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Area forecast discussion 
National Weather Service Taunton Massachusetts
945 PM EDT Friday Aug 1 2014

Synopsis...
scattered showers and thunderstorms should diminish early this
evening. A series of low pressure areas moving along a stalled
front will likely bring periods of rain to at least southeast New
England on Saturday and Sunday. Unsettled weather remains with US
much of the week as a couple of cold fronts move through.

&&

Near term /until noon Saturday morning/...

945 PM update...

***saturday washout expected for eastern new england***

Convection from this afternoon and early evening has come to an end
as instability was waning. Dry weather expected the rest of the
evening.

All attention now turns to plume of tropical moisture streaming
northward off the middle Atlantic coast. Based on some short term and
18z guidance...have expanded categorical probability of precipitation for Saturday into much
of eastern New England. Also...increased probability of precipitation some across the
interior. This still may not be enough...but wanted to trend in the
right direction. Appears steady rain will begin to work northward
into much of the region by 12z. Bands of heavy rain are expected
across eastern New England on Saturday given precipitable waters approaching 2
inches. This may result in typical nuisance poor drainage street
flooding...but do not see any major problems at this time. Marginal
instability may result in an isolated thunderstorm or two across the
southeast New England coast...but main story will be bands of heavy
rain and Saturday washout for much of eastern New England.

&&

Short term /6 am Saturday morning through Saturday night/...
***a period of steady and possibly heavy rain likely on Saturday
for at least eastern new england***

Water vapor imagery depicts rather impressive tropical connection along
the eastern sea board ahead of Ohio Valley trough. Models all depict in
some way shape or form a weak low pressure wave moving along a stalled
frontal boundary just offshore. Precipitable waters increase from a little over
1 inch this afternoon to 1.5 to 2 inches across much of southern
New England by Saturday afternoon. Noted that the 12z European model (ecmwf) was
especially bullish with the rainfall projection and also stronger
with the Saturday low pressure wave along the boundary. The NAM
seemed to be more inclined to put as much or more energy in a
second wave later in the weekend. The sref output depicts less
than 20 percent chance of an inch or more rain Saturday but the
sref is made up of a number of NAM members. In spite of some model
differences...see enough indicators to go with categorical probability of precipitation
across southeast Massachusetts and southern Rhode Island and likely probability of precipitation as far north and west
as Newburyport to the Worcester area to northwest Hartford County.
Indicated stratiform rain in the likely and categorical pop areas
and showery to the northwest. The water vapory imagery...increase in
precipitable waters ...deep synoptic scale SW flow with a tropical origin and
model consensus leads to believe a relatively high chance of 1 to
1.5 inches of rain across the southeast half of the area and tapering off
to quite a bit less to the northwest.

There is a low probability of embedded and elevated thunder across southeast
Massachusetts and southern Rhode Island where k indices may reach to near or just above 30.

Temperatures on Saturday are not likely to rebound very high due to
clouds and rain in much of the area. Warmest temperatures likely
in the Keene area. Most locations will probably not rise out of
the lower or middle 70s. Saturday night...have tapered probability of precipitation down as
surface wave moves NE out of area. Depending upon strength of the
surface wave of low pressure...may see boundary shift a little
further southeast. Have probability of precipitation tapering off to chance most areas except
kept them likely across the cape and islands closer to the frontal
boundary. Anticipate feed of tropical moisture will continue and
the air mass approaches conditionally unstable status over the
cape and islands. Fog may be more widespread late Saturday night
due to light winds and added moisture from the rain...especially
across southeast sections. Low temperatures Saturday night will depend to
some degree upon the strength of the surface wave and whether it
is able to draw any drier air into southern New England. Would
expect clouds to prevent much of a diurnal drop in temperatures
over the southeast half of the area.

&&

Long term /Sunday through Friday/...
big picture... upper trough over the east with upper ridge over the
western USA. One shortwave ejects from the eastern trough Sunday
while another shortwave drops in from Canada. The trough then
migrates east through New England Wednesday through Friday. This is
an unsettled pattern...but with near seasonable temperatures.

The dailies...

Sunday... cold front remains stalled off the South Coast. Meanwhile
the upper southwest jet remains to our west with southern New
England under the right entrance region. This will support upper
venting over the low level convergence along the front...resulting
in one or more waves moving along the front and producing areas of
isentropic lift that move up across parts of southern New England.
The most likely location for lift and resulting precipitation would be
across Rhode Island and southeast mass. Precipitable water values of 1.5 to
1.75 inches with 2 inch values just south of the coast...so
potential for a few heavy downpours.

With an ill-defined surface pattern and the upper jet right entrance
region also overhead...with cooling temperatures aloft as the upper
shortwave ejects...and with middle level lapse rates around 6c/km and
totals around 48 to our northwest...there is also potential for
afternoon/evening thunderstorms to develop over New York and Vermont. These could then
move into our western and northern areas...but the less favorable
environment here should cause that convection to diminish. Model quantitative precipitation forecast
fields also indicate measurable precipitation mostly outside of our area but
potentially clipping our northwest.

Temperatures aloft around 11c would support maximum surface temperatures around 80f.
Clouds in the east should keep temperatures cooler...while interior
sections may see enough sun to realize the warmer maximum temperatures.

Monday-Tuesday... instability lingers north and west...and across
our northern/western zones. On Tuesday...a cold front approaches
from the northwest. This should be enough to generate
afternoon/evening thunderstorms each day.

Wednesday through Friday... the cold front moves through New England
on Wednesday along with the colder temperatures aloft. This should bring a
chance of showers/tstms. Lingering instability Thursday with the
upper trough still in the neighborhood...but a drier airmass. Will
indicate slight chance probability of precipitation most places and chance probability of precipitation Merrimack
Valley. High pressure builds in place Friday with drier weather.

&&

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

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

Short term...tonight through Saturday night...

Tonight...moderate to high confidence. Mainly VFR conditions
except areas of IFR in fog especially in areas that have received
significant rainfall.

Saturday...moderate confidence. Expect a period of steady rain
along and southeast of a lwm-orh-bdl line with widespread MVFR ceilings and
visibilities developing. Generally VFR northwest of this line. Areas of IFR
ceilings and visibilities possible southeast Massachusetts and southern half of Rhode Island late morning
into the afternoon.

Saturday night...moderate confidence. Anticipating VFR conditions
northwest half of area and generally MVFR ceilings and visibilities southeast half. Areas
of fog and scattered showers persisting southeast half of area.

Kbos taf...high confidence in generally VFR conditions through
tonight. Most thunderstorm activity in the early evening should occur northwest
of the airfield but a low probability remains of one making a
close pass to the air field. Sea breeze from southeast will shift to be
from SW after about 00z. Any thunderstorms passing northwest of the air field
could cause a temporary outflow from the west. For Saturday...now
thinking that the rain shield will cover bos and probably cause
ceilings and visibilities to lower to MVFR levels. Rain may be heavy for a
time in the afternoon. Although rain should taper off to scattered
showers Saturday evening...MVFR ceilings will likely persist.

Kbdl taf...high confidence in mainly VFR conditions through
tonight. However...a thunderstorm complex approaching Litchfield County at
20z may cause a brief period of lower conditions sometime between
21z and 23z. For Saturday...moderate confidence in rain shield
reaching bdl and resulting in period of MVFR ceilings and visibilities. For
Saturday night...low to moderate confidence of MVFR ceilings and
visibilities in fog.

Outlook...Sunday through Wednesday...

Sunday...MVFR ceilings/visibilities southeast with IFR along the South Coast
and islands. Mainly VFR northwest with widely scattered MVFR in
afternoon thunderstorms.

Monday-Tuesday-Wednesday...mainly VFR. Scattered afternoon/evening
showers/thunderstorms with brief MVFR cigs/vsbys.

&&

Marine...
forecaster confidence levels...

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

Short term...tonight through Saturday night...

High confidence. Light winds and seas below Small Craft Advisory advisory expected
through Saturday night. Visibilities will generally be good
through tonight...but then lower to 3 miles or less in rain and
fog during Saturday and possibly lowering to a mile or less in fog
late Saturday night.

Outlook...Sunday through Wednesday...

Sunday...front remains stalled either over or just south of the
coastal waters. Areas of poor visiblity in rain and fog as
disturbances move along the front. Light winds shifting from
northeast to southeast. Winds and seas will remain below small
craft thresholds.

Monday-Tuesday-Wednesday...light winds Monday becoming southwest
Tuesday. Areas of poor visibility in fog each morning. Scattered
thunderstorms each day with highest chance along the North Shore.
Winds and seas remain below small craft thresholds.

&&

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

&&

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

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