Area Forecast Discussion
Issued by NWS Gaylord, MI

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FXUS63 KAPX 061704
AFDAPX

Area Forecast Discussion
National Weather Service Gaylord MI
1204 PM EST Tue Dec 6 2016

.UPDATE...
Issued at 1100 AM EST Tue Dec 6 2016

Deep vertically stacked low pressure system spinning over NW MN
with an occluded front extending south through WI/IL. A shortwave
with strong DPVA along and ahead of this front is working with
minimal moisture while lifting into western Lake Superior. This is
only resulting in a narrow corridor of light snow and rain
lifting eastward. Meanwhile, the deep moisture is associated with
a srn shortwave lifting through the SE states. Some of this H8
moisture was wrapping around the nrn periphery of the sfc low in
the mid Atlantic, and working toward Michigan via southerly winds.
This will likely result in a expanding swath of light rain/snow
with minimal QPF. Best chances over NE lower/Lake Huron where
there is longer time for moisture to advect there.

Heading into tonight, the occluded front will cross us by this
evening with the beginnings of the long talked about cold
advection. There is no pre-existing lake effect, and moisture is
shallow back behind the front. Thus, it`s gonna take time for the
lake effect machine to get going, but could see western Chip/Mack
seeing some flakes at nightfall. For tonight through Wednesday,
the pattern will be very stubborn to change, particularly the
1000-850mb flow which will hold in a WSW fashion, while
temperatures cool to -11C to -13C allowing moisture to easily
extend to where ice is introduced into the shallow convection
(inversion heights below 5500 feet). The WSW flow will pinpoint
rather light snow showers from north of M-68 in nrn lower through
eastern Chip/Mack counties of eastern upper, including the
Straits. Only an inch or two over this entire time period.

&&

.NEAR TERM...(Today and Tonight)
Issued at 305 AM EST Tue Dec 6 2016

...One more above normal day before cold air arrives tonight...

High Impact Weather Potential...Low. Increasing chances of mixed
rain and snow showers this afternoon and tonight.

Pattern Synopsis/Forecast...Narrow ridge axis continues to exit east
away from Michigan early this morning...as the far eastern edge of a
warm front/WAA wing lifts into NW sections of our CWA ahead of
deepening low pressure over the Northern Plains and Upper
Mississippi Valley. Latest base ref loop from KAPX shows a solid
area of mid level returns lifting NE thru Northern Lake Michigan
into Eastern Upper Michigan...with little in the way of any precip
actually reaching the ground attm. Deeper moisture and thus better
chances of precip remains further upstream in the vcnty of the cold
front which currently extends from NW Wisconsin thru Central Iowa
into Nebraska. Deepening vertically-stacked low will slowly make NE
progress out of the Northern Plains and into Southern Ontario over
the next 24 hours or so. Precip chances will be confined to Upper
Michigan this morning where deeper moisture will reside along and
ahead of the warm front. Better chances of precip will develop
across our entire CWA this afternoon and tonight as the cold front
and associated deeper moisture slide thru the area. CAA in the wake
of the cold front will begin to increase over-lake instability later
tonight...resulting in increasing chances of lake enhanced precip.

Primary Forecast Concerns...Precip type will remain an issue thru
this evening before CAA turns all precip over to all snow. Expect a
mix of rain and snow across Eastern Upper Michigan thru this morning
as temps hover in the upper 20s inland to the mid 30s near the
lakeshores...with little in the way of snow accumulation. As precip
chances increase across our entire CWA this afternoon and evening...
higher elevations will initially see a mix of rain and snow. This
mix will gradually spread into the rest of our CWA as the afternoon
and evening progresses with increasingly cold air sliding into the
area in the wake of the cold front. Precip will eventually switch to
all snow everywhere by around 06Z as temps fall into the mid to
upper 20s overnight. Although moisture will be sufficiently deep to
produce some light precip...narrow band of moisture will still be
limited and thus new snow amounts will remain minor...i.e. well
under an inch.

&&

.SHORT TERM...(Wednesday through Thursday night)
Issued at 305 AM EST Tue Dec 6 2016

...Colder with lake effect snow...

High Impact Weather Potential...Lake effect snow showers will
develop on Wednesday, becoming more intense Thursday and Thursday
night. Several inches of accumulation will be possible within the
typical snowbelts along Lake Michigan. In addition, gusty winds will
cause blowing snow and reduced visibilities, making for difficult
driving.

Pattern Synopsis/Forecast...An elongated upper trough will develop
over the Great Lakes as a cutoff low drifts east across southern
Ontario. A significantly colder airmass will be supportive for
persistent lake effect snow showers, but veering low level winds
will shift the impact of snow bands southward along Lake Michigan.

Primary Forecast Concerns...Wednesday morning a vertically stacked
low will be located over the northern tip of Lake Superior.
Ingredients are coming together for a prolonged period of lake
effect snow across the Upper Great Lakes. Low level RH Wednesday
into Thursday across northern Michigan will be plentiful with 925-
850mb RH averaging 90 percent or higher. With 850 mb temps around
-12C, delta Ts will be around 20 degrees. Winds for most of
Wednesday will be brisk out of the SW with gusts of 30-35 mph
possible, higher along the coasts. LES bands are therefore expected
to initially affect eastern Upper and northwest Lower. PoPs will
increase through the day as mid level lapse rates steepen and the
capping inversion weakens and rises, especially north of M-32. Some
enhanced lift will come from an H5 shortwave as it passes across
northern Michigan Wednesday evening, mainly influencing areas north
of M-32.

A surface trough/wind shift line will then drop south through
northern Michigan Thursday morning, causing winds to gradually veer
to the NW by Thursday evening and to the NNW Thursday night. Low
level convergence along this line is expected to enhance the snow
potential, as will the erosion of the temperature inversion aloft.
In addition, favorable low level negative omega will coincide with
the dendritic growth zone for much of Thursday and Thursday night.
Despite the favorable setup for very efficient LES, the gradual
shift in wind direction will cause the bands to shift their
orientation, becoming more focused further south along the Lake
Michigan coast as the day/evening progress. This will act to temper
snow amounts for any given location, but areas around and south of
the Grand Traverse Bay area could easily see 6 inches or higher by
Thursday night.

Temperatures Wednesday and Thursday will top out in the upper 20s to
lower 30s with lows in the lower to middle 20s...closer to seasonal
normals.

.LONG TERM...(Friday through Monday)
Issued at 305 AM EST Tue Dec 6 2016

High Impact Weather Potential...

High impact weather potential: Accumulating lake effect snow
continues with some significant accumulations possible west of I-75
Friday - Friday night. Additional snowfall is possible on Sunday
across all of northern Michigan.

Focus early in the extended period continues around the ongoing lake
effect snow across the snowbelts of northwest Lower and eastern
Upper Michigan. Fairly good guidance agreement in H8 temps around
-13 C (delta Ts 20-22 C) becoming progressively colder throughout the
day Friday. Predominately NW winds to start the day should focus the
greatest potential along and west of US-131 and far western
Chip/Mack before the threat are subtly shifts northward as winds
slowly back WNW/W by Friday evening (thus focusing the heaviest lake
effect north of M-72/west of I-75). Signals continue to hint at
potentially significant accumulations (although hampered somewhat by
the aforementioned shifting/backing winds), which include steep low
level lapse rates, impressive subsidence inversion heights (15-20+
kft), and negative omega squarely in the DGZ at various points
Friday into Friday night. Decreasing lake effect PoPs will be the
rule Saturday, although expecting locations at least near the tip of
the mitt and eastern Upper to see additional snow showers, as winds
weaken and begin to veer with associated WAA ahead of a system
developing over the Plains. This sytem may be the next to provide
impact locally with widespread snow during the Sunday timeframe,
although a lack of guidance consistency will preclude much in the
way of details at this juncture.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1158 AM EST Tue Dec 6 2016

Deep low pressure was in SW Ontario with deep moisture and another
low pressure in the SE states. Moisture from the srn system will
wrap toward nrn Michigan ahead of a cold front associated with the
Ontario low and could produce some light rain across the airports
later this afternoon and evening. The onset of cold advection then
begins this evening and spreads from west to east, ultimately
resulting in light lake effect snows in WSW flow regimes, which
will primarily impact PLN with periodic 1-2sm VSBYS.

CIGS are difficult to say the least. Cannot see the extent of
IFR/MVFR stratus underneath the mid and high level clouds. Gonna
have to tackle this with tempos for the next handful of hours,
until the low level saturate behind the cold front. Mainly MVFR
expected.

SE winds will gust this afternoon and turn out of the SW behind
the front.

&&

.MARINE...
Issued at 1100 AM EST Tue Dec 6 2016

Upgraded nrn Lakes Michigan and Huron nearshores, including the
Straits, with a Gale watch, as low level temperatures get colder
and colder with a tightening pressure gradient settling in tonight
through Thursday. Overlake instability will be large with deep
mixing tapping low end gale gusts tonight into Wednesday night.
Conditions will remain gusty, but at this point speeds are not
expected to reach gales Thursday.

Also, lake effect snows will become more and more common over this
time period, with the worst of conditions Thursday/Thursday night.

&&

.APX WATCHES/WARNINGS/ADVISORIES...
MI...NONE.
LH...SMALL CRAFT ADVISORY until 7 AM EST Thursday for LHZ348-349.
     GALE WATCH from 10 PM EST this evening through late Wednesday
     night for LHZ345>347.
LM...GALE WATCH from 10 PM EST this evening through late Wednesday
     night for LMZ323-341-342-344>346.
LS...SMALL CRAFT ADVISORY until 7 AM EST Thursday for LSZ321-322.
&&

$$

UPDATE...SMD
NEAR TERM...MLR
SHORT TERM...MEK
LONG TERM...MJG
AVIATION...SMD
MARINE...SMD


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