Scientific Forecaster Discussion
fxus61 kgyx 220106
Area forecast discussion
National Weather Service Gray ME
906 PM EDT Wed Mar 21 2018
a coastal storm developing south of southern New England tonight
will spread snow over parts of southern New Hampshire and
coastal western Maine. Snow will stay mainly south of the
mountains, with light to moderate accumulations near the coast.
Amounts of 2 to 4 inches will taper quickly to around an inch
in the foothills. Snow will come to an end Thursday, tapering
off to showers. A trough of low pressure will linger over the
area into Saturday with scattered snow showers in the mountains.
High pressure will build in from the north Sunday and Monday
and will hold over the area on Tuesday.
Near term /through Thursday/...
as of 9 PM... current forecast generally on track. Preponderance
of the high res numerical guidance suggests leading edge of the
snow moving into far southern New Hampshire and southwest coast of ME by
around 04-05z. Have only made some minor tweaks to the quantitative precipitation forecast/snow
forecast through 06z to better reflect current conditions. Winds
have been a little slow to pick up but have been slowly
increasing over the last couple of hours along the coast.
Remainder of the forecast looks fine for now.
shortwave trof exiting the southeast coast will cyclogenesis off the
mid Atlantic coast this evening. Low pressure will move newd
towards the Canadian Maritimes by Thu. Confluent flow aloft is
helping to keep low level flow nly...advecting dry air down thru
much of the forecast area. This has been eroding the leading
edge of snowfall for most of the day. It is just now that
stronger advection from the southeast shortwave is forcing the main
precip shield nwd. That slow and steady advance will continue
this evening...reaching srn New Hampshire by about 8 PM. Snow may hold off
until closer to midnight in the white mtns and parts of wrn ME.
Despite a fairly unconventional and complicated evolution...by
about 2 am tonight deep saturation and lift thru the snow growth
zone should allow for a band of moderate snow roughly parallel
to the coast of wrn ME. Snowfall rates from midnight to 4 am may
approach a half inch an hour at times. Hi-res model guidance
continues to be quite aggressive with snowfall amounts...pushing
warning criteria for the seacoast and some of the islands and
necks in wrn ME. On the other end of the spectrum the 21.12z
ecwmf and eps remain steadfast on meager quantitative precipitation forecast. Probabilities for
3 or more inches of snow are about 10 percent for the extreme
sern parts of the forecast area. Given the European model (ecmwf) held serve...I
have not changed the snowfall amounts from the midday update.
Winter wx advisories remain in effect...though the nrn parts of
those zones will be on the marginal side. The intensity of the
snowfall is forecast to rapidly weaken by around 8 am.
Short term /Thursday night through 6 PM Thursday/...
a weakening band of frontogenesis and deformation northwest of
departing low pressure will lingering precip into Thu. It will
gradually become more showery into the afternoon...with rain
possibly mixing in as temps warm. Some forecast guidance does
indicate a saturated snow growth zones lingering over the
forecast area well into the afternoon. So pop may need to be
increased for showers in later updates. Only light additional
accumulations are expected. Nly winds will remain gusty in the
wake of low pressure...with some gusts up to 25 kts possible.
Long term /Thursday night through Wednesday/...
upper level low will remain in place over the region through much of
Friday will be mostly cloudy with light northwesterly flow as the
upper level trough remains to the west. High temperatures will be
near average near 40 in the south and mid 30s in the north.
Saturday the low shifts overhead bringing an increased of
snow showers through the mountains.
By Sunday a stronger upper level vort Max will move in allowing the
trough to have a positive tilt as it moves out. The cold air will
push in possibly accompanied by a few snow squalls. High
temperatures will barely reach freezing.
High pressure moves in for the start of next week with clear skies
and near average temperatures. Another oceanic low will pass out to
sea in the middle of next week.
Aviation /01z Thursday through Monday/...
short term...expecting VFR conditions for another few hours
while dry air erodes the leading edge of the -sn. After 04z srn
New Hampshire will likely quickly go to IFR conditions in snow...closer to
06z across coastal wrn ME. In that 06-12z window...snow may become
moderate at times with LIFR conditions possible. Snow tapers off
mid morning Thu...though shsn may linger into the afternoon...especially
across nrn New Hampshire and wrn ME. North-northeast winds will also remain gusty...with
surface gusts approaching 20 to 25 kts at all terminals.
Long term... VFR conditions will prevail through the weekend
under mostly cloudy skies. Upslope snow showers may briefly
reduce ceilings to MVFR in the mountains.
short term...NE winds will continue to steadily increase thru
tonight ahead of approaching coastal storm. Expecting gales
outside the bays...with marginal gales in Casco and Penobscot
Bay. Seas are expected to increase to 15 to 20 ft tonight. Winds
and seas will slowly diminish Thu.
Long term... seas and winds will subside as the midweek storm
moves out to sea. Winds will again increase to near gales by
Sunday morning as another front moves through the waters. An
oceanic storm may generate large swell in the middle of next
a tidal anomaly at close to a half foot and an additional storm
surge up to two feet are expected tonight near the time of high
tide. This will combine with building nearshore waves of 10 to
15 feet to produce generally minor beach erosion and splash-
over flooding. Pockets of moderate impacts are possible, due in
part to the damage of sand dunes and infrastructure from the
storms earlier this month. Astronomical tides peak as well
There will also be coastal flooding with up to a foot of water
on the side streets near Portland Harbor and near Marginal Way.
Flooding can also be expected along Granite Point Road in
Biddeford and the backbay area of Hampton. Some streets in
Portsmouth may have minor flooding. Areas that have splash-over
will have also some flooding due to the pooling of water
landward of the dunes.
ME...Winter Weather Advisory until 11 am EDT Thursday for
Coastal flood advisory from 1 am to 5 am EDT Thursday for
New Hampshire...Winter Weather Advisory until 9 am EDT Thursday for nhz012>015.
Coastal flood advisory from 1 am to 5 am EDT Thursday for
Marine...Gale Warning from 1 am to 5 PM EDT Thursday for anz151-153.
Gale Warning until 5 PM EDT Thursday for anz150-152-154.