Area Forecast Discussion
Issued by NWS Portland, ME-- Highlight Changed Discussion --
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FXUS61 KGYX 250030
Area Forecast Discussion
National Weather Service Gray ME
830 PM EDT Mon Apr 24 2017
Today was the last warm, sunny day for a while. An easterly
onshore flow begins tonight and will transport cool, moist air
in off the Gulf of Maine. By Tuesday drizzle and rain will begin
to overspread the region with rain becoming heavier Tuesday
night into Wednesday as low pressure moves north along the East
Coast. The cool, moist maritime air may linger into Thursday
east of the mountains, but warm air will begin to move into
western New England and eventually even make it to the Maine
coast this weekend.
.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
-- Changed Discussion --800 PM Update...
Quick update to adjust temp/td/sky grids. High clouds will be on
the increase from the south and west through the remainder of
the evening and overnight as a storm system to the south slowly
lifts north. Area of stratus sitting offshore is also slowly
moving northward and will impact mid coast areas with low
clouds and possibly some fog by midnight or shortly after. May
see a few stray showers by morning in southern zones but the
main event will come tomorrow afternoon and overnight Tuesday.
No other changes planned attm.
High pressure lifts into Nova Scotia tonight while low pressure
drifts north along the Outer Banks of North Carolina. In between
the two, an easterly onshore flow will develop over New England.
This will begin transporting cool, moist air into the area
overnight tonight. Clouds will be on the increase and by morning
there will be a chance of rain and drizzle in southern areas.
Out in the Atlantic right now there are a few patches of low
clouds lurking well offshore, and these may get pulled westward
into New England by morning. Temperatures tonight will fall only
into the 40s across the southern half of the area where the
easterly flow will have the best moisture associated with it.
Further to the north across interior and northern Maine,
temperatures will fall below freezing as dry air remains in
place there and the sky will stay clear for a longer portion of
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.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Increasingly moist onshore easterly flow will promote thickening
clouds and a threat of drizzle or light rain from south to north
on Tuesday. Daytime temperatures will remain stuck in the 40s,
but on the northwest side of the mountains a downslope component
to the wind will allow things to be a bit warmer, in the mid
Slow moving cut off low pressure system begins arriving in New
England Tuesday night, bringing better synoptic scale support
for rainfall Tuesday night through the day Wednesday. With a
good moisture feed off the Atlantic, expect a more significant
rainfall to begin especially on the coastal plain east of the
mountains. Total rainfall amounts could exceed 2 inches along
the coast where some enhancement along a coastal front could
occur. The temperature will gradually warm out of the 40s and
into the 50s on Wednesday as moist advection forces the
temperature upward. Rain may come to an end during the day
Wednesday to the northwest of the mountains where temperatures
may pop up into the 60s.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
-- Changed Discussion --High Impact Weather Potential: Residual minor flooding /both river
and coastal/ to start the period. There is some threat in the
Friday-Saturday period for thunderstorms...although severe weather
appears unlikely at this time.
Pattern: A look at the pattern across the Northern Hemisphere early
this afternoon reveals a potent central/eastern Pacific jet largely
forced by an impressive cutoff low over the Aleutian islands
/+EPO/. Weak troughing exists along the west coast of North
America...which would imply downstream ridging to the east...but
this is only part of the story as a cutoff low meanders south of
this over the southeastern Untied States. The primary feature
controlling our sensible weather will initially be this cutoff...but
as this feature is eventually pushed east...we/ll see more impacts
from developing southwesterly flow aloft ahead of western trough
with embedded impulses providing sensible weather impacts.
Thus...after the initial impacts of the cutoff through
Thursday...the Friday-Monday period should be dominated by
southwesterly flow aloft /suggesting temps at or above seasonal
norms...teleconnecting well with the +EPO/ although the position of
the trough near the Rockies this weekend may allow some continental
polar air to sneak in from the north between shortwaves.
Model trends/confidence: Rather good agreement in the pattern exists
in the deterministic/ensemble guidance for Wednesday night through
the start of the weekend...with the largest differences showing up
to end the forecast period Sunday-Monday particularly in the details
of amplifying trough over central North America...with downstream
impacts on our region. Recent run to run changes have also been
rather small through the end of this week...although longer term
trends favor a slower exit to departing cutoff low. Overall
confidence is therefore above average for this forecast package.
Wednesday Night - Thursday Night: Cutoff low actually makes it/s
closest approach during this period with weak surface low expected
to be just off the southern New England coast 12Z Thursday. The
surface feature becomes absorbed in the lowering pressure field /due
to approaching shortwave over the Ohio Valley/ through the day
Thursday with mid level low slowly pushing east towards southern
Nova Scotia Thursday night. Ample moisture will remain and with
cyclonic flow gradually falling apart in the low levels...expect
numerous showers Wednesday night to gradually lessen in coverage
before ending by Thursday evening. Rainfall totals will not be
substantial at this range with little support for anything other
than broad ascent on the synoptic scale...with the likely presence
of a coastal front / cold air damming signature across coastal Maine
potentially providing assistance on the mesoscale. With weakening
precipitation forcing...expect conditions a good bit warmer with
some sunny breaks likely north of the mountains and down the
Connecticut valley. Temperatures aloft will be rather warm and so
there is certainly bust potential on temperatures depending on where
any sun is realized. Expect most locations to reach the 60s...but in
the aforementioned sunnier spots...lower and perhaps even middle 70s
is possible. With the warm airmass aloft and ample cloudiness...
overnight lows both nights look to remain well above /+10F/ seasonal
Friday: While our cutoff low will finally move east on Friday...our
weather remains potentially unsettled as low pressure moving north
from James Bay pushes a cold front into the region. T8s ahead of
this front will be +11 - +13C which indicates the potential for some
summer-like temperatures. Upstream frontal boundary will be in a
weakening phase as it arrives late in the day or on Friday
night...with rather lackluster forcing despite a narrow PWAT plume
around 1.25 inches /about +2 sigma/. Will carry a chance of showers
and possibly thunderstorms...with the greatest threat for rain being
over the mountains and points north...closer to the best mid and
upper level forcing. Would not be surprised if locations over
southern NH make a run at 80 if any clouds/storm activity hold off
until late in the day. Wind fields are fairly robust aloft...so a
strong storm isn/t out of the question but the lack of forcing
precludes much concern about a significant severe threat.
Saturday - Sunday: Given the deep layer west southwest flow across
the region...the aforementioned cold frontal boundary will struggle
to cross the region this weekend...and will eventually pull back to
the north by early next week as low pressure takes shape in the Mid
Mississippi valley. This leaves the front somewhere across New
England with modest agreement on a fast-moving impulse moving
through the region Saturday. Given timing uncertainty and the
frontal proximity...can/t rule out a shower either day...with
thunder possible south. As for temperatures...there/s a better
signal for warmth Saturday with the front sinking south of the
region on Sunday likely keeping us closer to climatology. Certainly
no reason to expect a washout...but feel that at least some
precipitation is likely for much of the area during the 2 day
Monday: Despite it being a week away...there is overall agreement
on a large Great Lakes low pressure system to end this forecast
period on Monday with a very warm signal over our region as warm
front pushes north of the area. At this range...plenty of things
can change...but starting this period with above normal temperatures
and a chance for showers/thunderstorms with the nearby warm front in
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.AVIATION /01Z TUESDAY THROUGH SATURDAY/...
Short Term...VFR conditions this evening will begin to
transition to MVFR and then IFR conditions from south to north
during the day Tuesday as low level moisture spills in off the
Gulf of Maine. There could be an area of lower clouds arriving
along the immediate coast as early as tonight which would bring
a threat for LIFR conditions. But the broader forecast is for
worsening conditions on Tuesday with just about all areas IFR or
worse by Tuesday evening in drizzle and light rain which will
likely last through Wednesday. To the northwest of the
mountains up in Whitefield, conditions may stay VFR to MVFR
until the better forcing for rain arrives Tuesday night.
Long Term...Lingering MVFR/IFR ceilings likely through Thursday
with nearby low pressure system. Improvement to VFR likely for
Thursday night into Friday...with occasional restrictions in
SHRA and possible TSRA possible Friday afternoon. Frontal
boundary remains across the region through the weekend with VFR
likely dominating...but scattered MVFR/IFR restrictions in SHRA
Short Term...Expect an increasing easterly flow tonight into
Tuesday, with winds as high as 25 KT beginning Tuesday
afternoon. Winds shift to the southeast on Wednesday and
decrease in intensity Wednesday night. The onshore fetch will
allow wave heights to built up to 5 to 9 FT through Wednesday
Long Term...While winds will subside...lingering wave activity
will necessitate SCAs through at least Friday...with quiet
conditions for the coming weekend.
Moist onshore flow will bring high humidity and a wetting rain
to all areas Tuesday through Wednesday.
Continued snow melt in the mountains will keep river levels
high. Rain expected Tuesday night and Wednesday will add run off
to area rivers already at high levels. While widespread flooding
is not expected, it is not out of the question that a few rivers
could reach minor flood stage.
High astronomical tides begin Tuesday night and last through the
weekend. The combination of high tides and an onshore flow with
the coastal low will bring the chance of beach erosion, splash
over, and minor coastal flooding during the time of high tide
Tuesday night and Wednesday night.
MARINE...Small Craft Advisory from 7 PM Tuesday to 6 PM EDT Wednesday
Small Craft Advisory from 2 PM Tuesday to 6 PM EDT Wednesday