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031
FXUS63 KGRR 221734
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
134 PM EDT Sun Oct 22 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 316 AM EDT Sun Oct 22 2017

Rain will move into Lower Michigan today with a slowly approaching
cold front. Potentially heavy rainfall is expected Monday afternoon
and Monday night before windy and cool weather arrives on Tuesday
and Wednesday. Temperatures through the week will decline steadily
from highs around 60 Monday to highs in the 50s this coming
weekend.

&&

.SHORT TERM...(Today through Tuesday)
Issued at 316 AM EDT Sun Oct 22 2017

Will issue a flood watch for Monday afternoon and Monday night
for the eastern two thirds of our forecast area. This is based on
very strong run to run consistency with the ECMWF/ECE and with the
Global GEM for deepening a significant low over Lake Huron and
providing a strong trowal/deformation area of heavy rain wrapping
back into the area. The upper pattern preceding this development is
a minor trough/PV maximum approaching the area. A larger and
broader trough/PV max farther upstream is digging towards the
minor trough. This often results in a quick and significant
transfer of energy from the upstream broad trough to the
downstream minor trough prior to phasing, which can be very
favorable for rapid cyclogenesis. ECE members show a much tighter
clustering of forecast low positions over Lake Huron which
reinforces the idea of a trowal in our eastern forecast area.

Juxtaposed with the prospects for rapid cyclogenesis is an
atmosphere that is anomalously moist. Upstream observed soundings
continue to show precipitable water values around 200 percent of
normal for this time of year.

Our initial forecast guidance typically is based heavily on the GFS,
which has itself been very consistent with placing precipitation
farther east. Consequently, we have made upward adjustments to
inherited PoP and QPF from Monday afternoon through Tuesday morning
to better agree with the watch. Even so, local precipitation amounts
of 5 inches or more are quite possible east of US-131 by Tuesday
morning.

.LONG TERM...(Tuesday night through Saturday)
Issued at 316 AM EDT Sun Oct 22 2017

The worst of the potential wet and windy period in the short term
should be over with by the beginning of the long term on Tue night.
Even though the more extreme weather will be moving out, we look to
remain rather damp into Wed before we see a temporary dry period.
This occurs as the long wave trough over the area will be reinforced
by one more wave coming in on the backside of the trough. Lake
enhancement will be a factor with H850 temps expected to be below
0C.

The dry period is then expected to be centered around the Thu-Fri
time frame. The last short wave coming in on the backside of the
trough Wed will sweep the long wave trough out of the region. An
upper ridge will traverse the area for Thu and Fri, allowing for
drier and somewhat milder weather.

The weather is expected to become somewhat wet once again next
weekend. The upper ridge will be pushed to the east, and a fairly
strong long wave trough looks to dig over the Rockies. This will
induce a low level srly flow ahead of the developing sfc low, along
with more moisture coming in from the Gulf. Temperatures should stay
somewhat mild next weekend, until the front pushes through. At that
time, another cold period will be likely after next weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 131 PM EDT Sun Oct 22 2017

Aviation conditions will deteriorate as we head toward this
evening. A large area of rain will move across lower Michigan as
low pressure moves north from the southern U.S. Cigs will fall to
IFR overnight as the rain moves in. A strong south flow aloft will
create wind shear at TAF sites that have weaker surface winds.
It`s also possible that there could be an embedded thunderstorm,
but chances were low enough that we left it out of the TAFs.

&&

.MARINE...
Issued at 316 AM EDT Sun Oct 22 2017

Latest WW3 guidance suggest waves will decrease to below Small Craft
Advisory levels this afternoon in the Holland-Grand Haven-Whitehall
region and then early tonight north of Whitehall. The SCA is set to
expire areawide at 8 PM tonight which is within a few hours on
either side of the forecast. Have no plans to get any more precise
than this.

Monday night and particularly Tuesday could see significant winds
and waves with northwest gales likely developing.


&&

.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...Flood Watch from Monday afternoon through Tuesday morning for
     MIZ039-040-045-046-051-052-057>059-065>067-072>074.

LM...Small Craft Advisory until 8 PM EDT this evening for LMZ846>849.

&&

$$

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