Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 101136
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
636 AM EST Sun Dec 10 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 300 AM EST Sun Dec 10 2017

Expect periods of snow and cold temperatures through this week.
Today a clipper system diving south southeast from northern
Minnesota will bring some light snow to the area today.
Accumulations will be mostly and inch or less.

The next system is significantly stronger and has a arctic front
attached to it. This will bring some light snow to the area Monday.
Once the arctic front comes through early Tuesday morning,
trailing the surface low, we will have a significant lake effect
snow event into Tuesday evening which will include blowing and
drifting snow. Tuesday will be the coldest day of the season so
far, highs may not get above the lower 20s in most area during
the day time hours. We get a break from significant snow
accumulation issues on Wednesday than another system moves
through Thursday with more cold air and lake enhanced snow into
Friday.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 300 AM EST Sun Dec 10 2017

The word SNOW comes to mind for what the highlights of this
forecast is. Expect some snow in every forecast period through
the short term. The heaviest snow will be the lake effect event
behind the arctic front Tuesday.

Today we have the exit region of the polar jet enhancing the lift
over Michigan. This results in an insentropic lift snow event
today. The good news if you do not want much snow is that there is
little moisture to work with so any snowfall today will be light.

Since this is a clipper system...winds turn to the north this
evening so expect some lake enhanced snow showers this evening
near the lake shore but amounts will be mostly less than an inch.
Inland there may be some clearing overnight which will result in
very cold temperatures, near 10 or maybe colder.

The clipper system on Monday is driven by a much stronger upper
jet, near 150 knots, so it will be more dynamic, having better
lift and thus better moisture to work with. All areas will see
some light snow by midday Monday that should come to an end by
Monday evening inland of US-131.

The arctic front assoicated with this system will bring in the
coldest air of the season. Highs Tuesday afternoon my not get
above the lower to mid 20s. It will be breezy too bringing wind
chills to near zero during the day Tuesday.

Winds will be north northwest in the cloudy layers near the Lake
and inversion heights will be in the 7000 ft to 10000 ft level.
That is more than enough to result in a significant lake effect
snow event for those near US-31. We may need some sort of winter
weather headline for this event.

The bottom line is on and off snow with temperatures staying below
freezing.

.LONG TERM...(Tuesday night through Saturday)
Issued at 300 AM EST Sun Dec 10 2017

Not much change generally in the forecast. Cold weather along with
intervals of snow showers expected through much of the period. We
will see a temporary moderation in the air mass toward next weekend,
however the weather will remain unsettled.

A fresh cold, canadian air mass will start out the period Tue night
in place over the area. This will bring additional lake effect to NW
flow favored areas, while inland areas avoiding the lake effect will
see a cold night. The lake effect will gradually diminish on Wed as
some ridging moves through the area.

The break will be short-lived as the next system will be diving into
the region with the long wave trough remaining entrenched over the
area. The trend for the next system has been for it to dive south of
the area, with only some of the snow on the northern periphery
potentially affecting the area. Yet another wave will track north of
the area Thu/Thu night. This one will allow for some lake
effect/enhancement ahead of it with a SW flow.

The moderation in temps begin on Fri, but will be more noticeable on
Sat. This temporary pattern change will come as the dominant upper
ridge out west get pushed east by strong energy coming onshore over
the West Coast. The next result will be the upper jet flattening out
and cutting off the flow of cold air from Canada. Pcpn will tend to
shift north also, closer to the Nrn branch of the jet. Pcpn that
does occur could become a mix with the warmer air moving in.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 636 AM EST Sun Dec 10 2017

Expect mostly MVFR cigs today as another low pressure system dives
south southeast across the western Great Lakes today. I am
expecting some areas of light snow over western TAF sites this
morning (mostly MVFR visibilities) but as the main shortwave moves
through the area mid to late afternoon all taf sites may well get
1 to 3 hours of IFR cigs/vsby in snow. After that MKG will likely
hold on to the MVFR cigs. However GRR, AZO and BTL should become
VFR for around 6 hours before the next systems low level moisture
field brings MVFR cigs back. LAN and JXN may see skies become
clear by 04z or so but the next systems clouds may not reach those
sites until after 12z.

&&

.MARINE...
Issued at 300 AM EST Sun Dec 10 2017

I will let the small craft advisory go as it is but it may have
to be canceled early as it is marginal. A more significant event
is expected Monday night into Tuesday as the arctic air surges in.
We may need gales for that event.


&&

.HYDROLOGY...
Issued at 1030 AM EST Sat Dec 9 2017

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

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

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 1 AM EST Monday for LMZ844>849.

&&

$$

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



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