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Area forecast discussion 
National Weather Service Taunton Massachusetts
1001 PM EDT Tuesday Sep 2 2014

a cold front will cross the region tonight and will be accompanied
by scattered showers and thunderstorms. Behind the front drier and
much less humid air overspreads the area Wednesday and Thursday.
Humidity will once again increase by the end of the week as the next
cold front brings the risk for more scattered showers/thunderstorms
Friday night and Saturday. More autumn-like weather follows Sunday
and into at least early next week with cooler and less humid


Near term /until 6 am Wednesday morning/...
10 PM update...
tricky forecast regarding what will occur with ongoing convection
in New York/NJ/PA this evening. Current timing has this at the box County Warning Area
border by about 0330z or about 1130pm. Latest mesoscale-scale guidance
continues to indicate about 1000j/kg of 100mb ml cape values but
this continues to decline considering we are over an hour from
sunset now. There is still modest low level jet energy mainly focused with
the activity across New Jersey...but there is also a significant dip in
k-values thanks to a broad dry layer across the area.
Therefore...feel that models may be on to something dissipating a
lot of this activity as it moves in despite the what is left of
the instability. Even what is left should be much diminished by
the time the front associated with it arrives. Therefore...will
gradually taper probability of precipitation from west-east from chance to slight chance with
the possibility that some areas east of the Worcester Hills see
little if any rainfall. Still...will need to monitor this activity
as it enters the west thanks to that noted spike in shear which
may at least help keep things modestly organized. Otherwise...will
also need to watch southern coastal waters should any convection move
across with that low level jet currently over New Jersey. In sum...could still be
some thunder left by the time it arrives in ares west of the
Worcester Hills...with only a very isolated severe threat...but
expect everything to rapidly weaken as the front swings across southern
New England.

Previous discussion...


Strong cold front will move across the northeast overnight. This
front is expected to move into the western zones around 02-04z and
offshore between 10-12z. Anticipate convection will be ongoing as it
approaches the region per the amount of surface heating and dynamics
out ahead of it in western cwa's. By the time it approaches the
region a good amount of instability will be loss so we may see dying
convection. However some models...especially the high-res guidance shows
about 500 j/kg of cape lingering by the time the front approaches.
On top of the possible instability...850mb low level jet does increase as the
front approaches and shear values increase as well. In
fact...guidance shows that 0-6km shear increases to 25kts...but it
is the 0-1 km shear that increases to 30-40kts. Lastly k-index plume
moves over the region during this timeframe giving the region the
moisture it needs for thunderstorms to continue.

Therefore believe that all the ingredient mentioned above may allow
for a few strong to perhaps a severe storm as the front moves the
region. Keep in mind that this is an overnight event which is not in
the northeast favor per climatology...but conditions do show that a few
strong storms are possible. Lastly...even though the region did see
some action today...the atmosphere did bounce back very well after
the storms passed.

As the front moves through the k-index value as well as the
instability do drop after 06z so that the chance for any convection
to occur dissipates significantly. In fact...the showers and
thunderstorms from the front will fragment and become very isolated in
nature by this time frame and may even struggle to reach the East
Coast. With the amount of moisture in the atmosphere believe that a
few showers will reach so continued the slight chance pop for now.

Overnight temperatures will struggle to drop as cloud cover will keep temperatures
from radiating out overnight and dewpoints will remain in the 70s
until frontal passage.


Short term /6 am Wednesday morning through Wednesday night/...
tomorrow and tomorrow night...

Surface front will move offshore by early tomorrow morning as upper
level trough will push through as well. 850 mb ridge will begin to
set up over the region allowing for dry air to move into the area.
This will keep the weather dry for both tomorrow and tomorrow night.
850 mb temperatures drop down to -14c and with mixing up to 850mb will allow
for temperatures to reach into the middle to upper 80s. Difference will be the
humidity as dry air will mix down dropping dews down into the upper
50s. Winds will be gusty reaching about 20 miles per hour.

Overnight...with clear skies and winds become light and
variable...anticipate radiational cooling. Lows will drop into the
middle to low 50s. A few sites could even drop into the upper 40s
across CT valley.


Long term /Thursday through Tuesday/...

* dry Thursday/Friday with warmer than normal afternoon temperatures
* more humid Sat with scattered storms possible
* trend toward cooler/less humid weather sun into early next week

Thursday and Friday...

Dry/quiet weather continues Thursday with high pressure and a dry/stable
airmass over the region. It will be warm with highs of 80-
85...cooler along the coast with afternoon seabreezes...however dew
points remaining in the 50s will provide comfortable humidity levels.

By Friday the subtropical ridge along the East Coast briefly builds
northward into southern New England ahead of the next northern
stream trough. This will result in warmer temperatures with highs well into
the 80s combined with more humid conditions as dew points climb from
the 50s into the 60s. Thus a warm and humid day Friday.

All model guidance including the 12z gefs and new 12z European model (ecmwf) suggest
jet dynamics/best shear/instability and convergence associated with
cold front hold off until Sat. Thus slight chance probability of precipitation confined to
western New England late Friday from a model blend seems reasonable.
Hence most of the area especially Rhode Island and eastern Massachusetts may remain dry


All deterministic and probabilistic /12z gefs and 12z ecens/ guidance
suggest cold front will move slowly into the region Sat. Temperatures are
projected to be in the 80s Sat along with dew points in the 60s to near
70 so this should yield at least marginal instability. The 12z gefs
suggest a 30-40% probability of up to 1200 j/kg of cape. This
combined with modest jet dynamics/shear and frontal convergence
expecting a risk for scattered thunderstorms along with a threat for
heavy rain as precipitable waters climb above normal. Could be a few strong storms
given the parameters mentioned above.


Likely our transition day as both ensembles /12z ecens and gefs/
suggest front may be slow to clear the coast Sun morning. Thus
showers may linger early Sunday especially South Coast with a trend
toward drier weather as the day progresses from northwest to southeast.

Monday and Tuesday...

Should be drier...cooler and less humid as front moves offshore.
However some of the guidance shows uncertainty how far offshore this
boundary gets. Thus clouds and possible unsettled weather may lurk
just offshore. Will keep the forecast dry for now.


Aviation /02z Wednesday through Sunday/...
forecaster confidence levels...

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

Short term /through Wednesday night/...high confidence in

7 PM major changes from 18z tafs. Showers and T-storms
over Massachusetts continue to erode. Next round of T-storms over western-
central New York and PA arrives in western New England 01z-03z then
weakens as it enters eastern New England. Fog at Nantucket will
persist and then begin to improve around 06z and especially 09z-
12z when frontal passage occurs. Earlier discussion below.


Tonight...a lull in activity after 00z. Before another round of
showers with isolated thunder moves through after 03/04z. This
convection is associated with an approaching cold front which
will pass through the region overnight. Expect convection to
weaken as it moves through with brief lowering in ceiling heights.
Patchy fog is expect ahead of the front and should dissipate as
it passes through.

Wednesday into Wednesday night...VFR. Patchy fog in CT valley Wednesday night.

Kbos terminal...high confidence in trends. May be a shower or
thundershower in the afternoon.

Kbdl terminal...high confidence.

Outlook...Thursday through Saturday...

Thursday and Friday...high confidence. Mainly VFR conditions other than
late night/early morning patchy ground fog in the typically prone

Friday night and Saturday...moderate confidence. A period of MVFR-
IFR conditions expected in low clouds/fog patches and scattered
showers/thunderstorms. Timing highly uncertain at this point.

Sunday...any MVFR along South Coast should trend toward VFR with
potential showers moving offshore. Otherwise VFR prevails


forecaster confidence levels...

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

High confidence through the period Wednesday night.

SW flow begins tonight before a cold front passes over the waters
overnight. Expect a few gusts to around 20 knots are possible as the
winds shift to the W-NW. A few showers and thunderstorms may move
over the southern waters by midnight lasting until about 3am. Seas
may approach 5 feet on the south ocean waters but have low
confidence and held off on issuing a Small Craft Advisory. Otherwise...some fog may
lead to visibility reductions tonight before the front passes.

Wednesday into Wednesday night...quiet boating weather. A few gusts near 15kts
possible during the day.

Outlook...Thursday through Saturday...

Moderate confidence through this period.

Thursday...high confidence. Weak pressure gradient yields light
winds and seas along with good visibilities should make for nice
boating weather. Afternoon seabreezes expected.

Friday...moderate to high confidence. Near shore southwest wind
gusts of 20 to 25 knots may develop in the afternoon with good
mixing over the land. SW wind waves develop especially near shore.
Dry weather will prevail with a chance of showers/T-storms
increasing after sunset.

Friday night and Saturday...moderate confidence. Areas of fog may
affect the southern waters. Southwest wind gusts around 20 knots
are expected ahead of an approaching cold front. There will be the
risk of scattered showers and thunderstorms during the day Saturday with a
few strong storms possible...wind and heavy rain.

Sunday...front begins to move offshore slowly but showers may linger
especially in the morning.


Boston Logan Airport reached 93 degrees today. This is the warmest
temperature in Boston so far this Summer.


Box watches/warnings/advisories...
New Hampshire...none.
Rhode Island...none.


near term...doody/dunten
short term...dunten
long term...nocera

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