Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 291818

Area Forecast Discussion
National Weather Service Grand Rapids MI
218 PM EDT THU SEP 29 2016

Long Term/Aviation

Issued at 323 AM EDT Thu Sep 29 2016

Low pressure will continue to spin near and over Lower Michigan
through the week and into the weekend. This will bring mostly
cloudy weather with periods of showers and a few thunderstorms.


Issued at 902 AM EDT Thu Sep 29 2016

Based on the latest radar trends and looking at an ensemble of
the HRRR, HRRRX, and RAP model (using time lagged ensembles) I
redefined the rain area farther north (by about a row of
counties) for today and also updated the QPF to match.

There is some through about flood advisories but at this point I
think the rain band near I-94 is going to break up enough over the
next few hours so that will not be a big issue. Jackson County has
had around an inch of rain so that would be the most venerable.
Again I think that rain band should be moving out of there in the
next few hours. Overall this still will have to be watched.


.SHORT TERM...(Today through Saturday)
Issued at 323 AM EDT Thu Sep 29 2016

Upper cutoff low spins across the Ohio Valley then moves north
back into Lower Michigan by the end of the period. Strong easterly
flow including low level jet will tap into tropical moisture
plume moving up the east coast by Thursday night. Showers will
become more widespread and persist into Friday before beginning to
taper off as the occluded low moves into the forecast area.

Marginally steep lapse rates late Thursday into Saturday make
diurnal enhancement of thunderstorms possible each day. The
persistent nature of the flow regime and the presence of tropical
moisture feeding in will mean that excessive rains are possible
and this will have to be monitored closely.

.LONG TERM...(Sunday through Thursday)
Issued at 215 PM EDT Thu Sep 29 2016

The main issue here is when does the rain from the current system
end? The good news is the models have been very consistent about
this for several days now, the rain should end by Monday morning.
Sunshine should return by Monday afternoon. The next period of
showers will be late next week into next weekend as the next
Pacific system slowly moves east through the area.

That next upstream Pacific system (using a Henry rule sort of
argument) gets close enough to our current system to "boot it
out" by Sunday night or early Monday. Then a larger upper ridge
develops over our area in response to the digging Pacific system.
In fact this creates a nearly closed off upper level high (seen
on both the GFS and ECMWF) by Wednesday. This will also mean
warmer tempeatures most of next week as the air behind this system
is of Pacific origin not from the polar regions.

Finally we get another "Henry Rule" closed upper low over the
Southwest CONUS being booted out by the next upstream Pacificshortwave
(no end to that sort of thing). This will bring the next threat
for rain to our area back by Thursday.


.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 215 PM EDT Thu Sep 29 2016

I expect MVFR/IFR cigs to prevail over all of our taf sites
through Friday and beyond that thanks to the stalled upper level
low pressure system. MKG has been in and out of the MVFR cigs
since mid morning but I expect the east winds over time to bring
solid MVFR/IFR cigs to all taf sites including MKG. The current
rain band lifting through the CWA will replaced by yet another one
later today into Friday morning. That will help bring a more solid
IFR cig/vis set up to our TAF sites.

Winds will continue to be gusty today but should diminish some
tonight and even farther on Friday as the surface system slowly
weakens with time.


Issued at 953 AM EDT Thu Sep 29 2016

We have added waterspouts to the forecast for the nearshore waters
south of Whitehall for today. This is where most of the showers
that could produce waterspouts will be, closer to the center of
the low, and under the cold pool aloft. Most of them should be
offshore with the offshore wind, but it is still enough of a
threat to mention.


Issued at 140 PM EDT Tue Sep 27 2016

River levels are around normal for this time of year. Lake enhanced
rain showers are possible through Wednesday. Rain showers continue
to be possible into Sunday. Rainfall totals into the weekend will
range from a half inch up to an inch and a half. Rivers and streams
should remain below bankfull through the weekend.


.GRR Watches/Warnings/Advisories...
LM...Small Craft Advisory until 6 PM EDT Friday for LMZ844>849.



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