Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 141130
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
630 AM EST Tue Nov 14 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 324 AM EST Tue Nov 14 2017

Areas of fog will lift by late morning, then it will be partly
cloudy and sightly warmer with highs in the mid 40s. A quick
moving frontal system will bring a period of rain to the area
during the morning hours of Wednesday and it will be breezy and
slightly warmer yet with highs in the upper 40s. Some lake effect
rain and snow showers are expected early Thursday. Then a strong
fall storm will bring showers and windy conditions later Friday
into Saturday morning. The cold front will bring sharply colder
temperatures and lake effect snow showers with some accumulations
Saturday night into Sunday.

&&

.SHORT TERM...(Today through Thursday)
Issued at 324 AM EST Tue Nov 14 2017

There are two main issues with this forecast, the dense fog
advisory for our southern 2 rows of counties this morning and the
rain threat Wednesday morning. There will also likely be lake
enhanced rain and snow showers Wednesday night into Thursday
morning as the cold air comes back in.

We have large surface high passing just east of Lower Michigan
this morning as a developing storm system heads for the Great
Lakes Wednesday. The storms center will cross Lake Superior
Wednesday morning which puts southwest Michigan on the warm side
of the system. It will tap into polar air to the north of the
system tomorrow, which will help strengthen the system
considerably.

With the surface high to our east we now have southerly winds at
the surface and there is a large area of dense fog over most of
northern Indiana as I write this. The southerly winds will bring
this fog into our southern 2 rows of counties (mostly west of
I-69) by sunrise this morning. This is shown well by numerous runs
of the HRRR, RAP and NAM model visibility forecasts. So, instead
of waiting for the fog to move in I issued the fog advisory now.
Since most areas that this fog will move into already have
temperatures below freezing we may have a black ice issue too. So
in the Dense Fog Advisory I addressed these issues.

Once the fog mixes out we may have a few hours of sunshine before
the mid and high clouds from the next system move in. This should
allow highs in the mid 40s today.

The rain Wednesday morning is well supported by 1000/850 moisture
transport, jet dynamics, isentropic ascent, and deep moisture.
There has been excellent model continuity on this system and good
ensemble support too. So expect about a 6 hour period of light to
moderate rain Wednesday morning. The instability for
thunderstorms stays south of Michigan tomorrow so no issues there.

Since this system taps into the polar air tomorrow as it is
developing we will get a period of lake enhanced rain changing to
snow showers Wednesday night into Thursday morning. Skies may
clear by late in the day Thursday as the system pulls out.

.LONG TERM...(Thursday night through Monday)
Issued at 324 AM EST Tue Nov 14 2017

A high pressure ridge will continue to bring fair weather Thursday
night. Southerly flow waa will develop on the back side of the
departing ridge Friday and scattered showers will develop as
moisture increases out ahead of the strengthening low pressure area
off to the west.

Rain will become widespread late Friday through Friday night and
there is also a chance for a few thunderstorms to develop Friday
evening if enough instability develops just ahead of the
strengthening low and cold front. However the better chance for
convection should stay south of our fcst area where there is
potential for greater instability to develop.

Windy conditions will develop Saturday with strong nw flow caa on
the back side of the strengthening sfc low. Winds will potentially
be strong enough to cause some impacts in the form of scattered
power outages. The brisk nw flow caa will cause pcpn to change over
to snow showers in the afternoon and evening.

It is noted though that fairly significant medium range guidance
discrepancies remain in terms of the eventual track and strength of
the sfc low and also with regard to timing of the system. So
forecast confidence at this time remains only moderate at best.

Lake effect snow showers are anticipated through Sunday with the
deep upper trough in place over the eastern CONUS and with delta t/s
well into the teens. However larger medium range guidance
discrepancies continue in terms of just how cold h8 temps will be by
Sunday and this has significant implications in terms of les
potential.  The wx pattern should become more tranquil by Monday as
any lingering snow showers taper to flurries as the ridge to the
west gradually begins to build in.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 631 AM EST Tue Nov 14 2017

There is an rather extensive area of low clouds and dense fog just
south of the Michigan/Indiana state line at 11z. The GOES-16 fog
product shows a narrow ribbon of dense fog along the county
boards of Allegan and Kalamazoo and Cass and St Joesph that was
spreading northward and expanding west and eastward. I still
believe that the HRRR, RAP and NAM fog forecast for this fog to
rapidly expand north around sunrise makes sense so I brought it
into AZO and BTL. By the time it would reach any of the other TAF
sites it should have mixed out.

I also believe, as suggested by the NAM3km that all of the low
clouds will blow out by late this morning leaving largely most
clear skies.

Tonight that system moves in from the west with increasing
southerly winds. This will keep low clouds and fog out of the
picture tonight. The rain should not move into the TAF sites until
after 12z. Possibly MKG and GRR may see rain by 12z, so those were
the only sites I brought the rain into by 12z Wednesday. Once the
rain moves in expect solid IFR for a few hours.

&&

.MARINE...
Issued at 324 AM EST Tue Nov 14 2017

Little question we will need Small Craft Advisory headlines this
afternoon into Wednesday. The only real issue is what to do about
the gales tomorrow night as the cold air comes in. After
coordinating with the Chicago office I decided to just go with a
small craft advisory for now and then we can issue the gale
warning tomorrow when the time gets closer.


&&

.HYDROLOGY...
Issued at 110 PM EST Mon Nov 13 2017

River levels are running near to above normal for mid-November.
Another quarter to half inch on Wednesday will keep the ground
soggy. A Fall storm system may produce rainfall totals around a
half to 1 inch Friday into Saturday, which would cause river
levels to rise again.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Dense Fog Advisory until 10 AM EST this morning for MIZ064>066-
     071>073.

LM...Small Craft Advisory from 11 AM this morning to 6 PM EST
     Wednesday for LMZ844>849.

&&

$$

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



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