Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 162338
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
638 PM EST Sat Dec 16 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 311 PM EST Sat Dec 16 2017

Relatively quieter weather can be expected across the area through
the first half of this week. Precipitation will be much more
infrequent, and the air will be milder compared to the past couple
of weeks. There will be a low chance of rain or snow from late
Sunday through Monday. This is not expected to cause any issues.

A stronger storm system is expected to impact the area in one way or
another from the Thursday to Friday time frame. The effects are
quite uncertain at this time, and could range from warmer and rainy
weather, to snow accumulations depending on the track of the low.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 311 PM EST Sat Dec 16 2017

There does not look to be much in the way of impact weather in the
short term, contrary to the past couple of weeks. The general theme
for much of the area will be plenty of cloud cover with some small
chances of pcpn from late Sunday through Monday.

All pcpn has moved out of the area, leaving mainly cloud cover north
of a line from South Haven to Ann Arbor. This line is likely the
location of the stalled out front that separates 20s up north, to
well into the 30s south of it. We expect that the low clouds will
expand/drop south through tonight.

A low chance of pcpn will move in across the southern half of the
area Sunday afternoon. This is the result of an upper wave currently
over the TX panhandle that will dampen and lift NE through Sun
afternoon. It will be quite flat by the time it gets here, and it
will have separated from the srn stream, keeping the Gulf moisture
from making it this far north. P-type will be snow early, but
diurnal heating and increasing melting layer will tend to change
anything over to rain late in the day. Moisture depth is fairly
limited, so it may end up just being drizzle.

The Sunday wave moves through, and then we see another northern
stream wave approach the area by Monday. The upper jet and best
dynamics stay north of the area. We will see just enough of the srn
periphery of the wave to try and squeeze a light shower out of it.
Again the moisture will be quite limited, in concentration and
depth, so it may end up just being drizzle. Temps should stay mostly
above freezing so freezing drizzle threat looks fairly low.

Once the wave moves out late Mon, mainly dry and mild weather will
be expected on Mon night.

.LONG TERM...(Tuesday through Saturday)
Issued at 311 PM EST Sat Dec 16 2017

The storm on Thursday into Friday is the focus of the extended
period. There is still sufficient spread in the ensembles and
volatility in the operational runs to keep this a low confidence
forecast.

The period begins with cold advection and lake effect snow showers
developing late Tuesday afternoon then tapering off Wednesday
afternoon as warm advection begins ahead of the next low. Bumped up
Superblend POPs for lake effect snow showers during this time period.

For Thursday and Friday...the 12Z ECMWF breaks with consistency and
shows a more amplified low while the GFS is consistent with a weaker
and further south, thus colder, storm. So given the continued
uncertainty, did not make any changes to the POPs or temperatures
during this time as the blend still looks like a good compromise.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 637 PM EST Sat Dec 16 2017

Currently skies are nearly clear or are clear at AZO and there is
only mid clouds at all of the other taf sites except for LAN which
still has and MVFR cigs. It seems to me over the next few hours if
anything LAN may break out of the MVFR clouds for a time. Later
tonight I do believe the MVFR cigs will overspread the rest of the
I-96 taf sites. The front will stage south overnight to south of
I-94. Still the dew points south of the front are not above the
surface temperatures in the cold air (for the most part). This
typically means little in the way of low clouds north of the
front. So I am not sold on widespread low clouds till the front
starts pushing back north as a warm front Sunday. Even then the
dew points in the warm air all still mostly near to below
freezing.

As for low clouds and fog for the I-94 taf sites winds do no go
calm at the surface tonight and the moisture content of the air
moving in for Sunday is not all that high. So I am not sold on
dense fog tonight for our I-94 tafs. Instead I bring in a IFR
cloud deck toward morning.

&&

.MARINE...
Issued at 311 PM EST Sat Dec 16 2017

It appears we should remain headline free through most of Monday
before the next advisory may be needed. Winds will be up some on
Sunday, but they should be offshore and should top out around 20
knots.


&&

.HYDROLOGY...
Issued at 311 PM EST Sat Dec 16 2017

Many rivers in Central and West Michigan have iced over due to an
extended period of freezing temperatures. A suspected ice jam on the
Grand River has caused the river level to rise above bankfull near
Eagle. While fluctuations in the river level should be expected over
the next few days, the general trend will be for the level to fall
through the weekend. Dry weather is expected for the remainder of
today. The next chance for rain and snow showers will be Sunday
afternoon and Sunday night, but liquid amounts should be very light
and less than 0.10 inches.

Warmer temperatures with daytime highs above freezing are expected
through the early and middle parts of next week. Water amounts
contained within the frozen snowpack range from roughly 0.5 to just
over 1 inch across the area, with highest amounts along and west of
US 131. The warmer temperatures are not expected to cause a full
melting of the snowpack this week, but we could see some of the
water released as runoff. This will be in addition to occasional
light rain and snow showers at times through the first half of the
week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...Ostuno
AVIATION...WDM
HYDROLOGY...HLO
MARINE...NJJ


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