Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 160413
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1113 PM EST Fri Dec 15 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 322 PM EST Fri Dec 15 2017

The lake effect snow showers that impacted the area this morning are
diminishing this afternoon. This will continue into early this
evening. Another band of light snow will develop this evening south
of I-96, and will migrate north through Saturday morning before
ending. 1 to 3 inches of new snow will be likely.

A lull in the snow is expected then Saturday night and early Sunday.
Precipitation chances will increase once again later Sunday. The
precipitation will start as some rain or snow as warmer air moves
in, and continue through Monday night before ending.

&&

.UPDATE...
Issued at 929 PM EST Fri Dec 15 2017

I have issued a Winter Weather Advisory overnight till 10 am
Saturday morning for 2 to 4 inches of snow. Nearly all of the Hi-
Resolution models for several runs in a row now are forecasting
around 0.30 inches of QPF from this warm advection/isentropic lift
event overnight. The 18z and 00z NAM and several runs of the HRRR
and RAP model show around 50 knots nearly perpendicular to the
anisotropic surfaces tonight (around midnight). The focus of the
heaviest precipitation is from Southeast Kent County northwest
across Ottawa into Southern Muskegon County. Since warm advection
events tend to over preform I though it best to issue the Winter
Weather Advisory for the counties with the heaviest snowfall
overnight.

&&

.SHORT TERM...(This evening through Sunday night)
Issued at 322 PM EST Fri Dec 15 2017

Our main focus of the short term is now the light snow expected
tonight, and then p-type questions for Sunday and Sunday night. The
advisory for today will be allowed to expire on time with the new
forecast package.

Lake effect snow showers are on the declining trend this afternoon
with regards to intensity and coverage as expected. The short wave
that helped the uptick today, is now moving east of the area. This
is driving down the inversion heights for a bit.

The break in snow will be short lived, especially for the Srn
portion of the CWFA. We will see a sfc trough/cold front over the
central portion of the area, drive south to I-94 this evening before
stalling. It stalls out as upper heights start to build also this
evening. We will see a wave of moisture transport translate along
the front and develop a band of light snow.

This looks to develop a NW to SE band of light snow later this
evening, that will gradually shift north to be centered around I-96
by Sat morning. This does not look to be significant enough to
warrant another advisory, but will drop 1-3 inches over a 12-18 hour
time frame. It will end as the wave moves east of the area Sat
afternoon.

The next situation will be incoming pcpn late Sun and Sun night. We
will see a short wave across the Central and Southern Plains lift NE
into the area. This wave will be different from the series of waves
we have seen over the past couple of weeks since it will be coming
from the SW. This will allow warmer air to push up into the area.
The pcpn may start out as some snow initially. It will tend to
change over with a SW low level flow advecting above freezing air,
and a sufficient melting layer depth to likely change it over to
rain for at least srn areas Sun night as temps warm.

It does not look like this system will produce significant impacts.
We will monitor it in case temps do not warm quick enough, and p-
type becomes a problem.

.LONG TERM...(Monday through Friday)
Issued at 322 PM EST Fri Dec 15 2017

Active pattern continues in the extended with two lows moving
through separated by a period of lake effect snow.

The first storm early in the week does not look to have much impact
as the southern stream moisture remains well separated from the
dominant northern stream and the sfc low passes well north of the
forecast area. Cold front comes through Monday night with cold
advection and lake effect snow showers Tuesday and Tuesday night
then tapering off Wednesday as warm advection begins ahead of the
next low. Bumped up Superblend POPs for lake effect snow showers
during this time period.

The second low comes through Thursday into Friday and continues to
have a lot of ensemble spread between a further north, stronger low
and a weaker, more southern track that would be colder with mostly
snow instead of mostly rain. The 12Z operational GFS and ECMWF have
flip flopped with each other since yesterday with the ECMWF now
having the less amplified, colder solution. Did not make any changes
to the POPs or temperatures during this time as the blend still
looks like a good compromise.

Looking beyond Friday and to December 25th...the overall synoptic
pattern shows a polar vortex across Eastern Canada and a SE CONUS
upper ridge. There is a very impressive temperature gradient between
the Great Lakes and Central Atlantic coast, with arctic air and 850
mb temperatures of -20C in Lower Michigan and a balmy plus 12C near
Cape Hatteras.

We will have to closely watch where the southern edge of the arctic
air and tropical moisture feeding up from the Gulf meet. At this
point it appears we will be solidly in the cold air with some lake
effect snow showers while there could be a band of heavy freezing
rain somewhere to our south...potentially across the Ohio Valley.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1100 PM EST Fri Dec 15 2017

IFR/MVFR conditions will continue at all the terminals overnight
due to significantly reduced visbys in light to occasionally
moderate snow. The steadiest snow band will shift slowly north to
northeastward and bring IFR conditions to KMKG... KGRR and KLAN
through around 12Z Sat while conditions should slowly improve to
mainly MVFR further south overnight.

Snow will gradually taper off during the mid morning hours
Saturday along the I-96 corridor and allow conditions to improve
a bit to mainly MVFR. MVFR conditions will then linger through
the rest of the day into Saturday evening due to low clouds and
patchy fog.

&&

.MARINE...
Issued at 322 PM EST Fri Dec 15 2017

We will be allowing the current Small Craft Advisory to go on as is
for now. Most of the current obs along the shore continue to support
advisory conditions. Winds will gradually ease up, and waves will
slowly follow suit. The 5 am expiration time looks good at this
point.


&&

.HYDROLOGY...
Issued at 322 PM EST Fri Dec 15 2017

Many rivers have iced over due to the snow and cold temperatures.
The Looking Glass near Eagle has risen above bankfull due to
downstream ice constricting the flow. It is expected to gradually
fall but some fluctuations are still possible. High temperatures
above freezing early next week will soften the ice a bit. Water
frozen in the snowpack ranges from 0.5 to 1 inch in most locations,
but a full snowmelt is not expected anytime soon.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 10 AM EST Saturday for MIZ050-051-
     056>059-064>067.

LM...Small Craft Advisory until 5 AM EST Saturday for LMZ844>849.

&&

$$

UPDATE...WDM
SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...Ostuno
AVIATION...Laurens
HYDROLOGY...CAS
MARINE...NJJ



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