Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 170829

Area Forecast Discussion
National Weather Service Grand Rapids MI
329 AM EST Sat Feb 17 2018

Synopsis/Short Term/Long Term

Issued at 310 AM EST Sat Feb 17 2018

A large area of high pressure centered over the Central
Appalachians will track rapidly east today. This will allow a weak
area of low pressure to track through the region this evening. A
round of light snow will accompany the passage of this system.
Another area of high pressure tracks up the Ohio Valley later
tonight into Sunday. This system will result in dry weather for
the area.


.SHORT TERM...(Today through Monday)
Issued at 310 AM EST Sat Feb 17 2018

The main challenge in the short term deals with the amount and
type of precipitation this evening...and also the impacts of the
storm system that moves in for Sunday night into Monday.

The first low pressure system moves in later this afternoon into
the evening. To start...the airmass will be relatively dry through
the morning into the early afternoon hours. This is forecasted to
delay the onset of the precipitation a few hours. This system is
loosely organized at this time so still a lot of uncertainty but
enough potential there and agreement in the models to support
relatively high POPs for a few hours around 00z. As for the
type...I featured more snow than rain. Forecast sounding are a
little cooler and considering wet bulb temps...any steadier
precipitation should be in the form of snow. As for
currently looks like surface temperatures may be a little above
freezing...which should limit the impacts.

The next storm system starts to move in later Sunday night into
Monday. A warm front tracks northward into the CWA during this
period. It appears that it will stall out over the CWA with the
thermal/moisture boundary strengthening on Monday. Still some
uncertainty where this will happen which impacts who will see the
most rain. An active right entrance region will support deep lift
with a low level jet underneath it. Abundant Gulf moisture spreads
in over the frontal heavy periods of rain are possible...especially
later in the day. May need to add thunder to Mon Aftn if the
elevated instability shown continues.

A period of freezing rain is still possible over interior Lower
MI Monday morning. This is not forecasted to last...unless the
surface frontal zone stalls out across Southern Lower MI...which
is not the most likely scenario. Will need to monitor trends.

.LONG TERM...(Monday night through Friday)
Issued at 310 AM EST Sat Feb 17 2018

A consensus of latest 00z medium range guidance continues to show
potential for a decent rain event associated with a very slow moving
frontal boundary Monday night through Tuesday. Several waves of low
pressure will move northeast along the boundary and enhance
potential for rain.

The combination of the significant rainfall (on the order of an inch
or two on average) and near record warm temperatures/melting snow
will result in an increased risk for some flooding. In addition
there is a chance of convection Monday night into Tuesday as some
elevated instability will develop over our southern fcst area
as h8 li/s go slightly negative.

Precipitation should gradually become mixed with then change over to
wet snow Wednesday before coming to an end as cold air advection
occurs on the back side of the departing system. Larger medium range
guidance discrepancies arise for late week but the most likely
outcome is that high pressure will build in at least briefly to
bring fair weather Thursday.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1127 PM EST Fri Feb 16 2018

VFR conditions will continue overnight through most of Saturday.
Conditions will deteriorate to MVFR at our western terminals late
Saturday afternoon into Saturday evening as snow moves in from the
west. Snow may mix with rain at times mainly at KMKG. Brief
reductions to IFR are possible when steadiest pcpn moves in.
Southwest winds will ramp up to 10 to 20 kts Saturday.


Issued at 330 PM EST Fri Feb 16 2018

There`s uncertainty how much water remains locked in the remaining
snowpack and saturated/frozen topsoil, but certainly not all of the
available 1 to 1.5 inches of snow water content has run off into the
rivers. Rivers have responded rather modestly to the runoff so far.
Some of the typical flood-prone rivers in the upper Grand basin are
near bankfull and will be susceptible to flooding early next week.

One to two inches of rainfall is becoming increasingly probable
Monday through Tuesday. A complete melting of the snowpack is a
virtual certainty for Central-Southern Lower Michigan (except for
perhaps the Ludington area which could remain in the cooler air) as
dewpoints over 50 degrees transported by steady winds impart
continuous condensation and latent heat release onto the snow. As of
now we anticipate a good number of rivers to climb out of their
banks, but a few could reach minor flood stage and create more
attention-worthy impacts to roadways and vulnerable property. Minor
ice jams are possible and short-term river level fluctuations may
also occur in spots.

Note: Current river level forecasts published on AHPS do not account
for precipitation expectations beyond 48 hours.




LONG TERM...Laurens
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