Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 181701
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1201 PM EST Sat Nov 18 2017

LATEST UPDATE...
Hydro

.SYNOPSIS...
Issued at 330 AM EST Sat Nov 18 2017

A deepening fall storm system will move through the Southern Great
Lakes today bringing another round of rain. Brisk conditions will
develop this afternoon and evening as cold air drives in on north
winds. Wind gusts will exceed 30 mph inland and 40 mph along the
Lake Michigan shore. Snow showers may mix in as early as this
afternoon, and fully change over tonight and Sunday morning. A
dusting to an inch or so of snow is possible, especially towards
Lake Michigan. High pressure moving through the area will bring a
dry period from Sunday evening through Monday night. It will turn
colder tonight, Sunday and Sunday night as cold air brought in on
the north winds swings through the region.

&&

.UPDATE...
Issued at 1008 AM EST Sat Nov 18 2017

Minor updates made to current forecast. Radar shows large area of
showers moving into West Michigan...with the strongest returns along
and south of Interstate 94. Fog indicated over the majority of the
area...with visibilities over one half mile at most locations.
Surface observations indicate snow on the trailing edge of the
precipitation area over Wisconsin...which will need to be watched
this afternoon.

Small craft advisory has been allowed to expire...with gale
warning in effect until Sunday morning. Dangerous conditions
expected to develop along the Lake Michigan shoreline this
afternoon.

&&

.SHORT TERM...(Today through Monday)
Issued at 330 AM EST Sat Nov 18 2017

The fall storm system moving through today and tonight is the main
focus of the short term, namely thunderstorm chances today,
increasing winds this afternoon and evening, snow chances both
synoptic and lake effect and colder air settling in.

As for thunderstorm chances, the best instability looks to remain in
our far south or even south of the Grand Rapids forecast area.
MUCAPE values are quite minimal in our south today around 200 j/kg.
That said it does not take much with a dynamic system to get a few
rumbles of thunder. Convection allowing models are showing some
cellular looking returns and have opted to keep slight chance
wording in our southern two rows of counties for thunderstorms.

Not thinking we will reach wind advisory criteria based on model
guidance. If we flirt with advisory criteria though it will be along
the lakeshore this afternoon and evening from Holland south down the
lakeshore. Most of the area should see some advisory criteria winds
with gusts less than 45 mph.

As for snow, models have been hinting at a bit of synoptic snow
mixing in before the precipitation exits to the east this afternoon
and evening. Some hi res guidance is suggesting and inch or so
inland as web bulb zero heights lower to below 1500ft. Did not
fully include this in the forecast but it will need to be watched.

Tonight lake effect snow should get going in cold air advection.
Expecting some decent bands of LES between 06z tonight and 18z on
Sunday. The heaviest will likely be between 09z and 15z up near the
points and from Holland to South Haven in northwest flow. Could see
an inch or two of accumulation in the heaviest bands but this will
be very localized.

Otherwise quiet weather the remainder of the period from Sunday
night into Monday.

.LONG TERM...(Monday night through Friday)
Issued at 330 AM EST Sat Nov 18 2017

Similarities and differences exist between the gfs and ecmwf during
the long term. Both show troughing developing Tuesday and Tuesday
night, but the ecmwf is much more amplified with the upper trough
Tuesday night over the state than the gfs is. This short wave is
pushing a clipper across Ontario and because the low is so far
north, we`re likely to not see much of anything with the system.
Dynamics with the upper wave may produce isolated rain or snow
showers but that`s about it. Boundary layer moisture is only 50-60
percent. Even though h8 temps fall to around -12c Tuesday night, the
lack of decent moisture will limit any snow showers to the far
northwest lake shore. A large area of high pressure is progd to
build south into the Ohio Valley toward the end of the week bringing
dry weather to the state.

Highs in the mid 40s Tuesday will fall to the mid 30s Wednesday and
Thursday after the cold front moves through. Temperatures will
remain below normal through Friday.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 754 AM EST Sat Nov 18 2017

Poor aviation conditions are expected today with IFR and LIFR
conditions expected much of the day. Showers are expected into mid
afternoon at least with a thunderstorm possible along I-94. Have
VCTS wording in the I-94 TAF sites mid morning through midday.
An improving trend should be noted tonight as ceilings lift to at
least MVFR. Lake effect snow showers will commence after about 03z
this evening. A strong north wind is expected to develop today in
the 14-30 knot range at most of the TAF sites.


&&

.MARINE...
Issued at 330 AM EST Sat Nov 18 2017

Maintained the current headlines, Small Craft Advisory in effect
until 10am this morning at which time it will be replaced by a Gale
Warning. The Gale stretching into and through tonight. Gale
conditions will likely lag the start time by a few hours, but it was
done to match up with the open lake headline. Looking at 40 knot
north gales that will back to the northwest and drop off into the 20-
30 knot range late tonight. Waves will peak this evening south of
South Haven around 12 feet or so. Some moderate beach erosion is
possible south of South Haven as large breakers sweep up on the
beach. The angle of the waves in our marine zones will be along
shore and which will lessen impacts slightly. Piers will be
overtopped by waves this afternoon and tonight and will be dangerous
places to be.


&&

.HYDROLOGY...
Issued at 1200 PM EST Sat Nov 18 2017

Observed rainfall amounts are on pace to be in fairly good
agreement with forecasts. As a result, several rivers are
expected to rise over the weekend. Most sites will remain within
banks, while those exceeding banks are within the Grand and
Kalamazoo basins. This is where most of the rain has fallen and is
also where the second wave of precipitation is expected to be
greatest. The only site projected to get close to flood stage is
Holt on the Sycamore Creek.

The positive news is that while minor flooding can be expected
immediately along streams and rivers, any larger impacts should be
mitigated by quieter conditions in the upcoming week.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Gale Warning until 7 AM EST Sunday for LMZ844>849.

&&

$$

UPDATE...MWS
SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...04
AVIATION...Duke
HYDROLOGY...JAM
MARINE...Duke



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