Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 140000
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
800 PM EDT Fri Oct 13 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 326 PM EDT Fri Oct 13 2017

Rain chances will be on the increase tonight, with periods of
showers and thunderstorms then expected through early Sunday
morning. The biggest threat with this will be heavy rain that could
cause some minor flooding, and strong winds Saturday evening through
Sunday.

Some lingering showers will be possible on Sunday before ending.
Cooler weather will come in for early next week along with dry
weather. Temperatures will then warm up gradually next week to above
average levels once again.

&&

.SHORT TERM...(This evening through Sunday night)
Issued at 326 PM EDT Fri Oct 13 2017

Our main concern in the short term is dealing with all aspects of
the incoming system which will impact the area from tonight through
Sunday.

A quiet weather day is in progress, or the calm before the storm.
The changes are on their way, with the leading edge of these changes
making their way through central Wisconsin as of 19z. Initial
moisture advection is ongoing, and will ramp up a bit this evening
as the front to our W/NW drops into the state. The showers this
evening will move in across the NW first and gradually sink south
through most of the CWFA by 12z Sat. Thunder does not look possible
until very late tonight, and just across the far srn half of the
CWFA based on elevated instability parameters.

We may see a brief lull in the morning as the front settles a bit
south, and the the initial surge of moisture moves east. This will
be short lived, if at all before the next surge of moisture advects
in with a 35+ knot low level jet. Thunder will become a little more
likely as more elevated instability moves in. Parcels below 850 mb
remain relatively stable through 00z Sun. Heavy rain will be the
main threat with pwat`s increasing to as much as 1.75 inches, which
is almost as high as it gets for mid-October. Some training of
showers/storms will be likely with only small shifts in the
front/rain axis.

Saturday night/early Sunday morning will be when the most impactful
weather affects Lower Michigan. This is when the main wave lifts in
from the SW and brings a strong slug of moisture via a 60+ knot low
level jet in addition to the strong dynamics via the RRQ of a 140+
knot upper level jet to our NW. Instability overall will not be
impressive with only a few hundred j/kg of MU Cape available. It is
the wind fields that are concerning. If any storms are able to
organize an updraft, the 50 knots below 1000 ft and 60 knots around
1500 ft could be brought down in the downdraft and cause damage,
even with little to no lightning. The strong low level wind shear
also could cause kinks on any convective lines, that could produce a
spin up tornado. The time frame for this would be roughly from 9 pm
Sat night to 3 am Sun morning. The best sfc based instability does
stay to our S and SW, keeping the best threat there.

We then turn our attention to the frontal passage which should occur
shortly after the worst convection. The frontal passage will bring
strong cold air advection and pressure rises, which could help bring
down the strong winds, albeit a little less than earlier with the
low level jet. We are looking at wind gusts of over 40 mph likely
behind the front. This may eventually require a Wind Advisory for at
least the lakeshore counties, and maybe also further inland.

Conditions will gradually improve later Sunday as the best gradient
moves out. We will likely have a few instability showers linger on
Sunday, with some lake enhancement likely off of the lake with h850
temps dropping to -1C or so, giving delta t/s over the lake of mid
to upper teens. Moisture will not be too shabby for lake effect
purposes, since it looks to be up to around 7k feet or so.

.LONG TERM...(Monday through Friday)
Issued at 326 PM EDT Fri Oct 13 2017

Lake effect showers will diminish Monday morning as inversion
heights lower following the departure of the upper trough. Monday
will be the coldest day of the week but clouds will scatter out.
Warmer air above the surface will already be flowing back over the
area Monday night. A strengthening pressure gradient between a low
to our north and high to our south is expected to maintain southwest
winds Monday night and prevent frost.

In the third week of October, climatological normal temperatures are
considered to be highs near 60 and lows in the lower 40s. We will be
5 to 10 degrees warmer than that from Tuesday through the rest of
the week. A stubborn anticyclone will develop over the eastern US
and Michigan will be under nearly continuous southwest flow. An
upper level wave and Hudson Bay low will pass by well to the north
of the Great Lakes on Wednesday, and precipitation chances here look
quite low. Overall, this looks like a dry and milder-than-normal
week with abundant sunshine.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 800 PM EDT Fri Oct 13 2017

A frontal boundary will become stalled across the area tonight
and remain in place on Saturday. The most likely position where it
stalls will be roughly from AZO to LAN.

MVFR to IFR conditions with numerous showers are expected north
of boundary and this includes MKG and GRR. Meanwhile predominately
VFR conditions should occur south of the boundary with a bit less
in the way of shower activity and this includes JXN.

The terminals close the boundary such as AZO/BTL/LAN will
probably waver back and forth between VFR and MVFR/IFR, with the
lower conditions more likely when heavier showers are moving
through.

Thunder possibilities will increase on Saturday afternoon, as will
the potential for torrential  downpours that could drop vsbys to
LIFR at times.

&&

.MARINE...
Issued at 326 PM EDT Fri Oct 13 2017

We are issuing a Gale Watch from 00z Sun through 00z Mon for the
entire nearshore waters with this package. This Gale watch will
encompass strong winds immediately ahead of the front, likely Gale
to potential Storm force winds along the front/with convection, and
the strong cold air advection coming in behind the front. Winds and
waves will then come down Sunday night as the gradient relaxes.


&&

.HYDROLOGY...
Issued at 326 PM EDT Fri Oct 13 2017

We continue to expect a large area of 1 to 2 inch rainfall across
much of the forecast area with the most intense rainfall occurring
Saturday evening. There is potential for significantly greater
amounts in excess of 3 inches in spots, which could produce moderate
localized flooding. There is some disagreement about exactly where
the heaviest rain will fall.

Some of the larger scale forecast models place a broad swath of 2+
inch accumulation totals from Muskegon to Mount Pleasant, which
would mostly affect the Muskegon and Chippewa river basins. Some of
the finer scale convection allowing models place the greatest
precipitation farther south towards the I-94 corridor.

Ensemble river forecasts suggest that even with the greater rainfall
amounts, there remains a limited chance for achieving flood stage in
mainstem rivers in the affected basins. River advisories seem much
more probable.

The overall environment highly favors training or back building
convection which could produce rainfall rates of 1-2 inches per hour
and totals well over 3 inches in spots. Smaller streams, basins, and
urbanized areas could see significant flooding if overlapped by
these local areas of enhanced rainfall. Another factor that may
exacerbate nuisance flooding is the falling leaves that might clog
smaller drainage systems.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Gale Watch from Saturday evening through Sunday evening for
     LMZ844>849.

&&

$$

SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...CAS
AVIATION...Meade
HYDROLOGY...TJT
MARINE...NJJ


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