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Area forecast discussion 
National Weather Service Taunton Massachusetts
342 am EDT Thursday Sep 18 2014

Synopsis...
a weak cold front will cross the area today...but moisture is
limited so it will come through dry. Cold high pressure builds
into New England tonight and Friday...possibly bringing a touch of
frost to the interior. The high will move offshore through the
weekend resulting in a warmup. A cold front moves through on
Monday...followed by cooler weather Tuesday and Wednesday.

&&

Near term /until 6 PM this evening/...
305 am update...

Skies remain clear across region early this morning and will
remain that way through sunrise. Expect patchy fog in low lying
areas due to good radiational cooling conditions. Lows will bottom
out in 40s and 50s.

Weak cold front remains draped across St Lawrence Valley and will
head southeast...crossing southern New England this morning. Cross
sections show moisture is limited with front...so only expecting a
period of cloudiness as it moves through especially due to passage
of upper trough. Could be brief sprinkle but radar trends suggest
keeping forecast dry.

Highs will top out in 60s and lower 70s...which is blend of MOS
guidance. NAM MOS seems a bit too warm with highs reaching middle 70s
near S coast considering expected cloud cover.

&&

Short term /6 PM this evening through 6 PM Friday/...
clearing will take place this evening from north to S as Canadian high
pressure builds across New England and subsidence increases behind
departing upper trough. This will set stage for a cold night as
lows drop back into 30s and 40s...with touch of frost possible
across parts of northwest Massachusetts and SW New Hampshire. Per collaboration with
neighboring offices there are a few concerns which may limit
widespread frost...including how fast clearing occurs and do winds
drop off enough to allow frost formation. Given these concerns we
will hold off on issuing advisories and allow day shift to get a
better look.

High pressure remains in control Friday. Onshore flow combined with
cooler airmass in place should keep afternoon highs in 50s along
immediate coast and higher terrain to middle 60s elsewhere.

&&

Long term /Friday night through Wednesday/...
overview and model preferences...
long range is likely to be defined by upper level longwave trough and
associated cutoff low pressure extending out over Hudson Bay and
baffin island. Initially...this trough is shunted to the north of New
England...as middle level ridge moves to the east...yielding zonal flow.
This is the weekend timeframe...and the rising heights and modest
SW low level flow will lead to moderate warming through Sat and sun.
However...strong cold shortwave associated with cp airmass near
Alaska...will once again dig and deepen the trough across the east
early middle next week...leading to another round of cooler temperatures
across the region. For the most part...these synoptic scale
features are reasonably well agreed upon...and the 18.00z GFS is
now more in line with European model (ecmwf) timing of the cold front associated
with the early week wave. Therefore...feel a blend of
deterministic guidance is a good starting point.

Details...

Friday night through sun...

High pressure will hold fast across the region...yielding an inverted
ridge as the center shifts offshore thanks to confluent zonal flow
aloft. Friday night...weak pressure gradient and near sky clear skies may lead
to enough radiational cooling in northwest Massachusetts and SW New Hampshire valleys for frost
development. Something to watch over the next 24-36 hours. For the
rest of the weekend...modest return flow and warm advection lead
to increasing 850 mb temperatures from +11c Sat to +14c sun.
Therefore...highs range in the low-middle 70s Sat...then middle 70s to
around 80 on sun. Precipitable water values look too low at this point to
support the light quantitative precipitation forecast GFS continues to show each afternoon...and
with subsidence in place beneath the inverted ridge...will opt to
maintain a dry forecast through most of the day sun.

Sun night into Monday...

Cold front is expected to cross the region as low pressure moves
through southern on and qc. Modest precipitable water values around 1.5 in...right
equatorward region of upper level jet streak and modest low level
f-general all support the possibility of showers across the region as
this front moves through. Models starting to line up on
timing...moving it through generally during the am hours Monday. This
timing will limit convective potential and soundings look
relatively stable...even k values remain mostly below 30.
Therefore...suspect mainly just widespread -shra activity to move
through during the morning hours with gradual improvement late day
Monday. This timing will also keep coastal low pressure offshore...so quantitative precipitation forecast
values should generally remain below a half inch given current
thinking.

Tuesday into Thursday...

This cold front will allow moderately cool airmass to begin to
build in early next week. In fact...guidance suggests that by
Tuesday...546dm thickness values may approach the northwest corner of the
forecast area along with 850 mb temperatures around +2c. Therefore...under
the cool northwest flow and cold advection...highs Tuesday may struggle to
break out of the 60s...with some 50s possible. Gradual moderation
is expected into Wednesday and Thursday...but with temperatures still leaning
slightly below normal. The high may not crest across the region
until Wednesday...only moving slightly east by Thursday...so suspect mainly dry
conditions prevail.

&&

Aviation /08z 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/...high confidence.

Patchy MVFR/IFR in valley fog through sunrise...otherwise VFR
today with scattered-bkn040-060 developing around midday. Weak cold
front brings wind shift to north/NE across most of area by late
morning or early afternoon. Possibility of brief sprinkle but
probably not much more than that and not enough to wet runways.

Clearing skies tonight. Possibility of patchy frost across valleys
of northwest Massachusetts and SW New Hampshire. VFR Friday with east/NE flow.

Kbos terminal...high confidence in taf...including timing of wind
shift today.

Kbdl terminal...high confidence in taf.

Outlook...Saturday through Monday...

Saturday and Sunday...high confidence.
Mainly VFR through the period although some ground fog is possible
Sat morning and Sun morning at typically prone airports. Winds
mainly S-SW...with gusts around 20 knots possible sun.

Sun night into Monday...moderate confidence.
Cold front crosses the region during the morning hours Monday.
Showers and possibly MVFR/IFR conditions may move through with
this front. Otherwise...improving back to VFR by late day Monday.

&&

Marine...
forecaster confidence levels...

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

Short term /through Friday/...high confidence.

Weak cold front crosses waters later this morning with wind shift
to N/NE. Swell from offshore Hurricane Edouard should bring 3-4 feet
seas to outer waters but conditions expected to remain below Small Craft Advisory.

Should see brief surge in north/NE winds tonight as cooler air arrives...
mainly from Merrimack River to Cape Cod/islands where 25kt gusts
likely as seen on forecast soundings. Small Craft Advisory being posted for all
waters except Boston Harbor... Narragansett Bay and Block Island
Sound.

Winds and seas diminish Friday as high pressure becomes centered over
Gulf of Maine...but will maintain Small Craft Advisory for outer waters where seas
will take time to drop below 5 feet.

Outlook...Saturday through Monday...moderate confidence.

Saturday-Sunday...winds swing around to southwest with daytime
gusts around 20 knots...possibly near 25 knots Sunday. Seas will
remain 2-4 feet. A cold front approaches late Sunday with a chance
of showers Sunday night.

Monday... a cold front crosses the waters early in the day. West
winds behind the front will remain less than 20 knots. Seas will
build to 5-7 feet on the outer and exposed waters...as a south swell
increases to 3-5 feet. Small craft advisories may be needed.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory from midnight tonight to 6 am EDT Friday
for anz231>235-251-256.
Small Craft Advisory from midnight tonight to 6 PM EDT Friday
for anz250-254-255.

&&

$$
Synopsis...doody/jwd
near term...jwd
short term...jwd
long term...wtb
aviation...doody/jwd
marine...doody/jwd

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