Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 202259
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
559 PM EST Fri Jan 20 2017

.SYNOPSIS...
A weakening weather system may produce some spotty light freezing
rain across northern Massachusetts tonight. Otherwise, tranquil
weather is expected Saturday into Sunday. A significant coastal
storm will push toward the region Sunday night, bringing a mix of
rain and/or snow, along with some icing conditions possible
inland. This storm may bring strong to possibly damaging winds to
coastal areas, and be potentially dangerous for mariners late
Monday into Tuesday. High pressure brings dry weather for
Wednesday.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...

Weakening short wave trough is getting stretched as it runs into
a quasi-stationary upper ridge across New England. Areas of light
rain may intrude into southwestern zones where temperatures are
expected to remain above freezing. However, cannot rule out a few
spotty areas of light freezing rain across Franklin, northern
Worcester, and far northwest Middlesex County tonight, generally
after 10 PM per a consensus of high resolution models. As of 3 PM
mesonet temperatures were mostly in the mid 30s with a few
readings of 33 and 34 across northern Massachusetts. It is a
fairly low probability event for any one spot but even a light
amount of freezing rain on a bare surface can be quite
treacherous. Hence, plan to issue a Special Weather Statement for
this relatively limited area of northern Massachusetts where even
under clouds may see temperatures slip a few degrees after dark.
Evaporative cooling if precipitation makes it into the region
could also help drop the temperature a degree or two tonight. QPF
where rain/freezing rain occurs should generally be under .05
inches. Have gone with the NERFC quantitative precipitation
forecast.

Otherwise, anticipate clouds to persist through the night with
some areas of fog developing. Do not think the fog will be as
significant as indicated by much of the guidance. The clouds will
also result in a narrow diurnal range with most temperatures
remaining in the mid to upper 30s overnight except for some lower
30s across northern Massachusetts.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...

Saturday...Main question is amount of lingering cloudiness during
the day. For now have gone with partial clearing. The air mass is
warmer than normal with 850 mb temperatures generally +2C to +4C
across the area. With modest mixing and light west to southwest
flow, anticipate high temperatures reaching the upper 40s to lower
50s across most of the area.

Saturday night...With elevated dewpoints and light wind flow
through a deep column, model guidance suggests areas of low clouds
and fog forming. The air mass remains considerably milder than
normal through the night, although colder air begins to sag into
northern New England very late at night. Anticipate most areas
will have low temperatures above freezing in the mid to upper 30s
with just a few spots near freezing along the interior northern
Massachusetts border.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...

Big Picture...

Storm off the Pacific NW coast moves ashore this afternoon and
evening, then moves across the Southern Tier of the USA during the
weekend, arriving at the Southeast USA coast by Monday morning. The
storm then moves up the coast Monday and Tuesday. Upper ridge then
moves across on Wednesday. Meanwhile the Wednesday-Thursday upper
flow shows a ridge along the Pacific Coast and positive tilt trough
from eastern Canada to the Desert Southwest.  A shortwave moves
through this progressive flow and across New England Wednesday night
or Thursday.

Model mass fields show fair agreement through Sunday night, then
differences in the positioning of the Monday-Tuesday storm as it
moves up the coast.  Mass field also show some agreement on the
general pattern Wednesday and Thursday, but again with differences
in details over the Midwest.

As noted earlier, the shortwave to be associated with the Monday-
Tuesday storm is still over the Pacific Ocean and outside of the
better sampling over North America. This reduces confidence in what
has otherwise been a consistent model solution. Expect that
confidence will increase later today and tonight as the shortwave
moves ashore in the West.  This assumes there are no significant
changes in the data as the storm moves ashore.

Details...

Sunday...

Weak cold front moves south through New England early Sunday.
Limited moisture with the front, enough for some clouds but not
precipitation. North to Northeast winds behind the front will
bring colder air into our region during Sunday. Lots of moisture
below 800 mb, but limited lift. Generally light low level east
wind expected over Srn New England during the day with a stronger
east wind of 25 to 50 knots focused on the Mid Atlantic coast and
possibly to Long Island late. So not a lot of low level lift
expected. Based on this, sky cover should be mostly clouds through
the day. The weak east flow may create some drizzle or sprinkles,
especially across RI and Eastern/Central MA. We will be trimming
model POPs to less than 25 pct.

Sunday night through Tuesday...

Our most significant weather system of the forecast period takes
place during this time. As noted above, a weather system off the
Pacific Northwest moves ashore later today and then moves along with
the southern jet stream across the southern tier of the country.
The developing surface low reaches the Carolinas Sunday night and
slowly moves up the coast Monday and Tuesday. The fast movement
across the nation followed by slow movement up the coast is
reasonable based on the developing negative tilt to the upper
trough as it reaches the East Coast.

The low will tap moisture from the southern stream, with
precipitable water values of 1 to 1.25 inches reaching Cape Cod
during Monday night. Meanwhile, cold air will be draining south due
to the building of Quebec high pressure over New England along with
a pronounced cold air damming signal in the pressure pattern. Low
level ageostropic flow remains a question mark with a strong NW flow
rather than the usual N drainage flow.

Model data shows very high POPs for Monday and Tuesday. As noted
earlier, the system that will give rise to the storm is still over
the Pacific Ocean. While it may appear to be a slam-dunk forecast,
there are enough scenarios still in play that could change that
forecast. As the system moves ashore and becomes better
sampled...and shows no changes in the forecast...our confidence will
grow to match the model confidence. Meanwhile, we have undercut POPs
to strong-likely.

Timing... Onset of snow remains uncertain, but likely during Sunday
night as persistent lift is expected to move into the region.
Onset of the heaviest precipitation should be late Monday or
Monday night as the strongest southerly winds move up over equally
strong east winds closer to the surface.

Precip types and amounts... Cold air is initially deep enough to
support a period of snow in Massachusetts, especially areas north
and west of the Mass Pike and I-495. Projected storm center tracks
over or just outside of Cape Cod support this notion. Amounts of 1
to 3 inches before changeover are reasonable in northern MA and
possibly an inch or two more in some spots.

Winds... Forecast data is advertising a strong low level jet. An
east jet of 50-70 knots at 925-950 mb moves over the region Monday
night and early Tuesday. BUFKIT shows east winds of 50 knots within
1000 feet of the surface Monday night. While not all of it may reach
the surface due to temperatures and time of year, it still suggests
potential strong wind gusts and possible damaging wind gusts. Best
chance of this would be along the coastal plain in MA and RI.

Model data shows some differences in timing the end of the storm,
but the surface low passes Tuesday evening. Would expect winds
to shift from the northwest and any pcpn to diminish at that time.

Wednesday through Friday...

Weak high pressure will build across the region Wednesday,
bringing dry and mild conditions.

Another fast moving front will cross the region during Thursday.
This system looks rather dry as most of the moisture and energy
moves into northern New England. However, can not rule out some
patchy light rain and/or snow across the E slopes of the
Berkshires, with light rain possible through the remainder of the
CT valley.

A persistent NW wind flow will be in place by Friday, along with
the chance for some isolated light rain and/or snow showers as a
digging upper level trough approaches during the day and into
Friday night. Temperatures will be at more seasonal levels for
late January.

&&

.AVIATION /22Z FRIDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Saturday Night/...

Rest of this afternoon...

High confidence. Will continue with a persistence forecast with a
continuation of high end MVFR or low end VFR ceilings. Light
winds.

Tonight...

Moderate confidence. MVFR with spotty IFR conditions anticipated.
Have leaned toward the higher end of the guidance envelope. Very
light rain or sprinkles possible western MA into CT. Spotty
surface icing is possible across Franklin, northern Worcester and
far northwest Middlesex County.

Saturday...

Moderate confidence. MVFR likely in the morning with spotty IFR
possible. Anticipate gradual improvement to VFR by afternoon in
most locations.

Saturday night...

Moderate confidence. Elevated dewpoints will help set the stage
for areas of fog and low cloud development. Forecast guidance is
fairly emphatic in lowering conditions to widespread MVFR ceilings
and visibilities with areas of IFR conditions. Our forecast once
again trended toward the more optimistic end of the guidance
envelope.

KBOS TAF...Moderate to high confidence in TAF.

KBDL TAF...Moderate to high confidence in TAF.

Outlook /Sunday through Wednesday/... Moderate confidence.

Sunday...
VFR to start Sunday morning, then lowering to MVFR from SW to NE
in light rain during the day. Light/variable winds shift to E-NE
during the afternoon. Gusts increase to 20-25 kt across outer Cape
Cod and Nantucket toward evening.

Sunday night through Monday night...
MVFR-IFR CIGS/VSBYS across the region Sunday night in rain and/or
snow, with some IP developing after midnight and continuing
through Monday. May see LIFR conditions Monday afternoon into
early Tuesday morning in patchy moderate rain and/or snow and
patchy fog. E wind gusts up to 40-50 kt likely along portions of
the immediate S coast, especially the outer Cape and Nantucket Mon
night. LLWS likely as E winds at around 2000 ft increases to 50-70
kt Mon night.

Tuesday...
Areas of IFR CIGS/VSBYS in RA/SN/FZRA early, changing over to RA
during the day. Precip mixes to FZRA/IP/SN over western areas
early Tue night before ending. IFR-LIFR CIGS linger Tue night
across eastern areas through midnight or so, then should improve
toward daybreak Wed.

Wednesday...
May see local MVFR-IFR conditions across the E slopes of the
Berkshires into the CT valley and central areas in SN/IP/FZRA as
cold front approaches. Most of the energy remains N and W of the
region, so looks like mainly VFR conditions elsewhere. NW winds
around 10 kt shift to SW during the day. Conditions should improve
Wed night.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday Night/...

Tonight through Saturday Night...High Confidence.

Light winds and seas under 5 feet expected to persist through
Saturday night. A 2 to 3 foot swell is expected to dominate
during this period.

Outlook /Sunday through Wednesday/...Moderate confidence.

Sunday...
Light N winds veer to E-NE during the day. Gusts will increase to
25-30 kt Sunday afternoon, with seas building to around 5 ft by
late in the day. Patchy light rain may reduce visibilities during
the afternoon along the S coast.

Sunday night through Tuesday...
Strengthening low pressure of the mid Atlantic coast Sunday night
will push NE. Expect E winds to rapidly increase as strong winds
will mix down from about 3000-5000 ft. Gusts will likely reach
50-60 kt across a good portion of the waters. Have already issued
storm watches across most of the waters due to the potential
threat of prolonged storm force winds. Seas will also build up to
15-20 ft. Expect the strongest winds and highest seas during
Monday afternoon and night. This is a potentially life threatening
storm for mariners. Visibility restrictions in rain and patchy fog
likely through most of Tuesday.

As the low passes across SE Mass toward the Gulf of Maine,
leftover gale force winds will push out of the eastern open waters
Tuesday morning. Winds will briefly diminish around midday and
through the afternoon Tuesday, then NW gusts could reach 25-30 kt
Tue night. Seas will remain at or above 5 ft over the open waters
through Tuesday night. Some visibility restrictions continue
early, then should improve from S-N.

Wednesday...
Winds will diminish as they back to W-SW during Wed. May see some
gusts up to 25 kt on the southern waters. Seas remain at or above
5 ft over the open waters. May see some visibility restrictions
on the waters S of RI in patchy light rain Wed afternoon and
night.

&&

.TIDES/COASTAL FLOODING...

Given potent easterly LLJ and period of strong wind gusts, expect
seas to probably build to 20 feet or higher across the eastern
waters late Mon into Tue. Fortunately, astronomical high tides are
about as low as they get, and the strongest winds Monday evening
will coincide with an astro high tide of only 8.2 feet in Boston.
Thus the risk of any significant coastal flooding is low. In fact,
it is hard to imagine a scenario of much worse than some
splashover for the Monday evening high tide, thanks to the lowness
of the astro high tide. The Tuesday morning high tide is higher at
9.4 feet. If winds are still blowing strong out of the northeast
along with 20+ foot seas just offshore, minor coastal flooding
could occur along the eastern MA coast. It would take a storm
surge of nearly 3.5 feet and waves greater than 20 feet to even
approach a moderate level of coastal flooding. Unless this system
progresses much more slowly than the current consensus of models
indicate, it is unlikely that we will experience anything worse
than minor coastal flooding and some beach erosion for the Tuesday
morning high tide in spite of such dangerous marine conditions
just offshore.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Storm Watch from Monday afternoon through late Monday night
     for ANZ232.
     Gale Watch from Sunday evening through late Monday night for
     ANZ233-234.
     Gale Watch from Monday morning through late Monday night for
     ANZ230.
     Storm Watch from Monday afternoon through Monday evening for
     ANZ231.
     Gale Watch from late Sunday night through late Monday night
     for ANZ236.
     Storm Watch from Monday morning through Monday evening for
     ANZ235-237.
     Storm Watch from Monday morning through late Monday night for
     ANZ250-254.
     Gale Watch from Sunday evening through Tuesday morning for
     ANZ251.
     Storm Watch from late Sunday night through Monday evening for
     ANZ255-256.

&&

$$
SYNOPSIS...EVT/Thompson
NEAR TERM...Thompson
SHORT TERM...Thompson
LONG TERM...EVT/JWD
AVIATION...EVT/JWD/Thompson
MARINE...EVT/JWD/Thompson
TIDES/COASTAL FLOODING...Thompson



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