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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KGYX 281051 AAA
AFDGYX

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
651 AM EDT Wed Oct 28 2020

.SYNOPSIS...
An area of light showers will move across southern portions of
the area today, while the day remains relatively cool even
outside of any precipitation. The approach of the remnants of
Zeta and a storm system currently affecting the southern Plains
will bring our next chances of precipitation. A generally strung
out area of rain will move into the southern half of the
forecast area Thursday afternoon. Thursday night rain will
change to snow as colder air moves in from the north. While snow
totals will be light, it will likely be the first accumulating
snowfall of the season for many locations. Cold weather lasts
through at least Saturday before high pressure shifts east and
allows temperatures to slowly rebound.

&&

.NEAR TERM /TODAY THROUGH TONIGHT/...
-- Changed Discussion --
Update...Minor changes to reflect latest observational trends.
Light rain is being reported across Srn NH...as far N as
CON...and no significant increase in intensity is expected this
morning.

Previous discussion...Zonal flow overhead today will allow a
weak S/WV trof to zip thru the area. WAA ahead of it will lead
to some showers along and N of the warm front. It remains pretty
dry aloft thru the day...so precip will probably tend more
towards drizzle than rain at times. I opted for a relatively
high PoP given the light QPF amounts expected. Clouds will try
to clear from the W in the late afternoon. Tonight clouds will
increase again from the W...which should keep temps from getting
too cold.
-- End Changed Discussion --

&&

.SHORT TERM /THURSDAY THROUGH THURSDAY NIGHT/...
Most impactful part of the forecast period expected late
Thu...as the first snowfall for many locations looks likely.

Undeniably the trend in guidance has been to keep the remnants
of Zeta and the ejecting upper low two separate features. As
such...fast flow and confluence over the Canadian Maritimes is
keeping the resulting system flatter. Less amplification is
leading to less QPF...so despite plenty of cold air advecting
into the system snowfall looks to be light.

Much of Thu should be dry...with precip moving into Srn zones in
the late afternoon. Initially the air mass will be relatively
mild...and so rain is expected. High pressure building to the N
of the system will set up a feed of cold and dry that will
gradually cool the column thru the night. Current forecast
soundings have changeover occurring after midnight.

The best lift looks to remain mainly S of the forecast
area...but may sneak into the Monadnock region early Fri.
Overall that limits potential heavy snow...and higher snowfall
amounts. I also think that dry air on the Nrn edge of the system
may limit QPF in those areas...and so ultimately a sharper edge
to snowfall than my current forecast is possible.

Trends today will be quite important...as ensemble sensitivity
has much of the forecast variance explained by a more amplified
system. If we can see observed heights to the NE of the upper
low come in lower than forecast we could see trends move back
towards more amplification and higher snowfall amounts over
parts of Srn NH and extreme SWrn ME.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
First things first, we start the long range forecast period wrapping
up the storm that is the main story for this forecast package. By
Friday morning, a broad low pressure area making up the post-
tropical remnants of Zeta will be draped west-to-east over the
Atlantic waters south of New England with upper level support in
the form of a shortwave trough lagging behind as it approaches
the coast. Upper level phasing between these features remains a
key source of uncertainty with this storm with widely varying
QPF solutions depending on how well forcing lines up over New
England... as was discussed above the general trend has been
toward weaker phasing.

One feature I`m keeping a close eye on in models is a mid-level
(around 7-800mb or so) trough axis and associated deformation/FGEN
zone on the backside of the departing surface system. Should
moisture remain available, this would give us one more ramp up of
forcing Friday morning into mid-day, potentially resulting in
another inch or two of wet snow before dry air filters in from
the north later in the afternoon. As has been noted in prior
AFDs... while the uncertainty in phasing is revealing a plethora
of deterministic model solutions... model ensembles have been
able to provide some confidence in some aspects of the forecast.
By 12Z Friday, cold advection will be the rule through the
column, so any sleety/mixed precipitation from the overnight
period would switch to all rain or snow depending on how low-
level temperatures shape up with the diurnal trend. Speaking of
temp profiles... there is model consensus in a fairly deep
isothermal layer of m5 to m10 C up through the mid- level lift
zone, so there shouldn`t be that much of a fluff factor as is
usually the case on the backside of synoptic systems unless
temperatures are significantly cooler (unlikely without also
going dry) or better phasing occurs and more lift is realized
in the DGZ higher up (trending less likely). Did hedge a little
on the higher side of QPF/snowfall with this last burst as
ensembles favor moist over dry low/mid- level profiles south of
the Whites.

As it sits right now, have additional snow up to about 1-2"
through mid-day Friday on top of what falls Thursday night. A
sharp northern terminus of precipitation remains in the forecast
with highest QPF around 1" at the Mass border and less than
0.1" north of a line from around Gorham NH to Waldo Co ME...
little change to the prior forecast other than to tighten it
further. All in all a couple slushy inches looks to be a fair
bet between Thursday night and Friday south of the Whites across
into extreme southwest Maine with storm totals up to around 4",
mainly in the higher terrain of southern New Hampshire. This
would make for messy Friday morning travel as commuters remember
how to drive in wintry conditions. Temperatures will hang
around freezing through the day, likely depending on
precipitation so regardless of if a wet snow or rain falls it`ll
be a raw day especially as a north wind picks up over Maine and
along the coast. Will also have to keep an eye on phasing and
coastal amplification to see just how strong this north wind
will be but for now have gusts to around 20-25 mph there,
favoring the less-phased solution.

The good news is that quiet (albeit cool) weather returns to the
forecast area at least to start the weekend with upper level ridging
and surface high pressure building into the northeast. High pressure
passes overhead Friday night into early Saturday, providing what may
end up being the coolest night of this season so far as temperatures
plummet into the teens (north) and 20s... depending on how quickly
we can scour out low level moisture in the wake of the messy late-
week system. Temperatures rebound into the 30s and 40s on
Halloween under clear skies with an increasing southerly flow
into Sunday as high pressure moves offshore.

Models are in good agreement that by Sunday afternoon a long-wave
trough axis will extend from the Arctic down to the Gulf of Mexico
Coast, setting up a broad warm and moist advection over the
eastern CONUS. Locally this likely spells increasing onshore
flow yielding increasing clouds, fog, and drizzle with
temperatures limited by SSTs currently in the low-50s. A cold
front associated with the long wave trough tracks into New
England around the end of Sunday with increasing chances for
rain showers, switching over to snow over the higher terrain
which may persist through Monday in upslope flow. A
significantly drier airmass filters in behind this front, then
high pressure builds in through mid-week with a dry and cool
forecast to start the work-week. One fly in the ointment is a
little clipper- type system in cold advection which could
deliver light showers on Tuesday... but confidence is low and
impacts would be minor. Stuck close to the consensus blend for
the forecast this weekend into next week given stronger than
usual consensus in the synoptic pattern.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Short Term...VFR conditions will give way to MVFR and eventually
areas of IFR CIGs this morning ahead of passing S/WV trof. I do
not expect much in the way of SHRA at terminals...but most
likely across Srn NH including MHT and PSM. VFR conditions
tonight into early Thu will quickly give way to IFR over Srn NH.
Still a fair amount of uncertainty how far N that makes it...but
any changeover to SN will lead to IFR or lower VSBY. Most likely
window for that will be 06z to 15z Fri.

Long Term...IFR or worse conditions in SN or RASN are expected
for most terminals except for KHIE and perhaps KAUG/KRKD on
Friday with a north wind. Conditions will improve to VFR from W
to E as the system pulls away during the second half of Friday,
prevailing through Saturday. Increasing onshore flow Sunday may
lead to lowered ceilings and fog.

&&

.MARINE...
Short Term...Offshore flow will increase today but is expected
to remain below SCA thresholds. As high pressure builds to the N
thru Thu the NE flow will increase over the waters. CAA and
favorable wind direction for overperforming speed...so some gale
force gusts are in the forecast. A gale watch may be needed on
subsequent shifts.

Long Term...Marginal gale force winds over the outer waters
gradually wind down Friday as low pressure passes south of the
Gulf of Maine and pulls away into the North Atlantic. High
pressure moves into the region by Saturday with winds weakening
and seas falling below 5 ft. Another period of SCAs is likely at
least over the waters with the next cold frontal passage late
Sunday into Monday.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.

&&

$$

NEAR TERM...Legro
SHORT TERM...Legro
LONG TERM...Casey
AVIATION...Casey/Legro
MARINE...Casey/Legro