Area Forecast Discussion
Issued by NWS Portland, ME-- Highlight Changed Discussion --
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FXUS61 KGYX 232353
Area Forecast Discussion
National Weather Service Gray ME
753 PM EDT Fri Sep 23 2016
Showers will continue along a cold front through this evening
over southern New Hampshire and far southwestern Maine before much
cooler and drier air arrives for the weekend as Canadian high
pressure builds into the region. This will bring the potential for
frost or freezing conditions Saturday and Sunday nights...
especially over northern areas. Gradually moderating temperatures
are expected Monday before another approaching cold front brings
the threat for showers on Tuesday.
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
-- Changed Discussion --8 PM Update...Narrow band of rain showers are progressing across
southernmost NH as forecast and should exit over the next 1-3
hours. Therfore, very little changes to the going forecast.
530 PM Update...Have updated the grids for the next few hours
based on latest trends in observations and mesoscale model
guidance. The result was little change to the going forecast.
However, we did tighten up the PoP gradient a little bit across srn
NH for this evening based on latest radar mosaic loops and HRRR
guidance. Narrow stripe of showers is beginning to take shape
across Upstate NY to the NW of ALB. This area of showers is
expected to mainly affect Cheshire and Hillsborough counties this
High Impact Weather Potential: Patchy frost in the far northern
Current Pattern: Water vapor imagery early this afternoon shows an
onslaught of northwesterly flow extending from Manitoba south and
east to a leading shortwave now pressing through New England. This
shortwave is interacting with a modest /although thinning/ moisture
plume along the east coast to produce multiple bands of showers as
it pushes south and east. Also of interest is an area of darkening
in water vapor imagery over western Ontario which represents another
upstream shortwave that will sharpen as it rapidly pushes southeast
into New England during the first half of the night. Beyond
this...sprawling 1030mb high pressure over northern Ontario will
begin to push toward the region...bringing a clearing/drying/
cooling trend to the area. Forecast concerns are thus focused on
precipitation potential through this evening...and temperatures as
the continental polar airmass arrives tonight.
Through Early Evening: Shower activity has dwindled markedly over
the past 1-2 hours...so through 6pm expect little more than a few
sprinkles/very light rain showers over southern areas with some
breaks in the clouds opening up over northern areas.
Tonight: High resolution near term guidance consensus has been
honing in on another period of more widespread shower activity over
southern NH and far southwestern ME this evening as aforementioned
shortwave over Ontario sharpens and interacts with H8 frontal zone.
The best window of opportunity for rain looks to be between 8pm
and midnight...generally south of a LEB-PWM line. This will
certainly help to boost meager rainfall totals in these areas thus
far...with another few tenths of an inch of rain quite possible.
Beyond this...cold and dry advection will be the rule with gradual
clearing from north to south as PWATs retreat back below 0.5".
Temperatures will fall rapidly...particularly with partial
clearing after midnight...with consensus lows in the upper 30s
north to mid/upper 40s south looking reasonable. Can/t rule out a
few patches of frost over the far north...but not widespread /or
confident/ enough to warrant a county-based headline.
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.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
High Impact Weather Potential: Frost/Freeze Potential Saturday
Pattern: The short term forecast period will be dominated by
continued northwesterly flow aloft given highly amplified
trough/ridge/trough pattern across North America...with large
surface high over eastern Ontario Saturday morning remaining nearly
stationary through this forecast period. T8s start the period near
the freezing mark...and gradually settle lower by Saturday night
with these values representing -1 to -2 sigma departures from
climatology. PWATs will similarly be in the -1 to -2 sigma
range...promising little in the way of sensible weather through the
period. Forecast concerns will center around temperatures Saturday
night as the coolest night of the early fall season is expected.
Saturday: Longwave trough will continue to deepen along the east
coast of NOAM as sharp longwave ridging over the central portion
of the continent slowly builds and moves east. This will maintain
northwesterly flow through the column over our region...although
any significant cool advection will be over by daybreak. BUFKIT
profiles agree with CU rule suggestions from the guidance
indicating some SCT cumulus development in the mountains...with
very little in the way of cloudiness making it to the coast.
So...expect a mostly sunny day for most spots. T8s will only be
around +1-3C...so expect northern areas to remain in the
50s...with areas south of the mountains to reach the lower/middle
60s. Given the robust gradient aloft...expect northwesterly winds
10-20kts for the day...adding a bit of chill to the air.
Saturday Night: High pressure remains north and west of the area
with PWATs falling to around 0.25" in continental polar airmass.
Expect any afternoon cumulus to clear...with a chilly night in
store across the region.
Primary factor potentially limiting the chill will be an ongoing
breeze as 1000mb geostrophic winds remain 20-30kts overnight likely
keeping many locations coupled. Honestly...this looks more like a
freeze vs. no freeze situation with perhaps too much wind to support
frost. Given the breeze...have kept overnight lows a bit higher
than statistical guidance. Breeze will also restrict widespread
fog...but given how cool temperatures will be...some CT valley
locations will still probably see some patches.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Chance of High Impact Weather: Minimal.
Where for the last few months 500mb blocking was hard to come by
south of 50 N, we are finally starting to see sow blocks develop
that will bring troughs down in the 30-40 N range. The models do
not show stable blocking, but it is enough to bring temps to
around normal with some fluctuations either side of normal through
the long range. However, with the instability of the blocking,
systems will be mobile and not very strong, so any chance for a
soaking rain remains low through next week.
The cold air will continue to stream in Sunday, as 500 mb trough
works through in the morning. this air is very dry, but should
produce some clouds in the mountains Sunday, maybe become mostly
cloudy for a few hours. The coastal plain will see more sun in the
downslope as N-NW winds pick up in the 10-20 mph with some higher
gusts. Highs will be cool, and a few degrees below normal ranging
from the low to mid 50s in the mountains to the low 60s in srn NH
and along the ME coast.
Sunday night will turn out to be an very good rad cooling night as
sfc high builds in ands winds die off quickly with clearing
skies. Lows will range from the mid-upper 30s in the mountains to
the low 40s along the coast and in urban areas of the Merrimack
valley. This will bring the threat of frost to almost everywhereexcept
the immediate coast.
Light winds shift to the SW on Monday as the high shifts offshore
and it looks to be mainly a sunny day with highs a few degrees
warmer than Sunday, bring temps to around normal, generally in the
60s. The next chance for rain will be in the Mon night thru Wed
time frame as 500mb closed low tracks NE out of the great lakes
and drags a cold front across the region. The Euro is a little
progressive the fropa, ad moving the rain out by Tue night, with
the GFS lingering it. However, the Euro also merges the two closed
500 lows to our north and spins the resultant low back SW and
brings it just N of ME. This would result in cooler temps late in
the week than the GFS. Went with Superblend, which split the
difference which keeps temps around normal Wed-Fri, though if the
Euro is right, they would be cooler, and if the GFS they would be
.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
Summary: A cold front will push south of the region this evening
with high pressure building in it/s wake through Saturday and
remaining over the region through Saturday night.
Restrictions: Band of showers will bring MVFR restrictions northern
terminals...with CON/MHT/PWM/RKD likely seeing brief drops to MVFR.
Conditions improve to VFR as showers depart this evening...with
LIFR/IFR fog likely HIE/LEB...and possible CON/AUG. VFR for Saturday
/ Saturday night with the exception again being overnight fog
Winds: Northwesterly winds 10g16kts this afternoon will diminish to
5-10kts /light and variable in the valleys/ for the overnight before
increasing to 12g20kts from the northwest for the day Saturday.
Northwest winds will diminish to below 10kts again Saturday night.
LLWS: No LLWS expected through Saturday night.
Thunder: Very slight chance of thunder MHT through 21UTC...but
chance not great enough to mention in TAF. No thunder beyond this
through Saturday night.
Long Term...Sunday could continue to some N-NW wind gusts in the
20-25 kt range, but VFR Expected Sun-Mon. Some showers are
possible Tuesday and Tue night, and could see a period of flight
restrictions ahead of cold front Tue afternoon and night.
Short Term...Strengthening northerly winds tonight will bring
winds/seas close to marginal SCA values for the overnight before
shifting northwest on Saturday...with winds/waves remaining near
marginal SCA values through Saturday night...after a brief lull
in the winds early Saturday.
Long Term...Borderline SCA possible Sunday in N-NW winds, but
winds diminish by Sunday evening and stay below SCA through Wed.
MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ150-152-
NEAR TERM UPDATE...Ekster