Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 171754
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1254 PM EST Sun Dec 17 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 316 AM EST Sun Dec 17 2017

Milder air will spread north into the area. Light rain will be
possible near I-94 tonight into early Monday, but temperatures
should be warm enough to keep precipitation all liquid. Generally
dry conditions are expected late Monday through Wednesday with
highs around 40 both Monday and Tuesday. Unsettled weather returns
towards the end of the week with cold temperatures making a
return for the weekend right before Christmas.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 316 AM EST Sun Dec 17 2017

No impact weather expected. Cyclone currently over the Southern
Plains will lift northeast towards the Great Lakes as it weakens,
brushing our southern forecast area with a chance of rain. As
noted previously, there is some question about the depth of
available moisture with this system. GOES-16 W/V channels
currently indicate a deep dry layer over TX in the lee of the
system. This could give us predominantly drizzle if it overspreads
our area. However, that is an uncertain outcome based on the
variety of simulated satellite NWP solutions from the NCAR
Ensemble and the NAM nest. Even without dry slotting,
precipitation should be light and not impactful given warm
temperatures.

Otherwise dry and milder weather expected from late Monday through
Tuesday.

.LONG TERM...(Tuesday night through Saturday)
Issued at 316 AM EST Sun Dec 17 2017

A high pressure ridge will build in and bring fair wx with
seasonable temperatures Tuesday night through Wednesday. The
weather pattern will become active once again late in the week as
one or a series of low pressure systems moves east from the Plains
states.

Also late in the week an upper level trough will begin to dig and
amplify over the northern plains and upper midwest. This in
conjunction with a southern stream system will set up potential for
a stronger low to potentially develop and move northeast through the
Ohio Valley region Friday.

This will result in potential for a wintry mix of precipitation
over our area Thursday and potential for accumulating snow Friday
depending on the exact track and strength of the main low pressure
system. It is still far too early to get into too many specifics
that far out in time and given considerable medium range guidance
spread/uncertainty in the evolution of pattern.

A much colder airmass will advect in on the back side of the system
for next weekend. Longer range medium range guidance continues to
show potential for an even colder arctic airmass to move in by
Christmas Day. H8 temps by then could be as cold as -22 to -24 C as
still suggested by the 00Z ecmwf.

High temps by next week may only reach the teens. Confidence is high
in much below normal temps for our area from Christmas Day and for
at least several days afterwards. This notion is also supported by
the latest 8-14 day cpc outlooks.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1247 PM EST Sun Dec 17 2017

The main challenge deal with the IFR and lower conditions and FZDZ
risk.

Widespread IFR was located over the TAF sites to start the period.
Conditions improve somewhat to MVFR near KLWA and KBEH.
Indications are that this axis of better conditions will track
slowly northeast and could make it into most of the TAF sites.
This is not certain to happen but enough confidence is there to
support including this trend in the TAFS for this afternoon.

This evening an area of light precipitation will be diminishing as
it move in from the southwest. Its a close call for freezing
drizzle. KGRR looks to be the coolest site...so I did include a
period of FZDZ. Will need to monitor trends through the period
closely. Still IFR and lower conditions should redevelop as low
level moisture increases...and there is snowpack on the ground.

&&

.MARINE...
Issued at 316 AM EST Sun Dec 17 2017

No concerns today with relatively light offshore winds. Winds flip
to southwesterly by Monday morning and then increase during the day,
bringing SCA waves by late Monday that last through Tuesday.


&&

.HYDROLOGY...
Issued at 311 PM EST Sat Dec 16 2017

Many rivers in Central and West Michigan have iced over due to an
extended period of freezing temperatures. A suspected ice jam on the
Grand River has caused the river level to rise above bankfull near
Eagle. While fluctuations in the river level should be expected over
the next few days, the general trend will be for the level to fall
through the weekend. Dry weather is expected for the remainder of
today. The next chance for rain and snow showers will be Sunday
afternoon and Sunday night, but liquid amounts should be very light
and less than 0.10 inches.

Warmer temperatures with daytime highs above freezing are expected
through the early and middle parts of next week. Water amounts
contained within the frozen snowpack range from roughly 0.5 to just
over 1 inch across the area, with highest amounts along and west of
US 131. The warmer temperatures are not expected to cause a full
melting of the snowpack this week, but we could see some of the
water released as runoff. This will be in addition to occasional
light rain and snow showers at times through the first half of the
week.

&&

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

$$

SYNOPSIS...TJT
SHORT TERM...TJT
LONG TERM...Laurens
AVIATION...MJS
HYDROLOGY...HLO
MARINE...TJT


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