, if not more.
This thinking is further supported by CIPS Analog Guidance.

Wednesday night through Sunday: We will see additional chances for
accumulating LES with periodic system snow through this time period
as we remain in a highly active pattern across the central CONUS.

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 120 PM EST SUN DEC 10 2017

Typical lake effect scenario playing out right now, but that will
gradually diminish through this evening. Synoptic snow will move in
late tonight into Mon morning, mainly impacting KIWD and possibly
KSAW. Behind that low pressure system, more lake effect kicks in
late Mon into mid-week.

.MARINE...(For the 4 AM Lake Superior forecast issuance)
Issued at 444 AM EST SUN DEC 10 2017

Northeasterly winds will gust to 20 to 30 knots this morning with
freezing spray possible, especially across the eastern half of Lake
Superior. Tonight, winds will weaken to below 20 knots but then
increase Monday afternoon into Monday night with gale force gusts to
40 knots and heavy freezing spray possible. A gale watch has been
posted for Monday afternoon into Monday night for central and
eastern portions of Lake Superior. Winds will then weaken to 20 to
30 knots on Tuesday and below 20 knots on Wednesday. The pressure
gradient will increase across Lake Superior Wednesday night into
Thursday, allowing winds to once again increase to 20 to 30 knots.

Upper Michigan...
  Winter Weather Advisory until 2 PM EST this afternoon for

  Winter Storm Watch from Monday afternoon through late Tuesday
     night for MIZ005-006.

Lake Superior...
  Gale Watch from Monday evening through late Monday night for

  Gale Watch from Monday evening through late Monday night for

  Gale Watch from Monday afternoon through late Monday night for

Lake Michigan...


LONG TERM...Ritzman
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