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Area forecast discussion 
National Weather Service Taunton Massachusetts
442 am EDT Wednesday Jul 23 2014

Synopsis...
weather will be hot and humid weather today ahead of an
approaching cold front. The front will trigger scattered showers
and thunderstorms late this afternoon and evening. A few strong
thunderstorms are possible west of I-495 and north of the Mass
Pike and I-84. Showers will likely linger into Thursday southeast
of the Boston to Providence corridor. High pressure builds with
drier weather for Friday and Saturday. An approaching warm front
Sunday will bring more humid unsettled weather for the early part
of next week.

&&

Near term /until 3 PM this afternoon/...

Should remain dry through the near term with a fair amount of
sunshine. 850 mb temperatures around 18c with at least hazy sun
should support temperatures reaching near 90f or a couple degrees
higher where the wind has a predominantly land trajectory. The
combination of temperatures near 90 and dewpoints near 70 will
certainly make it feel hot and humid but well shy of heat
advisory criteria.

&&

Short term /3 PM this afternoon through 6 PM Thursday/...
* a few strong to severe storms possible late this afternoon into
the evening hours *
* showers will likely linger across the cape...islands and
adjacent southeast Massachusetts during much of Thursday

Today and tonight...

We continue to anticipate strong thunderstorms to develop late
this afternoon in the west and move/redevelop further east this
evening. Looks like marginally strong enough wind shear and
sufficiently robust instability for a few thunderstorms to reach
severe limits...most likely in the form of pockets of damaging
winds...as a cold front approaches New England late today. Latest
thinking is that the severe risk will be mainly along and west of
i495 and along and north of the Mass Pike and i84. This is close
to although just a tad broader than the area outlooked by Storm Prediction Center.

Model forecast soundings...indices and cape all point to
sufficient instability for strong thunderstorms late this
afternoon and evening. Sbcapes of 1200 to 1800 j/kg indicated with
the 00z GFS and 2500 to 3000+ j/kg per the NAM...which has a
tendency to overdo the cape. Nevertheless...showaters drop to
around -3 and k indices reach into the middle to upper 30s and total
totals approach 50. Anticipate fairly strong low level lapse
rates and moderate middle level lapse rates around 6.5c/km late this
afternoon/early evening. There is a middle level cap in place around
11k feet at the start of the afternoon but lowering 500 mb heights
and the approaching cold front has the cap eroding by middle
afternoon in the west and late afternoon/early evening in the
east. At the very least the instability alone is enough to support
strong pulse thunderstorms across the northwest interior.

The 0 to 6 km bulk shear magnitude...however...is marginal at best...25
to 30 knots and even weaker to the southeast of the Worcester area.
Another factor to consider is the right entrance region of the
upper jet which approaches this evening but not ideally situated
over the area until Thursday morning when the surface cold front
reaches the southeast coastal waters. A more general synoptic recognition
of this event suggests the timing of the surface cold front should
support convection in at least western and central sections and
probably in a weakened state in the east later into the evening.
The axis of instability is initially across western Massachusetts and western
CT late this afternoon and then works across SW New Hampshire and central Massachusetts
during the early evening and more into NE Massachusetts later into the
evening. Overnight the axis of instability shifts to the southeast
zones but weakens. It currently seems doubtful whether deep
convection will survive past the bos to pvd corridor.

Putting this all together...we perceive a risk of severe
thunderstorms with damaging winds the greatest threat over a
sizable chunk of the northwest interior bounded to the east by i495 and
to the south by the Mass Pike and i84. The time frame looks to be
primarily 4 PM to 11 PM...starting in the west. Thunderstorms may
reach metropolitan Boston later in the evening but likely in a weakened
state.

With precipitable waters near 2 inches ahead of the cold front...the potential
also exists for intense rainfall and localized flash flooding.
This concern is heightened with the slowing of the frontal
boundary as it moves across southern New England tonight.
However...we do not see a strong tropical plume connection and so
continue to be inclined to hold off on putting a large part of
the forecast area under a Flash Flood Watch at this time.
Nevertheless...we will still need to be vigilant for the
possibility of localized flash flooding...a threat that may
persist tonight after the damaging wind threat has faded.

Thursday...

The front deaccelerates as it approaches southeast New England and
then a wave moves along the front during the day Thursday.
Thus...expect showers to linger Thursday morning across the
southeast half of the area and for most of the day across Cape
Cod...the islands and adjacent southeast Massachusetts.

Will be much cooler Thursday behind the front...and southeast of
the Boston to Providence corridor clouds will also act to cap the
temperature. For most location...expect highs in the 70s on
Thursday. The atmosphere may be marginally unstable enough to
support a few embedded thunderstorms over the cape and islands as
well as southeast coastal waters on Thursday.

&&

Long term /Thursday night through Tuesday/...
big picture...
four features in the large scale flow. Trough along the Pacific
northwest coast...ridge extending from The Rockies and plains to
northern Canada...closed low over northern Quebec with trough axis
across the Great Lakes...building ridge over the western Atlantic.

The Quebec trough hangs over northeast North America...the trough
axis and coldest temperatures aloft swing across New England Thursday
night/Friday. The Pacific northwest trough runs over the top of The
Rockies ridge...then merges with the Quebec low over the weekend.
This draws the closed low south over the Great Lakes/northeast early
next week. The Atlantic high pressure meanwhile blocks off the
eastward exit of this merger. The combination of upper low pressure
over the lakes and high pressure over the Atlantic creates a
southwest flow up the East Coast from the deep south for the first
half of next week.

Models have been showing consistency both run to run and with each
other. This increases confidence in the overall solution...but with
the usual uncertainty in timing shortwaves moving through the
flow. This is especially true for Monday and Tuesday.

The dailies...

Thursday night...cold front lingers offshore while the upper jet
right entrance region moves overhead. This may support a wave along
the front during the first part of the night...but the jet shifts
east after midnight as should the supporting dynamics. We will keep
chance probability of precipitation on the cape and islands early in the night. If the jet
position at 00z is as advertised...we may be a little conservative
on evening probability of precipitation. Otherwise the movement of the upper jet should
trend southern New England to end any remaining precipitation and bring
clearing skies.

Friday-Saturday... surface high pressure builds over the region with
dry weather and dewpoints in the 50s. Temperatures at the top of the mixed
layer will support upper 70s to middle 80s both days...a little warmer
Saturday.

One concern will be sea breeze development on Friday which may hold
down afternoon temperatures on the eastern coast. Other concern would be
the passage of the upper trough and colder temperatures aloft. This latter
concern could destablize the airmass and promote cumulus
development. But model relative humidity fields show a dry airmass with relative humidity values
below 700 mb below 50 percent. Will keep sky cover low Friday...then
increasing cirrus on Saturday afternoon.

Sunday... Pacific northwest shortwave digs as it approaches the
Great Lakes and generates a surface low. Strong low level jet is
expected to develop in the Ohio Valley Saturday with 35-40 knot
winds at 850 and 900 mb. As this moves east it will bring deeper
moisture to New England with precipitation water values climbing to 1.5
inches by Sunday. The lifting of this air over the departing surface
layer would generate some showers/scattered thunderstorms. We will mention chance
probability of precipitation later Saturday night into Sunday.

Monday-Tuesday...the merging low over the Great Lakes surface and
aloft will swing a front through New England. Timing is in question
at this point with the GFS showing frontal passage Monday night while the
European model (ecmwf) stalls the front over US. With no clear trend on a day 7
forecast...we will stay close to blended guidence solution of
diminishing probability of precipitation Tuesday. But with low confidence.

&&

Aviation /09z Wednesday through Sunday/...
forecaster confidence levels...

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

Short term /through Thursday/...

Generally moderate confidence on forecast.

Through this morning...generally VFR with any patchy fog or low
stratus burning off within a couple of hours after sunrise.

Late this afternoon into tonight...MVFR ceilings and visibilities in
vicinity of scattered thunderstorms ahead of a cold front. A few thunderstorms may
briefly lower ceilings/visibilities to IFR levels. Also...a few thunderstorms may
produce strong wind gusts. Sref probabilities are high for low
cloudiness to form over southeast Massachusetts and Rhode Island after 03z tonight.
This suggests a fairly high risk of areas of IFR ceilings forming
tonight across the cape and islands. MVFR ceilings and visibilities
anticipated in rain or showers along and southeast of a pym-uuu line
during much of Thursday...improving late.

Kbos taf...high confidence. VFR through most if not all of the
day. Thunderstorms possible after 23z to the north and west of bos and closer to
the airfield itself after 01z.

Kbdl taf...high confidence in taf. Thunderstorms possible after 21z. Any
thunderstorms could contain strong wind gusts.

Outlook...Thursday night through Sunday...moderate confidence.

Thursday night through Saturday...VFR. Local IFR possible in early
morning fog/low clouds. Potential sea breeze Friday afternoon.

Sunday...
VFR but with areas of MVFR in showers/tstms/fog.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday/...moderate confidence.

The SW flow will strengthen ahead of a cold front to produce gusts
approaching 25 knots across many of the waters. Seas may approach 5
feet east of Cape Ann...across the east portion of stellwagen
bank...and east of Cape Cod this afternoon. Although the wind wave
will be formed from the SW flow...there is also a swell that is
encroaching the coastal waters from the east. Although of fairly
low amplitude of less than 3 feet...the swell appears to be
enough to trigger a moderate rip current risk along portions of
the east facing coast. Areas of fog may develop along the south
coastal waters tonight.

Outlook...Thursday night through Sunday...moderate confidence.

Thursday night-Friday-Saturday... north to northeast wind Thursday
night becomes light Friday with afternoon sea breezes developing.
Winds become southwest Saturday but remain below 25 knots. Seas 4
feet or less through the period.

Sunday...a warm front approaches the waters with warmer more humid
air and with clouds/showers/tstms. Increasing south-southwest winds
with gusts 20-22 knots...approaching small craft threshold. These
winds will also build seas on the exposed waters to 5 or 6 feet. A
Small Craft Advisory may be needed.

&&

Equipment...
kbox radar may be in standby Mode this morning while construction
takes place nearby. The radar will return to service this afternoon
prior to the start of any convection in our forecast area.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory from 10 am this morning to 7 PM EDT this
evening for anz230>237-251.
Small Craft Advisory until 3 am EDT Thursday for anz250-254.

&&

$$
Synopsis...wtb/Thompson
near term...Thompson
short term...Thompson
long term...wtb
aviation...wtb/Thompson
marine...wtb/Thompson
equipment...Thompson

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