Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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000
FXUS63 KGRR 120617
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
117 AM EST Tue Dec 12 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 326 PM EST Mon Dec 11 2017

The next few days will be rather snowy. Another shot of arctic air
will move over the lake Tuesday and heavy lake effect snow along
with blowing and drifting is expected, especially closer to the lake
shore. Another clipper will move across the state Wednesday and more
accumulating snow can be expected.

Temperatures will fall through the day Tuesday as arctic air flows
south and end up mostly in the teens by late Tuesday. Temperatures
should rise above freezing by Saturday.

&&

.UPDATE...
Issued at 104 AM EST Tue Dec 12 2017

I extended the Winter Weather Advisory for our southeastern zones
for freezing drizzle. The DGZ does not get saturated over the
Jackson area until around 5 am. Locations north and west of there
will see the change to all snow sooner, but since it will take
till 5 am at Jackson I have the advisory till then. The other
headlines seems fine at this point.

UPDATE Issued at 935 PM EST Mon Dec 11 2017

The back edge of steady snow was shifting southeast...through
Battle Creek and Mason leaving behind some spotty freezing
drizzle. The 00z APX sounding captures this low level moisture
nicely. Thus I added freezing drizzle to the forecast for a few
hours this evening. The arctic airmass was dropping southeast out
of Northern Wisconsin and the Upper Peninsula. Radar loops in
these locations show bands of lake effect snow showers shifting
southeast. That will be the trend in Southwest Lower Michigan
after midnight.

UPDATE Issued at 429 PM EST Mon Dec 11 2017

Based on current radar trends, we canceled the advisory for
Montcalm, Gratiot, Ionia and Clinton counties. The heavy snow band
will remain south of those areas.

&&

.SHORT TERM...(This evening through Wednesday night)
Issued at 326 PM EST Mon Dec 11 2017

Forecast concerns deal with precipitation trends through the period.

Three systems will affect the cwa during the short term. The first
is a clipper that is just crossing the Mississippi near the Quad
Cities. Regional radar shows a deformation band of snow on the north
side of the track of the low and short range models such as the
HRRR, Nam3km, WRFARW all show this deformation snow moving across
the I-96/I-94 corridors this evening. It is the reason we extended
the winter weather advisory eastward this morning. Accumulations up
to 4 inches are possible with this band. However, deformation bands
tend to be narrow and it`s quite possible that only portions of
those counties will see the higher end of those totals, depending on
exactly where the boundary moves through.

Arctic air will flow in behind the departing clipper. H8 temps will
fall from -6c to -16c by 12z Tuesday. Lake effect snow will develop
and affect mostly the lake shore areas. Accumulations up to 5 inches
are possible along the lake shore tonight and up to 9 inches
Tuesday. Due to the NNW wind flow, locations east of US-131 will see
considerably less snow...perhaps up to 3 inches. Windy conditions
will develop tonight and significant blowing and drifting can be
expected along the lake shore.

The snow will diminish Tuesday night and Wednesday morning, but the
next clipper will be quickly moving toward the cwa. Another 1 to 4
inches of snow can be expected late Wednesday and Wednesday night.

.LONG TERM...(Thursday through Monday)
Issued at 326 PM EST Mon Dec 11 2017

Some sizable differences are noted between the GFS and ECMWF through
the long term period, so overall confidence in forecast details is
lower than average. Have opted to follow the ECMWF though for
guidance through the long term period.

Most of the long term will feature seasonable temperatures for mid
December with highs in the upper 20s to lower 30s. Tried to trend
temperatures towards the cooler ECMWF numbers over the weekend. A
parade of shortwaves are still forecast to move through the Great
Lakes region in the northwest upper flow. The first surface low is
forecast to move through our area from Thursday night into Friday
night. A lake enhanced environment is in place, so the highest
chances at accumulating snow will be towards Lake Michigan. The next
low pressure system is forecast out around the Sunday time frame.
The GFS is warmer, further north and quicker. Like the more southern
track of the ECMWF which would bring a good chance at accumulating
synoptic snow to much of our forecast area. Too early to nail down
details and our current forecast chances for snow will need to be
raised with time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 117 AM EST Tue Dec 12 2017

Areas of freezing drizzle were ending from the northwest and will
end over the JXN area around 09Z. The pcpn will either end or
become light snow showers.

The flow behind a passing front will be NNW. This means the TAF
sites are expected to largely stay out of the heaviest lake
effect snow showers. Have gone with a mix of MVFR and VFR, however
occasional IFR will be experienced closer to the lake in snow
showers. The winds are expected to become more NW toward mid
afternoon, allowing the lake effect to push a bit farther inland.
More impact to the TAF sites is expected after 19-20Z when this
happens. MKG, GRR, BTl and especially AZO should see an uptick to
the snow showers and occasionally lower vsbys. Somewhat drier air
arrives into the late evening (04-05Z) when the snow showers
should become less intense.

Bottom line, poor conditions will exist near the lake shore for
much of the next 24 hours. However the TAF sites will likely see
an increase in snow and lower vsbys between roughly 20Z and 04Z.

&&

.MARINE...
Issued at 326 PM EST Mon Dec 11 2017

No changes to the gale warning. An Arctic cold front will move across
the lake late tonight and gale force winds will develop and continue
through Tuesday.


&&

.HYDROLOGY...
Issued at 1023 AM EST Mon Dec 11 2017

Rivers are running near normal levels. No flooding or significant
rises in river levels are expected. Snow is forecast periodically
through the week.

Temperatures have been near to below freezing since last Tuesday
night and similar temperatures are expected to persist through the
end of this week. This should allow ice to begin forming on area
rivers, especially on the Muskegon River in Central Lower
Michigan.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Warning until 7 AM EST Wednesday for MIZ037-043-050-
     056-064-071.

     Winter Weather Advisory until 7 AM EST Wednesday for MIZ038-039-
     044-057-065-072.

     Winter Weather Advisory until 5 AM EST early this morning for
     MIZ066-067-073-074.

LM...Gale Warning until 7 PM EST this evening for LMZ844>849.

&&

$$

UPDATE...WDM
SYNOPSIS...04
SHORT TERM...04
LONG TERM...Duke
AVIATION...JK
HYDROLOGY...63
MARINE...04



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