Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
1036 PM EDT Thursday Mar 26 2015

Synopsis...
cold front crossing southeast New England will move offshore
overnight. Weak low pressure may bring scattered snow showers
Friday night and Saturday. High pressure will push south of New
England Sunday. Another batch of precipitation is possible late
Sunday night into Monday ahead of a cold front.

&&

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

Surface observation at 10 PM showed southwest winds across South Coast and
north winds across Boston and the interior. The cold front...with
a significant wind shift line...extended from Marshfield across
the city of Providence to the southwest Connecticut coast.
Observed movement of the front trends off Nantucket roughly 06z-
07z.

Radar showed at 10 PM one area of showers along and leading this
front. A second area of showers extended from southwest New Hampshire across
western mass and western CT to northern New Jersey...with a
northeast motion. Back edge of all the showers trends to the CT
valley around 1 am and to the East Coast of mass around 4-5 am. So
the current forecast of categorical probability of precipitation for showers looks fine.
Sufficient moisture and positioning of the upper jet /right
entrance region overhead/ to support scattered or widely scattered
showers beyond the rain shield.

Colder air on the edge of the rain shield has the potential of
bringing a rain/snow mix to the showers in western/central mass
and adjacent CT toward morning.

Temperatures in the southeast will remain steady until the cold front
slips through. Cooling trend north and west of the front with
temperatures dropping back into the 30s and lower 40s.

&&

Short term /6 am Friday morning through 6 PM Friday/...
Friday...

Middle level frontogenesis behind the actual cold front will allow for
rain to continue across eastern New England Friday morning. As we
mentioned in the previous section...it may become just cold enough
across the higher terrain of mass for a change to wet snow before
precipitation ends. Things should dry out for most of the region by
afternoon as best forcing departs...but some lingering showers will
remain possible over the southeast New England coast. Across the
rest of the region...plenty of clouds will linger even though the
rain will have come to an end. High temperatures mainly in the 40s.

&&

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

* scattered snow/rain showers Friday night into Saturday evening
* weak cold front will move through on Monday bringing scattered showers
* moderating temperatures by the middle of next week

Overview and model preferences...

Long term amplified upper level pattern continues into the early
portion of the long term forecast. Beyond this weekend...medium
range models and ensembles continue to signal the long wave
troughing that has been nearly a constant over the last couple of
months across the eastern U.S. Will shift slowly east...but will
bring a fast northwest flow aloft early next week. This will bring fast
moving weak systems across New England. Timing is in question due
to fast movement lending to wide operational model variance along
with lowered confidence from late Tuesday through Thursday.

Used a blend of available guidance through Tuesday...then leaned
toward the superblend along with some blending with the ensemble
means to try and resolve timing of systems toward the end of the
forecast period.

Models and ensembles continue to signal fast flow aloft with more
waves moving along into fast flow aloft into Easter weekend which
will need to be monitored.

Details...

Friday night through Saturday night...moderate confidence. Cold
front works through early Friday night as front weakens.
However...still good lift and forcing as upper trough works east out
of New York state overnight. This...along with cold pool aloft...will
cause light rain showers to change to snow showers overnight.
Generally a tenth of a inch or less of quantitative precipitation forecast expected...so not
expecting much in the way of accumulations at this point.
However...

Most models and ensembles continue to signal development of
inverted trough late Friday night through midday Saturday as another
low passes well S of the 40n/70w benchmark. With the low level
moisture in place along with steep lapse rates will allow for
enhancement of rain/snow showers. Expect precipitation to remain mainly
as light snow... though temperatures might rise enough /mid-upper 30s/ to
mix/change to rain around midday but change back snow before
tapering off from north-S late in the day or Sat evening.

Big question at this point will be whether and...if so where...
the inverted trough will set up to focus the instability and
precipitation. Generally...expect less than an inch. But...if this does
set up...some locations might see a slushy 1-2 inch accumulation.
Low confidence with this aspect but something to definitely keep
an eye on.

Overnight lows Friday night will be drop to the Lower-Middle 30s across
the higher inland terrain...ranging to around freezing along the S
coast. However...with cold air advection along with precipitation...temperatures
will not rise much on Sat. Highs will only rise to the middle and
upper 30s...more typical of January rather than the end of March.

Conditions will improve Saturday night for most areas...though
some leftover light snow showers might linger along the S coast. North
winds will be gusty...especially across the cape and islands.
Overnight lows will fall to the middle and upper teens inland ranging
to the upper 20s on the Outer Cape and Nantucket.

Sunday-Sunday night...high confidence.
As upper trough finally moves offshore...expect high pressure ridging
at the surface and aloft will bring sunshine across the region. As
fast northwest flow aloft sets up...ridge will build east during the
afternoon and clouds will begin to filter in. Will see some warm
air advection work in as winds back to W-SW...but not in time for
temperatures to recover so highs will only be in the upper 30s to middle
40s.

Temperatures will drop a few degrees Sun night as clouds continue to
increase. Next fast moving short wave in the upper flow
approaches. Timing is an issue...though looks like most of the
model suite holds precipitation off until Monday except for possibly a
few isolated snow showers toward daybreak across the east slopes of the
Berkshires.

Monday-Monday night...moderate confidence.
Short wave moves across bringing a period of snow showers
early...changing to rain showers by midday as the front crosses
the region. Temperatures will moderate ahead of the front. Expect highs
mainly in the middle 40s to around 50.

Some leftover rain showers will linger across central and eastern
areas Monday night before pushing offshore as front clears the coast.
Upper level short wave also pushes east with northwest flow kicking back in.
However...core of 850 mb cold air remains north of the region. Will still
see cooler temperatures...but lows will only fall back to the upper 20s
to middle 30s as skies become partly cloudy after midnight.

Tuesday through Thursday...low confidence.
Fast flow aloft continues...bringing one system across. With wide
timing issues amongst individual models...have rather low
confidence. For now...expect dry conditions Tuesday...then possible
showers Tuesday night into Wednesday. Looks to dry out again by midday Wednesday.
Warm front may bring clouds and the chance for precipitation Wednesday
night/Thu.

&&

Aviation /02z Friday through Tuesday/...
forecaster confidence levels...

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

Short term...tonight and Friday...

Tonight...moderate confidence.
Expect periodic showers through the night with mainly IFR
conditions. Low clouds will linger...areas of fog as well. Noting
areas of vlifr visibilities along S coast...Cape Cod and islands with
spotty visibilities at or below 1/2sm at times across CT valley into central Massachusetts
mainly across the Worcester Hills. Will see slow improvement but
remaining at least MVFR overnight across the region. Low level wind shear will
continue as SW low level jet at 60-65 knots across north CT/RI/E Massachusetts
will move offshore between 07z and 09z.

Friday...moderate to high confidence. Rain showers should come to
an end for most of the region during the morning...although may
linger a bit longer across the far southeast Massachusetts coast. Ptype may
actually change to a bit of wet snow before ending across the
higher terrain...but odds of any real accumulations on runways are
rather low. Otherwise...conditions should improve to mainly VFR by
afternoon although some marginal MVFR ceilings may persist at times.

Kbos taf...moderate confidence in taf.

Kbdl taf...moderate confidence in taf.

Outlook...Friday night through Tuesday...moderate confidence.

Friday night and Saturday...mainly MVFR ceilings/local MVFR-IFR visibilities
in rain and/or snow showers Friday night. Expect light snow Friday night
after midnight into Saturday morning. Less than an inch of slushy
snow accumulation expected...best chance at korh as well as
remainder of Worcester Hills. Precipitation briefly changes to -shra
around midday...then back to -shsn toward nightfall.

Saturday night...local MVFR ceilings/visibilities across north CT/RI/se Massachusetts early
in scattered snow showers before ending from north-S by midnight.

Sunday and Sunday night...VFR conditions. Clouds increase from
west-east Sun night...may briefly drop to MVFR across higher terrain of
west Massachusetts.

Monday-Tuesday...mainly VFR. Local MVFR ceilings/visibilities in scattered rain
showers. Chance of -shsn across central-west Massachusetts/north CT through midday
Monday. Conditions improve from west-east Monday night...VFR on Tuesday.

&&

Marine...
forecaster confidence levels...

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

Short term...tonight and Friday...

Tonight...moderate confidence. Cold front at 10 PM was over
southest mass and Rhode Island. Southwest wind gusts of 20 to 30 knots
ahead of the front followed by a shift to north winds behind the
front. Expect the front to move south/east of Nantucket by 3 am.
Seas linger at 5 to 9 feet overnight.

Friday...moderate to high confidence. Northerly winds of 10 to 15
knots with gusts to 20 knots expected behind the cold front. Left
over south to southeast swell will keep seas above small craft
thresholds across most open waters through Friday afternoon.

Outlook...Friday night through Tuesday...

Moderate confidence.

Friday night....N-NW winds gusting up to around 25 knots behind
frontal passage. Seas also build up to 6-8 feet on the outer
waters. Small crafts needed.

Saturday and Saturday night...northwest wind gusts may approach 30 knots on
the eastern outer waters during Sat...then slowly diminish Sat
night. Seas may approach 10 feet on the eastern outer waters
Sat...then will slowly subside but remain at or above 5 feet. Brief light
freezing spray possible on portions of eastern and southern outer
waters Sat night.

Sunday...expect winds and seas to briefly diminish below small
craft criteria as winds back from northwest to SW Sunday night. Seas
also subside below 5 feet.

Monday-Tuesday...SW winds pick up during Monday...gusting to 25
knots with seas building to 5-7 feet. Winds shift to west Monday night-Tuesday
with local gusts to 25 knots on the outer waters. Seas remain at or above 5
feet on the southern waters as well as portions of the eastern
waters.

&&

Hydrology...
basin average rainfall amounts will range from 0.5 to 1.25 inches
across the region through Friday morning. Nohrsc modeling adds 1
to 2 inches of water from melting snow due to dewpoints in the 40s
to near 50.

This will bring rises on rivers and streams through Friday...
especially smaller ones which may approach bankfull. However...
given rainfall has been spread out over a long duration with many
dry periods do not expect any significant flooding problems. Brief
heavy rainfall in showers may result in typical nuisance poor
drainage street flooding tonight...but again nothing significant
expected.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...dense fog advisory until midnight EDT tonight for maz020>024.
Rhode Island...dense fog advisory until midnight EDT tonight for riz005>008.
Marine...Small Craft Advisory until 6 am EDT Friday for anz232>234.
Small Craft Advisory until 2 am EDT Friday for anz231-236.
Small Craft Advisory until 8 PM EDT Friday for anz235-237-250-
254>256.

&&

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

National Weather Service Glossary of Abbreviations