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Area forecast discussion...updated
National Weather Service Gray ME
919 am EDT sun Oct 22 2017

Synopsis...
high pressure will gradually shift off to the east today. A
slow moving cold front will approach from the west Monday night
and Tuesday and will very slowly move across the region through
Thursday delivering beneficial rainfall. High pressure will
build in from the southwest on Friday and provide fair weather
over the weekend.

&&

Near term /until 6 PM this evening/...
update...mostly minor adjustments to temps...dew points...and
sky cover based on latest observational trends. NE flow is in
place over the coastal waters...and this will tend to keep some
relatively cooler air stuck in across wrn ME and maybe down into
the seacoast of New Hampshire...but wrn zones should see plenty of 70s
again today.

Original discussion...
high pressure slowly slinks off to the east today with a
light southeast flow developing over coastal areas today. There
is a pool of greater moisture which has developed over the Gulf
of Maine with dewpoints in the 50s noted. This will be pulled
inland today on the southeast flow. Temperatures today will be
similar to yesterday, generally in the 70s. The biggest question
on temperatures will be how much of an influence the southeast
flow has on temperatures over the coastal plain of Maine. The
met guidance suggests high temperatures around 60 due to the
southeast flow and some cloud cover. But this seems a bit too
drastic. At any rate, the greatest cooling influence will be
felt right along the coastline. It may be a bit too early to
expect marine cloud cover to develop which would be necessary to
keep temperatures that cool.

&&

Short term /6 PM this evening through 6 PM Monday/...
as noted before, a pool of moisture has developed over the Gulf
of Maine and will remain in place there and across the coastal
plain of Maine and New Hampshire tonight. This raises suspicions
of low cloud development as significant moisture over the cool
waters often leads to low clouds and fog. However, the moisture
seems quite limited to the lowest levels right near the surface.
Just above the surface at 925 mb it is much drier which suggests
it may be difficult to get a marine stratus cloud layer to form.
Instead it is more likely that radiational cooling will cause
fog formation over both the water and the land areas, with
valley locations the most favored. However, with the rapid
drying just above the surface it is possible that a good deal of
this moisture gets deposited as dew as temperatures cool tonight
with fog possibly limited to patch ground fog. Temperatures
tonight likely stay in the 40s for most of the area, but the
inland cool spots could cool into the 30s in spite of the moist
start to the night.

Temperatures Monday will again be warm, but perhaps not quite as
warm, only in the upper 60s and low 70s. With time, the moist
air mass in place will likely lead to low clouds developing over
the ocean and moving into the coastal plain, but this may hold
off until Monday evening.

&&

Long term /Monday night through Saturday/...
the long term period starts out with a marked pattern change
where deep troughing approaches from the west, likely giving US
some much needed rainfall. Not too much has changed in the
overall picture here - trough moves in Tuesday with heaviest
rain likely Tuesday night through Wednesday night. However,
there are still big questions about how amplified the trough
gets and therefore how long the forecast area remains in a
strong southeasterly inflow regime. This will determine how
much rain we eventually get. Either way, the prefrontal air mass
will be characterized by anomalously high precipitable waters streaming in on
a strong S/southeast low level jet. We'll get our rain - it's just a
matter of how much. 00z deterministic model runs are in
agreement that 1-3" of rain will be common at a minimum.
However, there are some - including the GFS and European model (ecmwf) - that
bring a widespread 3-6" with locally higher amounts - especially
in favorable upslope areas of the whites and western ME
mountains. While it has been very dry lately, some of these
higher end quantitative precipitation forecast outputs would likely pose a flooding risk for
smaller steams and other poor drainage areas. This outcome
wouldn't be all that surprising given the prolonged
southeasterly inflow with high precipitable waters . Again, this will depend on
how amplified the approaching trough gets.

In the dailies, we think Monday night will be the night that low
clouds come in and precip chances increase due to increasing
moist southerly inflow. Precipitation shouldn't be heavy though.
This general idea continues into Tuesday - pops increasing
during the day but heavy rain holds off.

Forcing for ascent really increases Tuesday night and Wednesday
as the cold front and dynamics with upper trough moves in. This
is when we should see rain, heavy at times, along with gusty
winds. Speaking of winds, the southerly low level jet will be
strongest Tuesday night and Wednesday. While there will be some
degree of a low level inversion, gusts up to 40 miles per hour cannot be
ruled out - especially in convective showers.

The weather Wednesday night and Thursday will depend on how
amplified the upper trough gets. Heavy rain could continue
Wednesday night into Thursday if the trof takes its time moving
east. We'll be able to Iron this out with time, but again, there
could be a substantial amount of rain by the time the system
departs. It's much needed. But there is a low probability that
it could be too much at once for small streams.

Fair weather finally moves in for Friday and the weekend.

&&

Aviation /13z Sunday through Thursday/...
short term...VFR conditions today with light winds. Could see
some ground fog developing tonight, with fog most likely in the
valley locations. VFR again on Monday but there will likely be
a developing layer of maritime stratus just offshore waiting to
move in. It seems likely this will hold off until Monday
evening.

Long term...IFR/LIFR ceilings Monday night through Wednesday.
MVFR ceilings Thursday.

&&

Marine...
short term...high pressure slowly moves east with south or
southeasterly flow gradually increasing with time.

Long term...sca's may be needed Monday night. Gales possible on
Tuesday into Wednesday.

&&

Gyx watches/warnings/advisories...
ME...none.
New Hampshire...none.
Marine...none.

&&

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