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Area forecast discussion 
National Weather Service Taunton Massachusetts
659 PM EDT Wednesday Sep 2 2015

Synopsis...
a cold front will sweep across New England Thursday ahead of which
there will be the chance of showers and thunderstorms. High pressure
from Canada builds over New England for the Labor Day weekend. A cold
front over Canada stalls over northern New England early next week.

&&

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

7 PM update...
generally clear skies at sunset with patchy clouds upstream that
should move through overnight. Minor adjustments to temperatures and sky
cover but otherwise the same forecast. Min temperatures near the dew
points...in the 60s...with a few urban spots in the lower 70s.

Previous discussion...mostly clear conditions under light winds
and high pressure makes for another evening of radiational cooling
and fog development in typically prone low-lying locations.
Increasing dewpoints will aid in this development. Likely to see a
combination of visibilities down to a half-mile or less and low
clouds of a few hundred feet.

&&

Short term /6 am Thursday morning through Thursday night/...
Thursday into Thursday evening...

Middle-level trough and accompanying impulse energy sweep southeast across the
NE Continental U.S. During the afternoon into evening hours. A surface cold
front along the leading edge of refreshing cool-dry airmass building
rearward will become the focus of showers and thunderstorms along
and ahead.

Evaluating the environment: decent low-middle level thetae convergence
ahead of the cold front yielding a moistening profile of precipitable waters 1.50-
1.75 inches. Conditionally unstable aloft...instability values build
with daytime heating with values of around 1-2k j/kg as boundary-
layer lapse rates steepen. Per BUFKIT...cape profiles deep and not
too thin. Inverted-v profiles at the surface. Yet some things Worth
noting is the rather weak shear. Better middle- to upper-level venting
resides more towards the NE. Predominant flow...even at h925-7...is
forecast out of the north and there is a lack of convergence along the
front...while also being weak around 10-15 miles per hour between 0-6 km.

Overall it looks like all the ingredients needed for showers and
thunderstorms appear to be there but this does not appear to be a
strong to severe weather event /echoed by cips analogs/ and nor does
it appear to be widespread. Certainly more focus along the north-facing
slopes of high terrain and as the cold front collides with expected
onshore sea-breezes during the day. Focus on the high terrain and
especially along the east/se-interior with the cold front / sea-breeze
collision.

Going back to cips analogs and the environment evaluation...should
any storms become strong can not rule out large hail and a damaging
wind threat though think this will be isolated. The main threat
appears to be heavy rain with the decent moist profile and forecast
slow-movement of the storms. Could be dealing with poor-drainage /
urban flooding issues. Will hold off on any enhanced wording in
the forecast with the isolated to scattered weather impact expected.
A hot and humid day. Highs near 90.

Thursday night...

The cold front pushes out of our area towards the short wave as a Canadian
high and cool-dry air builds into the region. Could be contending
with continued shower / thunderstorm activity towards midnight but
diminishing as the main middle-level energy pushes offshore away from
better dynamics aloft and we find ourselves beneath a dry-subsiding
airmass. Surface dewpoints crash. Lows dropping down around the low-
60s.

&&

Long term /Friday through Wednesday/...
big picture... long wave pattern this weekend will feature a ridge
over the central USA and troughs over the Maritimes and Pacific
coast. This flattens to a zonal flow over Canada and the northern
USA by midweek. Contour heights run normal or slightly above normal.
The shorter wavelengths show a ridge building over the northeast USA
with above normal heights. The ridge flattens a little for the first
half of next week but shortwave heights remain above normal.

Model mass and thermal fields are similar through at least Tuesday...
even Wednesday isn/T too bad. Using a blend of these would be reasonable.

The dailies...

Friday... the cold front moves off to the south while high pressure
builds down from Canada. This produces an appreciable north-S pressure
gradient across southern New England which bring increasing
northeast winds. The gradient diminishes in the afternoon...so
expect a breezy morning followed by lighter winds in the afternoon.

Moisture profiles show lingering moisture at or below 800 mb so
there could be some lingering clouds in the morning especially south
of the Mass Pike. But the trend will be toward a drying airmass...so
expect any morning clouds will dissipate during the day.

Mixing reaches to 900 mb...and temperatures at 900 mb will be at 13-15c.
This is similar to 8-10c at 850 mb...so expect maximum temperatures in the 70s.
Could be a few values at 80 in warm spots such as the CT valley. The
onshore flow should buffer values in eastern mass.

Labor Day weekend... high pressure moves overhead Saturday and shift
south of our area for Sunday and Monday. The air will be dry and
subsident through the period. So expect clear or mostly clear
skies through the period. Perhaps some increasing cirrus Sunday
night and Monday. Mixing continues to 900 mb Saturday and Sunday...
and to 850 mb Monday. Temperatures at these levels...converted to 850 mb
values...trend from 10-12c Sat to 13-15c Sunday to 17-18c Monday.
So expect values u70-l80s Saturday...80s Sunday...around 90
Monday.

Tuesday-Wednesday... a cold front stalls north of the Canadian
border Monday. A shortwave moving toward the Great Lakes drives the
cold front south into New England Tuesday night. The front stalls
over southern New England and possibly dissipates Wednesday. Limited
moisture is forecast with the front and most of that is below 750
mb. So look for a few clouds and maybe a stray shower late Tuesday
or Wednesday. For now we will mention slight chance probability of precipitation in northern
and western sections.

&&

Aviation /00z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through Thursday night/...high confidence.

Tonight...VFR. Areas of IFR fog or lower expected after midnight
in the CT valley...mostly mass...and other fog prone locations.

Thursday...
VFR. Afternoon into evening shra/tsra. Tempo MVFR impacts. Main
threat +ra. Perhaps some breezy winds out of the north and/or hail
should any thunderstorms and rain become strong. Thinking isolated impacts. Low
confidence. Sea-breezes developing along the coasts during the
day.

Thursday night...
VFR. Rain showers concluding. North-winds prevailing.

Kbos terminal...high confidence in taf. Sea-breeze for the midday
period on Thursday.

Kbdl terminal...high confidence in taf.

Outlook /Friday through Monday/...high confidence.

VFR through the period. Areas of early morning IFR ceilings/visibilities
possible in fog Saturday and Sunday. Strong northeast flow Friday
gives way to light flow over the weekend with sea breezes possible
Saturday. Increasing west-southwest flow Sunday-Monday.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday night/...high confidence.

A cold front will sweep the waters late Thursday into Thursday
night behind which NE-winds will increase with the potential for
gusts up to 25 kts towards Friday morning. It may be Worth
considering by this time a Small Craft Advisory as waves could
reach 5-feet especially over the E-waters. More in the outlook
discussion.

Scattered showers and thunderstorms expected ahead of the cold
front but are anticipated to be sub-severe. Can not rule out a
strong storm but the better chance is across the east/se-interior
away from the coast.

Outlook /Friday through Monday/...high confidence.

Friday... increasing northeast wind as high pressure builds south
from Canada. Moderate potential for 25 knot winds especially Friday
morning...which may drive sea heights of 5-6 feet. A Small Craft
Advisory may be needed especially over the eastern waters.

Saturday through Monday... high pressure builds over the waters.
Wind and seas diminish to light Saturday and southwest Sunday-
Monday.

&&

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

&&

$$
Synopsis...wtb/sipprell
near term...wtb/sipprell
short term...sipprell
long term...wtb
aviation...wtb/sipprell
marine...wtb/sipprell

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