Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 081813
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
113 PM EST Fri Dec 8 2017

LATEST UPDATE...
Hydro

.SYNOPSIS...
Issued at 251 AM EST Fri Dec 8 2017

Today should start out largely precipitation free and temperatures
will for stay near to below freezing all day. Tonight a
developing storm system will track quickly south southeast from
western Lake Superior this evening to central Lake Michigan by
midnight. This will bring a period of snow to the entire area
overnight. The snow will be heavy at times west of US-131 and
south of Muskegon. The system quickly moves to Lake Ontario by
Saturday evening bringing down polar air into the Great Lakes. A
narrow band of heavy lake effect snow should mostly stay off shore
during the day time hours of Saturday, impacting only extreme
western Allegan and Van Buren Counties. High pressure follows for
Sunday but the next system will bring more snow Monday into
Tuesday and bring down more polar air.

&&

.SHORT TERM...(Today through Sunday)
Issued at 251 AM EST Fri Dec 8 2017

I have issued a Winter Weather Advisory for Kent, and Ottawa
counties for the early to mid morning hours of Saturday for the
system snow enhanced by Lake Michigan. For Allegan and Van Buren
Counties the Winter Weather Advisory is from early Saturday
morning until Saturday evening. Here we see system snow early
Saturday morning but we the get a dominant lake snow band to form
will remain mostly off shore Saturday during the day but should
come on shore near and south of South Haven.

The snow event tonight into Saturday will be in two parts. There
will be the synoptic event from 9 pm till 7 am, then there will be
the lake snow band part from 7 am till around 7 pm (or so). The
latter event will be very narrow, mostly near the lake shore,
near and south of South Haven. The good news is the model
consensus on the track of the storm tonight which gives me more
confidence on what sort of headlines to issue.

This winter storm has significantly more dynamic energy going for
it. The lift in the clouds is much stronger and there is decent
lift in the DGZ. However it should be noted that the best lift is
below the DGZ (if it were in the DGZ I would have put a Winter
Storm Warning out instead of an Advisory). Being a southwest flow
event (low near MKG at midnight) that will bring heavy snow bands
into most of Van Buren, Allegan, Ottawa and western Kent Counties
between 9 pm and 5 am (or so). The model QPF under preformed in
the last event (yesterday) but is giving significantly more QPF
with this system, mostly in 6 hours from 1 am till 7 am Saturday.
The areas under those snow bands could get 6 inches in 6 hours or
even more. However the wind is light during this part of the event
and it is mostly when travel is a minimum (another reason I did
not go with a warning).

On Saturday, as the system moves quickly to the east across
central Lower Michigan there will be north wind behind the system.
That will force a north/south lake snow band which largely will be
off shore. It may clip the Big and Little Sable point and the area
near and south of South Haven. I am not sure if the band will
actually end up totally over the lake Saturday as there is some
suggestion of North Northeast winds in some models during the day
Saturday. In any event most of our CWA will not see more than
flurries Saturday during the day.

High pressure ridging moves in Saturday night lowering inversion
heights and turning winds more westerly after midnight. This may
bring the lake snow band inland but by then it should not be more
than flurries.

Sunday another system tracks east over the Northern Great Lakes,
having little impact on Southwest Michigan expect to result in
flurries and mostly cloudy skies.

.LONG TERM...(Sunday night through Thursday)
Issued at 251 AM EST Fri Dec 8 2017

We will see the cold weather generally hold in through the long
term, with periods of snow showers expected through the period. No
major snowstorms are expected, however the western half of the area
will see the snowpack gradually build through the week with the
periods of snow.

We will be watching a system diving SE toward the region late Sun
night, and more so on Mon. The models differ a bit on the track of
the upper wave and associated sfc low, which will have implications
on snow amounts. The fcst is a blend of the solutions which broad
brushes chcs and amounts a bit. We do think the more srn track of
the Euro is more likely due to systems tending to dive further
south, and due to the good track record of the Euro. This would
limit lake enhancement with a erly component to the low level winds.

No matter how the low tracks through, the lakeshore areas look to be
in for a good dose of lake effect late Mon into Tue. H850 temps will
be in the negative mid teens C, with upper cyclonic flow. The mean
low level wind flow should be NNW, favoring areas near the
lakeshore. This will likely move offshore Tue night with a weakening
wind flow, before moving onshore and dissipating Wed as the flow
picks up from the West.

Lake effect/enhancement will pick up late Wed after the sfc ridge
axis moves to our east. Upper cyclonic flow will persist, and
multiple short waves will rotate through the area. This will be the
case through Thu night.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 105 PM EST Fri Dec 8 2017

Widespread MVFR cigs this afternoon may retreat briefly this
evening. However the incoming snow system will send IFR vsbys into
the area from northwest to southeast later tonight, starting at
MKG/GRR around 04Z and reaching JXN around 09Z. The snow will be
heaviest at MKG/GRR where LIFR vsbys below one mile will be common
between 06z and 12z Sat.

The snow will taper off and vsbys will improve considerably
around 14Z at MKG and GRR as winds shift from southwesterly to
northerly, but the snow will not diminish/end until after 18Z at
JXN.

&&

.MARINE...
Issued at 251 AM EST Fri Dec 8 2017

I have extended the Small Craft Advisory until Sunday morning. I
though about a gale watch for Saturday during the day but just
about all of the models suggest the stronger winds will over Lake
Michigan (north wind down the long axis of the lake). I could see
a hour or two of gales and any one location as the cold air
surges in but for the most part winds will stay below gale force
(as seen in just about all of the models time/height sections near
the lake shore Saturday). Since any gales would be short lived,
seems the overriding Theme is for small craft advisory conditions
into Sunday morning.

&&

.HYDROLOGY...
Issued at 113 PM EST Fri Dec 8 2017

Rivers are running near or a little above normal. No flooding or
significant rises in river levels are expected. Accumulating snow is
expected tonight into Saturday and again late Sunday through Tuesday.

Temperatures have been near to below freezing since Tuesday night.
Similar temperatures are expected to persist through the end of next
week. This will allow ice to begin forming on area rivers.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 9 PM this evening to 7 PM EST
     Saturday for MIZ064-071.

     Winter Weather Advisory from 9 PM this evening to 9 AM EST
     Saturday for MIZ050-056-057.

LM...Small Craft Advisory until 9 AM EST Sunday for LMZ844>849.

&&

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...NJJ
AVIATION...Meade
HYDROLOGY...63
MARINE...WDM


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