Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 290215

Area Forecast Discussion
National Weather Service Gray ME
1015 PM EDT Thu May 28 2020

Warm and humid conditions remain in place, but relief is on the
way with a gradually crossing frontal complex which brings
widespread showers starting tonight with thunderstorms possible
through Saturday. Gusty winds and heavy rain are possible with
storms. Behind the front, a much drier and cooler airmass
arrives to start the next week.


1015 PM Update...Just a few minor adjustments to the going
forecast based on latest observational data. Lower clouds
continue to gradually work northward near the MA/NH border with
isolated showers/patchy drizzle from time to time. This activity
should move northward into southern zones over the next hour or

730 PM Update...Little change to the going forecast. A warm
evening to be expected. Increasing clouds will occur from the
south overnight as low level moisture continues to increase.


Expect another warm and humid night with high dew points leading
to more foggy night over the waters. Low temperatures tonight
dip into the 60s, except for over the MidCoast where southwesterly
flow brings in maritime air; expect lows there in the mid- to
upper- 50s with sea fog impacting the immediate coast, low
clouds and patchy fog several miles in.

Meanwhile aloft the ridge responsible for building the warm and
humid airmass shifts east with increasing baroclinicity ahead of an
approaching long wave trough over the upper Great Lakes region.
Lofting north ahead of the trough during this time will be the
remnants of tropical depression Bertha, riding a 40kt LLJ; so
while warm frontal forcing won`t be particularly strong, PWAT
will greatly increase tonight, to 1.75-2.0", so any shower does
have wetting rain potential. Greatest chances for rain tonight
is in the Connecticut River Valley and into the mountains.


On Friday, the forcing trough inches closer to the northeast.
Here in New England we remain squarely in the warm sector with
dew points well into the 60s. Am expecting more sunshine
tomorrow than we had today, so have temperatures reaching into
the mid- to upper-80s for the area corresponding to maximum heat
index values in the mid-80s to near 90.

With these temperatures and dew points we will develop quite an
unstable airmass... however there remains very little overlap
between favorable daytime thermodynamics and favorable dynamic
forcing closer/wind fields closer to the trough over the Great
Lakes. Any storm that does form during the day will have heavy
rainfall and gusty wind potential, but organization will be
lacking. The best chances for severe weather come later in the
evening as convection over upstate New York and VT track east
into the area. In general they will be on a down trend, but
isolated damaging winds will be possible in storms, primarily
over New Hampshire and into the western Maine mountains -
corresponding with a Marginal Risk from SPC.

Showers will continue through tomorrow night with thunder
gradually diminishing. Otherwise expect another warm, humid
night with a more southerly clip to flow allowing sea fog to
impact areas at least along the immediate coastline through NH
and ME, further inland along and east of Casco Bay/Portland.


High impact weather potential: Severe thunderstorms possible

A cold front will cross the region on Saturday setting off showers
and thunderstorms. This looks like a classic set up for summer
convection. Ahead of the front expect hot and muggy conditions;
overnight lows and dewpoints will be in the 60s with widespread
dense fog. That fog coverage will rapidly decrease through the
morning, with sun first appearing inland and eventually reaching the
coast by mid morning. The midcoast will be last to see the sun with
the potential for a low overcast to remain into early afternoon when
FROPA comes.

The cold front should just begin to move into the area by mid
morning, with plenty of forcing aloft to coincide with peak heating.
Aloft, a jet streak is collocated over the surface front with
wind speeds in excess of 100 kts. Forecast soundings indicate a
well mixed surface layer with CAPE reaching around 1000 J/Kg.
While mid- level lapse rates aren`t as steep as they could be
there is plenty of instability, and temperatures aloft remain
cold enough for some hail in the stronger updrafts. The greater
risk will be for damaging winds. Downdraft potential is quite
high with theta-e difference from the surface to 700mb around

While damaging winds is the main threat, the other severe
considerations, flooding or tornadoes are quite limited. For
flooding, the main risk would result from multiple rounds of storms
crossing the same location. Antecedent conditions are very dry,
and storm motion looks to be fairly fast, so it would take a
number of storms across the same area to result in minor
flooding, however with plenty of moisture and multiple rounds
forecast over the next few days it is worth a mention. Tornado
threat is also extremely limited, however if any cells are able
to form ahead of the main line, any differential heating
resulting from the eroding fog near the coast count help to set
up a boundary.

Bottom line: expect a broken line of strong to severe
thunderstorms to cross the region. Damaging winds and small
hail are the most likely threat.

After the frontal passage on Saturday cooler and dryer air will move
in. Temperatures on Sunday will be in the 70s south to low 60s
south. The cool high pressure will remain over the region through
Monday, a few mountain showers are possible in the afternoon. On
Tuesday another disturbance sets up over the Great Lakes and looks
to rotate around the upper trough bringing showers and rain to
northern New England on Wednesday.


Short Term...VFR for the remainder of the day with southwest
winds continuing through tonight. Marine clouds/fog are likely
to impact KRKD and eventually KAUG again tonight, lingering
past sunrise at KRKD. Further west, guidance points to the
development of MVFR stratus decks, possible IFR along the coast.
In generally these restrictions would form to the south and
move north into the area about 00-05Z...lifting within a couple
hours of sunrise. Tomorrow into tomorrow night, more
restrictions are likely with SHRA, TSRA later in the evening,
especially toward the west.

Long Term...Saturday expect a cold front to cross the region with
scattered thunderstorms impacting all terminals. Behind the
front cooler and drier air will move in, which should keep the
region in VFR through the start of the week. A few mountain
showers are possible. The next widespread precipitation will be
Tuesday night into Wednesday.


Short Term...Have added our southernmost marine zone to the SCA
as Isle of Shoals has been gusting 27-31 kt over the last few
hours. Maybe be able to lower this when the winds decrease in a
few hours. Have a SCA in place for outer waters east of
Portland through tomorrow, but this may need to be expanded into
the rest of the zones tomorrow. Showers and storms are possible
late Friday through the night with wave perhaps building to
near 7 ft by early Saturday.

Long Term...A cold front will cross the region on Saturday
bringing a brief period of NWly winds gusting to SCA through
Sunday morning. Beyond that high pressure builds in for the end
of the weekend with quiet conditions to start next week.


MARINE...Small Craft Advisory until 8 PM EDT Friday for ANZ150-152-154.


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