Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 210712
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
312 AM EDT Mon Aug 21 2017

LATEST UPDATE...
Synopsis/Short Term/Long Term/Marine

.SYNOPSIS...
Issued at 312 AM EDT Mon Aug 21 2017

Some high clouds and cumulus clouds will be around for the eclipse
event this afternoon. A small chance of a few showers and storms
will exist this afternoon, mainly away from Lake Michigan. Storms
are expected for most areas beginning overnight tonight and Tuesday
morning. Some of these could become locally strong to severe, with a
heavy rain potential also in place.

Rain will gradually clear out on Tuesday behind the frontal system,
with cooler air filtering in. Mostly drier and cooler weather will
then be in place for the remainder of the week. A brief shower can
not be ruled out, especially on Wednesday.

&&

.SHORT TERM...(Today through Wednesday)
Issued at 312 AM EDT Mon Aug 21 2017

First order of business for today is cloud cover trends with regards
to the eclipse event for this afternoon here. Right now, our highest
confidence thoughts are that we will be dealing with the blow off
clouds from the convection ongoing over Iowa this morning, along
with some cumulus development just inland from the lakeshore and to
the east around the time of the eclipse. This would lead us to
believe that the sun will be filtered a bit at least, with
variability of openings through the cumulus away from the lakeshore.
There should be less cumulus right at the lakeshore.

We then will focus on rain chances through Tuesday. We have the
potential of seeing a few spotty showers/storms after 18z for areas
mainly along and east of U.S.-131. The lake breeze/shadow will move
inland some this afternoon with a S/SW flow expected. This will help
to pop off a few storms with MU CAPEs possibly over 2000 j/kg. The
limitation today will be a cap that is evident on forecast soundings
around H850 and plenty of dry air around. Shear is not too
impressive this afternoon with values generally less than 30 knots.

Our best threat for storms will come after 06z tonight, and prior to
18z Tue. This is when we will see a 40 knot low level jet nose in
ahead of the incoming front, that will bring strong moisture
transport into the area. This will keep good instability in place
even at min heating with MU CAPEs expected to be up around 25000
j/kg. Shear will increase with values increasing to around 40 knots
by 09z. The strongest storms will likely remain west of the area
where sfc based instability will coincide with the better dynamics.
Some of the stronger storms could hold on into SW Lower.

The storms will likely be slightly elevated, based just above the
thin nocturnal inversion around 1k feet. This will limit the severe
potential some. CAPE profiles overnight are conducive to some large
hail with plenty of CAPE in the -10 to -30C layer. We can not rule
out some local strong wind gusts for the downbursts that may be able
to penetrate the inversion. Heavy rain potential also exists with
pwats up between 1.5 and 2.0 inches. Local training of heavy
downpours will be possible. We are not expecting any widespread
flooding with the progressive nature of the low level jet.

The front will push through on Tuesday, bringing an end to the
shower/storm activity. Skies will clear out, and winds will pick up
a bit with the cold advection taking place.

The forecast for Wed is a bit uncertain, which is dependent on how
far south a wave drops in across the area. The scenario could range
from a few cumulus clouds, to widespread stratocumulus with a few
showers possible. We have upped the cloud cover a bit inland, but
left the forecast dry for now.


.LONG TERM...(Wednesday night through Sunday)
Issued at 312 AM EDT Mon Aug 21 2017

High pressure will build into the Great Lakes for most of the long
term. Very little rainfall is expected from Wednesday night through
Sunday. Much cooler air with lower dewpoints is expected to give us
a taste of Autumn.

Thursday will be the coolest day with highs around 70. Thursday
night will be cool with lows in the 40s in many locations. After
that a slow warming trend will occur as the high drifts east and an
south winds develops, but temperatures will remain in the mid 70s
for the rest of the period and lows will be in the 50s.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday night)
Issued at 103 AM EDT Mon Aug 21 2017

An area of showers and thunderstorms is moving across northern
IL/IN. A few showers may move near the southern TAF sites
overnight and we added a vcsh comment to the early portion of the
TAF. From the afternoon through the evening, isolated
showers/storms are possible as low pressure approaches Lower
Michigan. More numerous showers/storms are expected after 06z.


&&

.MARINE...
Issued at 312 AM EDT Mon Aug 21 2017

The main period of concern for marine interests will be very late
tonight and Tuesday as winds pick up a bit ahead of the incoming
front, and then especially behind the front. It looks likely that a
Small Craft Advisory and Beach Hazards Statement will be needed on
Tuesday in the cold air advection behind the front.There is some
potential that this may linger a bit into Wednesday before the
gradient weakens by Thursday.

&&

.HYDROLOGY...
Issued at 312 PM EDT Sun Aug 20 2017

Area rivers are around normal levels for the time of the year and
well below bankfull. Though we are going to get localized heavy rain
of over an inch late Monday night into early Tuesday, most areas
will not see even minor river flooding. Some water covered roads are
possible.

&&

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

$$

SYNOPSIS...NJJ
SHORT TERM...NJJ
LONG TERM...04
AVIATION...04
HYDROLOGY...63
MARINE...NJJ


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