Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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326
FXUS63 KGRR 071631
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1131 AM EST Wed Dec 7 2016

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 333 AM EST Wed Dec 7 2016

We are entering a wintry period, especially for Western Lower
Michigan where lake effect snow will ramp up tonight and continue
into Friday. Accumulations in some parts of the lakeshore
counties will approach 10 inches by Friday evening. Blowing and
drifting snow is likely west of Highway 131 Thursday and Thursday
night. Temperatures will remain in the 20s for the most part the
next couple of days so road conditions are expected to
deteriorate. A more widespread snow is possible over the weekend
as a low pressure system moves through the region. Winter is upon
us.

&&

.UPDATE...
Issued at 1131 AM EST Wed Dec 7 2016

No changes planned with the current headlines.

Today will remain quiet as the inversion levels stay low,
generally 4K or less. Enough to produce a few flurries or light
snow showers is all.

A couple minor tweaks are perhaps seen with the new 12Z data with
regard to the lake effect event tonight through Friday. First,
the main surge of deeper moisture appears somewhat slower, so we
are probably looking at the more intense snows holding off until
close to daybreak Thursday. This probably isn`t significant enough
to adjust the headline timing, but the Thu morning commute may
not be that bad, other than vsbys beginning lower significantly.
At this point it appears the intense snow should pick up around
12Z Thu, so accums should be minimal prior to this time.

The other possible tweak may be to add Osceola County. However
this area won`t be highly impacted until Thu night as the flow
becomes more NW and streamers off of Lake Superior begin to come
in. Will be looking into this possibility more this afternoon.

The latest data suggests two main surges of intense lake effect.
The first should be toward 12Z Thu that should last into early
afternoon Thu. This will be influenced by the first short wave
coming through.  The flow will be westerly at this point,
impacting Ottawa, Kent, Allegan and Barry Counties most
significantly. Then we should see a slight decrease in intensity
through mid afternoon into early Thu evening as the wave departs.

The most intense and longest lasting portion of the event should
start Thu evening, continuing all night, and last through most of
Friday. The flow will be WNW to NW through this period, with
highest impacts over Mason, Lake, Oceana, Ottawa, Allegan, Van
Buren and Kalamazoo Counties.

Through this event we will see winds gusting as high as 35 mph.
This will cause blowing and drifting of the powdery snow,
especially over open areas.

&&

.SHORT TERM...(Today through Friday)
Issued at 333 AM EST Wed Dec 7 2016

Have opted to issue Winter Storm Warnings and Winter Weather
Advisories with this forecast issuance for lake effect snow combined
with blowing and drifting snow. Also factored into the decision is
the fact that it is the first significant winter weather event of
the season.

Winter Storm Warnings for Mason, Lake and Oceana Counties in the
northwest corner of the CWA and for Ottawa, Allegan, Van Buren and
Kalamazoo counties in the southwest corner. Winter Weather
Advisories for Muskegon, Newaygo, Kent, Barry and Calhoun.

The warning counties should have the worst of the conditions for the
longest period of time. Expecting a long duration lake effect snow
event with snow totals of 8-10 inches in spots in the Warning
counties and 4-6 inches in portions of the Advisory counties.

Wind will be a factor as there is solid 20-28 knot winds in the
lowest 4000ft in all of the BUFKIT overviews. Therefore, expect
blowing and drifting snow as the character will become more powdery
as we work through the event with colder air filtering in. With this
being the first event of the season, felt the combination of wind,
lake effect snow and road conditions warranted a Warning in some
counties. The snow totals will be border line, but the impacts
should be fairly significant.

The lake effect snow bands should be strong at times in a flow
regime that will be swinging from the west late tonight to the
northwest into Thursday afternoon and Thursday night. Given the
dominant banding structure at times expecting whiteout type of
visibilities especially up toward Mason and Lake counties and down
toward Allegan, Van Buren and Kalamazoo counties. The strength of
the wind should drive the bands inland toward and beyond Highway 131
at times.

The depth of the moisture increases to in excess of 10,000ft for
Thursday and Thursday night with deep vertical motion stretching
into and through the DGZ. The bands should be quite healthy during
this time frame and the 925-850mb omega fields bear this out with
dominant banding feel to them.

The lake effect will not pick up significantly until after dark
tonight, with it spreading from north to south with the increase in
deep moisture. By Thursday morning lake effect snow should be
widespread across Western Lower MI. The lake effect then continues
strong through Thursday and Thursday night into Friday morning. Kept
all of the times the same at this point for start and expiration,
but we may be able to drop some of the advisory early. For counties
like Kent there will be times where the lake effect may shift out of
most of the county with the exception being the far SW corner toward
Byron Center.

.LONG TERM...(Friday night through Tuesday)
Issued at 333 AM EST Wed Dec 7 2016

This weekend into early next week continues to look cold and snowy.
Impactful snow is possible late Saturday through Sunday.

Lake-effect snow will be undergoing a diminishing trend Friday night.
DGZ remains fairly saturated Friday night, but with moisture only up
to inversion heights around 4 kft. Upper level cyclonic flow also
exits by this time, but lake-850 mb delta Ts will remain large. Will
feature chance PoPs for WNW flow belts. Lows will fall into the
teens to lower 20s.

There is medium confidence that widespread accumulating snow will
fall across Southwest Lower Michigan later Saturday through Sunday.
Details on this synoptic event remain uncertain, but low pressure
kicks out of the Plains, and there should be a modest feed of Gulf
moisture.

After highs in the 20s on Saturday, temps should moderate some on
Sunday with highs around the freezing mark. This occurs as the low
pressure system draws up warmer air/moisture from the south, but not
enough warmth to consider any other precip type. Will feature all
snow.

Behind the weekend system, the coldest air of the season so far
pours into Michigan from early to mid next week. Highs drop back
into the 20s for Tuesday, and Wednesday looks colder. Lake-effect
snow is expected at times.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 656 AM EST Wed Dec 7 2016

A MVFR ceiling is being produced off of Lake Michigan and this
will continue to be the case the next 24 hours. Instability over
the lake is responsible for the lake clouds and no changes are
foreseen through tomorrow morning. There may be some flurries as
we head through today and into this evening, with lake effect snow
developing overnight. IFR visibilities due to snow will develop
late tonight at Muskegon and will likely hold at the other TAF
sites until just beyond the valid time of these TAFs, or after 12z
Thursday.

Gusty westerly winds of 15 to 25 knots will persist right through
the TAF period.


&&

.MARINE...
Issued at 333 AM EST Wed Dec 7 2016

Thinking we are going to remain below Gale criteria with high end
Small Craft Advisory criteria the next 48 hours or so. Extended the
SCA through 700am Friday at this point, but it will need to be
extended again most likely into Friday night. We are looking at
solid 20-30 knot winds into Friday morning which will produce large
waves of 6 to 10 feet in the nearshore waters. Piers and breakwalls
will be dangerous places the next couple days.


&&

.HYDROLOGY...
Issued at 1156 AM EST Tue Dec 6 2016

Near and above normal river levels continue to affect southwest
Lower Michigan today. The Looking Glass River at Eagle shows
notable rises, but should remain within banks. Slight rises have
also been noted on the Maple River at Maple Rapids, as expected.

Moist soils will provide opportunity for runoff with additional
precipitation this week. Lake effect snow will distribute most of
the precipitation to lake shore counties this week. The weekend
will feature more of a synoptic setup to provide inland locations
with precipitation. Overall, while high rivers may be a concern,
none of the precipitation is too heavy for any given site or
individual episode. Therefore, river rises are possible, but
flooding is of low concern at this time.

&&

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

     Winter Weather Advisory from midnight tonight to 4 PM EST Friday
     for MIZ044-050-057-065-073.

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

&&

$$

UPDATE...JK
SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...EBW
AVIATION...Duke
HYDROLOGY...JAM
MARINE...Duke



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