Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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368
FXUS63 KGRR 172325
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
625 PM EST Sun Dec 17 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 323 PM EST Sun Dec 17 2017

Milder weather can be expected much of this week, especially
compared to the last couple of weeks. there is a chance of some
precipitation tonight as a frontal system approaches the area.
Significant precipitation is not expected, although types could range
from rain, to snow, to some freezing drizzle depending on
temperatures.

The precipitation will clear out by Monday night, and some sunshine
and temperatures well into the 40s can be expected.

The end of the week will see chances for precipitation increase as a
major storm system affects much of the country. This system could
bring rain or snow to the area. Behind the system for the upcoming
Christmas holiday, colder weather is expected to settle back in with
chances for lake effect.

&&

.SHORT TERM...(This evening through Tuesday night)
Issued at 323 PM EST Sun Dec 17 2017

Our main issues in the short term are pcpn trends and type for
tonight and into Monday. The good thing is that impacts will be
limited to a chance of some light freezing drizzle tonight and Mon
morning where temps will be below freezing.

Pcpn has tried to make it up north into the area this afternoon, but
has not, and will likely not succeed as the deep moisture and wave
gets shunted away from the area. We will see better moisture advect
in later tonight from the SW, and interact with a weak front coming
in from the NW. This will help to develop some pcpn over the heart
of the area.

Pcpn will be light in nature. P-type issues are present as the
atmosphere is initially supportive of snow. The warmer air coming in
will try to change pcpn over to some light rain. The sfc temps will
drop off a couple of degrees this evening, before coming up
gradually overnight.

On either side of the best chance of pcpn (roughly from after
midnight to daybreak) the DGZ will not be saturated. This will lead
to some drizzle/freezing drizzle potential depending on the sfc
temp. This should be light and limited in time due to rising temps.
We do not think it will be significant enough for any headlines at
this time. Fog will become possible also overnight with saturated
lower levels.

Some light pcpn will linger into the first half of Mon before the
front clears out the moisture later in the day.

The remainder of the period from Mon night through Tue night looks
fairly quiet. There will be a system well north of the area that
could bring some flurries into the U.S.-10 corridor on the srn
flank. That will move out, and eventually we will see clearing take
place for Tue. Temps on Tue with sunshine and nice warm air
advection will likely climb well into the 40s.

.LONG TERM...(Wednesday through Sunday)
Issued at 323 PM EST Sun Dec 17 2017

The main challenge in the long term deals with the impacts from the
storm for the end of the week.

Models are in reasonable agreement in showing warm air advection
will be underway Wednesday night into Thursday. The combination of
lift and deepening moisture will result in some snow.  Overall the
impact potential looks limited as the low levels remain relatively
dry.  There are some indications that a warm above freezing layer
will try to move in aloft on Thursday.  If this happens we could see
more potential for liquid precipitation.

The GFS suggests a deeper further west track to the storm...bringing
the warm sector into Southwest Lower MI on Friday.  While the High
Res Euro tracks the low through the central parts of the CWA and
keeps the storm weak.  Either way any mixed precipitation Thursday
night should change over to all rain.  There is some risk for a
slick Friday morning commute...that will need to be monitored. I
would not rule out some freezing rain as well.

If the GFS is right...there could be thunderstorms around Friday.
Elevated instability is seen in the model with some suggestion of
surfaced based instability as well.  The High Res Euro shows much
less potential for that to happen.  Will not introduce storms at
this point...but will need to monitor trends closely in the coming
days.

As the colder air starts to work in behind the departing
storm...will feature falling temperatures.  At this point the
potential for impactful lake effect looks low for Saturday into
Sunday.  There are indications that a wave may form along the
frontal zone on Sunday which could lead to an increased risk for
snow.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 625 PM EST Sun Dec 17 2017

While it is hard to tell from IR satellite loops skies have
cleared below 12,000 ft at AZO, BTL, JXN and BIV and should soon
do the same at GRR. Maybe even MKG and LAN will clear of the low
clouds for a few hours this evening. This is shown well by the
HRRR Layered cloud forecast, which by the way is tracking nicely
over the past 3 hours. So, I used the HRRR Layered cloud forecast
to do the 00z TAFS.

The low clouds will fill back in by 06z or so at all TAF sites.
There is a wave on the front that comes through the area toward
morning. This should bring some light rain or drizzle to our TAF
sites. The glitch is temperatures are currently below freezing at
MKG and GRR and near freezing at LAN. The other TAF sites are 33
degrees. It would not be out of the question this could be
freezing rain. However the hi-res models show winds becoming more
southerly than easterly overnight. That will bring up the warmer
air from the south into our area mitigating the risk of freezing
precipitation. Still it is not out of the question. That should
more out of the area prior to noon, then just low clouds and some
light fog the rest of the day Monday.


&&

.MARINE...
Issued at 323 PM EST Sun Dec 17 2017

We will be issuing a Small Craft Advisory that will be effective Mon
afternoon through Tue. This will likely need to be extended out in
time eventually, but we do not want to go too long for the time
being. Winds pick up after the front/wave move through tonight into
Mon morning. A decent gradient sets up, and will likely last into
Tue night before diminishing enough. Waves will build up to around 8
feet or so.


&&

.HYDROLOGY...
Issued at 323 PM EST Sun Dec 17 2017

A few gauge sites remain ice affected, most notably the Looking
Glass at Eagle and the White River at Whitehall. Of the two, Eagle
is the only site with a level confirmed to be above bankfull.

All sites are stable at this point and the Looking Glass River
continues to recede. Improvement is expected to continue with warmer
temperatures over the course of the work week. Precipitation is also
manageable with only a few tenths of an inch forecast through the
next 7 days.

Flood risks are minimal. For now, concerns are largely focused on
behavior existing ice jams. Otherwise, no addition flooding is
expected at this time.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory from 2 PM Monday to 7 PM EST Tuesday for
     LMZ844>849.

&&

$$

SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...MJS
AVIATION...WDM
HYDROLOGY...JAM
MARINE...NJJ



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