Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 241721
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
121 PM EDT WED AUG 24 2016

LATEST UPDATE...
Hydro

.SYNOPSIS...
Issued at 328 AM EDT Wed Aug 24 2016

A frontal system moving into the area today will bring most
locations measurable rainfall by this evening. This will come
in the form of showers and thunderstorms. The risk for severe
storms is low but not out of the question. Highs will range from
the mid 70s to near 80. Showers and thunderstorms may linger into
this evening but a wave on the front will bring a period of
showers and thunderstorms Thursday during the daytime. Once this
is done the cold front comes through and skies will clear tomorrow
night. Mostly sunny skies are expected Friday and Saturday with
highs not far from 80.

&&

.SHORT TERM...(Today through Friday)
Issued at 328 AM EDT Wed Aug 24 2016

The main issue for today remains the threat of locally heavy
rainfall from thunderstorms plus the marginal risk some of the
storms could be locally severe. The other issue is on Thursday,
there is a significant risk for locally heavy rain (see latest
ECMWF) near and south of I-96 during the day time hours of
Thursday.

We have been following this set-up for some time now and it
continues to play out as expected (more or less). We have a
closed upper low that still looks rather impressive on water vapor
loops over Southern Canada, currently centered over southern
Manitoba. While that is happening the southern branch of the
polar jet is surging northeast toward the lower Great Lakes for
today into Thursday ( northern branch of polar jet entrance
region lift event). Note the tropical plum on the water vapor
loops. The initial surge of the deep topical moisture
(Precipitable water amounts area near 2 inches again) is helping
to fire the convection to our west at 2 am. This does move into
Michigan during the day time hours of today. Then if that were not
enough the the northern branch system`s jet entrance region
impacts our area Thursday. This would be frontogenesis sort of
event which would mean some areas will get more then 2 inches of
rain while most areas will get much less or none. It just depends
on how the frontogenesis event sets up. All of this moves out of
the area Thursday night as shortwave ridging builds into the area.
Friday should be sunny and dry with highs in the lower 80s.

As for our precipitation event today, we have a low level jet that
is feeding the complex to our west and that moves into Southwest
Michigan by late morning or early afternoon but it is weakening as
it does so. Not to worry through, by late afternoon a second low
level jet develops (seen on numerous runs of the RAP model) south
of CHI and that expands into the AZO/BTL area by early evening
with a 40 knot jet core. So I see and area of showers and
thunderstorms moving into our CWA by midday but it will not be all
that strong as the low level jet is weakening. However by late
afternoon the surge of the jet near I-94 will help to generate
stronger storms and that fits well with the SPC latest marginal
severe risk near and south of I-94. These storms will more than
anything else be heavy rain produces. The lack of strong effective
bulk shear will make it harder to organize these storms. Also the
extensive cloud cover will limit instability.

All of this will be east of the area by mid evening allowing for
the convection to quiet down overnight. However we still have the
jet entrance region lift event to deal with Thursday. The 00z run
of the ECMWF shows 1 to 2 inch in 6 hour rainfall amounts along
I-96 during the day time Thursday. This fits the jet lift
(frontogenesis)idea. Given the nature of this sort of event,
figuring out just where that band will be is not easy. So I have
only marginal high pops Thursday over southern and central area.

This all moves out tomorrow night so skies should clear and Friday
should be nice day with considerable sunshine and highs in the
70s (we are in the cooler air behind the cold front).

.LONG TERM...(Friday night through Tuesday)
Issued at 328 AM EDT Wed Aug 24 2016

The long term portion of the fcst continues to look like it will
start out dry initially. Rain chances will then increase for the
second half of the weekend. High pressure will slip east of the area
early Saturday, and allow return flow to start to set up. There is
some data bringing in rain chances as early as Saturday afternoon
now. The consensus is still that the best chc of rain will arrive
late Saturday night and into Sunday when better moisture advection
will take place after a couple of dry days.

A bit more uncertainty than usual exists for early next week. We
will see the deeper moisture be swept south of the state with a cold
front that pushes through. We will tend to dry out some, however low
chances of a few showers/storms will remain will a short wave trough
poised to move through.

A complicating factor with the pattern early next week will be the
potential tropical system that may move into the Gulf, and
eventually north out of it. The track of the tropical system will
have implications as to where the upper features will end up.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 730 AM EDT Wed Aug 24 2016

There is a large area of showers and thunderstorms with MVFR cigs
over WI and IL heading northeast toward Southwest Michigan at
1130z this morning. This area of showers will move through
southwest Michigan between 14z and 18z at MKG to 16z to 20z in
the LAN and JXN area. After that clouds will break up for awhile
but another surge of moisture will bring low clouds back by 06z or
so and more rain during the day time hours of Thursday.

&&

.MARINE...
Issued at 328 AM EDT Wed Aug 24 2016

We continue to expect 15 to 25 knot winds today diminishing this
evening so I will leave all marine headlines as is and call that
good.

&&

.HYDROLOGY...
Issued at 121 PM EDT Wed Aug 24 2016

A Flood Warning remains in effect for the Portage River near
Vicksburg into the weekend. More rain is expected for this area and
river levels will be slow to fall. River levels are above normal
across the area but falling. Additional rainfall may bring levels
near bankfull, but new river flooding is not expected through the
weekend.

&&

.GRR Watches/Warnings/Advisories...
MI...Beach Hazards Statement through late tonight for MIZ037-043-050-
     056-064.

LM...Small Craft Advisory until 2 AM EDT Thursday for LMZ845>849.

&&

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...NJJ
AVIATION...WDM
HYDROLOGY...63
MARINE...WDM



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