Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 201038
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
638 AM EDT Sat May 20 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 309 AM EDT Sat May 20 2017

A low pressure system developing in Kansas will track northeast
through tonight. This system will spread clouds and rain in our
direction. A cold front associated with this storm will push
eastward through Lower Michigan Sunday morning. High pressure then
builds in from the southwest on Monday.

&&

.SHORT TERM...(Today through Monday)
Issued at 309 AM EDT Sat May 20 2017

The main trend with this forecast is to reflect higher POPs for
later today and tonight.

There are numerous showers and thunderstorms upstream across parts
of Iowa...Missouri and into Arkansas. This area of precipitation
is in a region of deep moisture...lift and instability. The lift
is associated with an upper low. This feature is shown to track
northeast through tonight...through the Western Great Lakes
region. Favorable lift along with the axis of deeper moisture are
shown to arrive in the CWA this afternoon and through tonight.
This supports high POPs. The instability advects in from the south
later this morning and stays over the region through tonight. Thus
I put some timing in there for the arrival of the risk for storms.
The stronger mid level wind fields may support organized
storms...but instability is shown to be somewhat marginal. Will
need to monitor trends. I do see a potential for heavy rain...and
locally over an inch may fall. Will continue to highlight the
risks in the HWO.

Strong subsidence moves in for Sunday. Thus any risk for showers
and thunderstorms should only be in the morning. Monday morning
looks dry at this point...but showers may begin developing over
the region during the afternoon. Will only feature a low potential
for showers.

.LONG TERM...(Monday night through Friday)
Issued at 309 AM EDT Sat May 20 2017

Much of the long term looks to be fairly cool and unsettled. Maybe
even trending toward being more unsettled compared to the thoughts
24 hours ago.

We are still expecting rain showers to move in over the area Mon
night into Tue. This will come in/develop out ahead of an upper
trough that will be rotating SE toward the region. We will see the
associated sfc low to our west through most of Tue. This will help
to bring some Gulf moisture up, and increase the chances of rain
with the system. Sfc based and elevated LI`s indicate that the
situation is probably too marginal at this time to mention thunder
for now.

Additional short wave energy will dive in on the backside of the
larger upper low moving slowly across the area. This will induce
another sfc low to develop to our SE and wrap better warmth and
moisture back in across the area, even as better low development
happens over New England. The showers will likely linger into Thu as
the upper low and cold pool aloft moves directly over the area.

The chances for pcpn on Fri are not nearly as high as earlier in the
week, however they are still not zero. The upper low rotates east of
the area, but may be just close enough for a shower or storm to fire
during the peak heating portion of the day. This is likely not set
in stone yet with the exact details of Upper Lows still in question
depending on how it rotates and moves through.

&&

.AVIATION...(For the 12Z TAFS through 12Z Sunday morning)
Issued at 632 AM EDT Sat May 20 2017

With a warm front moving into the region during the forecast
period...we will see impacts developing at the TAF sites. The
initial MVFR conditions over southern Lower MI are forecasted to
break up over the next hour or two leading to VFR. However it will
not last long as showers begin to move in along with increasing
amounts of low level moisture. Timing of the arrival of IFR will
is somewhat uncertain but all sites should go down later this
afternoon or more so tonight as the showers become more numerous.


A risk for thunderstorms will develop later this morning and
persist through tonight.

&&

.MARINE...
Issued at 309 AM EDT Sat May 20 2017

Gusty easterly winds today may top 20 knots at times today. The
offshore component will act to keep the wave heights down in the
nearshore waters through tonight. The wind shifts on Sunday and
that may support building waves.


&&

.HYDROLOGY...
Issued at 1131 AM EDT Fri May 19 2017

Rivers levels have returned to near normal. Expecting rain totals
ranging from a half inch in Central Lower Michigan to over an inch
in Southern Lower Michigan today through Sunday with locally higher
totals possible. Another stretch of precipitation is possible Monday
night through Friday. Some river rises are likely with this rain,
but not expecting flooding at this time. Smaller streams and rivers
may need to be watched for above bankfull rises. Main stem rivers
should be able to handle this rainfall fine unless guidance trends
upward.

&&

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

$$

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


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