Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 230232
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
932 PM EST Sun Jan 22 2017

.SYNOPSIS...
Moist onshore winds will result in spotty light rain/drizzle
tonight. Subfreezing temperatures are expected across the high
terrain of Massachusetts. This will result in the risk for light
snow showers...sleet and even freezing drizzle. A significant
storm will reach the East Coast late Monday and move up the coast
Monday night and Tuesday. This will bring a mix of rain and/or
snow along with a period of icing well inland. The storm may also
bring damaging winds to the coast and the coastal waters. High
pressure then brings dry and mild weather Wednesday into Thursday.
A cold front will cross New England late this week, followed by
seasonably cold temperatures and blustery winds next weekend.

&&

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

930 pm update...

Moist low level northeast flow was resulting in areas of light
rain showers and drizzle along the coastal plain this evening.
Ptype was a mixture of sleet and light snow showers across the
higher terrain of MA. Continued the special weather statement for
the high terrain of the Worcester Hills and along the east slopes
of the Berks, where there may even be some pockets of light
freezing drizzle. Any amounts of light snow/ice will be very
light, but slippery travel is possible on untreated roads
overnight across the high terrain.

The rest of the region will remain above freezing, so just pockets of
light showers and drizzle but it will be raw with temps dropping
into the 30s. Moisture and lift is shallow, so not expecting any
significant precipitation overnight. Northeast wind gusts will be
on the increase to between 25 and 35 mph along the coastal plain
by daybreak.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
updated 440 pm...

Monday...

Models have slowed down onset of steady/heavier precip until late
in the day Monday and possibly not until early Mon evening. This
is due to northern stream short wave moving across Quebec into New
Brunswick. This keeps deep layer ridge over MA/RI and CT much of
the day. This will result in dry air in the mid and upper levels
of the atmosphere overspreading the area. This includes the snow
growth region. This combined with increasing moist onshore flow
will result in an abundance of low level moisture and will likely
yield spotty light rain/drizzle much of the day. Therefore not
expecting much if any snow and/or sleet until possibly late in the
day.

Much cooler day tomorrow with highs only in the 30s (seasonably
cold) to around 40 along the immediate coast. Concern again is
across the high terrain where temps may not get much above
freezing. Thus low risk for spotty light freezing drizzle across
the high terrain of MA.

Winds will be on the increase as low level jet approaches. By
sunset should see NE winds gusting up to 40 to 50 mph along the
south coast including Cape Cod and islands.

Monday night...

Deep layer moisture and lift overspread the area Mon night as
southern stream closed low comes rolling up the eastern Seaboard.
70+kt low level easterly jet off the Atlantic acts as a firehose
into the region Mon night. This combined with the likelihood of a
coastal front developing over eastern MA will result in a period
of very heavy rainfall. Good model agreement on a period of very
heavy rain over RI and eastern MA 06z-12z Tue with global guidance
offering qpf up to 1.5 inches over this region. As you expect the
high res guid is even heavier. Thus risk of significant
street/highway/urban flooding along with small rivers and streams
across RI and eastern MA. This supports our current Flood Watch
area and reasoning.

Farther inland very impressive warm nose above 850 mb with temps
in this layer warming to +5C or so depending on model of choice.
This will limit snowfall potential and have lowered snowfall
forecast after collaborating with WPC. However below 850 mb there
is a cold layer with temps as cold as -6C to -8C across northwest
MA depending on model of choice. This will likely result in a
period of freezing rain across the high terrain of MA especially
western MA. Somewhat concerned that there is a low risk of ice
accretion reaching or exceeding 0.50 inch. However surrounding
offices including WPC were not as confident and thus have opted to
go with a winter weather advisory for Monday night into Tue
morning. Although will have to watch this area for potential
upgrade to a warning if forecast confidence increases with time.
Until then expect a winter mix of snow and sleet Monday evening
changing to freezing rain across the high terrain with greatest
risk of significant icing over northwest MA.

Very strong winds remain in the forecast for Cape Cod and the
islands where sustained winds of 40+ mph and gusts near 60 mph are
likely. This is in response to a powerful 70+ kt low level easterly
jet moving across this region. More marginal for these high wind
speeds along the eastern MA coastline thus have continued the High
Wind Watch for those areas. May eventually need a wind advisory for
slightly lower wind speeds all areas including the high terrain as
model soundings show wind gust potential up to 50 mph.

Mild along the coastal plain as low level jet floods this region
with maritime airmass and temps in the 40s. Low level cold air will
remain entrenched northwest of I95 given CAD signature with high
pressure over Quebec and pressure falls to our south and east.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Overview...
Cutoff H5 low pressure and its nearby surface low will slowly
work up the eastern seaboard during Tuesday. The low will shift
E-NE during Tue, with high pressure ridge building across Wed with
dry and mild conditions as winds back to SW.

12Z models have some differences with approaching cold front,
along with NE-SW oriented long wave H5 trough that will slowly
cross the region late Thu or Fri. Noting differences in how
individual models handle any possibility of light precip moving
across as the front approaches. Cutoff H5 low sits across Hudson
Bay into northern Quebec, which will help drive the upper system
through late this week as H5 heights lower while another short
wave begins to shift SE out of Manitoba late week.

Once the upper system moves offshore, seasonably cold air pushes
in, but it will be blustery late Fri through next weekend as
another reinforcing shot of colder air works in on the NW flow at
the surface and aloft.

Details...

Tuesday...
Strong E winds will continue through midday Tuesday, highest
across coastal areas. The strongest winds will occur Tue morning.
Then, the low level jet associated with the strong winds will
shift E during the day, so the surface winds will diminish.

With the surface system trapped in the upper level cutoff flow,
will see more steady rainfall across eastern Mass, with another
0.50" or so of rainfall across Cape Ann likely during Tue. Will
also see mixed RN, IP and patchy FZRA across the higher inland
terrain linger early Tue, but should change over to RN by around
midday. May see a mix back to FZRA/IP Tue evening as the precip
tapers off Tue evening over the higher terrain. The steady rain
and patchy fog should exit E coastal areas after 07Z Wed or so.

The low should cross Nantucket and the outer Cape Tue evening,
while the cutoff upper system moves in tandem, passing near or
just S of the 40N/70W benchmark. Winds will back around to N by
Tue evening, with gusty winds across Cape Ann lingering.

Quite a wide range in temps forecast for Tue. Highs from 35-40
across the N central and W Mass and the higher inland terrain to
around 50 on the S coast. Readings then will fall back Tue night
as colder air starts to work in. Temps bottom out in the mid 20s
to around 30 across central and western areas, ranging to the mid
and upper 30s along E coastal Mass.

Wednesday-Thursday...
High pressure ridge at the surface and aloft crosses the region
Wed. As the ridge shifts E Wed night and Thu, may see scattered
rain and/or snow showers move in across the E slopes of the
Berkshires ahead of approaching weak cold front. Some timing
differences on both the frontal passage as well as how far S any
precip reaches Wed night or Thu. Temps will top off in the 40s
both days, though they will start to fall to more seasonal levels
Thu night as colder air starts to work in from Canada.

Friday through Sunday...
NW winds at the surface and aloft will usher in seasonable
temperatures for late January. Should remain mainly dry, but may
see some widely scattered snow showers over the E slopes of the
Berkshires. Otherwise, should see mainly dry but blustery
conditions during this timeframe. Some question about whether
there may be a quick passing weak low and trough late in the
weekend that might bring some light precip, but held off on this
feature for now.

&&

.AVIATION /02Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...Moderate confidence thru this
period.

Tonight...

Mainly IFR-LIFR CIGS across central and E areas, and around 1000
ft across CT valley. VSBYS locally IFR-LIFR across Worcester
hills, otherwise mainly VFR with some low end MVFR in patchy fog.
Patches of -RA/-DZ across E Mass/RI. May improve to MVFR all
areas by around 12Z. May see light -FZRN/ -FZDZ across higher
inland terrain as temps may fall to near or just below freezing
mainly after 04Z- 05Z. Elsewhere in the lower elevations temps
remain above freezing. NE winds increase overnight, gusting up to
35 kt along the south coast of MA and RI.

Monday...

MVFR with spotty light rain/drizzle...possibly freezing
rain/drizzle high terrain. Steady precip holds off until 21z-24z
from south to north. Strong northeast winds with gusts up to 50 kt
by late in the day across Cape Cod and islands.

Monday night...

MVFR/IFR in periods of heavy rain and strong winds across much of
the region with snow/sleet and freezing rain likely high terrain
especially northwest MA.

KBOS TAF...Moderate confidence in TAF. Greatest uncertainty
concerns areal coverage of light precip.

KBDL TAF...Moderate confidence in TAF. Greatest uncertainty
concerns areal coverage of light precip.

Outlook /Tuesday through Friday/...Moderate confidence.

Tuesday...

MVFR-IFR CIGS/VSBYS, with lowest conditions across central and E
Mass into RI. Expect mainly -RA across the coastal plain and lower
CT valley, with mixed -RA/-IP across N central and W Mass into far
NW CT early Tue, then should change over to all -RA from S-N as
temps increase to above freezing. Areas of fog. May see a mix with
FZRA across highest terrain of N Worcester county and the E slopes
of the Berkshires toward Tue evening as temps fall below freezing
again. Expect CIGS/VSBYS to improve to VFR across central and
western areas from 06Z-09Z.

Precip should taper off across S and W areas by around midnight,
though spotty RA/FZRA/fog may linger briefly after midnight.
Remainder of the -RA across eastern areas should end by 07Z-09Z.

E-NE winds gusting up to 25-30 kt inland, then up to 45-50 kt
across Cape Ann and outer Cape Cod Tue morning which will slowly
diminish by around midday. Gusts linger at around 25 kt through
the afternoon around Cape Ann and moving across Cape Cod Tuesday
night as winds back to N.

Wednesday...
VFR. An approaching cold front may bring a period of MVFR
CIGS/VSBYS in -SHRA/-SHSN late Wed/Wed night across the E slopes
of the Berkshires. Low chance for isold -SHRA and patchy fog
across the Route 2 corridor of N Mass. NW winds gusting up to 25
kt on outer Cape Cod and the islands early Wed, then diminishing.

Thursday-Friday...
Mainly VFR. Patchy MVFR CIGS linger across N Central and W Mass
Thu, along with sct SHRA/SHSN with local MVFR VSBYS Thu. SW winds
gusting up to 20 kt on outer Cape Cod and the islands Thu, then up
to 25 kt Thu night into Friday.

&&

.MARINE...
Forecaster Confidence Levels...

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

*** Storm Force Wind Gusts Monday Night into Early Tue ***

710 PM Update...Only a few adjustments to bring conditions
current.

Tonight...

Increasing NE winds overnight with gusts reaching 25-30 kt by
morning except up to 35 kt along the south coast. Vsby reduced at
times in areas of fog and drizzle.

Monday...

NE winds continue to increase to near 50 kt by sunset along the
southern waters...35 to 40 kt elsewhere. Vsby reduced at times by
drizzle and fog. Steady rain holds off until sunset across southern
waters...later elsewhere.

Monday night...

The brunt of the storm is Mon night with ENE winds up to 55 kt.
Strongest winds over eastern MA waters including Cape Cod and
Nantucket waters. Very heavy rain with isolated thunder possible.
Vsby reduced to less than a mile at times in fog and heavy rain.

Outlook /Tuesday through Friday/...Moderate confidence.

Tuesday...
Low pressure moves across Cape Cod Tuesday morning. E wind gusts
up to 50-55 kt on the eastern open waters early Tue, diminishing
Tue afternoon as winds back to N. Seas up to 25 to 30 ft early,
subsiding to 12-18 kt Tue evening. Rough seas will continue Tue
night as NW winds continue to diminish. Visibility restrictions
in rain and patchy fog into Tue night, then improving from S-N.

Storm Warnings continue from Monday afternoon through Tuesday
morning. This covers all waters except Boston Harbor and
Narragansett Bay, where Gale Warnings remain in effect.

Wednesday through Friday...
NW winds on Wed shift to W-SW Thu and continue on Fri. Gusts up
to 25-30 kt continue across most of the waters with seas remaining
at or above 5 ft. Seas may linger around 5 ft during Wed at the
outer entrance to Boston Harbor, otherwise winds and seas diminish
below small craft criteria on Boston Harbor and Narragansett Bay.

&&

.TIDES/COASTAL FLOODING...
440 PM Update...

Still looking at potential minor with pockets of moderate coastal
flooding along E MA coast during high tide Tue morning.

Despite low astronomical tides (8.2 ft at Boston Mon evening and
9.4 ft Tue morning), combination of persistent and very strong E
winds and high seas up to 20-30 ft offshore may lead to minor to
perhaps pockets of moderate coastal flooding with Tue morning high
tide. Surge guidance has values around 2.5 ft at the time of high
tide along with very strong wave action with seas 20-30 ft across
the eastern MA waters. This wave action ontop of the elevated storm
tide will likely be sufficient for a risk of minor coastal flooding
with a lower risk of moderate flooding. Still 36-48 hrs away and a
difference in 6 hrs of timing the strongest winds can have a big
impact on flood potential. Thus most likely scenario is a coastal
flood advisory event. Thus will hold off on any watches at the
moment.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Winter Weather Advisory from 4 PM Monday to 7 AM EST Tuesday
     for CTZ002.
MA...High Wind Warning from 9 AM Monday to 6 AM EST Tuesday for
     MAZ022>024.
     Flood Watch from Monday afternoon through Tuesday morning for
     MAZ005>007-013>021.
     High Wind Watch from Monday afternoon through late Monday
     night for MAZ007-018-019-021.
     Winter Weather Advisory from 4 PM Monday to 7 AM EST Tuesday
     for MAZ002>004-008-009.
RI...High Wind Warning from 9 AM Monday to 6 AM EST Tuesday for
     RIZ008.
     Flood Watch from Monday afternoon through Tuesday morning for
     RIZ001>007.
MARINE...Storm Warning from 1 PM Monday to 4 AM EST Tuesday for ANZ231-
     232.
     Storm Warning from 1 PM Monday to 3 AM EST Tuesday for ANZ233-
     234.
     Gale Warning from 7 AM Monday to 7 AM EST Tuesday for ANZ230.
     Gale Warning from 3 AM Monday to 4 AM EST Tuesday for ANZ236.
     Storm Warning from 7 AM Monday to 2 AM EST Tuesday for ANZ235-
     237.
     Storm Warning from 11 AM Monday to 7 AM EST Tuesday for ANZ250-
     254.
     Storm Warning from 2 PM Monday to 6 AM EST Tuesday for ANZ251.
     Storm Warning from 4 AM Monday to 4 AM EST Tuesday for ANZ255-
     256.

&&

$$
SYNOPSIS...Nocera/EVT
NEAR TERM...Frank
SHORT TERM...Nocera
LONG TERM...EVT
AVIATION...Nocera/EVT
MARINE...Nocera/EVT
TIDES/COASTAL FLOODING...Nocera


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