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947
FXUS63 KGRR 241914
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
314 PM EDT SAT SEP 24 2016

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

.SYNOPSIS...
Issued at 314 PM EDT Sat Sep 24 2016

Clouds and rain chances will increase Sunday with the approach of
cold front. Rain showers with perhaps a thunderstorm will pass
through the area Sunday night into early Monday. Breezy and cool
weather will follow behind the front on Monday with scattered rain
showers and possibly a thunderstorm. Unsettled and cool conditions
with highs in the 60s will persist until Wednesday. This will be
followed by warmer and drier conditions the rest of the work week
with highs reaching 70 or more.

&&

.SHORT TERM...(This evening through Monday night)
Issued at 314 PM EDT Sat Sep 24 2016

Main concern is precipitation chances Sunday into Monday. Really no
changes to the previous thinking. Maintained precipitation chances
greater than guidance from Sunday night into Monday night.

Marginal timing and instability with front should limit the
thunderstorm threat Sunday night. However rainfall could be
briefly intense with seasonably high PWAT values and some
convection allowing models suggesting a sharp cold frontal
rainband leading the area of precipitation.

Thunderstorm potential Monday looks a little better. A plume of
impressive instability from Lake MI //SBCAPE approaching 1000 J
per kg// should spread into far western Lower MI thanks to low
70s F water temperatures and a seasonably strong upper cold pool
moving over the area. Upper dynamics are also favorable with a
still substantial cyclonic upper jet positioning itself just south
of the forecast area / upper cold pool. Current SPC Day3 outlook
does not include our area in general thunder, but have discussed
this with SPC so future updates may reflect the thunder
potential.

.LONG TERM...(Tuesday through Saturday)
Issued at 314 PM EDT Sat Sep 24 2016

For Tuesday into Wednesday a mid level low over  Lake Superior is
shown to track south through Michigan.  The mid level height falls
coupled with lake enhanced moisture supports higher than normal
pops...especially Tuesday night.  Models are in very good agreement
on this scenario...which adds to the confidence for higher POPs.  I
also lowered temperatures during the daytime period given the
potential for some rain...and rather cloudy skies. We may need to
bump up POPs on Wednesday...especially near the lakeshore if the low
is still pushing through the state.

Mid level height rises then occur into the end of the week.  This
should support diminishing clouds and rain potential...along with a
moderating trend to the temperatures.  Above normal temperatures
become increasing likely as we go through the end of the week and
into the weekend.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 124 PM EDT Sat Sep 24 2016

A nearly stationary band of MVFR clouds over KAZO should gradually
diminish over the next couple of hours as mixing slowly erodes the
clouds. Otherwise...the low level moisture is forecasted to remain
limited...which supports a VFR forecast. Showers will be pushing
in from the west after 18z Sun as deeper moisture starts to
arrive.

&&

.MARINE...
Issued at 314 PM EDT Sat Sep 24 2016

Waves will begin to build Sunday night as winds shift to an onshore
direction. As noted previously, strong winds, cold advection, and
improved fetch with cyclonic flow will increase waves dramatically.
By Monday afternoon, convective equilibrium levels could approach
25000 ft, which is more than enough for thunderstorms and is
additionally favorable for waterspouts. The limiting factor for
waterspouts is strong winds, which will be around 35 knots in the
boundary layer. Frequent gusts over 35 knots will be possible,
which justifies the need for a Gale Watch should models continue
to indicate this.

&&

.HYDROLOGY...
Issued at 1140 AM EDT Sat Sep 24 2016

River levels are around normal for the time of year. Showers and
thunderstorms are expected Sunday into Monday night with additional
rain possible Tuesday into Wednesday. Up to around one inch of rain
is expected and therefore rivers and streams should remain below
bankfull through the week.

&&

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

$$

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