Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
1018 PM EDT Sat Aug 30 2014

Synopsis...
high pressure offshore brings south winds to New England. These
winds will bring warmer and more humid conditions overnight and
Sunday. An approaching cold front will bring showers and
thunderstorms with torrential downpours possible late Sunday
afternoon and evening especially over the interior. Scattered
on Labor Day the showers and storms will be focused across CT/Rhode Island
and south of the Mass Pike. Another cold front approaches later
Tuesday. Warm but less humid weather will then follow for middle
week.

&&

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

No significant changes with this forecast. Dewpoints continue to
slowly rise with values either side of 60. Dewpoints in the middle
60s to low 70s have reached western New York through the Chesapeake Bay
region and are moving northeast. The inflow of moist air...over
the airmass in place over US right now...will support developing
clouds overnight over our area. The moist layer will be relatively
shallow but could bring some fog and drizzle later tonight
especially along the South Coast.

With dewpoints around 60 and trending higher...expect min surface
temperatures to be in the 60s.

&&

Short term /6 am Sunday morning through Sunday night/...
*** potential for a few strong T-storms Sun afternoon and evening across
northern Massachusetts and southwest New Hampshire ***

Sunday...meridional flow across the MS valley into the Great Lakes is
yielding a plume of tropical moisture with precipitable waters +3 Standard streaming
northeast toward New England. 12z European model (ecmwf) MUCAPE climbs to about
1100j/kg at 18z Sunday across the interior. Even the low res 12z
gefs ensembles suggest 400-600j/kg of mean cape inland Sun afternoon.
Thus should be sufficient instability along with surface convergence
along a prefrontal trough to initiate convection across the interior
Sunday afternoon.

0-6km shear is initially weak at 25kt 18z Sunday but increases to 35-
45 knots by 00z across northern Massachusetts and southern New Hampshire. Thus this is
sufficient for a low risk for a few strong T-storms Sun afternoon and
evening with the highest risk across northern Massachusetts and southwest New Hampshire.
The main hazards will be torrential downpours with localized flash
flooding given the anomalous precipitable waters /dew points near 70 too/ and strong
winds given the increasing shear. Given the recent dry weather and
resulting high ffg /3.5-4" in 6 hours/ greatest risk of flash flooding
is the urban areas.

Not expecting a complete washout as there will be dry periods of
weather tomorrow. Once morning fog Burns off should see partial
sunshine and dry weather from middle morning into at least early if not
middle afternoon...as convection first fires northwest areas then
slides toward the coast by 00z. It will be warmer and more humid
than today with highs in the l80s and dew points in the mu60s to around
70.

Sunday night...

Scattered showers and T-storms linger into the early evening along
with the threats of heavy rainfall/localized flash flooding possibly
along with strong winds. Convection wanes as the evening progresses.
Warm and humid conditions continue and will lead to another round of
fog.

&&

Long term /Monday through Saturday/...
headlines...

* heavy showers/T-storms possible Monday...Tuesday evening into early
Wednesday
* brief break/dry weather Tuesday
* drying high pressure Wednesday through Friday

Overview...the overall pattern has the states containing a ridge in
the east and a trough in the west. The boundary in bewteen should
then bring in moisture along a cold front. After the rain-causing
trough moves through Wednesday...a ridge builds over the region for
the rest of the work week as a cold front approaches from the west
for Saturday. A negative nao forms blocking up the pattern late
while a negative pna stays over the United States keeping the ridge
over the East Coast.

Models have a good agreement on the first part of the period...however
the magnitude of the ridge over the Midwest comes into question
early Wednesday as the European model (ecmwf) has a digging trough on the West
Coast whereas the 6z GFS has more of a zonal flow pattern during
this period. With the quicker zonal flow pattern...the 6z GFS
brings the cold front through quicker than the European model (ecmwf)...which is
its normal bias...so have leaned against the 6z GFS for that
period. The 6z GFS is later than the European model (ecmwf) bringing that trough
down into the states as well so extra instability is into the
region Thursday night via the 6z GFS. Have leaned against that as
well and more towards the European model (ecmwf)...12z GFS and ensembles that have
a high still over the East Coast.

Dailies...

Monday...shortwave moves through the region with continued southwest
flow. This allows for precipitation and weak instability as shown by
precipitable waters 2 to 3 sds above normal and k-index above 30 in southern
portions of the region. MUCAPE is only just above 1000 j/kg. The low
level jet will be departing the region to the south so shear should
stay low however with decreasing 500 mb temperatures can/T rule out some
convection. Dew points will also reach the 70s so humidity will be
tropical in nature. Will have to watch for flooding...but region has
been dry for a while so localized flooding may be the only type that
is observed.

Tuesday...possibility of fog in valleys and other fog prone areas.
Otherwise areas of isentropic descension...rising heights and rising
700 mb temperatures to close to 10c allude to the possibility of a
lull in the precipitation during this period. Believe that occurs
from early morning to early afternoon. Dew points will again reach
the 70s during the day. Then...a cold front pushes through overnight
into Wednesday with a gerater possibility of showers and
thunderstorms ahead of it especially over northwestern Massachusetts. This is
where negative showalter values and better shear exists.

Wednesday...as the cold front pushes through in the morning...
expect to see any showers come to an end and drier air push in. A
high pressure system begins to take control with northwest winds.

Thursday and Friday...high pressure continues. Clearing skies and
drier air with dew points into the upper 50s.

Saturday...have to watch for an approaching cold front for this time
period.

&&

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

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

Short term /thru Sunday night/...high confidence in trends but
lower confidence in precise details and timing.

Overnight...VFR to start. Increasing sky cover with ceilings lowering
to MVFR after midnight. MVFR/IFR visibilities in patchy fog possible
after 2 am.

Sunday...marginal MVFR-VFR. Rain showers and thunderstorms and rain developing after 18z
west and then sliding east toward 00z. Activity most widespread
across northwest CT-Massachusetts into southwest New Hampshire. A few strong storms
possible with torrential downpours and gusty winds possible.

Sunday night...MVFR during the evening slipping to IFR. Rain showers and
thunderstorms and rain especially during the evening. A few strong storms possible
with heavy rainfall and gusty wind.

Kbos terminal...high confidence in taf regarding trends but lower
confidence on precise details and timing.

Kbdl terminal...high confidence in taf regarding trends but lower
confidence on precise details and timing.

Outlook...Monday through Wednesday...

Monday...moderate confidence.

MVFR-IFR ceilings/visibilities in areas of showers. Possible thunderstorms and rain especially
inland. SW winds.

Tuesday...moderate confidence.

May see patchy fog after midnight in normally prone valley areas in
the morning. A window of precipitation-free weather expected
sometime during this period. Otherwise another front pushes across
the region Tuesday night. MVFR conditions in showers and storms. SW
winds continue.

Wednesday and Thursday...moderate confidence.

Mainly VFR. Local MVFR-IFR visibilities from departing showers Wednesday
will improve by middle morning with cold frontal passage. Eventual
change from SW winds to northwest winds Wednesday. VFR conditions Thursday
with high pressure overhead.

&&

Marine...
forecaster confidence levels...

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

Tonight...

S-southeast winds become SW overnight. Good visibility should lower later
tonight patchy fog and drizzle.

Sunday...

Low clouds/fog and drizzle limit visibility early but improving by midday.
Dry weather thereafter with risk of showers and thunderstorms hold
off until at least 5 PM and possibly until after dark. SW wind 15-20
knots with a few gusts close to 25 knots near shore especially across Cape
Cod and the islands.

Sun night...

Scattered showers and T-storms during the evening. Patchy fog will
limit visibility. SW wind 15-20 knots.

Outlook...Sunday night through Wednesday...

Monday...moderate confidence.

Winds begin to relax during the day. Seas also relax from 5ft swell
to around 3 to 4 feet swell. Could be near Small Craft Advisory for decaying 5ft swells.

Tuesday through Thursday...moderate confidence.

Expect winds and seas below small craft criteria. SW winds will veer
to northwest late Tuesday night into Wednesday.

&&

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

&&

$$
Synopsis...nocera
near term...wtb/gaf
short term...nocera
long term...nocera
aviation...nocera
marine...nocera

National Weather Service Glossary of Abbreviations