Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 191113 AAB
AFDGYX

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
713 AM EDT Wed Sep 19 2018

.SYNOPSIS...
High pressure will build in from the north today and will hold
over the region on Thursday. A warm front will lift northeast
through the region Thursday night and early Friday followed by a
strong cold front Friday night. High pressure will build over the
area Saturday and will shift east into the maritimes late Sunday
and Monday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
711 AM Update: Refined cloud cover and inserted another few
hours of patchy drizzle wording over southeastern NH and coastal
ME given a few observations of such early this morning.
Otherwise...only minor tweaks to line forecast up with 7am
observations.

520 AM Update: Minor adjustments to cloud cover to match slowing
westward advance of stratus shield. Otherwise...have matched up
to current observed temperatures and dewpoints with no other
changes needed at this time.


Previous discussion below...
High Impact Weather Potential: Minimal.

Pattern: Complex surface pattern as of early this morning with
frontal boundary responsible for yesterday/s rainfall now south
of the New England coast and gradually sinking south and east.
North of this feature...somewhat of a back door cold front is
working south and west across northern New England...demarked
nicely by GOES-16 nighttime microphysics product. Behind this
boundary...low stratus has developed along with a decent
northeasterly wind. Ahead of this feature...skies are still
mostly clear with fog having developed in the valleys given
light winds. Early morning water vapor imagery shows another
band of upper level moisture to our west associated with an
upper level jet streak and very weak mid level disturbance.
Through today high pressure will continue to build in from the
north...but with only modest llevel dry advection our forecast
concerns center around low stratus and it/s evolution through
the day and impact on temperatures.

Through Daybreak: Backdoor cold front will slow as it moves west
across NH with patchy fog to it/s west and low ceilings to its east.
Have seen a few -RA reports /WVL-RKD/ which is more likely fog or
some light drizzle.  Temperatures will continue to fall a bit
more...particularly over eastern NH where the front has most
recently passed...with 50s to the east of the front and 60s to the
west.

Today: Southwestward moving frontal boundary reaches the CT Valley
before stalling...with mesoscale guidance suggesting a weak plume of
MUCAPE along the western edge of this boundary where there will be
some llevel convergence and implied moisture pooling.  Soundings
demonstrate a larger scale subsidence inversion...but there may be
enough instability below this to squeeze out a shower and have
included isolated PoPs for this potential. Cloud cover is going to
be stubborn to break up today given the trapped moisture and lack of
real robust dry advection in the low levels.  Expect some sun to
develop by afternoon...esp over western Maine and just west of the
front over the CT Valley with the potential for more clouds in
between.  Given the cloud cover and subsidence inversion aloft...we
won/t fully realize mixing to H8...with mixing to H9 suggesting
highs generally in the 60s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
High Impact Weather Potential: Minimal

Pattern: High pressure gradually building in from the
north...along with low pressure developing along the front well
south of New England will induce a strengthening northerly
gradient Tonight through early Thursday which will bring
increased drying to the llevel environment as mid level heights
build. Lower tropospheric ridge crests overhead by the end of
this period with warm advection encroaching from the west.

Tonight:  Only weak dry advection portends another night of stratus
and fog as llevel moisture re-solidifies beneath subsidence
inversion.  Where stratus does not fully develop a ceiling...there
should be plenty of moisture for fog development...which is most
likely over the CT valley where stratus may not impact.  Airmass
will certainly be cooler than 24 hours previous...with lows in the
40s north and lower 50s south...a bit closer to seasonal norms for
middle September.

Thursday: Strengthening northerly gradient which will get a boost
from northerly ageostrophic component of the flow at the leading
edge of 160kt H2 jet streak helping to scour more of the low level
moisture.  This should yield more sun than on Wednesday...but with
some remaining llevel moisture...and mid/upper level moisture
beginning to spill over arriving H5 ridge crest...still expect skies
to be no better than partly sunny.  Temps aloft will not change
significantly...so expect highs to only be a bit warmer given
more sun...still ending up being in the 60s in most spots...
with a few locations over the CT valley and southern NH around
70.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
High pressure will shift east Thursday night as a warm front
pushes north and a return south to southwest flow develops by
Friday morning. Associated with the passage of the warm front,
scattered showers are expected across the mountains while
elsewhere conditions should remain dry. During Friday all areas
end up in the warm sector with the increasing southwest flow and
warmer temps and increasing dewpoints. Preferred the warmer model
solutions for Fri into Fri night. Models all agree on the passage
of a strong Canadian cold front associated with a clipper late
Fri night. Although the wind field and shear ahead of the cold
front will be strong and favorable for tstorms, the timing of the
front later at night with the loss of heating will not be
favorable. Some strong tstorms still will be possible with the
showers but not expecting any severe storms to occur.

Behind the FROPA a cooler and drier air mass will move into the
region for the weekend as Canadian high pres builds east from the
Great Lakes. Much cooler temps expected Sat night due to
radiational cooling as high pres becomes centered over the area
allowing some readings to drop into the 30s in the mountains and
foothills and 40s elsewhere. Highs Sat will be generally in the
upper 50s mountains to 60s elsewhere. Sunday max temps moderate
some reaching back into the mid 60s to lower 70s.

It appears high pres will move into the maritimes Monday as the
next system approaches from the southwest. Timing on this
approaching system varies model to model so stayed close to NBM
solution.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Short Term...

Summary:  High pressure will exert an increasing influence over
flying conditions through Thursday...gradually scouring out low
level moisture currently in place across the region.

Restrictions: Largely IFR at the moment in low stratus with a few
patches of fog and drizzle out there as well as of 11Z.  Expect very
gradual improvement to VFR at the terminals today. However...
conditions will again deteriorate tonight to LIFR/IFR in fog and
stratus before more quickly improving to VFR at all sites on
Thursday.

Winds: Northeasterly winds around 10kts will continue today before
becoming light and variable tonight and continuing through Thursday
before turning onshore along the coast Thursday afternoon.

LLWS: No LLWS is expected through Thursday.

Lightning: No lightning is expected through Thursday.

Long Term...VFR conditions lowering to MVFR in any developing
scattered showers late Fri mainly in the mountains and foothills.
Widespread showers and scattered tstorms with the passage of the
cold front Fri night. High pres builds into the region for the
weekend bringing VFR conditions Saturday into Monday.

&&

.MARINE...
Short Term...

711 AM Update: Removed SCAs for all but Merrimack River-Cape
Elizabeth given weakning winds and lowering wave heights.
Remainder of SCA will likely be able to be taken down later this
morning.

Previous discussion below..
Marginal SCA conditions under northeasterly flow continue this
morning. Winds and waves in the far outer waters will remain
close to marginal SCAs this afternoon and tonight before
diminishing on Thursday as high pressure continues to build in
from the north.

Long Term...SCA`s likely Friday afternoon through Friday night due
to a strong south to southwest flow ahead of an approaching cold
front. Behind the front winds subside from the north Saturday.
Light winds and seas Sunday and Monday.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Small Craft Advisory until noon EDT today for ANZ154.

&&

$$
SYNOPSIS...
NEAR TERM...Arnott
SHORT TERM...Arnott
LONG TERM...Marine
AVIATION...Arnott
MARINE...Arnott/Marine


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