Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 180229
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
1029 PM EDT Fri Mar 17 2017

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 328 PM EDT Fri Mar 17 2017

Low pressure will move across Lower Michigan tonight. Mixed rain and
snow is possible as the system moves through. Another system will
move through Sunday night bringing light rain with it. Temperatures
will climb to around 50 Monday then fall to around 40 midweek,
before climbing into the 50s by Friday.

&&

.UPDATE...
Issued at 1029 PM EDT Fri Mar 17 2017

I updated the zones and grids to have a 100% chance of showers
snow showers toward morning near and south of I-96 and I also
added widespread fog (not necessarily dense).

Fog continues to develop over most of Southwest Michigan as I
write this. So...I updated the zones to include widespread fog
since nearly all the high rest models have visibility`s falling
to a 1/2 mile or less by midnight across most of the area. In
support of that AZO is already 1/2 mile in fog and more stations
are showing visibilities near or below 1 mile. With winds in the
boundary layer over 20 knots I do not think widespread dense fog
is likely but a general 1/2 to 1 mile visibility fog seems likely.
The fog develops as the pressure gradient decreases prior to the
cold front coming through toward morning. As the upper low and
surface low redevelop over SE MI and Northern Ohio by morning
winds become light for a few hours around midnight as this
transfer occurs. This will allow the fog to thicken.

Meanwhile as can be seen clearly on the IR satellite and radar
loops we have rather strong shortwave and assoicated jet exit
region over Srn MN and Iowa heading toward the I-94 area by 8 am.
This will mean the showers over WI will increase in coverage and
move into Southwest Lower Michigan by midnight or so. The air will
become cold enough (freezing level falls to around 1000 ft agl)
for snow to reach the ground by 3-4 am. So we may yet see around a
1/2 inch of snow in some places (between I-94 and I-96) toward
morning.

&&

.SHORT TERM...(This evening through Sunday night)
Issued at 328 PM EDT Fri Mar 17 2017

Forecast concerns deal with ongoing pcpn and then the next system
tonight, followed by another system late Sunday.

Some elevated instability exists across the southern cwa attm as
evidenced by the cellular nature of the radar echos and the sporadic
lightning strikes. There has been some small hail reported...pea
sized...but the convection will wane as the afternoon turns to
evening and the atm becomes more stable.

Low pressure moving from Minnesota will push a occluded front
through the cwa this evening. There is a decent short wave
associated with it, but upstream radars aren`t showing much attm. We
did boost pops a bit to account for this wave. Scattered snow and
rain showers are possible for most areas; the far northeast may see
patchy areas of light freezing rain for a few hours. The pcpn will
change to light rain Saturday morning as temps warm a bit.

Sunday, another cold front sweeps across the upper Mississippi
Valley. As it nears Michigan, a wave is progd to develop along the
southern flank and push some moisture northward. We`ll likely see
some light rain develop along the frontal boundary late Sunday
night.

.LONG TERM...(Monday through Friday)
Issued at 328 PM EDT Fri Mar 17 2017

Monday will start off the long term period with a weakening cold
front pushing through the region.  We will still hold onto a risk
for showers...but overall this system looks weak.

Cold air advection moves in behind the front for Monday night and a
strengthening high pressure system then builds in from the northwest.
This fair weather system then stalls out over MI for Wednesday and
pulls slowly east for Thursday.

Warm air advection strengthens for Thursday night into Friday. The
onset of the precipitation could be a mix.  By Friday morning it
currently looks like conditions will be warm enough for just plain
rain.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 811 PM EDT Fri Mar 17 2017

There is two issues to watch for tonight. The first issue the
risk of dense fog. All of our high resolution models are
forecasting dense fog (VIS <1/4SM)from 03z through 09z at most of
our taf sites. The problem with that is we have 20 knots in the
boundary layer tonight so that typically keeps dense fog from
forming. On the other hand AZO is already 1/2SM in FG. So, given
how well these models have done in the past with dense fog, I went
in that direction with the forecast.

The other issue is the band of showers (rain/snow) wrapping
around the west side of the system. Again all of the high
resolution models bring this precipitation into our TAF sites from
03z through 12z. Even if I accept that, we then have an issue
with the precipitation type. There is only a marginal case to
change this to all snow after 06z. Again since these models
typically do well with that to I also followed them there too.

Bottom line is expect IFR conditions at the SW MI TAF sites
tonight into early Saturday. A slow improvement is expected during
the daylight hours of Saturday.


&&

.MARINE...
Issued at 328 PM EDT Fri Mar 17 2017

We`ll leave the small craft advisories as is. Winds will increase
later tonight as high pressure builds into the upper Midwest and
waves will increase to 2 to 5 feet.


&&

.HYDROLOGY...
Issued at 1123 AM EDT Fri Mar 17 2017

River levels are running around normal to a little below normal.
With less than a half inch of precipitation expected through this
weekend and then no additional precipitation until Friday, no
flooding is expected across the area into next weekend.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 8 PM EDT Saturday for LMZ846>849.

     Small Craft Advisory from 4 AM to 8 PM EDT Saturday for LMZ844-
     845.

&&

$$

UPDATE...WDM
SYNOPSIS...04
SHORT TERM...04
LONG TERM...MJS
AVIATION...WDM
HYDROLOGY...63
MARINE...04



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