Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 212334
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
634 PM EST Mon Jan 21 2019

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 316 PM EST Mon Jan 21 2019

- Next system to bring a wintry mix/snow Tuesday into Wednesday

- Bitter cold air surges in behind Arctic Cold front Thursday

- Lake effect snow expected Thursday night into early next week

&&

.DISCUSSION...(This evening through next Monday)
Issued at 316 PM EST Mon Jan 21 2019

The main challenge deals with the potential impacts with the
next couple of storm systems to track through the area.

It looks like we will need WSW headlines for this next storm
Tuesday afternoon into Wednesday. To start a rather strong upper
level disturbance was tracking eastward through the Rockies. This
system shows up nicely on water vapor imagery. At the same
time...low level southerly flow was strengthening across the lower
MS Valley. This upper wave takes on a positive tilt as it enters
the Plains on Tuesday. The southerly flow will advect Gulf
moisture into this system as the upper level divergence
increases. In addition some weakness in the stability is
noted...especially up around 700 mb in the Great Lakes Region
which may allow for some of the precipitation to be moderate to
maybe heavy for a period. Thus an expanding area of precipitation
is forecasted to develop and spread into the CWA Tuesday afternoon
into the evening. It does look like this system will catch the
evening commute. Based on soundings it looks like the onset will
be snow...but a warm layer moves into southern parts of the CWA.
This should support a mixture of precipitation types during the
evening. Eventually the surface temperatures look likely to rise
above freezing for the southern half of the CWA Tuesday night
supporting a period of rain. Further north where temperatures are
forecasted to remain below freezing the impacts are expected to
continue into Wednesday. Guidance has been showing a mid level dry
slot working in from the southwest later Tuesday night. This
could lessen the duration of impacts then.

A fast moving surface wave drops down from the Canadian Prairies
on Thursday and slows down as it reaches Lake Superior.  A strong
push or arctic air moves in on the south side of this system
going into Friday. It appears the pattern will be favorable for a
drawn out accumulating lake effect/enhanced snow event given the
deep cyclonic flow and favorable low level lapse rates over the
unfrozen lake. Commonly in these setups where the arctic air wraps
in from the southwest around the south end of Lake MI...we see
really favorable low level convergence throughout Western Lower MI
that enhances the coverage of the snow. One potential limiting
factor for heavy accumulating snow will be the possibility of the
DGZ going into the ground. Impacts will continue even if this
happens right through Friday and perhaps through Friday Night.

The snow may wind down late Friday Night into Saturday but wind
chill values in some areas may support hazardous values.   Yet
another surge of arctic air is shown by the models to move in
for the second half of next weekend or early next week as a series
of storm systems track through. Snow impacts look likely and
there is some risk for moderate or greater impacts.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 635 PM EST Mon Jan 21 2019

Expect VFR conditions at all TAF sites till early afternoon
Tuesday. Then snow will develop andy spread west to east across
the TAF sites. Conditions will quickly become IFR in snow and
remain so into the evening.

We do have a band of warm advection lift clouds crossing the CWA
tonight between 03z and 09z. Some light snow is expected to fall
from those clouds but more than likely will not reach the ground
at our TAF sites. Cloud bases during this event should be around
8000 ft AGL.

Winds increase above 1000 ft AGL overnight, by 09z winds should be
out of the south around 25 knots,but by 12z or so winds will be
near 40 knots at 2000 ft so we have low level wind shear in our
TAFs during the daytime tomorrow.

&&

.MARINE...
Issued at 316 PM EST Mon Jan 21 2019

The combination of strong winds cold temperatures and waves
approaching 4 feet may support heavy freezing spray tonight into
Tuesday. Hazardous waves will continue through Tuesday night into
Wednesday...but climbing temperatures may limit the potential for
freezing spray.

A lull in the larger waves is forecasted for Wednesday night but
the arrival of another arctic airmass will support rather
hazardous boating conditions for Thursday and beyond.


&&

.HYDROLOGY...
Issued at 316 PM EST Mon Jan 21 2019

Very cold temperatures will persist through Tuesday morning, before
we see a brief warm up for the middle of the week. Freeze-up ice
jams will continue to be possible on rivers and streams tonight, and
may lead to rapid changes in water levels and localized flooding.
Currently, ice jams have been identified or highly suspected along
the Chippewa River near Mt Pleasant, the Flat River near Smyrna, and
the Looking Glass River near Eagle. No known flooding is occurring
at this time.

Warmer temperatures midweek will also arrive with our next chance
for precipitation. Light snow or a wintry mix is expected to spread
into the area Tuesday evening before changing to rain over the
southern Lower Michigan and continuing as snow over the northern
half of Lower Michigan. The exact line between rain and snow is
still a little challenging to predict, but will delineate where rain
and melted snow run into the river system and where water remains
locked in a building snowpack. The highest potential for rain will
be near and south of the I-96 corridor. Where precipitation falls as
rain the longest, we could see between 0.25 and 0.5 inches of water.
An estimated 0.2 to 0.8 inches of water equivalent lies in the
current snowpack over southwest Michigan, which could all melt over
the course of a few days where the rain falls. Runoff from rain and
melted snow may break up any ice jams that have formed, or could
possibly increase the risk for flooding if the river channel remains
blocked by ice and more water is added. Water levels will also rise
where we see rainfall, and a few of the smaller rivers and
tributaries may reach bankfull.

Temperatures will become very cold once again Thursday night through
Sunday as another blast of arctic air pushes into the region. This
will once again favor significant ice growth on rivers and bring the
threat for ice jams. We will also see several rounds of light
snow.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 1 PM Tuesday to 1 AM EST Wednesday
     for MIZ056>059-064>067-071>074.

     Winter Weather Advisory from 1 PM Tuesday to 10 AM EST Wednesday
     for MIZ037>040-043>046-050>052.

LM...Heavy Freezing Spray Warning until 1 PM EST Tuesday for
     LMZ844>849.

     Small Craft Advisory until 1 AM EST Thursday for LMZ844>849.

&&

$$

SYNOPSIS...MJS
DISCUSSION...MJS
AVIATION...WDM
HYDROLOGY...HLO
MARINE...MJS


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