Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 240918
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
418 AM EST Tue Jan 24 2017

.SYNOPSIS...

A significant coastal storm will result in a wintry mix and
hazardous travel for the morning rush hour across the interior,
while heavy rain, poor drainage street flooding, and damaging wind
gusts will impact the Boston to Providence corridor.  Precipitation
lingers into tonight with pockets of freezing rain possible into
this evening across portions of interior Massachusetts.  High pressure
builds in behind the departing storm and brings dry mild weather
Wednesday and Thursday. A cold front crosses New England Thursday
followed by cooling temperatures and blustery winds through the
weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

***Hazardous morning rush hour across the interior as sleet changes
to freezing rain and temps may not break freezing today in some
locations***

***Damaging wind gusts and heavy rainfall to impact part
of the morning rush our on the Boston to Providence corridor***

Widespread heavy rain and strong to damaging northeast wind gusts
between 45 and 55 mph were occurring along and southeast of the
Boston to Providence corridor early this morning.  Some poor
drainage sleet flooding may impact part of the morning rush hour
along with the damaging wind gusts.

The strongest winds and widespread/heaviest rainfall will exit the
region by mid morning as low level jet/stronger forcing lifts to the
northeast of our region. However, well developed surface low and its
closed mid level centers will slowly lift northeast south of Long
Island.  Associated comma head will result in periods of rain
continuing today, which may fall moderate to heavy at times.  In
fact, mid level dry slot and steepening mid level lapse rates will
result in a low risk for an isolated thunderstorm or two across
RI/SE MA where Showalter indices drop to around zero this afternoon
and early evening.

Meanwhile, across interior MA and even portions of northern CT
shallow low level cold air has allowed temperatures to fall into the
upper 20s to the lower 30s.  Ptype was mainly in the form of sleet,
but should see a transition to rain/freezing rain this morning as
850T rapidly warm well above 0C.

Now to the biggest weather concern today.  While even the high
resolution guidance suggests that ptype will be warm enough for
plain rain in all locations by mid morning, past experience has us
skeptical in that scenario. Low pressure passing well southeast near
Cape/Islands will keep surface winds north northeast.
In fact, pressure falls east of the region may allow winds to even
shift back 10 or 20 degrees more to the north.  Given the above
reasoning it may be tough to get the portions of interior MA above
freezing today, especially across the high terrain.

So in a nutshell, will continue the winter weather advisory through
late afternoon across interior MA for sleet changing to freezing
rain.  Latent heat release may allow portions of the region to sneak
just above freezing, but not sure given NNE surface winds
and dewpoints in the 20s still advecting from the north. Ice
accretion of one tenth to one quarter of an inch will be possible
today across the Worcester Hills/East slopes of the Berks. Even in
northern CT temps around 32 or 33 with sleet and perhaps some
pockets of freezing rain this morning will result in hazardous
travel. Will continue winter weather advisory through 10 am.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM WEDNESDAY/...

Tonight...

Periods of rain likely persist much of the night given proximity of
well developed low pressure near the southeast New England coast.
Ptype still may be freezing rain across interior southern New
England especially across the high terrain, so winter weather
Advisories may need to be extended and will have to watch for a
potential impact on the evening rush hour.  Temps should remain
above freezing along and southeast of the Boston to Providence
corridor.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...

Big Picture...

West Atlantic ridge in place midweek will shift out to sea. Pacific
high pressure builds east into the Western USA, kicking an upper
trough east to the Eastern USA over the weekend. Ridge west/trough
east pattern continues early next week.

Shortwave embedded in the upper trough will move northeast with the
trough, crossing New England during Thursday. Upper jet associated
with this shortwave sweeps east and crosses New England later
Wednesday night and Thursday, then remains well to  our south over
the weekend.

Contour heights at 500 mb will be above average during midweek, then
below average Friday through Monday. This means mild weather midweek
followed by colder weather over the weekend.

Details...

Wednesday-Thursday...

Low pressure moves off through the Maritimes Wednesday morning and
carries off any leftover clouds and showers. Atlantic high pressure
builds over Southern New England with dry weather. Mixed layer
reaches to at least 950 mb, possibly a little higher to 925 mb.
Temps in this layer would support max sfc temps of 45 to 50.

Not much sky cover in the evening, but the trend during the night
will be for increasing clouds as the upper jet approaches and
enhances upper venting. Previous model runs had weakened the jet as
it moved overhead, but this 00Z suite maintains the jet strength
while the moisture in the column increases. Expect a few showers
from late night into Thursday morning.

The upper jet drives a shortwave trough and surface cold front
through our area. Current suite suggests timing during the afternoon
but this may shift a little as the event gets closer. Colder air
moves in behind the front, and the cold advection should increase
mixing depth to 850 mb. Winds in this layer reach 25 knots, possibly
30 knots...which is higher than model guidance. We will adjust
forecast values higher. Timing of the cold front will determine the
high temperature.  If the cold air arrives mid afternoon then max
temps may again be in the mid 40s to lower 50s.

Friday through Monday...

Broad cyclonic flow with cold pool clouds and a cooling trend on
temps aloft. Temps aloft trend from around -8C Friday to -11C or
-12C Sunday and Monday, so max sfc temps trend from either side of
40F Friday to the 30s early next week.  Another shortwave moves
through the flow over the weekend and may increase the sky cover as
it passes, but current forecast data shows moisture too limited for
precipitation.

&&

.AVIATION /09Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Today and tonight...Moderate to high confidence.  MVFR to IFR
conditions will persist in periods rain along the coastal plain
although not as widespread/heavy as early this morning. Across the
interior, sleet will transition to rain/freezing rain. Temps may not
rise above freezing across parts of interior MA especially across
the high terrain. Therefore, we are concerned about freezing rain
persisting into this evening. Northeast wind gusts of 35 to 45 knots
with the strongest on the coast will gradually diminish this
morning, although it will remain breezy across northeast MA through
the afternoon.

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

KBDL TAF...Moderate confidence in TAF.  Sleet will change to rain by
12z.

Outlook /Wednesday through Saturday/...Moderate confidence.

Wednesday-Thursday...

VFR Wednesday. Ceilings and vsbys trend to MVFR after midnight in
rain and snow as a cold front approaches from the west. Areas of IFR
are likely Thursday morning. Conditions trend back to VFR after the
cold front moves through Thursday afternoon. West winds gust to 20
knots Wednesday and 25 to 30 knots Thursday afternoon.

Friday-Saturday...

VFR. West winds gust to 25 knots each day.

&&

.MARINE...
Forecaster Confidence Levels...

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

***Storm force wind gusts diminish from south to north this morning
but very high seas persist into the afternoon***

Today...Moderate to high confidence.  ENE wind gusts of 45 to 55
knots with seas building to 20+ feet across our outer-waters this
morning. Winds should drop below Storm Force from south to north
this morning as low level jet lifts north of the region. In fact,
other than our northern waters winds probably drop below small craft
levels this afternoon as well developed low pressure moves into the
region diminishing the pressure gradient.  Seas will also diminish
but will remain very high into the afternoon.

Tonight...Moderate to high confidence.  Northwest small craft wind
gusts of 20 to 30 knots will re-develop behind departing low
pressure.  Seas gradually lower, but still between 7 and 12 feet for
much of the night across the outer-waters.

Outlook /Wednesday through Saturday/...Moderate confidence.

NW winds diminish Wednesday and back from the WSW. Lingering gusts
to 25 knots.  A cold front sweeps across the waters Thursday,
followed by colder air and gusty west winds 25 to 30 knots.  West
winds gusting to 25 knots continue through Saturday.

Seas will diminish through the period, starting 5 to 10 feet on the
exposed waters Wednesday and diminishing Wednesday night. Seas will
rebuild reaching 5 to 10 feet Thursday in the west winds behind the
cold front, then slowly diminish with 5 to 7 foot seas lingering on
the outer waters through Saturday.

A Small Craft Advisory will be needed for some of all of the waters
through this time period.


&&

.TIDES/COASTAL FLOODING...

Core of strongest winds affects South Coast through 6 am and East
Coast until 10 am.

Latest tide gauge readings show about a 2.0 to 3.0 storm surge early
this morning, which continues to run above model storm surge
guidance. It seems reasonable to still expect this morning`s storm
surge to peak between 3.0 and 4.0 feet, although 4.0 feet is
probably a good upper bound but not all locations will experience a
surge that high.

Seas were running close to 20 feet offshore and should build a
little further this morning to around 25 feet. These high seas will
result in strong wave action along coast with potential for
significant beach erosion and possible damage to structures such as
vulnerable sea walls.

Upshot of all of this is we expect widespread minor coastal flooding
along the East Coast as well as portions of RI South Coast. However,
pockets of moderate coastal flooding are still likely along Eastern
Massachusetts coast near Newburyport/Plum Island, Scituate, and
Nantucket Harbor. Winds turn offshore later today which minimizes
coastal flood threat in later high tide cycles.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Wind Advisory until 7 AM EST this morning for CTZ002>004.
     Winter Weather Advisory until 10 AM EST this morning for
     CTZ002-003.
MA...High Wind Warning until 7 AM EST this morning for MAZ019-
     022>024.
     Coastal Flood Warning until 11 AM EST this morning for MAZ007-
     015-016-019.
     Flood Watch until 10 AM EST this morning for MAZ005>007-
     013>021.
     Wind Advisory until 7 AM EST this morning for MAZ004-009-
     011>013-017-018-020-021-026.
     Wind Advisory until 9 AM EST this morning for MAZ005-006-
     014>016.
     Winter Weather Advisory until 4 PM EST this afternoon for
     MAZ002>006-008>012-026.
     Coastal Flood Warning until noon EST today for MAZ022-024.
     High Wind Warning until 9 AM EST this morning for MAZ007.
     Coastal Flood Advisory until noon EST today for MAZ023.
RI...High Wind Warning until 7 AM EST this morning for RIZ008.
     Flood Watch until 10 AM EST this morning for RIZ001>007.
     Wind Advisory until 7 AM EST this morning for RIZ001>007.
     Coastal Flood Advisory until 7 AM EST this morning for RIZ006-
     007.
MARINE...Storm Warning until 7 AM EST this morning for ANZ231>235-237-
     251-255-256.
     Gale Warning until 11 AM EST this morning for ANZ230-236.
     Storm Warning until 11 AM EST this morning for ANZ250-254.

&&

$$
SYNOPSIS...Frank/WTB
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...WTB
AVIATION...Frank/WTB
MARINE...Frank/WTB
TIDES/COASTAL FLOODING...JWD


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