Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 211755
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
155 PM EDT Sat Oct 21 2017

LATEST UPDATE...
Aviation/Hydro

.SYNOPSIS...
Issued at 312 AM EDT Sat Oct 21 2017

After a very warm Saturday with highs approaching 80 in spots, the
forecast changes dramatically Sunday with rain showers. Precipitation,
possibly heavy in some spots, is expect to last well into the coming
week. Monday night through Tuesday looks particularly wet with
increasing winds associated with a low pressure system which may
strengthen over Great Lakes Region.

Regardless of the details, there is increasing potential for
significant rain beginning Monday night and lasting well into
Tuesday. We have bumped up precipitation totals over the eastern
forecast area with this update in response to these model trends.
strengthening fall storm. Temperatures next week will be
seasonably cool with highs in the upper 40s to mid 50s.

&&

.SHORT TERM...(Today through Monday Night)
Issued at 312 AM EDT Sat Oct 21 2017

Rain is still expected to overspread the area Sunday and prospects
are increasing for significant rain beginning Monday night and
lasting well into Tuesday. We have bumped up precipitation totals
over the eastern forecast area with this update in response to
these model trends.

High confidence for widespread rain moving across the area Sunday.
Impressive moisture fetch off the Gulf feeding into this system
is supported by Friday evening soundings with precipitable water
greater than 200 percent of normal in parts of the southern and
Central Plains. Favored ECE ensemble guidance shows some, but mostly
minor, variability with timing and extent of the rain showers moving
east into the forecast area early Sunday. This precipitation
should be an extensive north to south band that is slow moving and
likely to produce heavy precipitation. This character of
precipitation can often alter low level winds / moist transport
vectors via low level PV generation associated with significant
latent heat release. This subsequently can alter the evolution and
intensity of the precipitation band. Models can`t typically
account for these diabatic PV processes well, which might explain
the position and intensity differences seen in the ensemble
members.

We expect a bit of a lull in precipitation Monday as the
preponderance of ensemble members move this band just east of the
area. However, Monday night looks quite interesting with several
ECE members showing an impressive trowal with significant rainfall
wrapping back into the forecast area. Even though the 21/00Z
deterministic ECMWF is farther east and drier for our area,
several of the 20/12Z ECE members showed a wide variety of
solutions, many of which affect our area. This is to be expected
because the predictability of this feature depends on what happens
Sunday. The 21/00Z ensemble will be interesting to see, but the
expectation is that there will still be many members showing a
trowal over our area. Regardless of the details, there is
increasing potential for significant rain Monday night into
Tuesday.

Otherwise, today looks very warm again similar to Friday so have
bumped temperatures up accordingly.

.LONG TERM...(Tuesday through Friday)
Issued at 312 AM EDT Sat Oct 21 2017

This is the time of year when the jet stream begins to take some
plunges farther south across the country and synoptic systems become
much more amplified. That`ll be the case this week as a deep upper
trough develops over the Great Lakes Tuesday. A short wave over the
Deep South will get kicked northward by the stronger trough moving
across the Upper Midwest. The resulting surface system will move
northeast to southeast Ontario Tuesday morning. We`re going to see
showers Tuesday as the trough moves across the state. Temperatures
will be much cooler with highs in the lower 50s. Lingering rain
showers Tuesday night may mix with wet snow over the far northeast
cwa as temperatures fall into the 30s. A chilly day with highs in
the 40s is expected Wednesday.

Another short wave following closely on the heels of the deep trough
will keep showers in the forecast Wednesday, after which weak
ridging moves over the state. However, another series of short waves
will move over the Northern Plains and Great Lakes helping to keep
the long wave trough in place, which means highs will be closer to
normal in the mid 50s.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 155 PM EDT Sat Oct 21 2017

Gusty south sfc winds to 25 kts this afternoon will diminish to 5
to 10 knots after dark but since winds remain relatively strong
(35 kts) just above the sfc all night will include LLWS in the
TAFs after 00Z.

Cloud bases will remain aoa 12,000 ft through tonight, with VFR
conditions probably persisting through 18Z Sunday. Showers and
MVFR conditions associated with the approaching cold front are
expected to hold off until after 18Z Sunday when they`ll come in
gradually from west to east.


&&

.MARINE...
Issued at 312 AM EDT Sat Oct 21 2017

Based on WW3 guidance, have expanded Small Craft Advisory
southward to Holland. With strong and persistent winds from the
south, waves are expected to remain at SCA levels well into
Sunday. Even more significant winds and waves will be possible
towards the middle of next week due to increasing chances for a
fall storm centered around Lake Huron.

&&

.HYDROLOGY...
Issued at 155 PM EDT Sat Oct 21 2017

Showers will spread into the area from the west Sunday and Sunday
night as a cold front begins to track through the Great Lakes. Rain
may be moderate at times in intensity, but is not expected to lead
to flooding. Total rainfall through 8AM Monday will be up to one-
half inch.

An area of low pressure will lift northward along this frontal
boundary, allowing an area of heavier rainfall to develop late
Monday through Tuesday over the Great Lakes. While confidence in
this scenario is increasing, the precise location of where the
heaviest swath of rain will occur, where amounts could exceed 2
inches, is still uncertain. Latest forecast guidance places a north
to south-orientated band of heavier rain totals anywhere from Lake
Michigan to eastern Ohio. Localized flooding and rises in rivers,
creeks, and stream could occur in this swath. The potential for
heavy rainfall across the western half of Michigan late Monday
through Tuesday will continue to be monitored.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 8 PM EDT Sunday for LMZ846>849.

&&

$$

SYNOPSIS...TJT
SHORT TERM...TJT
LONG TERM...04
AVIATION...Meade
HYDROLOGY...HLO
MARINE...TJT



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