Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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866
FXUS63 KGRR 282344
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
644 PM EST Tue Feb 28 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 315 PM EST Tue Feb 28 2017

A low pressure system moving through the Great Lakes region will
bring rain and a chance of thunderstorms tonight. A much colder
airmass will gradually move in Wednesday and cause rain to
changeover to snow. Temperatures will average below normal for
this time of year Thursday through Friday with some lingering
light snow showers and flurries.

&&

.SHORT TERM...(This evening through Thursday night)
Issued at 315 PM EST Tue Feb 28 2017

Short term fcst concerns involve determining convective potential
tonight and potential for accumulating snow Wednesday.

Elevated convection moving east across our northern fcst area this
afternoon will continue to bring potential for hail in addition to
brief heavy downpours and frequent lightning. None of this
convection is or will become severe but hail up to around one
quarter of an inch in diameter remains possible.

A low pressure system currently centered over southern Iowa will
continue to move ene and bring rain and scattered convection into
our fcst area tonight and potential for some locally heavy
rainfall. Shear profiles will be very impressive but we expect
severe weather to stay south of our fcst area where instability
will be much more favorable. This far north we expect only weak
instability to develop tonight.

Brisk north to nw flow caa will occur on the back side of the
system Wednesday which will cause rain to change over to snow.
Strong mid level frontogenetical forcing will develop over our far
northern fcst area in the morning through the afternoon. Therefore
we expect snow to become heavy at times over our northern fcst
area tomorrow.

We will hoist a winter wx advisory for our northern two rows of
counties (with the exception of Isabella county) where 4 to 6
inches of snow is forecast tomorrow. Locally higher amounts seem
possible where mid level fgen banding is most persistent.

The combination of the snow strong n-nw winds and falling temps
through the day will produce hazardous travel conditions up there.
South of that area around an inch or two of snow is fcst tomorrow
through tomorrow night.

Light snow showers and flurries will linger Thursday and it will
remain cold with high temps only reaching the upper 20`s to middle
30`s.

.LONG TERM...(Friday through Tuesday)
Issued at 315 PM EST Tue Feb 28 2017

More than anything this is a progressive weather pattern and the
blocking over Greenland does not seem to be strong enough to stop
the progression. So, what we have had we will see more of.  That
means large temperature swings with convection on the warm side of
systems passing by and snow on the cold side.

Just about all the models have some sort of version of a band of
snow (warm advection lift event) Friday night into Saturday. Where
exactly that sets up and timing it to within 6 hours is questionable
but none the less there will be a event so I have pops for that.

The next Pacific systems goes well north of Michigan over the
weekend and Pushes a stronger warm front through the area late
Saturday into Sunday.  That will more than likely be a rain event
but it lacks Gulf moisture so I do not expect more than a quarter
inch in any one place from that event.

The big storm is the one on Monday into Tuesday. It will be like the
one last Fri and today into tomorrow as it will be deep, have strong
southerly flow ahead of it bringing Gulf of Mexico moisture north
with it with a strong upper level jet. It also taps cold air so that
will help it have a lot of wind too. Monday could get really warm if
the sun could come out even for a little while. The of course the
cold air surges back in and we get snow showers again.  How much
snow is hard to say this far out in time. It should be noted that
the ECMWF is about 12 hours faster than the GFS on this system so
the snow would start sooner if the ECMWF was to be correct.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 623 PM EST Tue Feb 28 2017

Ifr conditions will continue this evening due to low cigs and
patchy fog and reduced visbys in rain showers. Scattered
thunderstorms will also potentially affect most of the terminals
this evening and overnight.

Mainly ifr conditions will continue Wednesday and it will become
windy as north winds increase to 20 to 25 kts with gusts to near
30 kts toward midday. Rain showers will gradually transition to
snow Wednesday afternoon as much colder air moves in. Conditions
should remain primarily ifr through Wednesday evening due to low
cigs and reduced visbys in snow.

&&

.MARINE...
Issued at 315 PM EST Tue Feb 28 2017

We will issue a small craft advisory for early Wednesday morning
through mid morning Thursday as north winds will back to the
northwest and strengthen to around 30 kts Wednesday. This will
cause wave heights to ramp up considerably Wednesday into
Wednesday evening.


&&

.HYDROLOGY...
Issued at 315 PM EST Tue Feb 28 2017

Considerable rises are forecasted on the area rivers as a low
pressure system pushes through the CWA on Wednesday.  This system
will tap abundant Gulf moisture.  The frontal zone that this system
will be tracking along is already situated over the region.  What
this means is increasing coverage and intensity of the rain at the
system nears.  Multiple rounds of rain will likely occur and the
rain will be heavy at times.

Forecasted rainfall amounts range from  near an inch across the Pere
Marquette and Upper Muskegon basins to close to 2 inches near the
MI/IN border. The rivers can handle such rainfall amounts without
significant flooding.  If we see basin average amounts over 2
inches...or if the axis of heavy rain ends up being further
north...the risk will increase.  Those northern basins are elevated
because of the recent snow melt and as a result are more vulnerable
to flooding.

Higher rainfall rates associated with the downpours could cause some
urban and poor drainage flooding.  Rainfall rates could top a half
inch per hour in the heaviest storms.  Given the moist
conditions...small stream flooding will need to be monitored as well.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory from 9 AM to 9 PM EST Wednesday for
     MIZ037>040-043>045.

LM...Small Craft Advisory from 7 AM Wednesday to 8 AM EST Thursday
     for LMZ844>849.

&&

$$

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



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