Area Forecast Discussion
Issued by NWS Gaylord, MI

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639
FXUS63 KAPX 130514
AFDAPX

Area Forecast Discussion
National Weather Service Gaylord MI
1214 AM EST Wed Dec 13 2017

.UPDATE...
Issued at 941 PM EST Tue Dec 12 2017

North northwest flow lake effect banding continues at this hour.
Most of the bands continue to be rather weak mainly due to the
lack of moisture (mean 850-700 mb rh only about 50 percent).
However...a good connection of off Lake Superior and inversion
heights a decent 5500 feet or so has continued to feed a moderate
band near Traverse City where they have picked up a couple of
inches over the last couple of hours. Conditions grow increasingly
hostile to lake effect generation overnight with an increasing
subsidence inversion, drier air, lighter winds, and winds slowly
veering into the north and eventually northeast late tonight into
Wednesday morning. So still another couple of inches possible
under the better band but overall a diminishing trend with
activity likely headed toward the Lake Michigan lakeshores. Cold
tonight with a large range in low temperatures from the teens
above zero near Lake Michigan to several degrees below zero inland
(a few spots have already fallen below zero). Will have to adjust
some spots down toward 10 below.

&&

.NEAR TERM...(Tonight through Wednesday)
Issued at 311 PM EST Tue Dec 12 2017

...Lake snows end tonight...widespread system snow arrives
Wednesday...

High impact weather potential...Accumulating lake snows this
evening. Widespread accumulating snow Wednesday, especially south of
M-72.

Pattern synopsis/forecast: Surge of barely modified Arctic air
overhead, as evident by highs this afternoon in the single
digits/teens and wind chill values several degrees colder. Fully
mature north-northwest flow lake snow bands impacting northwest
lower and sections of eastern upper Michigan, the most persistent
activity west of Interstate 75. Despite phenomenal over-water delta
t`s, aggressive drying above a slowly shrinking subsidence inversion
has kept snowfall intensity somewhat muted. Still, excellent Lake
Superior connection has resulted in some decent snowfall rates,
likely nearing an inch per hour at times for areas southwest of
Grand Traverse Bay.

Very progressive pattern set to continue for the next several days.
High pressure will steadily build east into the northern lakes
tonight, helping further disorganize the ongoing lake snows as it
does so. Attention then turns to vigorous shortwave trough,
currently digging southeast across central Alberta. In true clipper
fashion, this system will continue to race southeast, reaching
northern Indiana by Wednesday evening. Attendant good Pacific
moisture plume and strong fgen signal north of the surface
reflection all point to accumulating snows returning Wednesday,
especially across central and southern lower Michigan.

Primary forecast concerns/challenges: Additional lake snows,
especially this evening. Placement, timing, and amounts of Clipper
snows Wednesday. Increasingly complicated winter headline management
concerns.

Details: First order of business deals with lingering lake snows,
especially this evening. Lake snow generation parameters all trend
steadily downward this evening, most notably being a lowering of the
subsidence inversion and a continuing scouring out of any synoptic
moisture contribution. Simple inertia of lake effect and excellent
Lake Superior connection still supports some additional
accumulations, on the order of a few inches, for areas southwest of
Grand Traverse Bay. Less intense snow showers for eastern upper and
along/west of US-131 in northern lower. Given downward trend in
intensity (worst is definitely over), will go ahead and cancel
remaining headlines. Wind fields go light overnight, and become
light easterly by morning in response to that approaching clipper.
This will focus weakening lake snows closer to the Lake Michigan
coast with time, and likely pushing snow entirely out of eastern
upper Michigan. Some clearing expected, especially across interior
regions. Maintenance of light winds and some lingering cloud cover
should prevent temperatures for tanking too much. Simple cold start
will allow lows by morning well down into the single digits away
from the big waters across northern lower, with a few degrees below
zero readings for some areas of eastern upper. If quicker clearing
and lighter winds are realized, could see temperatures several
degrees colder across the board.

No rest for the wary as that clipper snow arrives rather quickly
Wednesday morning. Latest trends support snow overspreading areas
primarily south of M-72 during the day, with the heaviest snows
focusing along and south of M-55. Still some lingering uncertainty
on the northern extent and exact placement of the heavier snows,
although a sharp northern cutoff is anticipated given aggressive dry
air feed from high pressure over southern Canada and downward branch
fgen dynamics. System has efficient snow producer written all over
it, with that fgen signal collocated in background isentropic
upglide field/deformation dynamics and high end snow-liquid ratios
anticipated with broad omega within the favored DGZ. Have
tentatively painted a stripe of 2 to 4 inches along and southwest of
a Traverse City to Tawas City line, with the highest amounts near
Manistee. No additional headlines forthcoming just yet, but
something to monitor for sure.

&&

.SHORT TERM...(Wednesday night through Friday)
Issued at 311 PM EST Tue Dec 12 2017

...More Cold Air Then Another Decent Clipper...

High Impact Weather Potential...Below Zero wind chills will impact
the forecast area again. A decent clipper, Friday, may have moderate
to heavy lake effect snow.

Pattern Synopsis/Forecast...It looks like the ridge/trough idea at
500 mb will continues through Thursday, before the last clipper, on
Friday leads to a temporarily zonal pattern. Again, energy topping
the ridge in Alberta and dropping into the trough
Wednesday/Wednesday night that is in the Great Lakes. The clipper
will be a little farther south and will bring NE to N winds to the
forecast area on Wednesday night before backing NNW by Thursday
morning. Models are drying a bit in the 850-700 mb layer so that the
snow amounts should remain minor to light. Moisture moves back in
ahead of the next clipper system Thursday night. Initially, the
winds are west through 09z/Fri, so would expect that the snow
amounts would be light to moderate, with the heavier bands North of
M-32, where there is a better connection to currently open Green
Bay. Friday morning, winds turn NW and somewhat remain that way
through at least 21z with good moisture and instability to produce
some decent bands.

Primary Forecast concerns...With the two coldest nights this season
(tonight the first one, see above, and Wednesday night the second)
impacts could be that the people that haven`t prepared for cold
temperatures may be shocked for below zero temperatures in some
locations where we have low near zero.

The other part is the LES...Wednesday night and Thursday don`t look
bad, but Friday as the moisture and lift from the synoptic system
moves into the region, the snowfall could increase substantially
with the -14c 850 mb temperatures the near 90% RH in the 850-700 mb
layer and a consistent NW flow over the lower peninsula and E Upper
Michigan. On paper, this looks good, but if the synoptic moisture is
less, we could get another "average" LES day with no big amounts. So
will go with the consensus of the models, that moderate to heavy
snow bands will be possible in the NW flow areas of NW Lower
Michigan.

.LONG TERM...(Friday night through Tuesday)
Issued at 311 PM EST Tue Dec 12 2017

With downstream blocking finally relaxing a bit, the persistent cold
air drainage we`ve been experiencing is expected to lessen a bit.
This should allow for temperatures to begin moderating over the
weekend. Some areas may even see temperatures a bit above normal,
which is in the upper 20s this time of year. The pattern remains
active; however, as a series of clippers is in line to pass through
the region bringing some snow chances and some shots of cooler
air.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1209 AM EST Wed Dec 13 2017

Lake effect snow showers will slowly wane early this morning at
KTVC and KMBL. Cigs will be on the cusp between high end MVFR and
low end VFR. A band of snow tied to low pressure passing off to
our southwest will likely bring deteriorating conditions once
again to KTVC and KMBL later Wednesday morning into Wednesday
afternoon. The storm track is a bit uncertain but if it`s farther
north, then it may affect KPLN and KAPN as well. Conditions slowly
improve later Wednesday evening as the snow moves away to the
east.

&&

.MARINE...
Issued at 311 PM EST Tue Dec 12 2017

Lingering small craft advisories early Wednesday as winds diminish
but choppy wanes linger. North to northwest winds transition to
easterly Wednesday as low pressure passes by to our south. More
small craft advisories will likely be needed Wednesday afternoon
and Wednesday night, especially on northern Lake Michigan.


&&

.APX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LH...SMALL CRAFT ADVISORY until 5 AM EST early this morning for
     LHZ345>349.
LM...SMALL CRAFT ADVISORY until 5 AM EST early this morning for
     LMZ323-341-342-344>346.
LS...SMALL CRAFT ADVISORY until 5 AM EST early this morning for
     LSZ321-322.
&&

$$

UPDATE...AS
NEAR TERM...MB
SHORT TERM...JL
LONG TERM...AM
AVIATION...AS
MARINE...MB



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