Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 301528
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1128 AM EDT TUE AUG 30 2016

.SYNOPSIS...
High pressure will be over the region today with seasonably mild
temperatures and continued dry conditions. Temperatures will warm
on Wednesday ahead of an approaching cold front with showers and
thunderstorms possible. This cold front will slowly move offshore
through Thursday with additional showers and a turn to cooler
temperatures. Outside of an isolated mountain shower Friday...
seasonably cool and dry conditions are expected before a dry and
seasonably mild holiday weekend as a slow moving high pressure
system moves into the northeastern United States.

&&

15z update...
Skies continue to clear across southern and coastal Maine late
this morning and this trend will continue into early afternoon as
high clouds push east. Clouds associated with weak over-running
across southern Canada will keep skies in northern zones mostly
cloudy through mid afternoon. Have adjusted temps/td`s/sky grids
based on current obs. No other changes planned attm.


Previous discussion...
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Update...Minor changes to adjust for latest observational trends
in temps and dewpoints.

Previous discussion...High pressure is settling into the region
this morning. Closer to the ridge center...valley fog will be more
common across NH this morning. It will be patchy dense at times
across the CT River Valley.

The high pressure center will slowly shift E thru the
day...allowing return flow to develop late. This should aid the
sea breezes by the afternoon as well. Clouds will also spill into
the area late as the upper ridge axis moves E too.

A continued long period SW swell will continue today as well.
Though the wave heights are around 2 to 3 ft...periods over 10
seconds will support a moderate risk for rip currents.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
A s/wv trof diving out of the Hudson Bay region will continue to
push a cold front towards New England tonight. The impact of this
trof looks to be a little blunted from a few days ago...thanks to
the potpourri of tropical systems out in the Atlantic. With no
room for the trof to amplify into...it just sort of sweeps thru
the region and lifts out. This will be good enough for scattered
showers and maybe an isolated thunderstorm. Initially there may be
some WAA type showers...mainly for Nrn zones...but eventually
convection along and ahead the front will move into the forecast
area. The jet streak aloft strengthens late Wed...so coverage of
precip should be highest later in the day versus the first half.
Shear does increase as winds aloft do too...but instability
remains rather meager...so the strong to severe threat seems
rather limited. If we can produce better moisture return there
could be a chance for a stronger storm or two however.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
High Impact Weather Potential: Some threat for onshore swell/rip
current risk early next week. Otherwise...minimal.

Pattern: A look at water vapor imagery early this morning across
North America reveals the development/deepening of a broad longwave
trough over the Gulf of Alaska with an even broader synoptic scale
ridge dominating much of the eastern 2/3rds of the United
States...with a relatively progressive flow along the northern edge
of this feature near the U.S./Canadian border.  For the remainder of
this week...the primary change to the upper pattern will be
downstream amplification in response to the Gulf of Alaska trough as
it deepens further and moves ashore along the west coast of North
America.  A wave train will develop downstream of this feature with
a rather potent trough arriving over the eastern United States
Thursday and Friday /with a return to temperatures near seasonal
norms/. However...with a lack of downstream blocking...the Omega
block in place Friday morning will continue to slowly progress east
through this weekend. While there are disparities in the model
forecasts by Sunday-Monday...there is broad agreement that as the
eastern trough exits our area...some energy /or a cutoff feature/
will remain over the southeastern states...yielding a Rex-block over
the region as impressive heights for early September build into the
northeastern United States/eastern Canada.  Were this to occur...we
could very well expect a return to much warmer than normal
temperatures next week...with another stretch of dry days likely.
Before this...eyes will be on frontal passage to start the period
Wednesday night-Thursday as this presents the only real
precipitation threat through the long term forecast.

Tropics: The developing eastern trough late this week...and ongoing
northwest flow behind this feature as it pushes east of the region
this weekend favors keeping tropical activity well to our south
through the long term forecast period.  The GFS is an outlier in
taking TD 9 and lingering it not all that far off the New England
coast early next week as a weak system...and will continue to not
buy into this idea.  Can foresee...however...a period of enhanced
swell early next week with renewed rip current threat certainly
possible for Labor Day...which given the warm temps will be worth
keeping an eye on.

Model Trends/Confidence: Strong deterministic/ensemble agreement
exists through Saturday...with some divergence Sunday/Monday
regarding how quickly/fully the northeastern trough exits the region
and where/if a cutoff low forms over the Mid Atlantic region.

Given that we have multiple tropical systems over the southeast to
start the period...have to be a bit cautious in just how confident
we are this weekend/early next week as...depending on any deepening
of these two depressions...they will have some potential impact on
the large scale flow.

The Details....

Wednesday Night: Pre-frontal trough and upstream cold front continue
to push south and east Wednesday night with height falls spreading
overhead ahead of impressive mid level shortwave.  While instability
will be lacking...plenty of moisture /PWATs around 1.5 inch...+1-2
sigma/ will be available...and with good dynamic support for ascent
expect at least scattered showers to continue through the night.
With ample cloudiness...expect overnight temperatures to remain
elevated...upper 50s north to upper 60s south...a bit above the
guidance consensus.

Thursday: With remnants of TD 8 well offshore...cold front will slow
it/s southeastward progress...bisecting the area in the
morning...but not clearing the coast until mid afternoon.
Thus...continued shower potential /and thus chance PoPs/ looks good
over southeastern areas.  Can see some showers further north as well
in the cold advection regime as temps aloft fall /below -15C/ near
the core of the trough at H5...with a collocated spoke of available
mid level moisture.  Add to this daytime heating and some upslope
help and isolated/scattered showers /rumble of thunder/ appear
reasonable here as well.  T8s straddling +10C suggest highs near 70
north...and upper 70s to around 80 south...in line with the guidance
consensus.

Thursday Night - Friday: Shower activity will come to an end
Thursday evening with the loss of daytime heating and arriving high
pressure from the northwest.  A much cooler night expected Thursday
night given the cooler airmass overhead and at least partial
clearing. Can/t rule out a couple of mountain showers on Friday with
some limited instability and upslope assistance beneath a developing
subsidence inversion.  Cu rule suggests we/ll have decent CU growth
even south of the mountains...but in the downslope regime do not
expect precipitation.  T8s around +8C will result in highs a bit
lower than Thursday...with upper 60s north to mid 70s south looking
on target...right in line with guidance consensus.

Friday Night - Sunday.  A beautiful /except for the continued
drought conditions/ weekend is in store as high pressure crests over
New England with light winds and T8s warming from around +7C
Saturday to +10C Sunday.  Dry wedge overhead suggests little/no
cloudiness...so full daytime heating should allow highs to range
through the 70s Saturday with some 70s and lower 80s on Sunday.
Comfortable sleeping weather in store both nights with dry airmass
in place...with lows in the 40s and 50s from north to south.  Expect
we/ll drop a few degrees below guidance consensus each night.

Labor Day:  The forecast period looks to end under a dry regime with
high pressure still anchored overhead.  Already a decent signal in
the NAEFS/EC-ENS for heights well above climatological norms...which
argues for lots of sun...and temperatures above seasonal norms under
light winds.

&&

.AVIATION /16Z TUESDAY THROUGH SATURDAY/...
Short Term...VFR conditions will dominate as high pressure slowly
shifts E today. CT River Valley fog will produce local IFR or
lower conditions at HIE and eventually LEB thru sunrise before
lifting. A cold front will slowly approach the area from the NW
tonight. Scattered SHRA and maybe an isolated TSRA will be
possible along and ahead of the front...with MVFR or lower
conditions in the heavier convection. Precip will be most likely
late tonight into Wed.

Long Term...Scattered showers Wednesday night and Thursday will
bring the threat of isolated/scattered brief MVFR/IFR
restrictions with VFR conditions dominating Friday through the
coming weekend outside of morning valley fog HIE/LEB.

&&

.MARINE...
Short Term...Winds and seas look to remain below SCA thresholds
thru Wed. Wave models do want to bring seas above 5 ft into the
coastal waters today. Given that these models are already running
a foot or so above observations...I knocked the guidance down a
foot thru Wed. This better matches the idea of a very long period
swell that the buoys are picking up.

Long Term...Southwesterly flow will shift northwesterly on Thursday
as a cold front moves through the waters...with winds turning light
for the coming weekend as high pressure move into the northeastern
United States.  Greatest threat of SCA conditions will come to start
the period with marginal SCA wave heights over the outer waters.
Beyond this...wind/waves look to remain below SCA levels through
the weekend.

&&

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

&&

$$


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