Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 181922

Area Forecast Discussion
National Weather Service Grand Rapids MI
325 PM EDT Tue Jul 18 2017

Synopsis/Short Term/Long Term/Marine

Issued at 325 PM EDT Tue Jul 18 2017

A weak cold front will bring potential for a few showers and perhaps
an isolated thunderstorm very late tonight into Wednesday. A very
warm and humid weather pattern will develop Thursday through
Saturday with several rounds of thunderstorms expected late this
week through the weekend.


.SHORT TERM...(This evening through Thursday night)
Issued at 325 PM EDT Tue Jul 18 2017

Primary short term fcst concerns involve determining convective
potential late tonight through Thursday night.

Fair weather will continue through the evening. A weak cold front
will move into our northwest fcst area very late tonight and bring a
few showers and perhaps an isolated thunderstorm. However potential
for pcpn/convection tonight is limited significantly by several
factors including unfavorable frontal timing... weak instability and
weak lift with the boundary. Dry wx should prevail across the vast
majority of our fcst area overnight.

This weak front will continue to sink slowly southeastward across
our area Wednesday before stalling out. A few showers are expected
with the front Wednesday morning with potential for isolated to
scattered convection to develop Wednesday afternoon as instability
increases a bit along and ahead of this boundary.

By Wednesday evening dry wx will briefly return. However a large
mcs is expected to develop over the upper midwest Wednesday night
on the nose of a 40-45 kt llj. The most robust convection should
stay west of our fcst area but there is potential for remnant
showers/storms from the dissipating mcs to move into our fcst area
late Wednesday night into Thursday morning.

Convective potential for Thursday is largely contingent on the
evolution of the mcs and associated clouds and pcpn in our area
Thursday morning. There is potential for a mesohigh to build in
behind this mcs as suggested by 12z ecmwf guidance which in
conjunction with a worked over atmosphere/lingering clouds would
result in minimal potential for convective redevelopment later in
the day.

However some medium range guidance solutions suggest convective
redevelopment may occur late in the day or Thursday night if
instability redevelops later in the day after the morning activity
moves out. If convection redevelops there would be potential for
very heavy rainfall with precipitable water values reaching 2-2.25
inches Thursday over our southern fcst area along with vigorous 1000-
850 mb moisture transport.

Given uncertainty regarding mcs evolution late Wednesday
night/Thursday morning and in turn how much the atmosphere will
destabilize later in the day only chance pops are warranted for
convection Thursday aftn/Thursday night at this time.

.LONG TERM...(Friday through Tuesday)
Issued at 325 PM EDT Tue Jul 18 2017

The long term period continues to look stormy at times. A low level
frontal zones moves into the region and remains there into the
weekend.   Deep layer shear continues to remain elevated into Sunday
and the atmosphere is shown to be unstable. One mid level wave
arrives early Friday.  Decent lift is noted with this system. This
feature could trigger some thunderstorms to start the day.  With
deep layer shear up over 35 knots there could be some organization
to the storms.

A stronger wave and associated cold front tracks into the area
either Saturday night or on Sunday. Deep layer shear continues to be
elevated. The low level jet is forecasted to climb up over 30 knots
ahead of this feature later Saturday or Saturday night.  This would
act get the storms going during the day and keep them around into
the night. Then the cold front pushes in.

A couple of factors could support locally very heavy rainfall rates.
PWAT values climb to over 1.75 inches.  LCLs are shown to be low
with relatively thin deep CAPE especially later Saturday into
Saturday night. With a nearly stationary boundary in place and a LLJ
bisecting the frontal elevated risk for flooding exists.

A stable airmass moves in for Monday into Tuesday.  Thus any
precipitation should be showers.


.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 144 PM EDT Tue Jul 18 2017

VFR weather if forecasted for the period. There will be some
showers around...especially after midnight...local impacts
possible. Generating the showers will be a weakening cold front
that will be dropping  into the region.


Issued at 325 PM EDT Tue Jul 18 2017

Wave heights in the 1 to 3 foot range are expected tonight into
Wednesday morning before subsiding to aob 2 feet Wednesday
afternoon. Isolated thunderstorms will pose a hazard to mariners
late tonight through Wednesday with a better chance for
thunderstorms late Wednesday night through Thursday which could
also cause briefly higher waves.


Issued at 325 PM EDT Tue Jul 18 2017

An active period is shaping up for the second half of the week.
Although most rivers are running near to slightly above average,
some may have considerable rises into the weekend with multiple
opportunities for thunderstorms. Favorable ULJ and LLJ dynamics will
be in play along with a weak surface boundary floating through the
region, coincident with some high PW values that may reach 2.00"-
2.25" at times. Any convection that develops could contain
torrential downpours with very efficient rainfall rates in this
environment. Exact placement is a tough call until the pattern is
captured by the high res convective allowing models. Interests along
area rivers and streams should pay close attention to trends later
this week.




SHORT TERM...Laurens
MARINE...Laurens is the U.S. government's official web portal to all federal, state and local government web resources and services.