Area Forecast Discussion
Issued by NWS Portland, ME

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729
FXUS61 KGYX 240226
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1026 PM EDT Fri Jun 23 2017

.SYNOPSIS...
Tropical moisture from the remnants of Tropical Storm Cindy...
and an approaching frontal system will result in showers and
thunderstorms overnight. A series of surface troughs and weak
cold fronts will cross the area this weekend into early next
week bringing us the chance of showers from time to
time...especially in central and northern areas.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
1020 PM Update...Have updated the forecast for current trends in
PoPs and temperatures. Have blended in the HRRRTL overnight for
PoPs as well. Mainly dry at the moment but a short wave trough
approaching from the west should result in an increase in
showers as can be currently seen over upstate NY. Have kept
isolated thunder in with weak elevated instability present, but
we don`t think thunderstorms will be commonplace overnight. A
few heavy downpours are likely in convection, but widespread
flooding issues do not appear likely at this time due to lack of
organization to the convection. Showers/isold thunder should end
from west to east during the early to mid morning hours on
Saturday.

7 PM Update...Have updated the forecast a bit, mainly for PoPs
over the next several hours based on latest trends in radar and
mesoscale model data. For the most part, have relegated PoPs to
the far west and north for the rest of the evening as points
east remain relatively stable and lack forcing for ascent.
Chances will once again increase after midnight. At this time it
appears the severe potential is done, but heavy rain later
tonight will have to be monitored.

Previously...

High Impact Weather Potential: Isolated severe thunderstorms
possible through this evening.  Localized flooding also possible.
Very minor coastal flood issues at the time of high tide
tonight.

Current Pattern: Early afternoon water vapor imagery reveals
deep...most southwesterly flow entrenched over the northeastern
United States with blended total precipitable water product
showing a warm sector featuring PWATS of 1.5 to 2 inches /+3 to
4 sigma/ having overspread all of the northeast except far
northern Maine. First band of shower activity along the surface
warm front has pushed north with southern portions of the GYX
forecast area having broken into the warm sector with
temperatures over southern areas racing through the 80s /and
even lower 90s/ with dewpoints rising towards 70F. Surface cold
front is gradually pushing into the eastern Great Lakes region
with areas to the east of this lacking much in the way of
organized forcing. The result has been isolated/scattered
clusters of showers and thunderstorms given modest isentropic
ascent of the warm/moist airmass in the warm sector. There are a
few forecast challenges through the near term period.
First...is whether we can see any significant convection through
the evening. Then...the focus turns to Cindy/s remnants that
will stream north and east along the approaching cold
front...with at least some potential for a period of heavy rain
later tonight.

Through this evening...Regional radar mosaic continues to paint a
band of showers along the international border close to the low
level warm front.  South of this...activity is much more
isolated...but feel that as the cold front begins to approach from
the west...showers/storms will become more numerous...particularly
over western areas /CT Valley/.  SPC mesoanalysis shows a blossoming
area of 1000 J/kg of MLCAPE which...along with the aforementioned
good PWATS should favor precip loading and the potential for gusty
winds...with a few severe storms not out of the question.
Otherwise...a steamy late afternoon as temps over northern areas
continue to climb.

Tonight...Expect scattered to numerous showers/thunderstorms
over western areas to gradually progress east...but with the
thunder aspect dwindling some as instability diminishes. High
resolution guidance /HRRR and the High Res window WRF runs/ all
keying in on taking Cindy moisture /over the Ohio Valley/ and
having it make a run at at least the southern half of the
forecast area well after midnight and towards daybreak Saturday.
PWATS will increase to well over 200% of normal...with warm
rain processes likely becoming dominant. Thus...see some
potential for heavy rain after midnight as this band of showers
/and some embedded thunder/ pushes in. Expect marine stratus/fog
to continue to expand inland and west as the night goes on.
Temperatures will be very mild ahead of the front...likely
remaining in the 70s over southern NH and upper 60s to around 70
elsewhere.

See tides/coastal flooding section for more details on high tides
tonight.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
High Impact Weather Potential: Locally heavy rain through the mid
morning hours over coastal areas. There is also a moderate risk
of rip currents along the coastline on Saturday.

Saturday: 12Z Guidance trend has favored a slower arrival of
the surface cold front with the front bisecting the forecast
area at 12Z Saturday. South and east of this feature...showers
/with a few embedded rumbles/ will likely continue through at
least mid morning before the shortwave associated with Cindy/s
remnants shifts east. Will have to keep an eye on heavy rain
potential...but confidence in any significant issues is not high
given that it looks to be a pretty quick hit...and due to poor
run to run model continuity. The story of the afternoon will be
robust dry advection which should take morning dewpoints that
will be near 70 over southern/coastal areas back into the upper
50s by late in the day. T8s remain +12-13C through the
afternoon...which will allow temperatures to reach well into the
80s south and east of the mountains...with mid/upper 70s to the
north.

Saturday Night:  Cold advection continues under northwesterly flow
aloft.  Secondary cold front nears the international border during
the evening...and may very well see some evening shower activity
along and ahead of this feature. With a digging shortwave over the
Great Lakes region...however...expect heights to actually begin to
rise after midnight. This...combined with the llevel drying should
slowly bring mountain shower activity to an end /as the front stalls
to our north. Expect temperatures able to fall back into the lower
50s in the mountains /decent gradient aloft should preclude all but
patchy fog/ with upper 50s to just above 60 to the south and
east.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
An approaching upper level trough will trigger scattered showers
on Sunday. A few locations over central and northern areas may
be sufficiently destabilized to trigger and thunderstorm or two,
mainly during the afternoon and early evening hours.

A broad long wave trough will remain over the region on Monday,
allowing for more clouds and the chance for a shower across the
region, again mainly during the afternoon and early evening
hours. Lapse rates will not be overly steep during the day,
which may minimize the threat for thunderstorms over the region.

On Tuesday, the long wave trough progresses eastward, allowing
for more in the way of showers and thunderstorms across the
region. The wind flow will shift to the west on Wednesday with
drier air entering the region, allowing for some clearing over
downslope regions. Upslope areas likely to have yet another
shower, mainly diurnally driven.

Towards the end of next week, the 12Z model suite diverges
somewhat with low pressure passing through southern New England,
central New England or to the north of New England depending on
the model of choice. This will have large ramifications on
temperatures and weather conditions late in the week next week.

&&

.AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
Short Term...

Summary:  A cold front will move through the region tonight with
scattered showers and thunderstorms...becoming more numerous after
midnight. Drier air moves into the region on Saturday and continues
through Saturday night.

Restrictions: VFR conditions should dominate into this evening all
sites except KRKD where marine stratus will bring a prolonged period
of IFR cigs.  Approaching cold front and very moist low level
airmass will allow for restrictions to expand...with MVFR/IFR
conditions likely after midnight as lingering Cindy moisture streams
up ahead of approaching cold front.  Expect improvement to VFR on
Saturday as cold front moves south of the region.  VFR expected
Saturday night.

Winds: Southerly winds strengthen to 10g15kts for this
afternoon...remaining near 10kts tonight before shifting west
northwest 12g18kts for the day on Saturday and diminishing to less
than 10kts Saturday night.

LLWS: 1kft winds to 35kts from the south tonight at RKD above stable
marine layer...causing a period of LLWS.

Thunder:  Can/t completely rule out a stray thunderstorm at any of
the terminals through this evening...although the best chances will
be at HIE/LEB. Tonight...a few thunderstorms may be embedded with
showers that move through the region associated with the cold
front.

Long Term...A dry west to northwest flow out of Canada will keep
conditions VFR late this weekend and much of next week. There
will be a chance of afternoon showers and possibly isolated thunderstorms,
with the best chance being across the interior.

&&

.MARINE...
Short Term...SCAs continue outside the bays for southerly winds
ahead of an approaching cold front. Winds will shift westerly
and diminish some on Saturday with waves gradually subsiding.
Winds/waves continue to subside on Saturday night.

Long Term...Winds will be fairly light through the coming week
in the absence of any strong pressure systems, with the daily
flow being dominated by the sea breeze.

&&

.TIDES/COASTAL FLOODING...
Astronomical high tides will continue through the weekend with
the possibility of minor flooding particularly with the tide
tonight which is 11:11 pm at Portland. The astronomical high
tide will be 11.8 feet with a storm tide overnight likely coming
in between 12.2 and 12.6 feet. With little in the way of wave
action, expect only very minor issues along the coastline.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Small Craft Advisory until noon EDT Saturday for ANZ150-152-
     154.

&&

$$

NEAR TERM...Ekster
SHORT TERM...Arnott
LONG TERM...Cannon
AVIATION...Arnott



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