Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 300231
AFDGYX

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
1031 PM EDT Wed Mar 29 2017

.SYNOPSIS...
High pressure will gradually build in from the northwest through
Thursday. Expect a mostly sunny day on Thursday with
temperatures near normal for this time of year. Low pressure
tracking through the Ohio Valley will track off the southern New
England coastline Friday night, spreading snow through northern
New England which will last into the day on Saturday. There may
be some rain near the coast. High pressure follows quickly on
the heels of the departing low Sunday into Sunday night. The
next low pressure system may impact our area on Tuesday with
another chance of rain or wintry precipitation.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
0230Z Update...
Area of convergence/fine line on radar over the Capital district
may be producing a few sprinkles but at this point am not
expecting enough to produce more than a trace. otherwise a few
upslope snow showers persist and this will be the case overnight
in the higher terrain. No major changes to the forecast for the
overnight period...watch was not altered at this time and will
wait for new model suite to do so.

2220Z Update...
Isolated rain/snow showers are extending a bit farther towards
the Augusta and Mid Coast region as well as the foothills.
Therefore PoPs/wx grids were adjusted for the next hour or so
until these showers dissipate in more aggressive downslope flow.
The only other changes at this time were in line with current
mesoscale temperature trends.

Previous discussion...
Clouds should break up a bit more tonight as downsloping
northwest winds provide subsidence to the coastal plain. Clouds
will linger to the northwest of the mountains in the upslope
areas. Temperatures fall into the 20s to near 30 which is near
normal for this time of year.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...
High pressure moves into the area during the day on Thursday.
Expect a mostly sunny sky with temperatures generally in the 40s
to possibly near 50 degrees.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...
Confidence continues to grow in a significant late season
snowfall for a good portion of the CWA Friday into Saturday.
However, there still remains several differences between
members in the 12z guidance suite which which lends to
continued uncertainty in snowfall amounts and the spatial
coverage of heavy snowfall amounts.

The ECMWF and the ECMWF ensemble has been the most consistent on
a run to run basis. The ensemble has shown a nice steady,
orderly uptick in 6" probabilities over the last 4 runs
(including the 12z run which now has a large area of 50-60%
probs of greater than 12" across southern NH). The NAM is in the
euro camp, but at this time I think the onset time of heavy snow
is too early and qpf too heavy. The GFS continues to be a
southern outlier due to non-existent help from a northern stream
short wave trough that most of the rest of the guidance suite
has. In essence, what the northernmost guidance members are
doing is digging the northern stream short wave trough into
northern New England which is helping to bring north the
moisture associated with the southern stream trough (sorta like
a partial phase). This process is helping to bring April-like
PWATS into a winter storm system which is resulting in pretty
high QPF in several models. We took a blend of the two camps,
but gave a bit of an edge to a ECMWF and GGEM blend.

Some concerns with mixing with sleet and rain still exist. On
Friday, boundary layer temps may be warm enough, especially in
Maine and the Merrimack valley of New Hampshire to the Seacoast.
Snow accumulations through mid afternoon Friday may be limited
due to melting and a mix with rain. However, when the higher
UVVs move in, we expect snow to become dominant over rain. The
other issue is that some warmer air may advect in from the south
above the boundary layer to the mid levels. This may allow for
some sleet across southern NH for a time Friday night which
would also limit accums. Therefore we are more conservative in
our snowfall accums than what the Euro has.

The upshot is that enough confidence exists in a heavy snowfall
event that a winter storm watch be issued for central and
southern NH at this time. Will hold off on Maine at this time as
this would be more of a Friday night - Saturday event there.
However, the snowfall may indeed be significant over a good
portion of southern/central ME as well; We just have a little
more time to look at the uncertainties there.

Snowfall consistency should be heavy and wet - especially across
southern NH...at least for several hours Friday into Friday
evening. This may lead to power outages as many late-season
snowfalls do. Otherwise, winds shouldn`t be too much of an issue
with this one, but will keep an eye on trends.

In a nutshell - confidence is increasing in a significant winter
storm for a good portion of the forecast area - especially
central and southern zones. However, several uncertainties
remain as outlined above.

The storm pulls out by later Saturday. Thereafter, there is the
potential for something else around Tuesday, but that may remain
well south of us.

&&

.AVIATION /02Z THURSDAY THROUGH MONDAY/...
Short Term...Expect ceilings above 3000 feet through the
remainder of the evening with clouds gradually breaking up to
the south and east of the mountains. Winds should diminish this
evening and only come back at about half strength tomorrow as
high pressure builds in.

Long Term...We expect IFR conditions to develop in snow and rain
on Friday, first in the morning across southwest NH then
spreading eastward through the midday hours. IFR or lower Friday
evening through Saturday morning in potentially heavy snow.
Improvement is foreseen Saturday late afternoon through Monday.

&&

.MARINE...
Short Term...Offshore northwesterly winds will gust to 25 or 30
KT tonight into early Thursday. Should see diminishing winds
Thursday afternoon into Friday morning as high pressure moves
through. Wave heights will be highest further from shore where
the fetch will be enough to possibly generate some 5 FT waves.

Long Term...SCA conditions likely Friday night through Saturday
night with coastal low pressure. Gales possible for a time
Friday night and Saturday.

&&

.TIDES/COASTAL FLOODING...
A very high astronomical tide will occur early Friday morning
with 11.1` expected in Portland at around 06Z. Onshore winds and
seas will just be beginning to develop at this time. Northeast
flow will be more pronounced during the Saturday afternoon high
tide which is lower at 10.3`. Expect about a 0.5 to 1.0 storm
surge at that time which would bring the storm tide to around
11.0 feet or so. With waves running 5 feet or so at the time,
splash-over nomograms suggest to only expect little in the way
of issues along the coastline.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...Winter Storm Watch from Friday morning through Saturday
     afternoon for NHZ003>013-015.
MARINE...Small Craft Advisory until 8 PM EDT Thursday for ANZ150>154.

&&

$$

NEAR TERM...Hanes
SHORT TERM...Ekster
LONG TERM...Kimble


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