Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 230044
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
744 PM EST Wed Feb 22 2017

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 329 PM EST Wed Feb 22 2017

Mild weather will continue with highs in the 50s both Thursday and
Friday, and lows mainly 40 to 45.  However rain showers will arrive
Thursday night and linger through Friday night.  Some thunderstorms
will also be possible, with some of these storms possibly severe
into Friday evening.

Colder air moves in late Friday night into Saturday, and the rain
will be switching over to snow.  Some light snow accumulation is
possible Saturday.  Another chance of rain and snow moves back in by
Tuesday night and Wednesday.  Cooler for the weekend and into early
next week with highs mainly be 35 to 40.

&&

.UPDATE...
Issued at 743 PM EST Wed Feb 22 2017

I increased the chance for measurable rain t around 40 pct over
our NW and N CWA this evening into tonight. The latest radar image
loops shows showers north of Hart moving east northeast. There is
a strong low level jet feeding these showers, if there were a
little more moisture in the 850 mb to 700 mb layer than there
actually is we would have much larger coverage and maybe even a
few thunderstorms. I continue to believe the dry air in that layer
will prevent the showers from becoming more widespread this
evening.

&&

.SHORT TERM...(This evening through Friday night)
Issued at 329 PM EST Wed Feb 22 2017

A few light showers will be possible tonight with a cold front
passing through.  It won`t be as warm Thursday behind this front,
but it will still be in the 50s.  Then a wet and stormy system
arrives for Thursday night through Friday night.

The moisture does not look that impressive with tonight`s front and
most areas probably stay dry. but will carry low POPs along and
north of I-96.  A northerly wind moves in for Thursday, keeping us
slightly cooler.  Mid 40s for Central Lower to around 60 for South
Central Lower.  After a cloudy start on Thursday we should see some
sun into the afternoon, but no pcpn is expected.

Main story will be the system approaching and moving through
Thursday night through Friday night.  Still appears to be potential
for severe storms, with the evening looking most favorable.  We see
a coupled jet move in by late afternoon, with a low level jet
arriving into the evening.  CAPE values are only modest, but with
the amount of dynamics believe we will see storms.  It seems like
the highest potential for severe storms will be along and south of I-
96 where dew points will manage to reach the 50s as this area gets
into the warm sector.  However it may be somewhat questionable,
whether the best dynamics reach us during the highest instability.
If the LLJ is delayed too much, much of the action could be to our
south.  If we see severe storms, damaging winds will be the main
hazard.

It will be a wet system with rain totals expected to be around an
inch.  Perhaps higher in West Central Lower.  At the onset of the
pcpn Thursday night we may see a bit of rain/snow mix over Central
Lower, but this should be brief and not cause issues.

Later in the event, the colder air arrives in the west just prior to
daybreak Saturday, so have included the chance of snow mixing in
late Friday night there.

.LONG TERM...(Saturday through Wednesday)
Issued at 329 PM EST Wed Feb 22 2017

The long term period will start off with the low pressure system
pulling away from the region on Saturday.  After being dry
slotted...the wrap around moisture will pivot through the CWA during
the day and into the evening.  Thus...higher POPs are warranted.
Based on the thickness values this looks to be a mainly snow
event...but some rain could mix in especially early on. Surface
temperatures will largely be above freezing...so impacts will likely
be limited.  It appears later Saturday...as the temperature starts
to drop off below freezing...that the risk for impacts will be
elevated. This is also when the precipitation will be winding down.

Sunday into Monday the risk for precipitation looks low. One storm
tracks up the OH valley for Monday.  Overall the trend of this track
has been further south...so this system should miss our region. Then
on Tuesday another storm will be approaching from the west.  Will
feature increasing POPs for Tuesday.  This system has trended
warmer...so it is currently looking like a rain event.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 700 PM EST Wed Feb 22 2017

There are two issues tonight to consider. First is the threat for
fog again tonight and second is the chance of showers and
thunderstorms. There is still some fog along the lake shore from
Grand Haven north and with the front coming through shifting winds
to the west it may well come on shore. However I would think since
this is a cold front and there is 40 knots as close to the ground
as 500 ft in the 06z to 09z time frame there would be way to much
wind to get dense fog. None the less the RAP and HRRR show this
impacting MKG and those models did very nicely last night so I did
put dense for in the MKG TAF after the winds let up (09z-14).
Those same models show the fog reaching GRR toward sunrise. Once
again this is behind the front (cold front) but winds do let up
toward morning so I did put a few hours of dense fog there too but
I am not 100% on this happening. Elsewhere I should expect the
mixing behind the cold front to keep fog becoming to dense but I
do believe we will be seeing a period of low clouds till mid
morning then skies should clear.

As for convection...there is 40kt to 50kt low level jet that puts
southwest Michigan to it`s left, which is a great place to get
storms to happen. There is 300 to 500 j/kg of most unstable cape
too but the instability is elevated above 700 mb. Also there is a
considerable amount of dry air once one gets above the 850 mb. So
I put VCSH in all the TAFs in the 03z to 06z time frame to cover
this.


&&

.MARINE...
Issued at 455 PM EST Wed Feb 22 2017

Webcams show vsbys have improved at Holland, Grand Haven and
Ludington over the last couple hours, but vsbys where still very
low at the Muskegon waterfront. The trend will continue to be
improving vsbys as winds increase, so will allow the fog advisory
to expire at 5 PM.


&&

.HYDROLOGY...
Issued at 1201 PM EST Wed Feb 22 2017

Snowmelt in the headwaters of the Muskegon River basin has caused
the river to rise to above normal for late February. The Grand and
Kalamazoo rivers are running near normal. Showers and storms on
Friday may put down swaths of rainfall in excess of a half or
perhaps 1 inch. Some rivers would be susceptible to rising above
bankfull if that amount of rain falls in their respective basins.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...Small Craft Advisory until 4 AM EST Thursday for LMZ845>849.

&&

$$

UPDATE...WDM
SYNOPSIS...JK
SHORT TERM...JK
LONG TERM...MJS
AVIATION...WDM
HYDROLOGY...CAS
MARINE...JK



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