Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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474
FXUS63 KGRR 270730
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
330 AM EDT SAT AUG 27 2016

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

.SYNOPSIS...
Issued at 330 AM EDT Sat Aug 27 2016

Low pressure will move northeast from Iowa this morning to Northern
Lower Michigan tonight. Southerly winds will stream moisture into
Southwest Lower Michigan on southerly winds which will bring rounds
of showers and thunderstorms today. Some strong storms may develop
today with high winds and hail. Locally heavy rain is possible as
well. A cold front will sag south through the area tonight with
additional scattered showers and storms expected along and ahead of
it. Sunday into Monday is expected to be mainly dry with high
pressure settling into the area. Seasonable temperatures are
expected the next few days with highs in the upper 70s to lower 80s.

&&

.SHORT TERM...(Today through Monday)
Issued at 330 AM EDT Sat Aug 27 2016

Main focus in the short term period of Today through Monday is on
thunderstorms today and tonight and the possible threat of severe
weather.

First item of note in assessing storm chances and severity today is
the fact that we will see a warm front lifting north through the
area. The warm front is located across Illinois and Indiana early
this morning but it will steadily work north this morning and
afternoon. Deep moisture will advect in on southerly winds with dew
points rising into the lower 70s and PWAT values approaching 2.00
inches. Instability may be the question mark today given the amount
of showers and storms that will likely be around this morning. A low
level jet on the order of 20-30 knots will be sufficient to make
showers/storms likely. A 500mb jet will peak this morning around 50
knots, weakening a bit this afternoon. We will be positioned well
southeast of the main upper wave across the Minnesota and Northern
Wisconsin, but we will see embedded convective vorticity maximums in
the southwest flow. Deep layer shear is high this morning at 40-50
knots but it diminishes to 20-30 knots this evening.

So, the bottom line is we are expecting fairly widespread showers
and storms as we go through the day as the LLJ interacts with the
warm front. The threat for severe is a bit less likely as the
parameters are a bit disjointed. Best dynamics may be in place this
morning but the better instability will likely hold off until this
afternoon. With a warm front lifting through the area however we
will need to keep an eye on storms as they may acquire rotation and
will likely be organized at times. Heavy rain may end up being the
biggest threat with the warm front and high PWAT air.

Showers and storms will likely taper off from northwest to southeast
tonight. Used 50 pops in the forecast tonight. High pressure builds
in for the Sunday through Monday time frame which should provide
pleasant and dry weather. Cannot rule out an isolated shower or
storm early Sunday morning towards I-94.

.LONG TERM...(Monday night through Friday)
Issued at 330 AM EDT Sat Aug 27 2016

A fairly significant pattern shift is progged by the middle of next
week after a cold front comes through on Tuesday.

Upper ridging will still be in control by Monday night, but it
should gradually be losing it`s grip on the northern states, with
the westerlies taking over.  This pattern will bring a cold front
through here on Tuesday.  This does not appear to be an overly
impressive front, but it should bring a few showers and storms
mainly late Monday night into Tuesday.

The big shift will come in the form of cooler air behind this front
for Wed and lasting through Friday.  The upper flow should become
northwesterly which brings a large Canadian high pressure into the
Great Lakes region.  After 80s on Tuesday, daytime temps will drop
into the 70s for Wed through Fri.  A few nighttime lows in the 40s
will be possible Wed and Thu nights.  With high pressure in control
will have a dry forecast for much of the area from Tuesday night
through Friday.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 140 AM EDT Sat Aug 27 2016

A round of showers will be moving into the TAF sites early this
morning. These should be relatively light, with VFR continuing.
However thunderstorms will also gradually move/form over the area
after sunrise, perhaps impacting all the TAF sites by 15Z as we
become increasingly unstable. These storms will continue over the
region until somewhat drier air arrives by late afternoon/early
evening.

Ceilings and vsbys should lower with the rain. Expect some MVFR
will be around by late morning, most likely over far Western
Lower, including MKG and GRR, where the steadiest rains can be
expected. This too is expected to improve toward late afternoon
as the bulk of the rain moves out.

&&

.MARINE...
Issued at 330 AM EDT Sat Aug 27 2016

We will need to keep a close eye on the forecast today out on Lake
Michigan. Most models keep winds and waves in check, however the NAM
deepens a low as it crosses Central and Northern Lake Michigan. The
bulk of models have winds that would generate waves in the 1-3 foot
range today. If the NAM verifies we could be looking at 3-6 foot
waves north of Holland. The peak in the waves looks to be in the
800pm to 200am time frame this evening. High pressure builds in for
Sunday and Monday which should provide much calmer conditions.

&&

.HYDROLOGY...
Issued at 328 PM EDT Fri Aug 26 2016

The Portage River is slowly falling and is expected to fall below
flood stage tonight or tomorrow. The only fly in the ointment is
likely rainfall on Saturday. Any thunderstorms will be rather
efficient rainfall producers as humidity increases through the depth
of the troposphere. Locally over an inch of rain is possible.

&&

.GRR Watches/Warnings/Advisories...
MI...None.
LM...None.
&&

$$

SYNOPSIS...Duke
SHORT TERM...Duke
LONG TERM...JK
AVIATION...JK
HYDROLOGY...63
MARINE...Duke



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