Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 170439

Area Forecast Discussion
National Weather Service Grand Rapids MI
1139 PM EST Fri Feb 16 2018


Issued at 330 PM EST Fri Feb 16 2018

After a quiet and seasonable period for tonight through Saturday
morning, a brief period of light snow is expected to overspread much
of the area late Saturday and Saturday evening. Accumulations of
around an inch will be likely, especially for areas west of U.S.-131.

Another dry period will ensue then for Saturday night into Sunday
night, before precipitation moves back in over the area early
Monday. Most of this precipitation will be rain, although there is a
small chance that some brief mixed precipitation will be possible up

A wet and warmer period is expected for at least the first half of
the week next week. Temperatures could warm into the 50s down south,
with 1 to 2 inches of rainfall likely everywhere. Cooler air will
move back in for the end of next week.


.SHORT TERM...(This evening through Sunday night)
Issued at 330 PM EST Fri Feb 16 2018

Our two periods of concern in the short term are snow chances late
Sat/Sat evening, and rain/mixed pcpn late Sun night. At this time we
do not expect that either of these events will create significant

Lingering lake effect clouds/flurries this afternoon are expected to
dissipate this evening as the trough passing through moves east, and
ridging builds over the area. Over-lake instability will also wane a
bit as temps at H850 warm from around -12C to less than -10C this

We will start out with filtered sunshine early on Saturday with high
clouds moving overhead. These clouds will thicken through the day,
with light snow breaking out later in the afternoon/evening from
West to East. We will see a fairly robust short wave emerge from the
Rockies into the Plains later tonight. This wave looks to briefly
tap enough sufficient moisture from the south for light snow to
break out before 00z Sun.

Right now, it looks to be a quick hitter, with up to an inch
possible at most areas. The best amounts should be west of U.S.-131
where some enhancement could occur off of Lake Michigan with delta
t/s around 10-11C. We do not expect this to be an issue as inversion
heights should cap the convective potential of this, and due to the
short duration of this (3-6 hours?).

We will then see another break in the activity from Sat night
through much of Sun night before the next chance of precipitation
moves in. The upper flow is expected to take on a SW to NE
orientation toward the area as a strong upper low dives south along
the West Coast. Short wave energy lifting out from the western long
wave will be able to tap better Gulf moisture and bring it into the
far southern counties late Sun night.

This should be mostly rain. We can not rule out a brief mixture
across the far north if the pcpn can make it up there before 12z Mon
with temps lingering around freezing there. The chances of this are
too low to make a big deal out of it for now.

.LONG TERM...(Monday through Friday)
Issued at 330 PM EST Fri Feb 16 2018

Main focus of the long term remains squarely on the rain event for
early next week. The GFS and ECMWF ensembles continue to show a
frontal boundary situated over the area from Monday into Tuesday.
Both also indicate waves of low pressure moving along the front
during this time frame right through Lower Michigan. The ECMWF
ensembles in fact show a couple waves moving through versus one main
wave via the GFS ensembles. The deterministic models show the same
differences with a second wave of low pressure in the ECMWF hanging
the boundary up over our area longer into Tuesday night. The GFS
shows more swaths of heavy rain through the area, while the ECMWF
has a more widespread heavy rain.

In terms of some of the typical parameters that we look at...PWAT
values are forecast to reach 1.20 inches which would be a daily max
via the DTX sounding climatology. 850mb dew points rise to around
+9, with strong 850mb moisture transport noted from Monday afternoon
into Tuesday. Parameters are certainly in place for the potential
for heavy rain.

The bottom line is confidence continues to build for a heavy rain
event from Monday into Tuesday of next week. 1 to 2 inches of rain
look quite possible with the event, and if the ECMWF verifies,
potentially more.

The event isn`t a clean rain event given there may be a bit of a mix
on the leading edge across Central Lower Michigan Monday morning.
The mix would be brief though as warmer air will be surging in. Snow
will try to mix back in on the back side Tuesday night into
Wednesday, especially if the quicker GFS verifies.

High pressure should bring fair weather later Wednesday into
Thursday. Another low may bring a mix of precipitation to the area
on Friday as snow trends toward rain in a developing southerly flow.

Temperatures will be up and down with the potential of reaching 60
degrees ahead of the early week system, across South Central Lower
Michigan especially.


.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.




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