Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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874
FXUS63 KGRR 101755
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1255 PM 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 an 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.

&&

.UPDATE...
Issued at 1003 AM EST Sun Dec 10 2017

We will update the fcst late this morning to feature less in terms
of snow accumulations today. We only expect trace amounts to
perhaps a few tenths of an inch of snow late this morning through
this afternoon as a result of an upper level disturbance and
isentropic lift. However moisture is quite limited and does not
extend through the dgz. As such we expect only very light snow
showers/flurries with very little to no snow accumulation. Pops
were also tweaked down a bit accordingly.

&&

.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 18Z TAFS through 18Z Monday afternoon)
Issued at 1255 PM EST Sun Dec 10 2017

Widespread MVFR cigs - bases around 2000 ft and tops around 6000
ft - along with some light snow showers or flurries, will impact
the airport terminals through early evening. Some icing will be
present in these clouds.

We should see cigs improve to VFR by mid evening with the exception
of MKG where MVFR may linger all night.

Widespread snow will move into the area toward, or just after,
12Z Monday - producing a period of IFR vsbys through 18Z Monday.

&&

.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 1000 AM EST Sun Dec 10 2017

Rivers are running near normal levels to a little above normal. 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...None.
LM...Small Craft Advisory until 1 AM EST Monday for LMZ844>849.

&&

$$

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



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