Area Forecast Discussion
Issued by NWS Portland, ME-- Highlight Changed Discussion --
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FXUS61 KGYX 210441
Area Forecast Discussion
National Weather Service Gray ME
1241 AM EDT SAT MAY 21 2016
-- Changed Discussion --High pressure will slide offshore overnight and early Saturday. A
cold front will drop south from Canada and into northern New
England Saturday night while a coastal low pressure system
develops south of the New England coast. This system will drift
north over the area Sunday and into early next week likely bringing
rain and cooler weather to the area late Saturday night through
early next week.
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.NEAR TERM /UNTIL 6 AM THIS MORNING/...
-- Changed Discussion --1241 AM...minor ESTF update to ingest the current mesonet values
into near term grids.
10 pm...Just made some adjustments to T and Td based
on current obs...but no real changes to forecast.
655 pm...Made a few minor tweaks to most of the grids based on
current obs, but not much change overall, as we look for a clear
and seasonably cool night, with lows 40-45 in the north, and maybe
a few readings in the upper 30s, ranging to 45-50 in the south.
Previously...High pressure will dominate once again tonight as it
crests over the area. Skies will be clear and winds near calm.
Overnight lows will be in the 40s over most areas except some
lower 50s over southeast NH.
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.SHORT TERM /6 AM THIS MORNING THROUGH 6 AM SUNDAY/...
655 pm...Have made some adjustments to Sat and Sat night, mainly
to scale back pops. GFS is really only model to bring precip into
the CWA before 06Z Sunday. The other models, including NAM12,
ARW, NMM, GEM, SREF and ECMWF all hold back precip until early
Sunday, except maybe for a few isolated showers in the mountains
late Sat into Sat evening.
Previously...High pressure moves off the coast Saturday as a weak
cold front drifts south from Canada. With the cold front dropping
south across the region in the afternoon some showers may develop
over the mountains and foothills. Meanwhile to the south of New
England a developing coastal low off the mid atlantic coast will
slowly drift north Sat night spreading rain to develop from south
to north. Models differ on timing of onset of rain Sat night so
used an equal weight on a series of models. Temperatures will be
mild once again Saturday and with clouds and developing rain Sat
night overnight temps will remain mild with lows in the upper 40s
to lower 50s.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Confidence increasing in a Wet and Cool Period...
High Impact Weather Potential: Minimal.
Pattern: A look at early afternoon water vapor imagery over North
America shows significant troughing along the west coast /-PNA/ with
somewhat flatter flow to the east...with a smaller scale upper low
centered over the Mid-Mississippi valley and slowly translating
east. Through early next week...strengthening jet energy over the
Gulf of Alaska will help gradually break down the trough over the
west /good ensemble agreement in a shift towards a neutral PNA/.
While this does open the door for more Pacific flow over the western
states, our weather will first be dictated by the interaction of
the aforementioned slow moving upper low now crossing into the
eastern states with energy now seen in water vapor imagery
crossing into the Dakotas from Canada. These two will combine and
form a cutoff off the east coast by the end of the weekend...with
this initially helping to develop another omega block over the
United States. By the second half of next week...with the western
trough eroding...we should see the eastern cutoff gradually be
pushed east as the flow slowly becomes more progressive.
This overall forecast idea suggests that at least the first half
of the long term forecast period will be dominated by unsettled
weather /numerous precipitation chances/ and cooler than normal
temperatures. This idea is well-agreed upon by extended
Model Trends/Confidence: The most significant trend in the guidance
over the past 24 hours has been to push the cutoff low over the
eastern states further north more quickly. This trend is evident
both in run to run changes in the ECMWF as well as a look at the
most recent GFS compared to the GEFS ensemble mean. Unfortunately
this spells a cooler and wetter solution for our forecast
area...but...given that we are talking about cutoff lows /a
notorious challenge for operational guidance/ I wouldn/t be
surprised to see some additional changes in the guidance envelope
before next week.
The guidance envelope is gradually collapsing around low pressure
developing off the Delmarva Saturday evening...moving near the
benchmark by Sunday morning before filling over the Gulf of Maine
through Monday. Given this increasing consensus...will continue to
trend PoPs up for Sunday...with a widespread wetting rainfall
looking likely. With this slow evolution into a developing cutoff
aloft...the dynamic forcing is not overly impressive...with most
guidance scenarios suggesting only modest low pressure development
off the coast. This will limit the overall impacts...but still
paints some potential for heavy rains /especially along the coast/
and marine impacts with possible gales. The period of greatest
impact looks to be from 12Z Sunday - 00Z Monday. GEFS/SREF members
paint a 30-50% likelihood of 1"+ QPF over southern New Hampshire
and possibly coastal Maine...with a weakening signal further north.
Heading into Sunday night and Monday we should see a transition to
more showery precipitation as the nearby surface low fills.
Temperatures will be largely dependent on clouds and
precipitation... with the potential for coastal locations to be
stuck in the 50s /lower 50s along the coast?/ especially on
Sunday...with some moderation on Monday /especially north and
west/ as the surface low fills.
The cutoff low remains pretty much overhead for Tuesday with
additional potential for surface low development /albeit weaker/ off
the east coast. The spread in guidance solutions increases greatly
at this range /particularly with the details of a secondary surface
low/...but the idea of a cold pool of air aloft and moist low level
cyclonic flow portends at least scattered shower activity with some
daytime enhancement. The more robust solutions would suggest /yet/
another wetting rain. Will not move pops into the likely category
yet given the guidance disparity in a cutoff low scenario. Do
expect temperatures to remain depressed...however...likely remaining
in the 60s.
While guidance can be too fast to break down blocking scenarios
will at least cautiously be more optimistic as we reach Wednesday
and Thursday...with the cutoff low over the east starting to
weaken and reconnect with the westerlies...with temperatures aloft
moving back above seasonal norms /+1-2 sigma/. Would not be
surprised if we actually realize a dry spell /!/ for 24-36 hours
in this period...and given the temperatures aloft...this would
suggest highs that could make a run through the 70s /even 80
south?/ by Wednesday and particularly Thursday. Can/t eliminate
PoPs at this time for either day...however...as nearby upper low
on Wednesday and developing warm advection pattern by the end of
the week...indicated at least some chance for additional
showers...and possibly thunderstorms by week`s end.
Chance PoPs and mild temperatures will continue to be the rule as
we end the forecast period Friday...with broad agreement on a cold
frontal passage during this period.
.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
Short Term /through Saturday/...VFR tonight and saturday.
Conditions will become mvfr and then IFR later Saturday night in
developing areas of rain from south to north.
Long Term...Low pressure moving off the New England coast will
likely bring a period of MVFR/IFR restrictions /worst at coastal
locations/ for Sunday with some improvement heading into early next
week. However...through next Wednesday there will be an elevated
potential for occasional MVFR/IFR restrictions along the coast and
Short Term /through Saturday/...Winds and seas should remain
below Small Craft threshold through Sat night.
Long Term...Low pressure tracking from the benchmark into the
Gulf of Maine on Sunday will bring the potential for gale force
wind gusts...especially outside of the bays. Winds will gradually
diminish beyond this...but expect residual small craft conditions
/due to both winds and waves/ to persist through the first half of
High pressure across the area with dry weather, warmer
temperatures, and light winds into Saturday. A coastal low will affect
the area with wet weather Saturday night through early next week.