Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 181610 AAB
AFDGYX

Area Forecast Discussion...UPDATED
National Weather Service Gray ME
1210 PM EDT Mon Jun 18 2018

.SYNOPSIS...
A Southwest flow ahead of an approaching cold front will bring
very warm and humid conditions to the area today. Some areas on
Monday will see heat index values break 95 degrees. The
atmosphere destabilizes significantly during this afternoon
with widespread thunderstorms expected. Once the front moves
though tonight, temperatures will moderate a little bit.
Another cold front moving through on Wednesday will push
temperatures down to near normal before they start rising again
toward next weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1159 AM Update: Noontime regional radar imagery shows a broken
line of convection extending from southern Quebec south and west
into far upstate New York. This convection is well ahead of the
surface cold front...and lines up with a subtle surface trough
and mid level impulse. The airmass south and east of this
feature is destabilizing...but slowly with both temperatures and
dewpoints lagging previous forecast. Thus...expect this activity
to gradually increase in intensity as it moves towards our
forecast area...reaching northern zones between 1 and 2pm. Deep
layer shear is ample...generally around 40kts in the 0-6km
layer. This will support some organized updrafts if we can
realize the instability...with the best threat for this being
over the southern two thirds of NH and far southwestern Maine.
Biggest threat is certainly wind with the strong winds aloft but
the aforementioned updraft organization...some hail is possible
and an isolated tornado is not completely out of the question
although pretty large surface dewpoint depression negates this
being a significant threat. More likely is heavy rainfall given
PWATs now moving above 1.5" and some potential for training
/albeit fast moving/ storms as their motion increasingly becomes
aligned with the mid level flow.

As for changes...have lowered temperatures and dewpoints given
late morning observational trends...with the remainder of the
forecast in good shape at this time.

853 AM Update: Updates this hour include adjusting PoPs to
increase them in shower/storm activity over northern sections in
line with morning KGYX 88D radar trends. Otherwise...have issued
a beach hazards statement along the coast for cold water
hazards. Other changes were minor and to match the near term
with morning observational trends.

615 AM...HRRR doing a solid job with morning showers moving
through, and based on it, should see the showers and weak
convection in Quebec cross the CWA by 12-13Z. After that thing
should heat up quickly, and rest of forecast looks good. HRRR
showing two rounds of convection, one around 19-21Z and another
around 00Z, both have severe potential although first line may
end up better severe threat, and second line may be the flooding
threat, but this remains to be seen.

Previously...In the immediate future, a couple of of bands of
showers will moves across the nrn and ern zones thru early this
morning, these are depicted fairly well by the HRRR, which clear
them out of the ern zones by 12Z. I suspect the HRRR may be
hanging on them too long, but wither way they should be
weakening as they move thru and not amount too much by the time
the cross the CWA, despite some lightning currently N of KBTV/.
Further west toward Ottawa, the front has been fairly active
overnight as well, and the HRRR does bring some SHRA/TSRA across
the far nrn zones this morning, but the concern for severe will
be this afternoon into this evening.

But first, let`s discuss the heat. Pretty decent low to mid
lvl WSW jet moving through today with 850 temps surging to
around 20C this afternoon, so, mixing down these warm temps
should not be a problem, and wind direction should provide
additional in the southern areas due to the downslope. The
downslope will help keep Tds down a bit, but they will still get
well into the 60s to around 70 in some spots, and this will
produce heat index values in the mid 90s to around 100 in much
of the southern half of NH, and SW ME. HEat indices will
probably reach into the 90s across much of the CWA today, with
only the mid-coast, and the ME mountains missing out. I did
extend the heat advisory into Cumberland county as interior
areas , especially on the coastal side of Sebago Lake, should
get into criteria, as well and the coastal areas around
Portland. Probably on the other side of Casco Bay, near
Brunswick and definitely Harpswell, HI values will stay below
90.

On to the t-storms this afternoon. Most of the storms associated
with the immediate front will likely not move into the northern
until late in the afternoon, and we`ll talk about these in the
tonight part of the discussion. Still will have to watch for any
storm that do pop up this afternoon. Good capping with warm mid
lvls should keep them from getting too strong, but if anything
can break the cap, they will have plenty of fuel to work with,
and a rotating supercell is not out of the question, given
decent shear and plenty of instability, and despite weak mid
level lapse rates.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Line of storms associated with frontal will likely cross into
the mountains between 4 and 6 pm. This line will be the best chc
for any severe weather as it traverses the CWA from NW-SE
during the late afternoon and evening. Best chance will be away
from the coast in ME and central and northern NH, but cannot
rule out a line that makes pretty close to the coast. Wind
damage will be on threat from these storms, as will lightning,
as there should be a fair mount of it. The other issue the flash
flood potential as air mass will be quit moist / PWs of 2+
inches/, and with front slowing as it crosses the CWA, should
set up an env that favors back-building of storms, especially
this evening.

As the line of showers heads into srn zones late this evening,
should the threats diminish, although will likely see another
weaker line move into the mtns mid-late evening, before that
washes out. Models suggest decent NW flow behind the front, ,
which should dry out the mountains, but as is often the case,
will the winds down south of the mountains? My guess is probably
not until sunrise, and this will keep the low level moisture in
at the surface, and produce fog, and a muggy night, with lows
ranging from around 60 in the north, to the mid to upper 60s in
the south.

Any early morning clouds and fog should mix out quickly Tue
morning, with dry NW winds picking up, and generally sunny
skies. The downslope should allow for temps in the lows 80s in
the south, and generally 75-80 in the north.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
A cold front with very little moisture associated with it will
move across the region late Wed. Behind the front temps will be
cooler and near normal with dry conditions through the end of
the work week. On Sat a warmer return southwest flow will
develop in advance of the next system approaching from the
midwest. More humid conditions will develop late Sat into Sun.
By Sunday the upper trof is progged to move across the region
producing unsettled weather.

&&

.AVIATION /16Z MONDAY THROUGH FRIDAY/...
Short Term...Coastal fog/stratus should mix out after sunrise
this morning with VFR expected into the afternoon. a Line of
TSRA with strong wind gusts will move thru during the late
afternoon into the evening, and conds behind the line should
keep MVFR to IFR flight restrictions thru a good part of the
night at all terminal except for KHIE, and maybe KLEB. VFR
expected Tuesday.

Long Term...VFR conditions Tue through Fri.

&&

.MARINE...
Short Term...No real changes to forecast thinking, with SCA in
place for later today through most of tonight in SW flow ahead
of a cold front.

Long Term...light winds Tue night will increase from the
southwest Wed ahead of an approaching cold front. Winds may
approach SCA conditions briefly over the outer waters late Wed.
A cold front moves across the waters Wed night shifting winds to
the northwest and continuing Thu. High pressure becomes centered
over the waters Thu night and Fri with light variable winds.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...Heat Advisory until 7 PM EDT this evening for MEZ018-019-023-
     024.
     Beach Hazards Statement until 8 PM EDT this evening for
     MEZ023>028.
NH...Heat Advisory until 7 PM EDT this evening for NHZ005>015.
     Beach Hazards Statement until 8 PM EDT this evening for NHZ014.
MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ150-152-154.

&&

$$

NEAR TERM...Arnott


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