Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 172347
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
647 PM EST Fri Nov 17 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 317 PM EST Fri Nov 17 2017

Rain will move into the area beginning this evening. Isolated
thunderstorms are possible near and south of I-94 Saturday
morning. Otherwise, Saturday will be cool with highs in the 40s.

The heavier precipitation exits to the east early Saturday night.
This will be followed by colder air will spreading into the area.
Some lingering light rain showers will change to flurries towards
Sunday morning. Flurries are possible near and west of US-131 on
Sunday with highs only in the 30s. Afterwards, the coming week
looks mostly dry with highs generally in the 40s.

&&

.UPDATE...
Issued at 602 PM EST Fri Nov 17 2017

We have added a mention of thunder for this evening over Lake MI
and additional areas south of I-96. This is based on thunderstorm
development over north central IL in a region of 500+ J/kg MUCAPE,
per RAP model estimates. This plume of instability and attendant
potential for thunder is expected to overspread far southwestern
Lower MI later this evening before peeling off to the southeast
Saturday morning.

&&

.SHORT TERM...(This evening through Sunday night)
Issued at 317 PM EST Fri Nov 17 2017

Little change to the previous forecast regarding precipitation
tonight into Saturday. The most likely scenario is that thunderstorms
will remain south of the area Saturday based on the SREF and
several convection allowing models. However, for the sake of
continuity, have kept a slight chance for the I-94 corridor,
especially since elevated instability isn`t always handled well by
the models. Precipitation totals are expected to range from
around 0.75 inches northwest to 1.5 inches southeast near Jackson.

A more uncertain but potentially impactful aspect of the forecast is
wind Saturday afternoon into Sunday morning. It is possible that
advisory headlines will be required with the strong post-frontal
cold advection, but this is a bit too uncertain to nail down now
with an advisory.

Finally, we expect lake effect rain showers to change over snow
Saturday night into Sunday, but any snow accumulations should be
curtailed be limited moisture depth into the DGZ. Some snow
showers/flurries will continue through Sunday.

.LONG TERM...(Monday through Friday)
Issued at 317 PM EST Fri Nov 17 2017

No big storms in the extended but a clipper and trailing cold front
could bring a quick shot of light snow on Tuesday afternoon or
evening followed by some lake effect snow showers into Wednesday.

Added some low chance POPs Tuesday through Wednesday and trended
temperatures a bit colder during this period. A clipper tracks north
of the Lakes Tuesday with enough low level convergence along the
trailing cold front to bring a brief period of snow or mixed rain
and snow late Tuesday afternoon into early Wednesday evening.

Strong cold advection follows behind the front with 850 mb temps
falling to minus 12C Wednesday night. Model soundings show inversion
heights around 7.5 kft AGL through Wednesday with northwest flow.
ECMWF is already showing warm advection by Wednesday afternoon which
would limit lake effect snow showers. Went with low chance POPS
across the favored NW flow lake effect areas Tuesday night and
Wednesday but it appears the highers chances will be Tuesday
night.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 641 PM EST Fri Nov 17 2017

An area of showers and thunderstorms across portions of WI to IL
will continue to track east through the evening. Thus a potential
is there for some thunderstorms at the TAF sites. I added this to
the forecast for all sites. By 06z the axis of instability pushes
east of KLAN so less of a risk by then.

Widespread IFR and lower conditions will develop as we go through
the night. These low conditions will persist likely through
Saturday.

Colder air will filter in from the north Saturday afternoon and
evening. There are some indications that the airmass will become
cold enough for snow to mix by 00z. Given that there is some
uncertainty...I only featured a r/s mix. Will need to monitor
trends as a sooner transition to snow could lead to a period of
heavy wet snow.

Gusts over 25 knots from the north look likely to develop for
Saturday afternoon and evening.

&&

.MARINE...
Issued at 317 PM EST Fri Nov 17 2017

Will keep the small craft advisory intact with a transition to a
gale warning at 10 AM EST Saturday and lasting through Saturday
night. This timing is to align with the gale warning issued for the
open waters, although it is more likely that gale force gusts over
the nearshore waters will commence in the early afternoon. Gale
conditions across the northern marine zones could end as early as
Saturday evening.


&&

.HYDROLOGY...
Issued at 1042 AM EST Fri Nov 17 2017

Better confidence in precipitation forecasts has resulted in
additional river advisories this morning. Widespread rainfall will
affect Lower Michigan tonight into Saturday with precipitation
totals of 0.75+ inches by early Sunday morning. Amounts will
increase from north to south with the greatest totals expected
south of I-96.

River rises are forecast through the weekend and should stabilize
into next week as precipitation vacates the area. Though current
forecasts anticipate drier conditions, rivers will remain elevated
and responsive. Any changes to include additional precipitation
may need to be closely monitored.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 10 AM EST Saturday for LMZ844>849.

     Gale Warning from 10 AM Saturday to 7 AM EST Sunday for
     LMZ844>849.

&&

$$

UPDATE...TJT
SYNOPSIS...TJT
SHORT TERM...TJT
LONG TERM...Ostuno
AVIATION...MJS
HYDROLOGY...JAM
MARINE...TJT



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