Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 220603
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
103 AM EST Wed Nov 22 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 300 PM EST Tue Nov 21 2017

Light lake effect snow showers and flurries will develop tonight
but with very little to no snow accumulation. A weakening clipper
system may also bring a few light snow showers and flurries mainly
north of Muskegon during the early morning hours of Thanksgiving
Day.

&&

.UPDATE...
Issued at 945 PM EST Tue Nov 21 2017

Really for the most part our on-going forecast is working out
nicely. We had and still have a 50% chance of snow showers along
the lake shore and we are seeing very light snow showers on the
radar and here at GRR we are getting flurries (10SM -SN in GRR
obs). Also the latest run and numerous previous runs of the HRRR
and for that matter the NAM3km and the RAP model, not to mention
the HiRes ARW and HiRes NMM all show light snow showers mostly
west of US-131 and really near US-31 most of the overnight. There
is decent lift seen near the lake shore in those models and while
the -12c does not get reached the -10/-11 does with RH values over
90%. Since we are already seeing snow, that is clearly close
enough to believe our on-going forecast is on track.

The one thing I did change is I added more cloud cover to
Wednesday forecast and that is based on the 1000mb to 900mb RH,
which is mostly above 90% to US-131 nearly all day. Typically that
sort of RH means nearly overcast. That being so I lowered the
highs about 2 degrees due to more cloud cover.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 300 PM EST Tue Nov 21 2017

A relatively tranquil wx pattern will continue through the short
term fcst period. A few light northwest flow lake effect snow
showers and flurries will develop tonight as h8 temps fall to -11
to -13 C yielding delta t/s in the upper teens.

However lake effect snow shower potential is mitigated by several
factors including lack of deeper moisture and low inversion heights
of only around 4-5 kft agl. Areas mainly near to west of US-131 will
have very light snow accumulations of under an inch. Very little to
no snow accumulation is fcst over our eastern fcst area.

Flurries or very light snow showers will linger near to west of US-
131 into early Wednesday morning before ending. Dry wx is forecast
Wednesday afternoon through Wednesday evening.

A weakening clipper system will bring a few light snow showers
and flurries to our northwest fcst area (mainly north of KMKG)
mainly between 06Z and 12Z Thursday with only a few tenths of an
inch of snow accumulation up there. Other than that tranquil and
dry wx is forecast for the Thanksgiving holiday and for Thursday
night.

.LONG TERM...(Friday through Tuesday)
Issued at 300 PM EST Tue Nov 21 2017

No major systems are expected in the long term but the pattern of a
cold front every few days will continue. No significant cold air
outbreaks will occur since the polar jet stays north most of the
time which also means minimal lake effect precipitation.

Out next cold front is progged to come through on Friday night and
out ahead of it guidance brings in H8 temps around 10C on Friday.
That should lead to high temps at or above 50 although breezy
conditions will make it feel colder.

Model consensus is for high pops for light rain showers with fropa
on Friday night, then a chance of mixed rain and snow showers in the
colder air behind the front Saturday. Little or snow accumulations.

Sunday and Monday look dry with moderating temperatures as sfc high
pressure moves from the Plains to the mid Atlantic. Another cold
front approaches on Tuesday although timing is low confidence at
this time. ECMWF suggests highs in the 50s in the warm advection
ahead of this next front.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 103 AM EST Wed Nov 22 2017

Plenty of lake effect cloud cover shrouds the area this morning,
with some flurries floating around. These flurries have not
impacted any ob sites, with vsbys only dropping to 8-9SM under the
flurries. We expect the cloud cover to persist much of the day at
the lakeshore. Skies will likely partially clear inland prior to
daybreak as drier air is moving in, and the flow is veering to the
NW.

Some diurnal cu is possible where the lake clouds are lacking.
Cigs should remain mainly VFR. We can not rule out some vsbys
dropping to MVFR later this afternoon as inversion heights lower a
bit. Mid/high clouds will move in toward evening ahead of the next
wave.

&&

.MARINE...
Issued at 300 PM EST Tue Nov 21 2017

A small craft advisory remains in effect through tonight into
early Wednesday morning. Wave models continue to suggest wave heights
will finally subside to below sca criteria after 13Z Wed. However
wave heights will ramp back up on Thanksgiving day when another
small craft advisory will likely be needed.


&&

.HYDROLOGY...
Issued at 100 PM EST Tue Nov 21 2017

Several rivers remain at advisory/action stage level as runoff
continues from rain that fell over the weekend. Water levels in
these and other rivers will remain fairly steady or slowly fall
through the end of the week. Rain and snow showers today and tonight
will only provide light amounts of precipitation, and will not have
an impact on water levels in rivers, creeks and streams. Additional
flooding is not expected.

A few more rounds of light rain/snow are expected through the end of
the week, but should not have a significant impacts on river levels.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 7 AM EST this morning for LMZ844>849.

&&

$$

UPDATE...WDM
SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...Meade
AVIATION...NJJ
HYDROLOGY...HLO
MARINE...Laurens


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