Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 170258
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
958 PM EST Mon Jan 16 2017

.SYNOPSIS...

Low pressure will move from the Great Lakes to New York state
with secondary low pressure forming near Long Island Tuesday
and passing east of Nantucket by Wednesday. A mixed bag of
wintry weather will impact much of southern New England from
Tuesday afternoon into Wednesday. Could see leftover spotty
showers along the shore late this week. High pressure will build
along the eastern seaboard with dry, milder conditions into next
weekend.

&&

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

10 pm update...

Monitoring closely mid-level banding structures associated with
the lead isentropic ascent ahead of the Tuesday into Wednesday
disturbance. Seeing WSR-88D returns with some of that precip
reaching the ground across SW CT / Mid and Upstate NY / NE PA.
Top down moistening but the mid to lower levels of the column
still dry and above freezing as discerned from the ALY sounding.
Can not rule out some precipitation reaching the ground perhaps in
the form of freezing rain into S/W New England. However a lot of
dry air to overcome, so greater confidence towards this precip not
reaching the ground. Rather trhe returns from WSR-88D are simply
denoting the aforementioned top-down moistening. Surface dewpoint
depressions also remain large across the region.

High pressure in control. Increasing clouds overnight from the SW.
Mid to high level cloud decks. A blanket, should limit radiational
cooling given the light winds. Continuing forecast of lows down
into the 20s for most locations. Low 30s along the S-shore and
islands.

&&

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

Low pressure moving eastward from the Great Lakes to New York
state will spawn a secondary low pressure area near Long Island
Tuesday morning, which will head east toward Nantucket late
Tuesday night. It will cause a wide variety of messy weather
across our region.

All models are coming into good agreement on the timing.
Precipitation is expected to break out in western MA and northern
CT in the afternoon and spread eastward toward evening.
The most difficult part of this forecast is the precipitation
types.

For this forecast package, we have expanded the Winter Weather
Advisory to include central Middlesex and western Essex Counties
in northwest MA. We have upped snowfall totals significantly over
interior and especially northern MA.

Although the typical 1540m value for 850-700 mb partial thickness
will be to our northeast through the bulk of the event implying
warm air aloft, cold air will remain in the low layers of the
atmosphere. The 1000-850 mb partial thickness values remain below
1310m over a large part of MA through Tue. night. In fact, the
ECMWF and GFS show a pronounced lowering of the 1540m line down
to 1535m in northeast MA as heavier precipitation moves in Tuesday
night and evaporational/dynamic cooling takes place. All model
snowfall algorithms and the NCEP WPC forecast have at least 3
inches of snow, with perhaps as much as 6 inches occurring in the
Route 2 corridor of northern MA. Sleet and freezing rain will be
the predominant type of precipitation in northern CT and in
western and central MA south of the MA pike, before likely
changing to all rain. Up to one-tenth inch of ice accretion is
expected, although we cannot rule out local two- tenths amounts,
especially in the slopes of the Berkshires.

In northern RI and interior eastern MA, an inch or two of snow
is possible before turning to rain. All rain is expected in
southern RI and southeast MA.

A lot could change with this situation. A small change in
temperatures, both at the surface and aloft, could result in
big changes of precipitation type and corresponding snowfall
amounts.

A thin coating of ice on untreated roadways can be dangerous.
Please slow down and use caution if driving late Tuesday and
Tuesday night.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...

Highlights...

* Precipitation may linger across Wed into Wed night, with change
  back to light snow or wintry mix across the interior
* Low pressure passing off the mid Atlantic coast may push light
  precipitation into the region Fri night into Sat
* Another low may push slowly up the coast next Monday

Details...

Wednesday-Wednesday night...Moderate confidence.
Low pressure will pass S of New England during Wednesday, while
high pressure remains across the Canadian Maritimes. This will
keep colder air across inland areas where a mix of rain, sleet and
snow will continue for most of the day. However, expecting only
light amounts of melted precip, generally around 0.15 inches or
less. With temps close to freezing, will see low snow to rain
ratios, so may receive an additional dusting to 0.5 inches of
snow, with the best shot across northern Mass along and N of the
Route 2 corridor as well as across the higher terrain of northern
Worcester county along with the E slopes of the Berkshires.

With deep low level easterly winds, could see spotty light rain
and/or snow linger through Wed night across E Mass into RI. Should
see precip tapering off over central and western areas Wed evening.

Highs on Wed will range from the mid 30s well inland to lower-mid
40s along the immediate coasts. Readings Wed night will fall back to
the mid and upper 20s inland, ranging to the mid to upper 30s along
the coastline.

Thursday...Moderate confidence.
May see a few lingering rain and/or snow showers across eastern
areas early Thu, then precip should end by mid to late morning.
However, with continued N-NE winds, clouds will linger through
most of the day before starting to break across the lower CT
valley toward sunset.

Some question as to weak low pressure that may rotate SE out of
northern New England across NE Mass around midday or so. The light
onshore wind flow may enhance any precip and possibly bring a brief
period of rain showers across Cape Ann. Will not last too long,
though.

Expect temps to top off in the upper 30s across the E slopes of the
Berkshires to the lower 40s over the coastal plain.

Friday...Low to moderate confidence.
With a relatively progressive steering flow continuing across the
lower 48, will see another low develop across the south central
U.S. With a general W-SW wind flow aloft, this low will work
toward the region. Still some model solution spread as this low,
along with its H5 short wave, shifts mainly E toward the mid
Atlantic coast. The 12Z GFS remains on the fast and further N end
of the model suite.

Have kept only slight chance POPs in the forecast for most areas,
but best shot will be along the immediate S coast and the islands.
Then, as winds shift to E Fri night, could see some spotty precip
lingering across eastern areas as well as along the S coast.

Saturday-Monday...Low confidence.
For now, looks like the low should pass offshore during Saturday,
but some precip may again linger during the morning across
central and eastern areas before heading eastward.

High pressure looks to build across the region Sat night through
Sun, so should see mainly dry conditions.

Models and ensembles signaling development of cutoff H5 low
pressure developing Sat/Sat night across the lower Mississippi
Valley, then will shift E-NE late Sun and Mon. At this point, have
brought in CHC POPs late Sun night through Mon. With the heart of
winter in place, may see PTYPE issues as well. Still way too early
to tell exactly when, if any, wintry precip will take place and
what effect this could have on the region. Still quite a bit of
time to see the evolution of this system.

&&

.AVIATION /03Z TUESDAY THROUGH SATURDAY/...

Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High Confidence.

Tonight...VFR.

Tuesday...VFR to start. MVFR developing from W to E in the
afternoon. Wintry mix of SN, IP, FZRA, and RA spreading east
during the afternoon. Conditions then becoming IFR throughout the
region. Snow may be locally heavy reducing visibility in the
higher terrain areas of northern and western MA.

Tuesday night...IFR in SN, IP, FZRA interior, changing to rain and
in RA in RI and southeast MA. Easterly winds gusting to 20-30 kt
near the coast late at night.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Wednesday through Saturday/...

Wednesday-Wednesday night...Moderate confidence.
IFR-LIFR CIGS and MVFR-IFR VSBYS in patchy RA across the coastal
plain and mixed RA/SN/IP inland. Precip tapers off late Wed/Wed
night. Locally reduced visibilities during Wed. E winds gusting to
25-30 kt on outer Cape Cod and Nantucket Wed, diminishing as winds
shift to NE Wed night.

Thursday...Moderate confidence.
MVFR-IFR CIGS early, improving to mainly VFR. May see local MVFR
CIGS lingering across higher inland terrain during the day.

Friday-Saturday...Low confidence, mainly for exact timing of lower
conditions.
VFR early Fri, then conditions may lower to MVFR in patchy
RA/SN/IP, mixed inland Fri afternoon through Sat. May improve
inland during Sat. Light N-NE winds Fri shift to E Fri night, then
back to N-NE later Sat.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...High Confidence.

Tonight...Winds and seas below small craft criteria. Dry weather
and good visibility.

Tuesday...Winds becoming easterly and increasing to 10 to 15 kt.
A chance of rain late in the afternoon, especially along the RI
coast.

Tuesday night...Low pressure will be passing near or just south of
Nantucket. Small Craft Advisories have been issued for the overnight
hours into Wednesday for the eastern outer waters. Seas will be
increasing to 3 to 6 ft as easterly winds persist...and gusts
increase to 20-30 kt over the eastern outer waters.

Outlook /Wednesday through Saturday/...

Wednesday-Wednesday night...Moderate confidence.
Strong E winds gusting up to 25-30 kt on the eastern waters. Seas
build up to 7-10 ft on the eastern waters, then increase to 5-8 ft
on the southern outer waters Wed night. Low risk of gale force
gusts. Locally reduced visibilities in light rain Wed, then in
scattered rain and/or snow showers Wed evening across the eastern
waters. Winds and seas begin to diminish Wed night as winds back
to NE.

Thursday...Moderate confidence.
N-NE winds gusting to 20 kt early, then becoming W-NW around 10
kt. Seas remain above 5 ft on the outer waters through Thu, then
slowly subside Thu night but remain around 5 ft on the eastern
outer waters. May see briefly reduced visibilities in isolated
showers on the waters E of Cape Ann late Thu morning into the
afternoon.

Friday-Saturday...Low to moderate confidence.
Light NE winds shift to E Fri night. May see gusts to 20 kt Sat
on the waters from Cape Cod southward Sat. Seas may approach 5 ft
during Sat. Visibilities may be locally reduced in patchy light
rain Fri afternoon through Sat.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST
     Wednesday for CTZ002>004.
MA...Winter Weather Advisory from 1 PM Tuesday to 7 AM EST
     Wednesday for MAZ003>005-010>012-026.
     Winter Weather Advisory from noon Tuesday to 7 AM EST
     Wednesday for MAZ002-008-009.
     Winter Weather Advisory from 4 PM Tuesday to 7 AM EST
     Wednesday for MAZ006.
RI...None.
MARINE...Small Craft Advisory from 1 AM to 5 PM EST Wednesday for
     ANZ250-251-254-255.

&&

$$

SYNOPSIS...EVT/GAF
NEAR TERM...Sipprell/EVT/GAF
SHORT TERM...GAF
LONG TERM...EVT
AVIATION...EVT/GAF
MARINE...EVT/GAF


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