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Area forecast discussion 
National Weather Service Taunton Massachusetts
401 am EDT Thursday Aug 21 2014

Synopsis...
a few showers are expected across mainly the western half of New
England through early Friday. Drier and cooler weather follows
this weekend. A warm-up is possible next week prior to an
unsettled pattern ahead of a frontal boundary sweeping south out
of Canada.

&&

Near term /until 6 am this morning/...
a tale of two low pressure systems will define the weather through the
day today. One...an upper level disturbance is currently weakening
and is associated with a warm front expected to remain stalled to
the west. The other is a slowly deepening weak wave of low pressure
expected to slowly move east of the 40/70 benchmark into the weekend.
This latter feature remains weak...and except for an inverted trough
evident in the flow pattern hanging back toward the islands...is
expected to have little effect on the region. Only expect a few
showers for mainly ack through about the afternoon timeframe.

The former...the upper level disturbance continues to weaken too as
it interacts with a blocked flow pattern and cutoff high pressure to
the north. In fact...it will continue to transition from a weak cutoff
to an open wave through the day. Flow through the column above southern
New England remains weak...with both upper and low level jet/S
remaining well to the west through the day. The approaching warm
front and modest lift provided by weak differential vorticity
advection from the wave will provide some focus for showery
activity to work on a moistening column mainly across CT and western
MA/NH. The inverted ridge already in place across the region has
also led to east flow which will suppress any attempt at
destabilization across the area. Therefore...with the bulk of the
energy and potential instability remaining to the west...will keep
probability of precipitation focused mainly across the extreme west portions of the area as
well at least through much of the daylight hours.

Regarding temperatures...with a fair amount of cloud cover expected
through the day between these two systems...highs are likely to
remain below normal and in the low-middle 70s.

&&

Short term /6 am this morning through 6 PM Friday/...
tonight...
slightly increased threat for overnight showers...but still mainly
across the west thanks to some leftover subsidence closer to the
gradually rebounding inverted ridge. The remnant vorticity maximum from an
upper level disturbance will be moving across the area through the
overnight hours. Jet energy throughout the column is weak and what
is left of a surface warm front will likely have washed out for most
part. Therefore...it is only the vorticity maximum which will be providing
lift to work on the 1.5 or less precipitable water airmass in place across the
region. Drier air still remains mainly across the east...so will
once again focus mainly from the Worcester Hills and points west for
the showers. No thunder with this update...as the airmass remains
quite stable throughout. Guidance has backed off on the quantitative precipitation forecast
bullseyes that it was showing in previous runs for the most
part...but these are likely just a convective feedback issue.
Otherwise...mild overnight mins with a fair amount of cloud cover.
May need to also monitor especially the east and southeast coastlines for
fog and stratus thanks to continued east flow and slightly higher
dewpoints within the moistened column in comparison to previous days.

Friday...
upper level vorticity maximum slides to the southeast during the morning...combining
with leftover cutoff energy across Atlantic Canada. While this
will form a hanging trough with axis from the middle Atlantic through
the 40/70 benchmark...middle level ridging will then be able to regain
control into western New England. This will also allow the inverted
ridge to build back in across the region. This should allow
showers across the area to gradually dissipate through the day as
the column dries from increasing subsidence. Have probability of precipitation diminishing
through the day across the area. It will likely take a bit more
time for the clouds to dissipate given moisture is likely to be
trapped beneath the subsidence inversion and east flow continues.
In fact...looks like an upper level backdoor cold front will lead
to decreasing 850 mb temperatures. Have held highs back...mainly in the 60s
to around 70. East coastal regions are likely to take the longest to
see any clearing.

&&

Long term /Friday night through Wednesday/...
highlights...

* quiet weather expected through much of this period
* uncertainty increases middle of next week with an approaching
front

21/00z models are in good agreement through about Tuesday. Then
significant differences arise for the latter half of next week...
making for a lower confidence forecast from the middle of next week
Onward.

A persistent upper level trough over the northeast will slowly make
its way east...moving just offshore this weekend as an upper level
ridge tries to push northeastward from the southern USA. This upper
trough is rather persistent though...with each successive model run
lingering this trough over the North Atlantic longer and longer.

Cooler...less seasonable weather is expected while high pressure in
Quebec will keep things relatively dry. Still agree with the
previous forecaster...thinking 21/00z guidance continues to struggle
with convective feedback into early next week. As such...kept a dry
forecast across most of southern New England.

A high pressure will sink south into early next week...eventually
moving south and east of southern New England and returning US to a
more westerly flow. A low pressure in Ontario may eventually move
east into Quebec...swinging a cold front through southern New
England Wednesday or Thursday. This is where models start to diverge
more significantly.

The European model (ecmwf) looks a bit quicker than the GFS with the approach of this
front...but neither is as progressive as the Canadian. Will take a
blended approach with the timing for now. Given the strength of the
surface ridge expected to be near the coast...would not be overly
surprised if the ultimate timing turns out to be slower than the
GFS. Depending on available moisture...we could see some showers and
thunderstorms ahead of this front.

&&

Aviation /07z Thursday through Monday/...
forecaster confidence levels...

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

Short term...through Friday...moderate confidence.

Through 12z...
mainly VFR except some IFR/MVFR possible at some sites across east
Massachusetts in fog. Some MVFR ceilings possible mainly across ack and the outer
arm of Cape Cod.

Today and tonight...
mainly VFR. Except for ack...most of the rain remains to the west
through the day. Then...better chance for showers across western
Massachusetts/CT/NH overnight especially...with occasional MVFR conditions
possible. Some low MVFR/IFR possible mainly across the East Coast.

Friday...
conditions improve through the day. Some MVFR/IFR possible in the
morning gives way to VFR through the day...but may take most of
the day along the East Coast.

Kbos terminal...moderate confidence. Main concern is whether a
brief period of IFR conditions move in early this morning
briefly...and then returns late Thursday night. Otherwise VFR with east
flow assisted by seabreezes expected.

Kbdl terminal...high confidence in taf. -Ra chances begin towards
Thursday morning along with low-end VFR ceilings. MVFR ceilings possible
though mainly tonight...but remains low-confidence at this time.

Outlook...Friday through Monday...

High confidence VFR. Low probability of patchy MVFR/IFR fog each
night.

&&

Marine...
forecaster confidence levels...

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

Short-term...through Friday...moderate confidence.

Low pressure will gradually strengthen offshore through the day...but
then move east into Friday. A few showers are likely to impact mainly
the waters S of The Vineyard and ack into middle day until the low
moves further east. The weak low pressure will also generate a modest
swell...but it should generally remain below 4 feet into the day on
Friday. Winds too may gust to about 15-20 knots at times out of the
east...but should remain below Small Craft Advisory levels through
the period.

Outlook...Friday night through Monday...

High confidence. Quiet boating weather is expected for much of this
period with high pressure across the region. Seas could approach 5
feet across the outermost southern coastal waters this weekend. Not
enough confidence to issue a Small Craft Advisory though. Low
probability of patchy overnight fog limiting visibilities.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...Belk/doody
near term...doody
short term...doody
long term...Belk
aviation...Belk/doody
marine...Belk/doody

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