Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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370
FXUS63 KGRR 261041
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
641 AM EDT Wed Apr 26 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 306 AM EDT Wed Apr 26 2017

A low pressure system developing in the Southern Plains will take
a track through the Western Great Lakes region on Thursday. This
system will bring with it increasing chances for showers and
thunderstorms along with locally heavy rainfall. A stronger storm
arrives over the weekend with a risk for heavy rain and
thunderstorms.

&&

.SHORT TERM...(Today through Friday)
Issued at 306 AM EDT Wed Apr 26 2017

The warm and moist air advection is underway. This was leading to
a few showers closer to the lakeshore...and the deeper moisture.
Models are generating some instability during the daytime period
today. The cap that has been in place also is shown to diminish
somewhat. Mesoscale models are trying to develop a few rw/trw
today. Will feature a low risk for thunderstorms today...with the
highest pops closer to the lakeshore.

The upper level wave moves in tonight and Thursday. It takes on a
negative tilt. Combine that with an unseasonably moist airmass and
instability and you typically end up with plenty of showers and
thunderstorms along with locally heavy rain. The main period for
showers and thunderstorm should be from about 03z to 18z. This is
when the strengthening low level jet tracks through. Deep layer
shear does increase through the night and into Thursday morning.
This may act to organize the convection. Considering the stronger
low level wind fields that will be in place...stronger wind gusts
look like the greatest risk.

We will need to monitor Friday for possibly another wave of low
pressure to track through. Models are in poor agreement on
this...with some showing almost no rain to others showing a risk
for flooding. Nam is the wettest while the High Res Euro the
driest. The GFS is between the two and generally what I used.


.LONG TERM...(Friday night through Tuesday)
Issued at 306 AM EDT Wed Apr 26 2017

Active weather will continue through much of the long range forecast
period. A few light rain showers are possible Friday night into
Saturday before a strong low pressure system with abundant gulf
moisture over the Arklatex region Saturday night moves north into
the western Great Lakes region.

1000-850 mb moisture transport will increase significantly late
Saturday night through Sunday night when widespread rain which could
be heavy at times will develop over our fcst area. Some convection
is anticipated as well.

Rainfall totals over our fcst area of one to two inches are likely
Saturday night through Sunday night with locally higher amounts
certainly possible if not probable. Temperatures should average
fairly close to normal for this time of year through the long range
fcst period.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 638 AM EDT Wed Apr 26 2017

Today is forecasted to be mostly a VFR day. A shower or
thunderstorm could develop especially as we go through the
afternoon. The coverage is not expected to be widespread enough to
put them in the forecast for now.

This evening...an increased risk for a band of thunderstorms to
move in from the southwest exists. Will mention them in the
forecast and impacts could occur as well. Moisture will be on the
increase tonight...which should lower the cloud bases as well and
MVFR conditions are certainly possible.

&&

.MARINE...
Issued at 306 AM EDT Wed Apr 26 2017

The wind and waves are currently lower than models were
forecasting. The pressure gradient is tightening which should
support an increase in the winds. Thus wave heights will be on
the increase. With a slight offshore component...the highest waves
will be primarily closer to the 5 nm mark. Will keep the headlines
going.


&&

.HYDROLOGY...
Issued at 330 PM EDT Tue Apr 25 2017

In spite of conditions drying out over the last few days, rivers
remain higher than normal across the area and a few decent
widespread rain events may prime them once again for a flood risk.
The Muskegon and Grand River basins in particular are running at
about the 75th percentile for this time of year. Rainfall amounts
Wednesday into Thursday could be locally heavy and the western
reaches of those basins may experience rises. Additional significant
rain this weekend over especially the central and western reaches of
those basins may cause some above bankfull rises and we will have to
monitor closely for a flood risk on the main stem rivers.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 8 AM EDT Thursday for LMZ844>849.

&&

$$

SYNOPSIS...MJS
SHORT TERM...MJS
LONG TERM...Laurens
AVIATION...MJS
HYDROLOGY...Hoving
MARINE...MJS



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