Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 121125
AFDGRR

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

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 326 AM EST Tue Dec 12 2017

The first intrusion of true arctic air of the season will result
in very typical Southwest Michigan winter weather today tonight.
However for locations near and mostly west of US-131, today will
be anything but typical. Very strong winds will combine with bands
of heavy snow to create snow squalls that will make travel
challenging at best.

The next storm moves into the ares Wednesday with a system type
snowfall that will bring most areas some snow but the I-94 area
will see 3 to 5 inches of snow by late Wednesday as that system
moves through. Then there will be more lake effect snow showers
Thursday only to have another system bring more snow Friday.

Yet another system moves through the area this coming weekend and
that will bring warmer temperatures but not warm enough for rain,
so we get yet another snowfall event Saturday into Sunday.

&&

.SHORT TERM...(Today through Thursday)
Issued at 326 AM EST Tue Dec 12 2017

Clearly the issue today is the heavy snow bands with winds gusting
to near 40 mph at times (near the lake shore). All of the high
resolution models are consistent with this event and just looking
at our 88D radar, it is clear the models will be correct about
this too. I have expanded the Winter Storm headlines eastward into
Lake, Newaygo and Kalamazoo Counties. This is because the winds
support a Lake Superior Snow band that will make it into western
Lake and western Newaygo Counties during the day time hours
today. Similarly we see another area of enhanced snow bands from
south of Grand Haven into western Kalamazoo Counties.

This is a very dangerous travel day over our western counties due
to the strong winds, heavy snow bands, and temperatures falling
into the teens resulting in wind chills near zero today and
tonight.

With the inversion height near 8000 ft and the max lift in the DGZ
(which will be below 5000 ft) this will be a good set up for large
snow flakes. All of the high resolution models are showing QPF in
the .5 to .7 inch range in the heavier snow band today into
tonight. Using a 20 to 1 ratio for snow we could see 14".
Typically through the true ratio on lake effect is frequently 30
to 1 or higher. However today with the strong winds, there will be
some compacting of the snow that falls. So I still believe we will
see some locations getting over a foot of snow from this event.

No sooner than the system that is creating todays weather moves
out then the next system moves in by midday Wednesday. There is
some issue with the track of this storm but the ensembles of the
ECMWF from the 00z yesterday, 12z run yesterday and the 00z run
tonight have shown northward progression of the area of heaviest
snow. That would be between I--96 and I-94. The GFS is similar to
this as is the NAM, and WRF models. The ensembles of the GFS also
go along with this idea. So this may need a headline (Winter
Weather Advisory) but I will let the next shift deal with that
issue as this storm is still developing in our area.

.LONG TERM...(Thursday night through Monday)
Issued at 326 AM EST Tue Dec 12 2017

Little change in the overall pattern through the work week, then we
see a slight warming trend into the weekend and lasting into early
next week.  The accumulating snow will be more commonplace until the
weekend too.

Upper troughing and cyclonic flow will result in a continuation of
the cold and snow Thursday night through Friday night.  Another
clipper will spark more widespread snow Friday into Friday evening.
This should bring an inch of snow to inland areas, and as much as an
additional 6 inches near the lake.

A bit of a pattern change is indicated as we reach the weekend
however.  The flow should become more zonal resulting in the arctic
air sliding to our east.  H8 temps bump up to -5 to -6C which will
cause the lake effect to end. Temps should moderate closer to
normal values.  However light snow will likely continue late
Saturday night into Sunday as a cold front comes through, with a
wave of low pressure sliding to our south.  Another low passes to
our south Sunday night/Monday.  Not very good model agreement by
this time frame, but it seems this system may stay south.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 625 AM EST Tue Dec 12 2017

The lake enhanced snow showers have developed and continue to
expand. For the most part that TAF sites will miss the heaviest
and most persistent snow showers. Any snow shower today at any of
the TAF sites will bring brief LIFR with snow and blowing snow. I
did not put this in any of the taf as it would be to transient.
MKG and AZO are the TAF sites most likely to have the stronger and
more persistent snow showers. Tonight the snow showers will
mostly be near the Lake shore leaving only MKG in the snow shower
activity. Even then, MKG will mostly stay out of the better snow
showers.

The next storm will bring solid IFR to all TAF sites Wed but just
beyond the 12z to 12z time frame.

&&

.MARINE...
Issued at 326 AM EST Tue Dec 12 2017

I have made no changes in our marine headlines. The latest marine
reports show frequent gusts to near 40 knots. South Haven is
showing gusts to 45 knots frequently. Winds will subside during
the afternoon into this evening so letting the gales expire at 7
pm seems good to me. We would then need a small craft advisory
but I will let later shifts do that since we do not typically like
to have multiple headlines out.


&&

.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 MIZ037-038-043-
     044-050-056-064-071-072.

     Winter Weather Advisory until 7 AM EST Wednesday for MIZ039-057-
     065.

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

&&

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...JK
AVIATION...WDM
HYDROLOGY...63
MARINE...WDM


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