Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 182340
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
740 PM EDT Tue Apr 18 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 300 PM EDT Tue Apr 18 2017

A weak cold front will bring scattered rain showers tonight.
Scattered showers and isolated thunderstorms are expected
Wednesday. A warm front and stronger low pressure system will
combine to bring more widespread rain and thunderstorms late
Wednesday night followed by more showers and thunderstorms for
Thursday as a cold front moves through.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 300 PM EDT Tue Apr 18 2017

Primary short term fcst concerns involve determining convective
potential Wednesday through Thursday.

A weak cold front will bring increasing cloud cover and scattered
light rain showers tonight. Isolated thunderstorms may also
develop during the early morning hours Wednesday near to just
ahead of the front over our southern/se fcst area as elevated
instability increases a bit as h8 li/s fall to -1 to -2 by 12Z
Wed.

The frontal boundary will stall out across northern IN/OH
Wednesday. It will be in closer enough proximity to our fcst area
to cause scattered showers and isolated thunderstorms to develop
Wednesday morning and afternoon over our southern fcst area. Any
convection that develops through Wednesday afternoon will not be
severe.

Widespread rain and convection will develop over our northern fcst
area late Wednesday night as the warm front lifts north and lift
and moisture and instability increase. Deep layer shear will
increase significantly Wednesday night and strong 1000-850 mb
moisture transport suggests potential for locally heavy rainfall
over our northern fcst area. Some of this convection may become
strong to potentially severe late Wednesday night with large hail
being the primary svr wx threat with elevated convection north of
the warm front.

12Z guidance trends suggest a somewhat slower movement of the low
pressure system and cold front eastward Thursday. As such we
expect an axis of weak to potentially moderate sfc based
instability to develop just ahead of the sfc low and cold front
with strong deep layer shear in place.

Therefore some strong to potentially severe storms are possible
Thursday along and just ahead of the cold front. The relatively
best chc for strong to severe convection would be over our
southeast fcst area. However the overall severe threat Thursday is
mitigated by potential for extensive cloud cover and areas of
showers in the morning that would tend to keep instability in
check.

.LONG TERM...(Friday through Tuesday)
Issued at 300 PM EDT Tue Apr 18 2017

The current forecast will reflect a relatively quiet pattern for the
extended period.  The cold front will be pushing east of the region
on Friday...allowing a ridge of  high pressure to move in from the
west. This fair weather system will act to support dry weather for
the area this weekend.

There could be some frost around Saturday and Sunday
mornings...especially up north toward Harrison and Mount Pleasant. A
cool airmass will be building eastward through Ontario.  The flow
here in MI is forecasted to be northerly.  Thus some shallow cool
air could filter down supporting this potential.

It is not until Monday night or Tuesday when a mid level wave is
shown to be tracking in from the west.  This system may support the
potential for a few showers.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 740 PM EDT Tue Apr 18 2017

High confidence for VFR through 06Z, with widespread fuel
alternate MVFR cigs developing by 12Z. Brief rain showers will be
possible in the 09-12Z time range accompanied by a wind shift.
Shortly after this, IFR cigs just below 1000ft are quite probable
at the AZO, BTL and possibly JXN terminals along the I-94
corridor. IFR and low MVFR cigs could last across the area well
into the afternoon.

&&

.MARINE...
Issued at 300 PM EDT Tue Apr 18 2017

Wind speeds and wave heights will remain below small craft
criteria through midweek. However thunderstorms will pose a hazard
to mariners. The best chance for thunderstorms will come late
Wednesday night into Thursday morning.


&&

.HYDROLOGY...
Issued at 300 PM EDT Tue Apr 18 2017

River levels are slowly falling toward more normal levels across
Southwest Lower Michigan, except for the Muskegon River basin. The
Muskegon River near Evart is now forecast to peak below flood stage
by Thursday afternoon at 10.6 feet. Light rainfall is possible
Tuesday night. This should not impact the rivers. Heavier rainfall
will be more likely Wednesday night and Thursday. This could result
in elevated river levels once again.

&&

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

$$

SYNOPSIS...Laurens
SHORT TERM...Laurens
LONG TERM...MJS
AVIATION...TJT
HYDROLOGY...63
MARINE...Laurens



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