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Area forecast discussion 
National Weather Service Grand Rapids Michigan
717 PM EDT Sat Oct 25 2014

Latest update...
aviation

Synopsis...
issued at 330 PM EDT Sat Oct 25 2014

A high pressure ridge will continue to build in tonight through
Sunday and bring fair and seasonably mild weather. A warm front
moving northward across the Great Lakes region will bring potential
for some showers and thunderstorms very late Sunday night and
through the day Monday. Showers and thunderstorms are likely
Monday night out ahead of an approaching cold front. A few showers
may linger into Tuesday morning before fair weather returns for
midweek. A much cooler airmass will move in late in the week.

&&

Short term...(this evening through Monday night)
issued at 330 PM EDT Sat Oct 25 2014

Primary short term forecast concerns involve determining convective
potential from late Sunday night through Monday night.

Elevated instability will ramp up during the early morning hours
Monday to the north of the warm front. 800 mb Li/S are prognosticated to fall
to -1 to -2 by 12z Monday by 12z GFS guidance. 12z NAM guidance is
just a bit slower with the arrival of decent elevated instability
but also shows that 800 mb Li/S will fall to -1 to -3 during the day
Monday.

The combination of increasing elevated instability and forcing from
the warm front and increasing 1000-850 mb moisture transport will
result in development of showers and storms from early Monday
morning through the day Monday.

Showers and thunderstorms are likely on Monday night with plenty
of moisture in place and with precipitable water values quite high
at 1.25 to 1.5 inches. The triggering mechanisms for convective
initiation will come from a 40-45 knots low level jet and then later in the night
from lift/forcing from the approaching cold front. Severe weather is
unlikely given the cold frontal timing and weak instability.

In the nearer term fair and tranquil weather is anticipated tonight
through Sunday afternoon as a high pressure ridge gradually builds
in.

Long term...(tuesday through saturday)
issued at 330 PM EDT Sat Oct 25 2014

It is looking more and more like we will see our first taste of
winter in the Halloween time frame. I am now thinking we may even
see some accumulations inland of the Lake Shore from snow showers
Friday night.

The cold front has been speed up by the models so most of showers
will be done by sunrise Tuesday morning...so southwest Michigan will
get a dry slot during the afternoon which would mean some sunshine.

As is typical for these types of forecasts... the key to what
happens later this coming week is what happens over the central
Pacific Ocean. Actually there is one of those system currently near
Kamchatka that rotates around an upper level system just south of
the North Pole. It then deeps significantly by Tuesday near the
dateline. After that is moves east and deepens further (ecwmf and
GFS both agree on this ) and ends up over the Gulf of Alaska by
Wednesday afternoon. This then builds an upstream ridge over western
North America which in turns deeps the East Coast trough. The models
do not do well with this type of digging Arctic origin upper wave.
So I am thinking whatever will happen will be more dramatic then any
of the models are currently showing.

What we will see from this is the primary digging northern stream
wave comes through southwest Michigan with showers on Thursday. But
that is not when the really cold air gets here. That happens with
the digging 110 knot polar jet dives south Friday into the west side
of the eastern trough. That brings -10c 850 air into lower Michigan
during the day time hours of Friday. Given the +12c lake temperatures...
this would be rather unstable and add a significant upper level wave
I have to believe we will be seeing rain showers change to snow
showers by Friday afternoon or evening. Snow showers will continue
over night Friday night (west of US-131) with some accumulation more
than possible. Very close the lake...near and west of US-31 due to
the Warm Lake... would see mostly rain.

So the bottom line is expect showers Thursday during the day... then
rain showers changing to snow showers Friday afternoon into Friday
evening.

&&

Aviation...(for the 00z tafs through 00z Sunday evening)
issued at 714 PM EDT Sat Oct 25 2014

High pressure will be gradually building into the taf sites
through Sunday. With the pressure gradient weakening and the
temperatures cooling off...the wind will be decreasing by 01z.
VFR weather will prevail through the period.

&&

Marine...
issued at 330 PM EDT Sat Oct 25 2014

A Small Craft Advisory is in effect from late this afternoon
through Sunday afternoon. Brisk west-northwest winds behind the cold front
will cause wave heights to build to 4 to 7 feet this evening. Winds
and waves will gradually subside Sunday afternoon as high pressure
builds in.



&&

Hydrology...
issued at 1157 am EDT Sat Oct 25 2014

Dry conditions will hold through much of the weekend while the
upcoming week gets a bit more active.

Precipitation chances will increase going into late Sunday
night. Showers and a few storms may develop which could total one
tenth to one quarter of an inch of rain. A brief lull is expected
during the day Monday...ahead of a strong cold front that is
anticipated to move through late Monday night and Tuesday morning.
This should bring additional precipitation to the region with
another quarter inch or so of rain. Precipitation totals will
be dependent on any thunderstorm development...meaning that
thunderstorm affected areas have a greater chance and exceeding
the aforementioned expectations. That said...recent dry weather
and probabilities of any big impact precipitation is low. No river
flooding is anticipated at this time.

&&

GRR watches/warnings/advisories...
Michigan...none.
Lm...Small Craft Advisory until 5 PM EDT Sunday for lmz844>849.

&&

$$

Synopsis...Laurens
short term...Laurens
long term...wdm
aviation...mjs
hydrology...jam
marine...Laurens

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