Area Forecast Discussion
Issued by NWS Portland, ME

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FXUS61 KGYX 240455
AFDGYX

Area Forecast Discussion
National Weather Service Gray ME
1255 AM EDT Mon Apr 24 2017

.SYNOPSIS...
A cold front will drop south into northern New England tonight
into Monday and stall. Low pressure will slowly move up the
Eastern Seaboard Tuesday and Wednesday with drizzle and periods
of rain. A cold front will slowly approach from the west
Thursday into Friday, before finally crossing the region on
Saturday.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
1255 AM Update...
Have updated the grids to account for current conditions. Mainly
clear skies continue over much of the forecast area except the
far north. Temperatures remain very mild over southern New
Hampshire with 30s elsewhere. Clouds from cold front will
attempt to slip southward over central portions of the forecast
area late tonight.

1048 PM Update...
Forecast was updated to include latest temperatures and dew
point information. Lowered overnight lows in a couple spots
that should be favored for radiational cooling outside of the
cloud covered areas. Winds have diminished at the surface quite
a bit but latest KGYX VAD profile is showing ~45 kts at 1 kft.
This should only impact aviation at this time with an inversion
having set up...but this as well as increasing cloud cover over
the south indicates the speed of the incoming system may be
faster than the models have indicated of late.


820 PM Update...
Only minor changes to the forecast at this time. Quiet weather
persists this evening with a weak cold front drifting towards
our northern borders. Some associated cloud cover will arrive
for northern sections later tonight, with a little blow off
reaching southern NH as well from warm air advection over the
mid Atlantic states. Temperatures should drop quickly tonight
with dew point depressions in the 10-20 degree range.

Previous discussion...
A fine Sunday afternoon will translate into a fine Sunday
evening across the region with little in the way of clouds. A
weak cold front will drop into ME and NH overnight, but won`t
have too much of an affect on our sensible weather other than a
couple of light showers, if any, across the western ME
mountains. There will be an increase of clouds later tonight
elsewhere but that`s about it.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM TUESDAY/...
A weak cold front will be draped across or just south of the CWA
on Monday. However, this front will remain moisture starved and
offer little adverse affects to our sensible weather. There will
be more clouds around than what we saw on Sunday but
temperatures still should make it into the 60s most everywhere,
with temps around 70 likely across southernmost NH.

Cooler temperatures will push into our region Monday night as
high pressure ridging builds southwestward out of the Maritimes.
Clouds will be on the increase also, but overall it should be a
nice night for most.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
The continental view of the forecast shows plenty of warmth
building in the oven of the continent over the southwestern
deserts where the temperature is warming well into the 90s each
day. Meanwhile cold lingers in the ice box over Hudson Bay where
subzero readings are still observed. The heat from the deserts
gets advected eastward by the westerlies into the Great Plains in
the form of an elevated mixed layer. As moisture from the Gulf
of Mexico occasionally streams northward beneath this mixed
layer, thunderstorms form, often violent, and serve the purpose
of translating this heat into moisture at all levels of the
atmosphere and releasing the excess heat as precipitation. This
then gets absorbed into synoptic scale pressure systems which
mix the warm, moist air with the remaining Arctic air. For this
coming week there will be a prevailing upper trough over the
western United States which will advect the warmth from the
southwestern ovens into the Great Plains and Great Lakes where
an upper ridge will develop in response. Meanwhile well to the
southeast of this developing ridge, a remnant pocket of cooler
air over the southeastern United States will slowly make its way
northward along the East Coast, cut off from the westerlies by
the developing ridge. So while our area will be broadly beneath
the developing northern stream ridge and protected from the
Arctic cold to the north, for most of this week we will not see
much of this warmth due to the cut off low slipping through the
region.

While the cut off upper level low remains well to the south over
the Outer Banks on Tuesday, there will be a persistent onshore
easterly wind developing between high pressure near the Canadian
Maritimes and the low over the Outer Banks. This will bring
increasing clouds and an increasing chance of drizzle and rain.
Forecast low level moisture profiles east of the mountains
favor drizzle from Tuesday evening right through Wednesday night
and even into Thursday for some areas. With Gulf of Maine
temperatures in the low to mid 40s and low level moisture feed
from the Western Atlantic arriving in the form of 50s
dewpoints, expect this to readily condense into fog and drizzle
as it arrives onshore and gets squashed into the terrain. As
the upper low gets closer we will get a little better synoptic
scale support for rain with the ascending quadrant of the low
positioning itself to advect moisture in off the Atlantic and
wring it out over New England likely in several batches. The
heaviest rain will be Tuesday night into Wednesday as the upper
low eventually makes its way toward Cape Cod. Have decided to
word the forecast for the Tuesday night through Wednesday time
period as "periods of rain and drizzle" as there will likely be
several batches of steadier rain interspersed with plenty of
drizzle. Total rainfall could exceed two inches along the
coastal plain.

Models are now suggesting that low level onshore flow could
continue right into Thursday for the eastern half of the area
especially. Cut off low is not moving as quickly to the north as
previously anticipated, which makes sense considering model
tendencies to get rid of these lows too quickly along with the
intensifying ridge to the north depriving it of the kick it
needs to get out of here. Could be a fair amount of bust
potential for the temperature forecast on Thursday considering
how much warm air is available in the southwesterly flow beneath
the ridge and the potential for onshore flow and clouds keeping
this warmth aloft only. Have tilted the forecast toward mid 70s
in the west on Thursday where onshore influences will be felt
the least, and low 60s (or even upper 50s) across Maine where
the coastal influences will be the strongest.

The wave that develops within the broader trough tracks north
through the Great Lakes Thursday and then to the east of Hudson
Bay on Friday. This means that while the broader scale
southwesterlies ahead of the trough will be over our area, the
low level cold front will have difficulty pushing east so far to
the south of the parent wave. Models have thus delayed the
advance of the cold front again, with its most likely arrival
now appearing to be Friday or Friday night. There could be
showers and thunderstorms as the front moves through considering
the warm, moist, moderately unstable air mass ahead of it. But
whether this occurs will be dependent on whether there will be a
supporting shortwave trough within this southwesterly flow. At
any rate, the southwesterly flow should finally kick out the
maritime air mass over coastal Maine and temperatures should
warm into the 70s for most of the forecast area on Friday, with
80s possible if we can get some good mixing.

While there is some indication that a piece of the remnant
Arctic cold could get shoved toward our area behind the front
for this weekend, the best cold air will be moving more east
than south and will not be strongly noticed in our area. This
cold will also be tempered by downsloping flow behind the front
and a strong April sun, yielding temperatures in the 60s this
weekend which is a few degrees above normal. Ridging begins to
develop across the center of the continent again late this
weekend and will make another attempt at venting the oven
warmth toward our region early next week.

&&

.AVIATION /06Z MONDAY THROUGH FRIDAY/...
Short Term...VFR conditions are expected tonight through most of
Monday night. A brief period of MVFR conditions will be possible
at RKD overnight tonight. We may start to see some MVFR cigs
develop across southern NH and coastal ME late Monday night.

Added LLWS due to KGYX VAD winds showing about 45 kts at 1 kft
associated with a LLJ. This will only be a concern for the next
couple of hours.

Long Term...Should see increasing clouds and lowering ceilings
on Tuesday as an onshore flow moistens up the low levels. IFR
conditions will be likely by evening with an increasing chance
of drizzle and rain. Conditions to the west of the mountains
will fair a little better at least initially. Onshore flow,
drizzle, and IFR or worse conditions are likely to continue
through Wednesday and Wednesday night. Could see this continue
into Thursday for much of Maine while New Hampshire may finally
begin to see breaks in the clouds and a return to VFR
conditions. VFR expected to return areawide on Friday.

&&

.MARINE...
Short Term...A brief period of marginal SCA conditions will be
possible tonight in advance of a cold front. However,
probability is pretty low, so will hold off an advisory at this
time. Otherwise sub-sca conditions are expected through Monday
night.

Long Term...East to southeast flow increases between high
pressure over the north Atlantic and low pressure tracking up
the East Coast. Winds could gust to 25 KT at times Tuesday night
through Wednesday. With the onshore fetch expect wave heights to
be on the increase, rising to 5 to 10 FT on Wednesday and
potentially staying above 5 FT into Friday.

&&

.FIRE WEATHER...
RH values will be generally in the mid 30s to mid 40s on Monday
with relatively light winds. The exception looks to be in the
western ME mountains where min RH values should be in the 20s.
However, winds are still expected to be light. Rain is expected
to develop Tue and last into Wed and Thu.

&&

.TIDES/COASTAL FLOODING...
We are entering a period of high astronomical tides which will
coincide in part with the weak storm system moving up the coast.
There are two primary tides we are monitoring for minor coastal
flooding, Tuesday evening and Wednesday evening, though high
water conditions may continue during the high tides into the
weekend.

For Tuesday night the astronomical tide at Portland is 11 FT.
Winds over the waters will be out of the east at about 20 KT
with waves building to 5 or 6 FT. This would cause about a 1 FT
surge along coastal New Hampshire and southwest Maine and create
the potential for minor coastal flooding, splash over, and beach
erosion.

The tide Wednesday night is 11.5 FT at Portland. At this time
winds will be out of the southeast at about 15 to 20 KT which
will not promote a strong surge on its own but will likely keep
a residual surge in the Gulf of Maine of up to 1 FT with waves
of 5 to 9 FT. Minor coastal flooding, splash over, and beach
erosion is possible with this tide as well.

The next two tidal cycles will see tides of 11.7 FT at Portland
Thursday night and Friday night, but atmospheric conditions
contributing to storm surge will be on the decrease and we are
not expecting significant issues with these tides.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...None.

&&

$$
JC


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