Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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360
FXUS63 KGRR 132349
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
649 PM EST Wed Dec 13 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 329 PM EST Wed Dec 13 2017

Low pressure tracking near the southern Michigan border will
bring heavy snow to parts of southern and central lower Michigan
this evening before ending overnight. Additional lake effect snows
will occur on and off through the remainder of the week as cold
air continues to pour south into the Great lakes Region.
Temperatures will moderate above freezing however for Sunday and
Monday.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 329 PM EST Wed Dec 13 2017

Still not planning any changes to the current winter headlines.

Latest trends in satellite and radar show pivot point has taken
place around GRR. This is where former warm advection snow which
halted northward progress earlier has been curling back southward
and being pulled in with the incoming upper level system while
intensifying.

The remainder of this event will now feature heavy bursts of mid
level deformation zone snows which will impact the I-96 corridor
and areas just north/south of there. The current warning depicts
this area quite well and this system will be peaking coincident
with the evening rush, so no adjustments needed. Also there have
been some upstream snow totals of 6 inches in northern Wisconsin
which supports keeping the warning going.

Inch per hour snows will be possible within heaviest bands
through about 7 pm while deep isothermal layer and omega are
present, then quick diminishing/ending trend expected after that.
Most of the snow should be done by 10 pm. Lighter snows, but
still impactful, are expected north and south of the axis of
heaviest snows (storms totals of 5-8 inches) within the warning
area.

For later tonight and early Thursday, the low level flow is NNE,
which means the only risk for any lake effect snow is around
Ludington. However as the flow goes westerly by Thursday
afternoon, they should starting penetrating farther east. little
or no accums expected from these, but potential for some
localized decent accums and impacts arrive late Thursday night
into Friday as next shortwave dives sewd through the region.

.LONG TERM...(Saturday through Wednesday)
Issued at 329 PM EST Wed Dec 13 2017

Fast flow regime continues through the extended with the usual model
differences in timing and amplitude of embedded shortwave features.

Some light snow is possible Saturday in area of isentropic ascent.
This is out ahead of a Plains low that makes a run at Lower Michigan
on Sunday into Monday. Forecast confidence decreases here as low
track is uncertain and whether Gulf moisture transport reaches here,
along with precip type if it does. Did not deviate from blended POPs
for Sunday and Monday as the GFS has trended closer to the more
consistent ECMWF, which continues to have higher chances across the
southern half of the forecast area.

The ECMWF had the low moving east on Monday with colder temperatures
arriving Monday night. The GFS was slower. and preference is for the
ECMWF here...so Superblend temperatures were lowered and POPS for
lake effect snow increased for Tuesday. Lake effect should be ending
on Wednesday as warm advection begins.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 632 PM EST Wed Dec 13 2017

The axis of impactful snow at all sites will shift southeast with
time...pushing through the KJXN site by 04z. Thus conditions are
forecasted to improve from NW to SE through the evening and it
appears that VFR weather should prevail after midnight.

A drier airmass dominates through Thu...but we will need to
monitor the snow band that will be to the west of KMKG through the
day. Local MVFR clouds could also develop during the day.

&&

.MARINE...
Issued at 329 PM EST Wed Dec 13 2017

Will extend the small craft advisory into Thursday morning for
now, but further extensions will probably be needed.


&&

.HYDROLOGY...
Issued at 329 PM EST Wed Dec 13 2017

Snow and cold temperatures the next few days will promote ice
development on rivers. Some river gauges will likely become affected
by ice and produce spurious readings. Freeze up ice jams are
possible and could produce minor upstream rises, but flooding is not
anticipated. Above-freezing temperatures are expected on Sunday.
Snowpack water equivalents across much of the area range between a
quarter to half inch, but a full snowmelt is not expected any time
soon.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 11 PM EST this evening for MIZ039-
     040-056-064-065-071>074.

     Winter Storm Warning until 11 PM EST this evening for MIZ037-038-
     043>046-050>052-057>059-066-067.

LM...Small Craft Advisory until 11 AM EST Thursday for LMZ844>849.

&&

$$

SYNOPSIS...Meade
SHORT TERM...Meade
LONG TERM...Ostuno
AVIATION...MJS
HYDROLOGY...CAS
MARINE...Meade



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