Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 201905
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
305 PM EDT Thu Sep 20 2018

.SYNOPSIS...
High pressure builds into New England tonight bringing clouds
but drier weather. The high moves off to the east Friday,
bringing gusty southerly winds and warmer temperatures. A cold
front swings through the region Friday night with showers and
scattered thunderstorms. Large high pressure will then bring
dry but cool conditions through the weekend into early next
week. Another warm front approaches late Tuesday into Wednesday
with scattered showers and possibly a few thunderstorms.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
High pressure over the Maritimes builds south over New England.
Onshore flow becomes light and variable during the night. Models
show increasing low level moisture, which would support
developing patches of fog. Otherwise dry weather. Observed dew
points in the 50s should provide a lower bound for temperatures,
so min temps in the 50s.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
High pressure moves offshore Friday morning. This should bring
winds around out of the south, with milder air drawn north over
Southern New England. Low level winds aloft increase to around
25 knots, so expect gusty winds near that speed late in the day.
Instability remains west of us in NY through the day, so expect
a rain-free day. Temps aloft reach an equiv of 7-9C at the top
of the mixed layer, so max temps should be in the 70s.

Cold front moves across Southern New England Friday night.
Theta-e ridge is projected to enter Western MA/CT around
midnight and move off Cape Cod toward 8 AM/12Z. Some stability
parameters are favorable for thunder, others are mediocre. Most
favorable would be K indices as high as the mid 30s. We will
continue to mention showers and a chance of thunder. The south
to southwest flow supports increasing dew points with values in
the 60s, so min temps should also be in the 60s. Strong south to
southwest winds aloft will support gusts 25 to 30 knots until
the cold front moves through.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...

Highlights...

*Saturday through Monday dry and cooler
*Warm front Tuesday brings wet weather Tues- Wed
*Cold front late week brings cooler air to end the week

Overview...

Saturday through Monday...Our late week cold front will have moved
on out of the region by mid morning on Saturday taking the rain with
it and leaving us with dry and cooler conditions from mid morning
onward. Behind the front a large high pressure builds in from
eastern Canada over the next three days bringing us light winds and
a drier atmospheric column with generally zonal mid level flow.
Saturday though Monday we`ll see mostly sunny skies and overall a
nice weekend. High temps will trend downward each day as surface
winds turn out of the north then northeast late in the weekend -
generally mid 60s to low 70s, with widespread low to mid 60s by
Monday. Sunday and especially Monday mornings have potential to be
quite cool given the potential for a decoupled surface with light
winds and partly clear skies allowing for good radiative cooling
overnight.

Tuesday through Thursday...

On Tuesday morning a breakdown in the upper ridge starts to
make it`s way eastward toward us associated with a surface low
moving out of the central US into Canada. This low will drag a
warm front across the region during the day Tuesday, turning
surface winds to the south and advecting warm and moist air
back into the region. Dewpoints go from the 40s on Monday to the
mid 60s by Wednesday. Meanwhile high temps top out on Wednesday
back in the upper 70s. The warm sector will bring showers to
southern New England from as early as Tuesday morning through as
late as Thursday morning. We could even see thunderstorms on
Wednesday as instability increases ahead of the trailing cold
front which moves through in the Wed night/Thur morning
timeframe. Models are in agreement on this general evolution,
but not settled on exact timing of the fronts and start/stop of
the rain. Overall Tuesday and Wednesday look wet while Thursday
and beyond are somewhat drier, at least for a time. Beyond the
cold front, both the EPS and GEFS indicate an extended period
of upper level troughing over the east coast. There is a good
deal of uncertainty in this period but it is likely that we
remain unsettled with periodic rain chances.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Through tonight...Moderate confidence.

Surface flow from the northeast will diminish and become
light/variable early tonight. Lingering moisture both in the
surface layer and about 5000 feet above sfc. We will continue to
indicate clouds during the night. Also potential for MVFR/IFR
cigs/vsbys in fog after midnight, especially in the interior.

Friday...Moderate confidence.

Lingering MVFR/IFR clouds early, but all areas should trend to
VFR during the morning as winds turn from the south. Winds at
2000 to 4000 feet will reach 25 knots mid to late afternoon, and
will generate gusts at those speeds at the surface.

Friday night...Moderate-high confidence.

Cold front moves east from NY and crosses Southern New England
between midnight and sunrise. VFR much of the time, but areas of
MVFR cigs/IFR vsbys in showers and scattered thunderstorms
between midnight and sunrise.

KBOS Terminal...High confidence in trends. Lower confidence in
timing.

KBDL Terminal...High confidence in trends. Lower confidence in
timing.

Outlook /Saturday through Monday/...

Saturday: VFR. Breezy. Slight chance SHRA.

Saturday Night through Sunday Night: VFR.

Monday: VFR. Breezy.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...High confidence.

Tonight...Northeast winds diminish and become light as high
pressure builds over the waters. Seas linger near 5 feet on the
eastern outer waters through the night. Small Craft Advisory
continues on those eastern waters.

Friday...Increasing southerly winds with gusts to 25-30 kt
developing by late afternoon, especially eastern MA waters.
Seas remain near 5 feet on the eastern outer waters.

Friday night...Winds from the south and southwest much of the
night. Winds at 2000 feet reach 35-40 knots, and some of this
may be brought to the surface in gusts. Seas build to 5-7 feet.
A cold front crosses the waters late at night and should produce
a few showers, possibly a thunderstorm.  A Gale Watch will be
issued for our eastern outer waters and for Massachusetts/Ipswich
Bays.

Outlook /Saturday through Monday/...

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of seas approaching 5 ft. Slight chance of rain
showers.

Saturday Night through Sunday: Winds less than 25 kt.

Sunday Night: Winds less than 25 kt. Seas locally approaching
5 ft.

Monday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 AM EDT Friday for ANZ250-254.
     Small Craft Advisory until 8 PM EDT this evening for ANZ251.

&&

$$
SYNOPSIS...WTB/BW
NEAR TERM...WTB
SHORT TERM...WTB
LONG TERM...BW
AVIATION...WTB/BW
MARINE...WTB/BW


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