Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 120516
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
116 AM EDT Thu Oct 12 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 329 PM EDT Wed Oct 11 2017

Widespread soaking rain is ending this evening, but mostly cloudy
skies will be a common sight through the weekend. Near-normal
temperatures on Thursday will be replaced with warmer air Friday
through Sunday (at least in southern Lower Michigan) as a frontal
boundary sets up over the area. Another soaking rain event is
expected over the weekend.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 329 PM EDT Wed Oct 11 2017

It appears there will be enough low-level cloud cover tonight to
limit the fog potential. Winds just off the surface will remain
around 25 mph under a slight amount of cold air advection, keeping
the surface mixed.

Low level moisture will stick around through Thursday night as
well  as troughing below 850 mb. The atmosphere above 850 mb will
be warm and dry tonight through Friday. Models are showing light
precipitation developing late Thursday night as the 850 thermal
trough comes through, however, with no saturated layers colder
than about +3 C, it would be difficult to develop more than
drizzle or very light rain.

The upper levels will moisten Friday night and rain will develop
along a strengthening baroclinic zone ahead of the next wave of
low pressure.

.LONG TERM...(Saturday through Wednesday)
Issued at 329 PM EDT Wed Oct 11 2017

The main story for this forecast is the fall storm that comes
through Saturday into Sunday. We are still looking at the threat of
significant thunderstorm activity with the front and heavy rainfall.
Behind the system we will have the typical gales over Lake Michigan
(Sunday) and lake enhanced rain showers Sunday night.

The setup for the system over the weekend still looks good for a
major fall storm to impact the Great Lakes. The ECMWF and the GFS
now agree on the timing and strength of the system. Like the system
we had today, this too is a shearing out upper wave. Like this
current system the models tend to weaken the upper level wave too
quickly so I would expect what actually happens this weekend will
be more amplified than any of the models currently show us.

Even so, if the models were correct as is we would still get an
impressive early fall storm.  Unlike the current system the one over
the weekend will tap some northern stream cold air (though it does
not really bring the core of the coldest air into our area), which
will help it have more wind.That will help transport more
subtropical air north in front of the system (1000/850 moisture
transport into Southwest Michigan Saturday night is very
impressive).  The precipitable water values forecast by the models,
just like with todays system approach daily record high values for
mid-October, but this time due to deeper southerly flow, it gets
up to around 1.7" instead of the 1.4 with the current system.
Helping all of this is the entrance region to a 130 knot northern
stream jet core. The 850 LI values go negative and the TQ index
(15 is the threshold for thunderstorms) gets into the lower 20s!
The bulk shear to 6km is around 60 knots to that helps the cause
too. Thus I believe there is a significant threat for strong to
severe storms Saturday night into early Sunday. Plus the threat of
heavy rainfall.

Once the cold front comes through we get a brief surge of cold air
so lake enhanced showers should be expected Sunday night into Monday
morning but by Monday afternoon noon the upper jet is out of the
area already so that will end any lake enhanced showers.

Beyond that a large surface high will be over the area under an
upper ridge so expect quiet weather till late in the week when the
impact of that 200 knot extended East Asian jet gets closer to
the CONUS.

I still see no sign of an overall, large scale freeze across all of
southwest Michigan, through next week. The cold air that does come
down is transient since there is no large scale blocking expected in
the northern hemisphere flow.  It should also be noted the axis of
the polar jet for the most part stays north of 45 degrees north
latitude all of next week and likely beyond that. There is a brief
dip in that flow Monday but it is not enough to bring down enough
cold air to cause a freeze.

Bottom line is warmer and stormy over the weekend then cooler and
quiet weather most of the rest of next week but the cold air will
not be colder than normal and threat for a widespread freeze seems
very low.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 115 AM EDT Thu Oct 12 2017

Much of the region was blanketed by MVFR ceilings, with some
patchy IFR. This trend should continue much of the next 24 hours.
However expect more IFR as we see a nocturnal lowering of the
ceilings through the early morning.

Then ceilings should trend upward a bit mid/late morning to mostly
MVFR. However due to the lessening winds and strengthening
inversion, these clouds/ceilings should remain over the region
through the day and into the evening.

&&

.MARINE...
Issued at 914 PM EDT Wed Oct 11 2017

Canceled the small craft advisory a tad early this evening as
wind speeds/gusts have diminished below advisory criteria at all
the buoys and pierheads.

&&

.HYDROLOGY...
Issued at 329 PM EDT Wed Oct 11 2017

Widespread rainfall of one half to inch fell across West
Michigan overnight into this morning...with isolated amounts in
excess of two inches west of US-131. Even with the recent dry
weather...runoff was sufficient to produce rises on area
rivers..but we do not expect any of the area rivers to reach
action stage.

While the latest model data suggests scattered showers will be
possible Thursday...coverage and amounts are not expected to
produce impactful rises on area rivers. Precipitation associated
with a cold front this weekend has a greater potential to produce
significant runoff...which could produce stronger rises on area
rivers. This situation will need to be watched closely.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...CAS
SHORT TERM...CAS
LONG TERM...WDM
AVIATION...JK
HYDROLOGY...MWS
MARINE...Meade



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