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-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KGYX 180028
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
828 PM EDT Sun Jun 17 2018

.SYNOPSIS...
Southerly flow ahead of an approaching cold front will bring
very warm and humid conditions to the area tomorrow. Some areas
on Monday will see heat index values break 95 degrees. The
atmosphere destabilizes significantly during the afternoon
tomorrow with widespread thunderstorms expected. Once the front
moves though tomorrow night, 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 AM MONDAY MORNING/...
-- Changed Discussion --
800 PM Update...
High clouds continue to spill into the region from the northwest
as a warm front moves through southern Quebec and Northern Maine
early this evening. Radar showing associated rain shower
activity mainly confined to northern Maine but a few showers are
dying out as they drift into northern Somerset County so have
increased pops in this area for the next 3 to 4 hours.

Elsewhere....strong marine layer has spread well inland in
southern Maine and and southwest New Hampshire dropping temps
into the 50s and 60s. Have tweaked temps and dew points based on
current obs and adjusted RH and Apprnt temp grids....but no
major changes planned for current forecast.

Prev Disc...
It should be a fairly quiet night across most of the area.
Increasing low level moisture could result in some patchy areas
of fog, but shouldn`t result in anything widespread at all.
Temperatures will be a bit on the warm side, with most places
only seeing lows in the 60s to 50s over the mountains.
-- End Changed Discussion --

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
-- Changed Discussion --
Tomorrow will definitely be the focus of the forecast as it
looks to be a very busy day. Increasing dewpoints and
temperatures ahead of an approaching cold front will set the
stage for convection in the afternoon and evening.

Heat...strong southwesterly flow will allow temperatures to
climb to around 15 degrees above normal. The warmer than normal
temperatures will combine with rising dewpoints to generate some
very high heat index values. With the new criteria of two hours
or more of 95F degree heat index now in place, we will be
hoisting a heat advisory for tomorrow afternoon over southern
New Hampshire and York county Maine.

Convection...The entire forecast area is now in either a slight
risk or marginal risk on the day two convective outlook.
Although the cold front won`t arrive until sometime during the
overnight, models are consistent in developing convection well
ahead of the actual frontal boundary. Expect convection to
develop first in New Hampshire and the mountains of Maine, then
move southeast toward the coast. SBCape values over most of the
area approach or exceed 2000 j/kg by early afternoon. There are
also some very respectable shear values that develop during the
afternoon as well. PWAT values are expected to exceed 2", which
is at/above SPC daily climatology max values. Once convection
fires, there should be plenty of moisture and instability to
work with. What is less certain, is how much if any convective
debris will affect storms later into the evening. Expect
convection to continue into the overnight hours, but the severe
threat should diminish dramatically as the evening progresses.

Due to the abundant low level moisture in place ahead of the
cold front, there could be some fog development Monday night. It
may be short lived though as the cold front moves through and
scours it out.
-- End Changed Discussion --

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Looking at the big picture across North America, we see a large
upper level ridge of high pressure building all the way north
along the West Coast even up into the Yukon Territory of
Canada. Beneath this ridge exists a cut off low over the
northern Rocky Mountains of Idaho and Montana, helping to
sustain the high amplitude ridge above it while also keeping the
hottest air baking over the Sonoran Desert and southwestern
United States. Meanwhile to the south and east, the subtropical
ridge is a bit squashed to the south by the downstream trough
extending from that western Canada ridge. This trough meanders
largely in place across Labrador and northern Quebec, with
occasional spokes of energy rotating around this trough sending
cold fronts through New England. Since the broader flow pattern
features northwesterly flow extending from deep in the Canadian
Arctic and traversing the still-frozen Hudson Bay, the overall
pattern across New England will stay largely as it has been for
some time: Mostly dry with only brief periods of more warm/humid
air arriving just in time for another cold front to usher in
the cooler/drier air again. In order to get any relief from the
developing drought especially across coastal areas, we will need
to get this weather pattern to shift and allow a coastal storm
or two to bring a good dose of rain.

By Tuesday morning the cold front will have moved offshore,
ushering in a cooler and drier air mass on a northerly breeze.
With cool advection and downsloping winds, expect the warmest
temperatures to be along the coastal plain extending all the way
to the beaches where another day in the 80s is forecast with
noticeably less humidity. High pressure building out of the
Great Lakes and into southern New England will allow for a clear
and mostly calm night Tuesday night, with low temperatures
dropping into the 40s and 50s. It will be a good night to open
windows and let out the heat and humidity of the previous few
days!

High pressure cresting to our south will bring a brief shift to
southwesterly flow on Wednesday. This brings a bit warmer air
and a slight increase in humidity. However, the rich Gulf of
Mexico moisture remains locked in over the Great Plains and
Mississippi/Ohio River Valleys. Thus even as another cold front
pushes into the area on Wednesday, there won`t be a lot of
convective fireworks with it and thus no drought relief
expected.

Behind this cold front exists a cooler and much drier air mass
originating from the Arctic coast of Alaska and traversing the
still frozen Hudson Bay on its way here. While the cold
temperature of this air mass will modify with each day of June
sun on its route, it will still have those dry Arctic dewpoints.
Raw data from both the GFS and the ECMWF indicate dewpoints
dropping down into the 30s Thursday afternoon for much of the
area. This was significantly lower than the SuperBlend which we
typically initialize the forecast with, so I deliberately
blended in some of these drier models to get lower dewpoints
into the forecast. This is a significant factor for fire weather
concerns, but also for the temperature forecast Thursday night
as high pressure slides out of the Great Lakes and across New
England. This sets up ideal radiational cooling conditions on
the shortest night of the year. Although the short nighttime
period does limit how cold it can get, we saw just last week how
cold it can get in a dry air mass under the right conditions
even well into June. Thus we cannot rule out the possibility of
frost in the northern valleys on the first day of summer.

High pressure slides offshore on Friday with temperatures
mainly in the 70s and dewpoints slowly recovering. Meanwhile,
that cut off low over the northern Rocky Mountains will have
slowly drifted east across the Great Lakes this week. What form
the resulting surface pressure features take is still not well
agreed upon by the models, but generally we should see a
southerly flow begin again on Saturday with warming temperatures
and increasing moisture. Then we could see some showers on
Sunday. It`s worth noting, though, that although most models
paint precipitation over the entire forecast area on Sunday,
they don`t have a great handle on the finer details of it yet
and thus are likely too broad with the precipitation. It is
likely that rain will be showery in nature and focused along the
accompanying frontal boundaries, so it may not be a washout for
everyone or the drought relief that we need.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
Short Term...Mainly VFR tonight. Showers and thunderstorms
tomorrow will produce some aviation issues. Expect to see IFR
conditions as storm move through.

Long Term...VFR conditions expected as several dry air masses
move through the region this week.

&&

.MARINE...
Short Term...Will hoist a small craft advisory for increasing
seas and winds tomorrow morning throughout the day. Advisory
will only be for the outer waters as the bays should remain
protected for this event.

Long Term...After the cold front moves through early Tuesday
morning, expect a shift to northwest winds gusting to 20 or
possibly 25 KT. High pressure slides by Tuesday night with light
winds becoming southwesterly on Wednesday ahead of the next
front Wednesday night.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
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ME...Heat Advisory from noon to 7 PM EDT Monday for MEZ018-023.
NH...Heat Advisory from noon to 7 PM EDT Monday for NHZ005>015.
MARINE...Small Craft Advisory from 8 AM Monday to 8 AM EDT Tuesday for
     ANZ150-152-154.
-- End Changed Discussion --

&&

$$