Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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727
FXUS63 KGRR 080238
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
938 PM EST Wed Dec 7 2016

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 325 PM EST Wed Dec 7 2016

Much colder air will sit over Michigan and the Great Lakes tonight
through Friday.  This will bring periods of intense lake effect
snowfall down-wind from the Lakes, including Lake Michigan. Winter
storm warnings and advisories have been posted for much of Western
Michigan as a result.  Areas west of a Houghton Lake to Grand Rapids
to Battle Creek line will see the highest impacts, where 4 inches,
to as much as a foot of snow is expected.  West to northwest winds
gusting to as high as 35 mph will cause blowing and drifting,
particularly Thursday and Thursday evening.  Areas east of this
line will see 1 to 3 inches of snow.

Another snowy period is expected Saturday night through Monday with
additional accumulations likely.

It will remain cold with highs only around 30 Thursday through
Sunday. Then we should become colder into next week, with highs only
20 to 25 Monday through Wednesday.

&&

.UPDATE...
Issued at 938 PM EST Wed Dec 7 2016

I have no plans to changer our forecast or headlines. The going
forecast looks on track to me.

I like to look at the lift in the DGZ (saturated) and the latest
run of the NAM (00z) shows feeble lift from 7 am till noon
tomorrow (Thursday) along the lake shore south of MKG but then
that goes away from noon till 5 pm. It is after 5 pm the lift goes
crazy from HOLLAND to KALAMAZOO and that actually continues most
of the day on Friday, it is not until 5 pm Friday is starts to
diminish. So, I get the impression the REAL event will be tomorrow
night into Friday, that is went the best lift is in the DGZ. We
likely will see some snow showers around sunrise but they at this
point, to me do not look like they would be all that heavy. At
this point I do not see anything to gain by delaying the start of
the event in the headlines.

&&

.SHORT TERM...(This evening through Friday)
Issued at 325 PM EST Wed Dec 7 2016

Only change to the headlines will be to add Osceola County to the
advisory.  This county should see about 3 inches over SE portions,
to as high as 8 inches over the NW.  This area, along with Mason,
Lake and Oceana Counties will see the most intense snows Thursday
night into Friday morning as the mean low level flow becomes
northwesterly.

Otherwise little overall change in the thinking with the forecast
over the next 48 hours.  All elements come together with climbing
over-lake instability, increasing moisture depth, and a deep DGZ
with occasional intense lift within it.

Expect this event to unfold in two waves.  The first advances our
way toward daybreak Thu with a short wave moving in.  This brings
another shot of cold air, along with deepening moisture.  As
explained with the morning update, the main surge of intense
snowfall should get generated about 12Z and last until about 18Z Thu
as this wave passes.  This period also shows the best chance for
strong gusty winds to as high as 35 mph producing occasional
whiteouts. So expecting a quick deteriorating of conditions around
daybreak and thereafter. Will keep the timing of the headlines as is
(starting 05Z), since it will snow prior to 12Z, but highlight more
intense snows starting around daybreak and throughout the daylight
hour of Thu morning.

The lake effect snow will continue Thu afternoon, but we should see
a brief lull in it`s intensity as the best lift from the short wave
moves away from the lake.

The flow will be west to west northwest Thursday, so Kent and Barry
Counties should see the bulk of their snow in this period.

Then the flow becomes more northwest by late in the day Thursday and
lasting through Thursday night, before going back to WNW mid day
Friday. Surface and upper troughing will keep the deep moisture in
place, and this troughing will also help increase the lift.  This
will be the period when the longest and most intense snow can be
expected as the DGZ is saturated with strong lift within it.
Dominate bands should set up by early evening and we will also see a
Lake Superior connection to the bands into West Central Lower.
Occasional whiteout conditions can be expected, however model trends
do show the winds should slowly relax overnight, so the blowing and
drifting should become less of a factor toward daybreak Friday.
Friday will see a continuation in the intense banded snowfall.  With
a diminishing trend late in the day as the deep moisture lifts away.

All told, areas in the warning and advisory will see 4 to as much as
12 inches of snow by Friday evening.  Locally higher amounts will be
possible.  Wind chill readings will be holding down around 10 to 15
above, so will highlight this too in the WSW since we have not felt
that so far this season.

.LONG TERM...(Friday night through Wednesday)
Issued at 325 PM EST Wed Dec 7 2016

Several rounds of snow are forecasted in the long term...the most
significant of which looks to be Saturday night into Sunday.

To start...the period starts off with moisture and temperatures
profiles still supportive of snow showers Friday Night.  Cyclonic
low level flow may enhance the snow showers somewhat...especially
Holland to South Haven.  I featured POPs much higher than what the
computer models were forecasting.  The snow showers should diminish
somewhat later Friday Night and more so Saturday morning because the
moisture depth decreases.

A Pacific mid level wave arrives Saturday Night and pushes through
the region on Sunday.  Lake moisture will enhance this system as it
pushes through.  Model forecasts show very deep moisture along with
lift...and that supports widespread accumulations.  Models also show
a coupled upper jet and moist fgen in the dgz.  So plenty of
ingredients there for heavier snow. Confidence has increased with
the risk for more than 6 inches region wide with higher amounts
possible.

Another wave of low pressure is possible on Tuesday or Wednesday.
This one will be riding on the Arctic front.  So once it goes
by...the potential is there for a very cold airmass to settle in.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 648 PM EST Wed Dec 7 2016

I expect mvfr cigs past 06z then snow showers will start to
develop near MKG and spread to GRR by 10z or so. The heaviest snow
showers Thursday for the TAFS will be between 12z and 16z when the
moisture is deepest. The snow showers will let up some during the
afternoon then intensify even more that Thursday morning after 00z
on Friday. LAN and JXN are not expected to be significantly
impacted by the snow showers, MKG, GRR and AZO should see the
lowest conditions.


&&

.MARINE...
Issued at 325 PM EST Wed Dec 7 2016

Extended the small craft advisory into Friday and it appears we may
be able to continue it through Friday night before conditions
finally improve.  6 to 8 foot waves will be commonplace.


&&

.HYDROLOGY...
Issued at 1240 PM EST Wed Dec 7 2016

Rivers have stabilized with levels running flat or slightly
falling. We continue to see near to above normal conditions
throughout southwest Lower Michigan.

Multiple rounds of precipitation are expected over the next week,
coming in different modes (lake vs. synoptic). Much cooler
temperatures point to all snow, which would result in slow runoff.
The greatest amounts will stretch down the Lake Michigan
bordering counties, where synoptic and lake effect precipitation
will combine to result in weekly totals that could exceed one inch.
The synoptic system for the weekend will likely be the biggest
contributor for inland locations, but still has many questions
with it.

Flooding is currently not a high concern. However, high river
levels will respond quickly to any runoff and should be given
extra attention, provided any significant storms impact the area.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Storm Warning until 4 PM EST Friday for MIZ037-038-043-
     056-064-071-072.

     Winter Weather Advisory until 4 PM EST Friday for MIZ039-044-050-
     057-065-073.

LM...Small Craft Advisory until 7 PM EST Friday for LMZ844>849.

&&

$$

UPDATE...WDM
SYNOPSIS...JK
SHORT TERM...JK
LONG TERM...MJS
AVIATION...WDM
HYDROLOGY...JAM
MARINE...JK



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