Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 231110

Area Forecast Discussion
National Weather Service Gray ME
710 AM EDT Mon Oct 23 2017

High pressure will hold over the maritimes tonight through Monday. A
slow moving cold front will approach from the west Monday night and
Tuesday and will slowly cross the region Wednesday through Thursday.
A ridge of high pressure will build over the region on Friday
and will shift offshore on Saturday before the next low pressure
system approaches from the southwest.


710 AM Update...Little change to the going forecast. Stratus and
fog are well represented in the forecast and will likely hold
tough on the coastal plain and southernmost nh today.


Low stratus has overspread a good portion of the forecast area
this early Monday morning except for the northern mountains.
Most locations with stratus and some fog are in the 50s, while
temperatures in the 30s to mid are common northwest of the
cloud shield.

The main question today is how much of the low clouds burn off.
There isn`t much of a scouring wind today with low level
inversion likely to hold firm for much of the day. The
northwest/west edges of the stratus deck should erode with
daytime heating, but the coastal plain may be stuck with it much
of the day. Even there, it should go BKN for awhile this
afternoon with some heating, but then quickly fill in again by
early evening. Therefore, we think mostly cloudy should do it
for the coastal plain, with sunnier skies expected the more
inland one is. Despite some early morning drizzle, it should be
dry today all areas with highs well above normal values.


Low clouds will once again push westward this evening and
overnight along with some fog and drizzle. Temperatures will
remain much warmer than normal overnight due to the cloud cover
and increasing low level jet and associated moisture advection.

The pressure gradient tightens up on Tuesday as a cold front
approaches from the west with strong high pressure centered
south of the Maritimes. This will result in increasing southerly
winds during the day, with gusts around 25 mph likely by
afternoon. Cloudy skies will be the norm with low level moisture
continuing to stream on in on a steadily strengthening low
level jet. Any heavy rain will hold off until Tuesday night but
there will be showers around during the day.


A long awaited pattern shift finally arrives later this week as
a deep trough develops just to our west. The trough will dig
south out of the Canadian prairies and form a cut off low over
the great Lakes by Wednesday. This trough will then moderate
and shift eastwards on Thursday. A second reinforcing cold air
mass dives south for next weekend.

Tuesday night will see a strong cold front on our door step to
the west. Ahead of it we`ll have strong southerly flow bringing
warm moist air into the region. Despite the calendar saying
late Fall, the moisture will run up to 4 standard deviations
above normal which should be enough to provide around 500 J/kg
of CAPE. Along with the cold front comes a very strong low level
jet. Still some variation with the strength of this jet in the
forecast models but expect at least 50kts within 3000ft of the
surface to provide plenty of wind shear. See SPC day 2 outlook
for marginal risk of severe thunderstorms.

As the front moves through we`ll see two main effects, wind and
rain. Wind gusts to 30kts are possible along the coast,
especially just ahead of the front itself. Still some
uncertainty in the timing and extent of these stronger gusts and
so have not issued a wind advisory for the moment, however one
may be needed.

With the front taking over 24hours to clear our area this looks
like an excellent producer of much needed rain. Storm total
rainfall will reach 2-3 inches across the region and higher
amounts are possible with any heavier more convective lines. The
highest amounts of rainfall will be in the upper kennebec basin
in Maine where the southerly flow runs into the moutnains as
well as along the immediate coastline where we may see some
coastal front effects. Both these regions could use the rain and
the coast remains in D1 drought. With rivers low the only chance
for flooding would be in the flashier basins where the rainfall
rates could briefly exceed the ability of the water to be
absorbed. The Pemi at Woodstock and Swift river at Roxbury are
the only points near the headwaters flash flood guidance.

After the front finally clear the region on Thursday a brief
ridge of high pressure will build back in before the next system
begins to move in for the weekend.


Short Term...IFR conditions across much of the region early this
morning should gradually improve a bit as the morning
progresses. However, MVFR cigs are likely today on the coastal
plain. Cigs and vsbys are expected to come down to IFR or lower
tonight and remain through Tuesday.

Long Term...Slow moving cold front will move through the region
on Wednesday into Thursday. This will bring widespread rain
with a rumble of thunder possible to all terminals. Expect a
decrease to IFR conditions in heavy rain as the front moves
through. Morning heavy rain, combined with a strong LLJ, as high
as 60kts at 3,000ft will impact the region ahead of the line
with conditions at or near airport minimums possible through
Wednesday morning. Despite fairly gusty surface winds overnight,
this strong jet will still result in low level wind shear
through the coast early Wednesday morning.


Short Term...SCA conditions are expected to develop Tuesday in
response to increasing southerly winds. Until then winds and
seas should remain below SCA thresholds, but marine fog will
likely limit vsbys.

Long Term...
 Southerly flow will continue to increase into Tuesday night out
ahead of the cold front. With high pressure parked off shore
there will be a long southerly fetch which will help to increase
the winds and waves. A low level jet of over 50kts will move in
just in front of the front and wind gusts to gale force are
expected across the outer waters. A gale watch has been issued
for Tuesday night. With the southerly flow, expect a large
portion of penobscot bay to also be exposed to the winds and
have issued a gale watch for them as well. Casco bay will remain
more sheltered and thus will likely only see small craft




A slow moving cold front will provide a prolonged period of
onshore flow. Seas will build to around 12ft in onshore flow.
While this is a descent wind wave, all other factors for coastal
flooding remain low. We are at an astronomical low point in the
tidal cycle with high tide at Portland in the 9ft range.
Additionally all of the wave is in the wind wave at around 8
seconds with no long period swell. The low tide and short period
means that even though there may be around a foot of storm
surge it will not be enough to cause problems.


MARINE...Small Craft Advisory from 8 AM to 8 PM EDT Tuesday for
     Gale Watch from Tuesday evening through Wednesday evening for



NEAR TERM...Ekster
LONG TERM...Curtis is the U.S. government's official web portal to all federal, state and local government web resources and services.