Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 220919
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
419 AM EST Sun Jan 22 2017

.SYNOPSIS...
After fog burns off this morning, onshore winds will result in
cloudy skies with spotty light rain this afternoon. A significant
storm will reach the East Coast 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 crosses New England late week followed by seasonably
cool temperatures and blustery winds.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
335 AM UPDATE...

Dense Fog Advisory continues from SE MA to South Coastal RI/MA and
Cape Cod/Islands. Observations and webcams indicate near zero
visibility across most of these areas. Dense fog has also pushed
into BDL and PVD areas, but webcams indicate this isn`t as
widespread. For now we`ll handle it with a Special Weather
Statement. Visibilities should improve by mid morning as weak cold
front drops south and turns winds to N/NE.

One other concern is for spotty light icing early this morning
with temperatures bottoming out near freezing away from immediate
coast. However, ground temperatures are a bit warmer thanks to
recent stretch of mild weather, so this should not be a
significant issue.

Flow turns onshore today with high pressure over Quebec and low
pressure off Mid Atlantic coast. Forecast soundings show plenty of
low level moisture so a cloudy day is in store after fog
dissipates. We may see some spotty light rain or drizzle,
especially near east slopes of Berkshires and Worcester Hills
which will provide some added lift.

Temperatures will run a bit cooler than our preview of spring
yesterday, but still above average for late January, generally in
40s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Downstream ridging aloft is present over southern New England as
mid level trough closes off over Carolinas. As a result, models
have slowed down onset of precipitation associated with developing
coastal low, aside from spotty light rain/drizzle from persistent
onshore flow.

Since lower levels cool with high pressure ridging down from
Quebec, we still cannot rule out a light wintry mix across much of
region, especially from Berkshires to Worcester Hills. Temperatures
are borderline for any accumulation near I-95 corridor between
Providence and Boston, but it will be a little colder farther
north and west where we could see light coating of snow or ice by
daybreak.

Lows will fall back into the low to mid 30s, with some upper 20s
possible in NW MA.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Big Picture...

Deep trough digs over the Western USA while a ridge builds over the
West Atlantic.  In between the two, a Pacific shortwave rides the
flow and deepens to a closed low as it crosses the mid-South. By
this point, the developing storm taps moisture from the Gulf of
Mexico and draws it north. The storm runs into the West Atlantic
ridge and slows, taking on a negative tilt as it moves across the
Eastern States. The Pacific trough then shifts east across the
nation, reaching the Northeast USA by late week.

The GFS and ECMWF mass fields are similar until Thursday, with a
similar broad scale flow Friday-Saturday and differences in details.
The GGEM is noticeably farther offshore with the Monday-Tuesday
storm, taking the 500 mb center well south of New England while
matching the surface low position of the other models. An overall
model blend should work, especially with focus on the GFS and ECMWF.

The Monday-Tuesday storm will lower 500 mb heights to near normal
over the region Monday-Tuesday.  Height may recover to above normal
Wednesday and Thursday, but then the advancing trough from the
Western USA will bring below normal heights for the latter part of
the week. All of this suggests a mild period following the storm,
then seasonable cold for Friday and Saturday.

Details...

Monday-Tuesday...

Onshore east flow will maintain clouds over the region Monday
morning. Winds will increase during the day and may supply enough
low level lift for areas of sprinkles or drizzle. More organized
precipitation will be difficult due to dry air at the nucleation
zone aloft between -10C and -15C. This will moisten during the
afternoon as deeper moisture advects in from the south, at which
point precipitation will be more likely.

Low pressure reaches the Carolinas Monday morning and model
consensus brings the system up the coast Monday evening and night.
Meanwhile, Quebec high pressure settles south into the Maritimes and
Northern New England. The low brings plenty of moisture, with precip
water values of 1.0 to 1.25 inches, while the high brings cold air.
As the two collide, the pressure gradient will increase and generate
strong winds both at the surface and for several thousand feet above
the ground.

Upper low becomes negatively tilted Monday, suggesting a slow
movement of the system up the coast Monday night and Tuesday. The
high will slowly move off, allowing the coastal low to move past New
England.  Model consensus brings the surface center across SE Mass
near Buzzards Bay Tuesday afternoon/evening.

Wind... As the pressure gradient increases on Monday, so will east
winds. Low level east winds will start Monday focused on the Mid
Atlantic coast, but will edge north to the New England South Coast
during the afternoon/evening. Winds in the core at 2000 feet AGL
are forecast at 65 knots. These winds will move across south
coastal areas early Monday night and sweep north of the MA North
Shore by Tuesday morning. This will produce strong wind gusts
later Monday and Monday night, primarily over coastal areas
including Cape Cod and the Islands. There is also a concern for
east coastal MA which will be most open to the east winds off the
ocean. Gusts to 60 knots are most possible here. Strong winds may
reach farther inland, but at lesser speeds.

Precipitation onset... Upper venting starts Monday focused over
eastern PA and adjacent NY/NJ. Low level east flow starts the day
south of New England aimed into the Mid Atlantic. Expect the
precip shield to start the day in this area and then shift
north/east as the upper venting and low level jet also shift. We
estimate starting time around midday along the south coast and
early evening along the NH border.

Precipitation type... Cold air drainage is still indicated by the
surface pressure pattern, but ageostrophic pattern still shows a
northwest flow rather than the typical north flow for cold air
drainage. Nevertheless, low level temperatures will be cold enough
for a period of snow/sleet over parts of central and western MA, and
possibly briefly in parts of Northern CT. Farther east and south the
precip should be mainly rain.

Rainfall amounts...Lots of precipitation moves in with the weathersystem,
with precipitable water values as noted. Highest values should be
near the storm center and a little to the left of its track. So
expect highest rainfall amounts in RI and Eastern MA. Overall,
amounts over Southern New England should range from 1 to 3 inches.
This should not be a cause of any river flooding. But urban and
poor drainage flooding may be a concern in any heavier downpours.
A Flood watch continues in RI and Eastern MA where the heaviest
rainfall is expected.

The storm center moves off along the coast of Maine Tuesday night.
Expect precipitation to diminish at that time, followed by slow
clearing overnight.

Wednesday-Thursday...

Generally dry weather through this period. Weak high pressure builds
over the region Wednesday. A weak cold front moves across on
Thursday, but starved of enough moisture for precip most places.
There remains a chance of patchy light rain or snow in Northwest MA.

Friday-Saturday...

No significant changes were made to the forecast due to our focus on
the Monday-Tuesday storm. Dry and cooler weather moves in behind the
front for Friday and Saturday.

&&

.AVIATION /09Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...

Moderate confidence.

Widespread LIFR from SE MA to South Coast, Cape Cod, and Islands
in fog. Conditions slowly improve after 12z but IFR ceilings
should persist all day. Elsewhere, areas of MVFR/IFR ceilings
develop by daybreak. Patchy fog diminishes during the morning.

Ceilings should lift to MVFR tonight with increasing E/NE flow.
May see spotty light rain near coast and light wintry mix farther
inland, especially after midnight. Gusts to 30kt possible near
immediate coast overnight.

KBOS TAF...Moderate confidence in TAF. Ceilings may not lower as
quickly as forecast early this morning.

KBDL TAF...Moderate confidence in TAF. Patchy ground fog may bring
large variations in visibility, but mainly in LIFR-IFR range.

Outlook /Monday Night through Thursday/... Moderate confidence.

Monday-Tuesday...

MVFR cigs will slowly lower to IFR/LIFR as precipitation moves in
from the south.  Patchy MVFR VSBYs may occur in drizzle Monday
morning. As the precipitation moves in from the south, this will
become a more general MVFR with areas of IFR vsbys in fog and rain.
Snow and sleet will be possible in the interior, with a period of
freezing rain before it changes to rain.  Best chance for snow and
sleet/freezing rain will be north of the Mass Pike, although
Northern CT has a smaller chance. Rain and fog farther south and
east.

Increasing winds above the surface Monday, with speeds reaching
their maximum Monday night and then diminishing Tuesday. Winds at
1000-2000 feet should reach 40 to 60 knots Monday afternoon and
continue Monday night. East winds of 65 knots are possible over Cape
Cod and Islands. Expect low level wind shear during this time along
with strong wind gusts at the surface. Winds then diminish during
Tuesday as the low level jet shifts north into Maine. Surface winds
start from the east, then from the Northeast on Monday night and
turn from the Northwest Tuesday. CIGS and Vsbys improve to VFR
Tuesday night.

Wednesday-Thursday...

VFR. An approaching cold front may bring a period of MVFR cigs/vsbys
in rain showers for parts of Northwest Mass.

&&

.MARINE...
Forecaster Confidence Levels...

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

High confidence through tonight.

Increasing N/NE winds today with gusts reaching 25kt during
afternoon. Winds should not reach Gale Force until later tonight,
probably around midnight or so, but seas will steadily build
today and tonight, reaching 10-15 ft offshore by daybreak Mon.

Dense fog will gradually lift this morning, followed by spotty
light rain or drizzle at times later today and tonight.

Outlook /Monday Night through Thursday/... Moderate confidence.

Monday-Tuesday...

Strengthening low pressure off the Mid Atlantic coast will move up
the coast Monday and cross near Cape Cod Tuesday afternoon/evening.
Expect increasing winds Monday as the low runs up against Maritime
high pressure creating an increased pressure gradient over the New
England waters, as well as 1000-4000 feet above the surface.
The strongest winds will move across the waters from Monday
afternoon and night, diminishing Tuesday. East winds above the
surface will reach 60 to 70 knots during this time, and have the
potential to produce storm force gusts at the surface. The exception
may be Boston Harbor and Narragansett Bay, where somewhat lesser
east gales may be the rule.

The Storm Warning continues from Monday afternoon through Tuesday
morning. This covers all waters except Boston Harbor and
Narragansett Bay. These waters will have a Gale Warning.

These east winds will build seas and push them toward the eastern
shoreline. Expect highest values of 15 to 20 feet over the most
exposed waters during Monday evening and night.

Winds will shift on Tuesday, becoming North during the afternoon and
Northwest Tuesday evening as the storm center moves past.

Wednesday-Thursday...

Winds diminish and back from the WSW. Lingering gusts to 25 knots.
Seas will subside through the day, but with lingering 5 to 6 foot
seas on the exposed waters.

&&

.TIDES/COASTAL FLOODING...
Still looking at potential for splashover or minor coastal
flooding along E MA coast during high tide Mon evening and 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 20-25 ft offshore may lead to splashover
with Mon evening high tide. Minor coastal flooding could occur
with Tue morning high tide since it`s a bit higher, although winds
would have to still be from NE which is questionable.

Overall, 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...High Wind Warning from 9 AM Monday to 6 AM EST Tuesday for
     MAZ022>024.
     Dense Fog Advisory until 10 AM EST this morning for MAZ017>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.
RI...High Wind Warning from 9 AM Monday to 6 AM EST Tuesday for
     RIZ008.
     Dense Fog Advisory until 10 AM EST this morning for RIZ006>008.
     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...WTB/JWD
NEAR TERM...JWD
SHORT TERM...JWD
LONG TERM...WTB
AVIATION...WTB/JWD
MARINE...WTB/JWD
TIDES/COASTAL FLOODING...



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