Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 041209
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
709 AM EST Sun Dec 4 2016

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 330 AM EST Sun Dec 4 2016

Light snow will spread across the area during the midday hours
peaking in intensity this afternoon and evening. By about midnight 1
to 3 inches of wet snow will have fallen, mainly on grassy areas.
Temperatures today in the middle 30s will keep the snow on the
slushy wet side. Quiet weather will return for a day on Monday
before rain and snow return to the forecast for Tuesday. A bigger
change in the weather will come mid week when colder air pours in on
northwest winds, which will get the lake effect snow machine going.
Accumulating snow is likely Thursday and Friday of this upcoming
week in the lakeshore counties especially.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 330 AM EST Sun Dec 4 2016

Latest model runs remain consistent that we should see a burst of
light to occasionally moderate snow that will move through Southwest
Lower Michigan this afternoon and evening. Still expecting a 1 to 3
inch snowfall across the area between about noon and midnight.

Moisture becomes deep, to around 20,000ft, for about an 8 hour
period with deep lift as well. Weak to moderate lift extends all the
way into a fairly shallow DGZ up around 15000ft. The high based
shallow DGZ will aid in keeping snowfall accumulations in check. The
500mb shortwave associated with today`s snow is still fairly potent
looking both in water vapor imagery at the present time and in model
data. The shortwave goes negative tilt as it swings through the area
before it shears out this evening to the east. Bottom line a burst
of snow is expected with the heaviest precipitation occurring this
afternoon and evening.

Model soundings and wet bulb zero heights (mainly below 1500ft)
still support a mainly snow event. Could see a bit of light rain
mixing in towards BTL and JXN, but kept all areas snow at this
point.

In terms of impacts and headlines, think two mitigating factors are
in play. The first will be marginal air temperatures that even
tonight will likely remain near or above freezing. Second, ground
temps remain around 40 to the lower 40s. So, pavement will likely be
wet to at worst slushy in the heavier snow bursts that move through.
An inch or two expected east and 2-3 inches to the north and west.
The NAM is indicating 4 inches possible to the northwest of GRR, but
this seems a bit high given the mitigating factors above. No
headlines planned for today`s event.

Otherwise, there may be a bit of drizzle in the wake of the snow
tonight, but it should be light and ending before we approach
freezing.

Quiet weather Monday and Monday night before our next precipitation
event on Tuesday. Tuesday`s event will slide through the southeast
CWA with mainly rain. The precipitation may start off as some wet
snow Tuesday morning before changing over to rain.

.LONG TERM...(Tuesday night through Saturday)
Issued at 330 AM EST Sun Dec 4 2016

A significantly colder airmass will move in mid to late week along
with potential for accumulating lake effect snow. Dry weather will
briefly return Tuesday night into Wednesday as the Ohio Valley low
pressure system continues to move northeast and away from our region
Tuesday night. A weak high pressure ridge will then build in to
bring continued dry wx with seasonably cool temps Wednesday.

A low pressure system will get organized in the Arklatex region
Wednesday night and move northeast into the Ohio Valley and mid
Atlantic regions. The most significant synoptic pcpn with this
system will likely stay se of our fcst area. However the
northwesternmost periphery of the pcpn associated with this system
will likely affect our area as light snow Wednesday night into
Thursday.

In addition we expect lake effect snow showers to begin to ramp up
late Wednesday night as h8 temps fall to around -12 to -14 C by 12Z
Thursday. The lake effect snow showers will then continue through
Thursday and Friday into Saturday with deep low level moisture in
place and strong lake induced instability with delta t/s reaching at
least the upper teens to lower 20`s.

One potential limiting factor to greater les accumulation potential
will be fairly low inversion heights generally in the 5-7 kft range,
but some 00z guidance model fcst soundings do suggest that inversion
height may briefly reach 9-10 kft late in the week which would
support heavier lake effect potential mainly west of US-131.

Some accumulations and travel impacts are likely in this time frame
particularly in our favored wnw flow snow belt regions especially
near to west of US-131. High temperatures Thursday through Saturday
will only reach the middle 20`s to lower 30`s with mins in the teens
to lower 20`s by then. In addition there is potential for a synoptic
low pressure system to move east from the Plains states and bring
our area more snow late Saturday through Sunday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 709 AM EST Sun Dec 4 2016

12z conditions are a mix of MVFR and VFR ceilings across the area
with no precipitation. Light snow was pushing our way from
Illinois and Wisconsin and should overspread the area in the
midday and early afternoon hours. Deteriorating conditions are
expected this afternoon and evening with IFR conditions developing
as the precipitation intensity picks up. Ceilings will likely dip
below 700ft with visibilities dropping around 3/4 of a mile.

A fairly quick improvement to MVFR should be noted tonight as the
precipitation lets up from west to east between 04z and 07z.

&&

.MARINE...
Issued at 330 AM EST Sun Dec 4 2016

Main marine concern is a spike in westerly winds tonight behind the
passage of a surface trough. Winds for a brief time will rise to
near 20 knots but the time frame is only about 3 hours. So, thinking
we will build waves into the 2 to 4 foot range but they will subside
again towards morning. Thinking right now is we will approach Small
Craft Advisory criteria but remain just below it. Winds and waves
otherwise both Today and again Monday and Monday night will remain
below 20 knots and 4 feet. No headlines planned.


&&

.HYDROLOGY...
Issued at 150 PM EST Sat Dec 3 2016

The Maple Rapids remains about a half foot below bankfull stage.
One to three inches of snow is expected on Sunday, containing
about 0.2 inches of meltwater equivalent. Much of this snow should
melt on Monday and Tuesday, which may push the Maple River above
bankfull. Additional chances for precipitation will arrive the
latter part of next week, most likely in the form of lake effect
snow. This moisture should have little impact on rivers in the
short term.

&&

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

$$

SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...Laurens
AVIATION...Duke
HYDROLOGY...CAS
MARINE...Duke



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