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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
700 PM EDT Mon Jul 22 2019

Synopsis...
low pressure will track along a frontal boundary draped across CT,
Rhode Island and southeast Massachusetts tonight and will be accompanied by scattered
showers and thunderstorms with heavy downpours possible. The front
slides along or just offshore Tuesday with showers then focused
across southern Rhode Island and southeast Massachusetts. The low and frontal boundary
will move offshore Wednesday morning, followed by a stretch of dry,
seasonable weather along with comfortable humidity arriving
Wednesday afternoon and lingering through the weekend with
temperatures gradually warming through the period.

&&

Near term /until 6 am Tuesday morning/...

*** Severe Thunderstorm Watch remains in effect until 10 PM ***

7 PM update ...

Warm front draped across Long Island to just south of bid-mvy-
ack. How far north this warm sector tracks will dictate area of
potential strong storms. Greatest risk for rotating storms will
be along and south of this boundary. Storm Prediction Center mesoanalysis has true
warm sector airmass denoted by 0-3 km cape with its axis across
Long Island at 630 PM. The rap model advects this warm sector
airmass across south coastal Massachusetts/Rhode Island including Cape Cod and the
islands later this evening. However outflow from convection may
drive warm sector farther south. If this occurs threat of severe
weather will remain farther south/southwest. Thus lots of
uncertainty.

Nevertheless Severe Thunderstorm Watch remains in effect until
10 PM. Main threat will be along the South Coast of Massachusetts/Rhode Island for
strong to damaging winds with these storms along with torrential
downpours and localized flooding with pwats near 2 inches and
dew pts into the low and mid 70s in the warm sector.

Elsewhere lighter much appreciable rainfall tonight. Expecting a
bit of a lull in the rain second half of the night with next
round of rain Tuesday morning into midday with trailing short
wave and secondary wave of low pressure tracking along or just
offshore. Heaviest rain will be confined to southern Rhode Island/Massachusetts and
possible just the immediate South Coast per hi res guidance.
Earlier discussion below.

=================================================================

Scattered rain showers and a few thunderstorms continue across
southern New England early this afternoon. At the synoptic level we
have a shortwave approaching in the mid levels generating a frontal
wave to our southwest that will track northeast and bring strong to
severe storms this afternoon and evening. Surface observations show
the front over southern CT with an impressive convergence boundary
in southeast New York/southwest CT firing off severe storms to our west.
This is the location of the best low level turning in the atmosphere
together with ample instability. As southern zones warm sector with
the advancing warm front dewpoints in the upper 60s will increase to
the low 70s, and MUCAPE increases on the order of 1000-2000 j/kg.
Though mid level lapse rates are poor, shear values are quite good,
which should allow for prolonged updrafts and organized, rotating
storms. 0-6km bulk shear values will reach 40-50kts. Good low level
rotation will increase tornado potential as well; 0-1km shear of 25-
35kts and 0-1km srh approaching 150. The area of strongest updraft
helicity (a good proxy for severe storm potential) continues to hit
on southwest CT reaching into northern CT and Rhode Island. A Severe
Thunderstorm Watch has been issued for all of CT and Rhode Island through 10
PM. The main threat will be from damaging winds with the potential
for an isolated tornado.

Additionally these storms will come with some locally heavy
downpours which could cause some urban and small stream flooding.
Anomalous pwats approach 2.25" with plenty of lift ahead of the
shortwave with placement under the entrance region of a 110kt 300mb
jet. Heavy rain is possible through the overnight hours, especially
over CT and western-central Massachusetts. A bit of a break comes after
midnight before our next round arrives early Tuesday.

&&

Short term /6 am Tuesday morning through Tuesday night/...

The trough axis remains to the west on Tuesday with another frontal
wave moving up the coast bringing additional heavy rainfall. This
time the axis of greatest pwats shifts southeast as the best lift
from low level convergence and favorable upper jet dynamics aligns
to bring the heaviest rainfall to Rhode Island and southeast Massachusetts. Another 1-2"
is possible with locally higher amounts up to 3" leading to the risk
of urban and poor drainage flooding. A 35-45kt low level jet moves over
southern Rhode Island and southeast Massachusetts which could mix down gusty winds in
heavy downpours as well, while retreating instability should keep
the threat of thunderstorms to the South Coast. The low lifts out of
the area bringing rain to an end from west to east Tuesday evening
into early overnight. Tuesday night should be mostly dry, but a
third and final low crosses further to our southeast keeping a
chance for some rain showers in the forecast for the South
Coast/Cape Cod/islands.

&&

Long term /Wednesday through Monday/...

Highlights ...

* dry stretch of weather Wed afternoon into the weekend
* seasonable temperatures and comfortable humidity Wed & Thu
* gradually becoming warmer and more humid Fri/Sat/sun/Mon

Long wave trough and attending weak surface low south of Long Island
12z Wed advects eastward giving way to a drying trend Wed afternoon.
Temps warm into the low 80s Wed afternoon but maybe only into the
70s eastern Massachusetts given later departure time of rain and cloud shield.

Then a beautiful stretch of dry weather and comfortable humidity Thu
and Fri as upper trough lifts/exits to the northeast and is replaced
by rising heights and anticyclonic mid level flow. As 1026 mb high
builds from the Ohio Valley into New England expect warm days and
comfortable nights with dew pts in the 50s. Weak pressure gradient
will support afternoon seabreezes each day.

High pressure remains anchored over southern New England Fri and Sat
but strong July sun will gradually warm airmass with humidity
remaining tolerable. By Sunday and Monday high moves offshore and
return flow will yield increasing humidity, although typical for
late July with dew pts in the 60s.

Ensembles suggest mean trough sets up over the Great Lakes Sunday
into early next week with subtropical ridge offshore. This will
likely support dry weather here in southern New England with frontal
boundaries remaining northwest of New England or fronts dissipating
eastward into New England. Thus seasonably warm and humid Sunday
into early next week at least by late July standards.

&&

Aviation /00z 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/...moderate to high
confidence.

23z update ...

Only change is strongest storms appear to track across south
coastal Massachusetts/Rhode Island this evening. Weaker storms northward. Torrential
downpours possible especially with stronger storms across
southern RI/MA. Earlier discussion below.

==============================================================

This afternoon...mostly cloudy at most taf sites with VFR
conditions early, but bases lower by mid afternoon. Showers
with isolated thunderstorms continue through the afternoon into
the evening. Areas of MVFR- IFR conditions in any convection.
Potential for strong wind gusts and heavy rainfall in any
thunderstorms, mainly across north CT into Rhode Island. Light east-NE winds,
with southeast along the S coast during the afternoon.

Tonight...showers and thunderstorms linger into early tonight
with areas of MVFR-IFR conditions. Showers linger through the
night across central and southern areas, but conditions may
improve to locally VFR after 06z across north central and west mass but
could still see scattered showers. Light S-SW winds becoming north
after midnight.

Tuesday...showers remain across the region, with isolated
thunderstorms possibly along the South Coast through midday.
Could see heavy downpours as well. Conditions slowly improve
across the north central and western mass during the afternoon.

Tuesday night: mainly VFR, with areas IFR possible. Slight
chance shra, patchy br.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Wednesday through Saturday/...high confidence.

VFR.

&&

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.

7 PM update ...

Strongest thunderstorms this evening appear to track across the
Rhode Island and southern Massachusetts waters. Storms move east and offshore after
midnight. Earlier discussion below.

================================================================

Today...showers and isolated thunderstorms move in through the
evening. Visibility restrictions in patchy fog and heavy showers.
Seas 3 ft or less.

Tonight...winds become SW up to 15-20 kt this evening on the
southern waters, but remain east-NE on the eastern waters early.
Seas build up to 5 ft on the outer waters as well as bi and Rhode Island
sounds, so small crafts continue. Expect visibility
restrictions in patchy fog and locally heavy downpours. Showers
through the night, but any thunderstorms should end from north-S
during the night, though may linger on the southern waters.

Tuesday...expect north-NE winds up to around 10 kt. Seas remain up
to 5-6 ft on the southern waters through the day. Visibility
restrictions continue in showers and patchy fog, with local
thunderstorms during the morning along with locally heavy
downpours.

Outlook /Wednesday through Saturday/...high confidence.

Wednesday: winds less than 25 kt. Areas of seas approaching
5 ft.

Wednesday night: winds less than 25 kt. Seas locally
approaching 5 ft.

Thursday through saturday: winds less than 25 kt.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from 8 am Tuesday to 2 am EDT Wednesday
for anz235-237.
Small Craft Advisory from 2 am Tuesday to 2 am EDT Wednesday
for anz250-254.
Small Craft Advisory from 11 PM this evening to 8 am EDT
Wednesday for anz255-256.

&&

$$
Synopsis...nocera/bw
near term...nocera/bw

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