Area Forecast Discussion
Issued by NWS Boston, MA

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267
FXUS61 KBOX 181102
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
702 AM EDT Fri Oct 18 2019

.SYNOPSIS...
Strong low pressure across New Brunswick early this morning
will cross the Maritimes during the afternoon, bringing dry,
cool conditions and brisk winds across the region. High pressure
will slowly build east through the weekend into early next
week, with seasonably mild and pleasant weather. Low pressure
across the Great Lakes with another low well south of Nantucket
will likely bring showers and gusty onshore winds Monday night
into the mid week timeframe. Another high builds to the mid
Atlantic coast with more dry weather around Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

645 AM Update...

Bands of low clouds seen moving across central and western areas
in the NW wind flow in place as seen on latest GOES-East
Nighttime Microphysics RGB satellite imagery. Downstream from
the cloud cover, skies were mainly clear across E Mass into RI,
though some clouds develop from time to time across Mass Bay and
push across portions of the outer Cape.

Temps were mainly in the 40s across the region at 10Z, with some
locations across the lower CT valley and the S coast, Cape Cod
and the islands into the lower 50s. Wind gusts were up to 25-30
kt along the S coast, namely KBID and KPVC.

Near term forecast pretty much on track except for some minor
tweaks to bring conditions current. Increasing low level mixing
should cause W-NW wind gusts to pick up once again by late
morning or midday.

Previous Discussion...

Today...

Upper air pattern is progressive, so deep closed low over Nova
Scotia will continue to lift northeast into the Maritimes this
afternoon. This results in rising heights across southern New
England promoting dry weather. However one more potent short
wave has to rotate thru the mid level flow across NY into New
England and is accompanied by -18C air at H5. This cold pool
combined with cyclonic flow should result in at least scattered
diurnal cu/scu and perhaps becoming broken at times. Some modest
lift associated with this short wave but column is pretty dry
especially above 850 mb. So other than a spot shower/sprinkle
this afternoon dry weather prevails. Clouds will erode toward
sunset as lapse rates ease.

Good mixing given cold air aloft and this will support highs 55-60,
low 50s high terrain. It will feel cooler given the modest NW winds.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...

350 AM Update...

Tonight...

Deep layer ridge advects eastward into the region. Thus dry weather
continues. Chilly given dry airmass, light winds and mostly clear
skies. Given these parameters derived min temps from coldest MOS
guidance yielding lows in the 30s regionwide, few upper 20s possible
northwest MA and lower 40s outer Cape and Nantucket. Given growing
season continues across CT/RI and southeast MA and expecting temps
to fall into the low and mid 30s in this region, will hoist a frost
advisory. Given dew pts are in the 30s, sufficient moisture for a
widespread frost all areas so have those ice scrapers ready for
windshields Saturday morning.

Saturday...

After a chilly/frosty start temps will rebound Sat afternoon thanks
to sufficient NW flow developing after sunrise and eventually mixing
out shallow cold air from blyr decoupling tonight. Deep layer ridge
remains west of New England so anticyclonic flow and associated
subsidence into MA/RI/CT providing plenty of sunshine. Subsidence
inversion limits blyr mixing to below 850 mb but +6C at 925 mb and
modest NW blyr winds will support highs from the upper 50s to lower
60s, which is on target for this time of year. However it will feel
milder given full sunshine and light winds. A real gem of a day for
late Oct.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Overview...

Expect the elongated N-S ridge to shift slowly across the region
overnight Sat night, which will bring light winds and mostly clear
skies. By mid morning to midday on Sunday, will likely see Post TC
16 push off the Carolina coast, but will linger well S of the
region. Will need to monitor this system, however, in case some
moisture tries to push into south coastal areas late Sunday or into
Monday.

While high amplitude mid level ridge builds E across the northeast
and into Quebec by the start of the work week, another deep H5
trough digs across the central U.S. Noting some timing and track
issues with its associated strong surface low which crosses the
Great Lakes around mid week. Big question with this system is whether
the frontal system will wrap around toward the E coast and allow
another low to develop close to or across the region, bringing
another shot of precip across Monday night, possibly lingering into
Wednesday. However, 00Z models signaling a rather wide solution
spread with this feature, so lower than average confidence for this
portion of the forecast.

Details...

Saturday night through Monday...

Will see dry and seasonable conditions through most of this
timeframe as high pres ridge builds slowly across. With the
ridge axis across the region Saturday night, will see excellent
conditions for radiational cooling especially across the
interior. Will likely see patchy frost develop across the
normally susceptible valleys across interior E Mass and the CT
valley. Lows will fall back to the upper 20s to lower 30s in
those areas.

Should be a rather pleasant day on Sunday with mostly sunny
skies to start, but may see some mid and high clouds work in
during the afternoon. Some question whether the northern edge of
moisture from Potential TC 16 tries to push N as it moves off
the mid Atlc coast, then slows to a creep well S of the region
Sun night. May see gusty E-NE winds develop during Monday along
S coastal areas but, at this point, conditions should remain
dry. However, some 00Z model members try to push some moisture
toward southern areas, but kept a dry forecast going for now.
Expect highs on Monday to reach the lower- mid 60s, except
holding in the upper 50s across Cape Cod and Nantucket.

Tuesday through Thursday...

Still some uncertainty in the sensible weather especially
during the Tuesday and Wednesday timeframe, which will depend
what will happen with the remains of Potential TC 16. May see a
secondary low spin up near the remains of the system as S-SE
winds increase, as well as an approaching cold front wrapping
around the low moving across central Canada.

Looks like best chance for showers occur from Tue afternoon
through Wed morning, but timing and track of the front and the
low to the S remain in question. There is the potential that
this low could become stronger than currently forecast, so keep
an eye on this portion of the forecast.

Should see the front sweep through sometime Wednesday afternoon
or evening, then high pres builds off the mid Atlc coast which
should bring dry conditions and seasonable temperatures.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Saturday/...

11Z Update...

Today...High confidence.

VFR conditions, except for a few spotty MVFR CIGS across the
lower CT valley and lower Cape which should improve by mid
morning. A few spotty showers/sprinkles. West winds 20-30 kt,
easing a bit this afternoon to 15-25 kt.

Tonight...High confidence.

VFR and dry conditions along with NW winds diminishing further
to 10-20 kt.

Saturday...High confidence.

VFR, dry and light NW wind.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...High confidence.

Saturday Night through Sunday Night: VFR.

Monday: Breezy.

Monday Night through Tuesday: Breezy. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Saturday/...High confidence.

350 AM update ...

Today...

WNW winds 20 to 30 kt diminishing this afternoon. Rough seas
continue over the ocean waters as westerly wind waves oppose SE
swells. Dry weather and good vsby prevail as powerful low over
Nova Scotia this morning exits into the Maritimes.

Tonight...

West winds continue to diminish as high pres builds into New
England from the west. Dry weather and vsby continue.

Saturday...

High pres builds over New England with winds diminishing
further. More dry weather and good vsby.

Outlook /Saturday Night through Tuesday/...Moderate confidence.

Saturday Night through Sunday Night: Winds less than 25 kt.

Monday: Winds less than 25 kt. Local rough seas.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Chance of rain showers.

Tuesday: Winds less than 25 kt. Rough seas up to 9 ft. Chance
of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Frost Advisory from 3 AM to 8 AM EDT Saturday for CTZ002>004.
MA...Frost Advisory from 3 AM to 8 AM EDT Saturday for MAZ017-018.
RI...Frost Advisory from 3 AM to 8 AM EDT Saturday for RIZ001-003.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for
     ANZ230>235-237-250-251-254>256.

&&

$$
SYNOPSIS...Nocera/EVT
NEAR TERM...Nocera/EVT
SHORT TERM...Nocera
LONG TERM...EVT
AVIATION...Nocera/EVT
MARINE...Nocera/EVT



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