Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Taunton Massachusetts
107 am EST Sat Dec 20 2014
a weak disturbance and onshore flow will result in a period of
light snow/rain showers mainly across eastern New England this
weekend into early next week. Low pressure will pass south of New
England...bringing another period of precipitation. A much
stronger storm passing west of New England will bring heavy
rainfall...gusty winds and coastal flooding concerns to the
region Wednesday into Wednesday evening. Dry but very windy
conditions follow for Christmas day into Friday.
Near term /until 6 am this morning/...
105 am update...
Low clouds beginning to back in off the ocean very early this
morning. This in response to an approaching shortwave resulting in
winds at 925 mb becoming east. Should see low clouds continue to
expand across eastern New England through daybreak...while the
interior has clear to partly cloudy skies. Temperatures will remain in the
20s through daybreak...except lower to middle 30s across portions of
the cape/ack with an onshore component to the wind.
Short term /6 am this morning through 6 PM Sunday/...
high pressure builds across north Maine and the Maritimes with low level
flow veering to the east while surface winds mostly NE. This will result
in low clouds expanding west across southern New England during Sat as low level
moisture increases. Not really looking at a pure ocean effect
situation as instability is marginal. However...hi-res guidance
picking up on a developing coastal front across Cape Cod vicinity
with east winds to the east of the boundary and north/NE winds to the west.
Low level forcing combined with deepening moisture as middle level
shortwave moves in from the west will result in rain/snow showers
expanding west from Cape Cod into eastern half new eng. While some
light precipitation is possible in the morning...especially Cape
Cod...bulk of precipitation occurs later Sat into Sat night associated with
deeper moisture and low level lift from the coastal front/inverted
looking at mostly rain for cape/islands and possibly into southeast
coastal Plymouth County where east winds will sufficiently warm the
boundary layer keeping temperatures in the middle/upper 30s. However...just
inland and north from the immediate southeast coast and west of the
coastal front where winds are forecast to be more
northerly...expect mostly snow although there could be a mix at
the start. Freezing drizzle is not likely as the column moistens
late Sat into Sat night with temperatures -10c in the moist layer which
is cold enough for ice Crystal production.
NAM is picking up on some enhancement in North Plymouth County into
Norfolk County with quantitative precipitation forecast up to a third of an inch. This likely
associated with a 20-25 knots east/NE low level jet that the NAM is
forecasting. However...this might be overdone as hi-res WRF quantitative precipitation forecast
is less and other models are weaker with the low level NE flow.
Dont want to completely ignore the NAM as it sometimes can pick
up on heavier mesoscale precipitation in these situations so used a
blend of NAM/GFS quantitative precipitation forecast to temper NAM output. Mainly looking at a
coating to an inch for eastern new eng...highest amounts across east
Massachusetts away from the immediate coast. Localized amounts up to 2 inches
possible...especially just SW of Boston.
Long term /Sunday night through Friday/...
* scattered snow/rain showers linger on Sunday with slightly below
* weak low pressure will pass south of New England Monday night and
Tuesday bringing light precipitation...as well as a wintry mix
to the interior
* strengthening low pressure will wrap up around anomalous vortex
across the Great Lakes...bringing heavy rain...gusty winds and
coastal concerns to the region late Wednesday into Christmas evening
Overview and model preferences...
Medium range model suite continues to signal development of
amplified upper level pattern across North America through this
forecast period. An 500 mb cutoff vortex will dominate across the Great
Lakes and its extending long wave trough wrapping around to the
eastern Seaboard as it becomes negatively tilted. With overall SW
flow in place...will see weak systems work toward the region early
in the period. A stronger low will develop near or just off the southeast
U.S. Coast around early Wednesday and pass west of the region Wednesday night...
bringing gusty winds...heavy rainfall with a few thunderstorms
possible as well as possible coastal concerns.
While there are still some timing and track variances with the middle
week system...models and ensembles remain in good agreement with the
development of this amplified pattern...as well as the upper
level vortex across the Great Lakes and surface system that will
more than likely affect the region during the Wednesday-Wednesday
Used a blend of available guidance as well as GFS/ec ensemble means
for this forecast which showed good continuity to the previous
Sunday and Monday...moderate to high confidence.
Leftover moisture will linger Sunday morning with light rain
and/or snow showers mainly across east Massachusetts/Rhode Island though could see some
isolated activity further inland as coastal front weakens. Ridging
will nose down the coast from high pressure over northern New England
and Quebec Sun night into Monday...bringing a brief dry period
though clouds will linger with a continued onshore wind
flow...albeit weak. Expect temperatures to run near or slightly below
normal...though will be a bit milder along the S coast during Monday.
By late Monday...may start to see some precipitation work into the lower
CT valley as a weak low moves off the middle Atlantic coast.
Monday night-Tuesday...moderate confidence.
Weak coastal low will pass off the South Coast...but will bring
enough moisture and lift for precipitation to move in from S-N. Temperatures
will still be chilly though...especially inland there the colder
air will be trapped... so will see some mixed precipitation /sleet and/or
freezing rain/. Even though quantitative precipitation forecast will be light...could be enough to
cause some light glazing problems late Monday night into Tuesday morning.
Still some timing issues as well as questions with thermal
profiles...but may need winter weather headlines during this timeframe.
Something to monitor closely.
By Tuesday afternoon...temperatures should recover with gradually increasing
east-southeast winds...so will see all rain by then. Highs on Tuesday will
range from the upper 30s across the higher inland terrain to the
middle and upper 40s along the coast.
One other issue that may need attention will be the long easterly
fetch and increasing astronomical tides on Tuesday. Boston
Harbor/S high tide occurs at 1142 am at 11.6 feet. Surface winds
look to remain at 15 knots or less but...with the mainly easterly
wind direction over several days...may see some ground swells work
in which could cause some possible splashover during the midday
Wednesday and Thursday...high confidence in system development and
effects...low to moderate confidence on timing details.
Will see strong surface system and its associated 500 mb cutoff
vortex will be intensifying during this timeframe...while long
wave troughing will become negatively tilted around the middle Atlantic
or southeast U.S. Coast. A secondary low...with energy and tropical
moisture working in from the Gulf of Mexico...will intensify
during Wednesday as it wraps around the larger upper level circulation.
Looks like this low will pass west of the region...but will be in the
area of best moisture convergence and transport up the eastern
Seaboard. The 12z GFS is bullish in bringing slug of heavy quantitative precipitation forecast
across the region. Both the 12z ec and ggem also signal this
influx though not quite as high on the precipitation amts.
Still noting precipitable waters at 1.4 to 1.5 inches...running 2-3sd above
seasonal normals at about the time of the heaviest precipitation late
Wednesday into Wednesday evening. On top of this...some good instability moves
across with k indices in the Lower-Middle 30s and total totals in the
lower 50s. Would not be surprised to see some isolated thunder so
mentioned in the forecast mainly during Wednesday night...along with
heavy rainfall from the convection. Low level jet on order of
40-55 knots at 850 mb and 50-60 knots at h925...so could also see gusty
winds especially with any storms. Ohio...one more thing...there
will be a brief burst of milder air that works in with this
system. Expect temperatures to rise to the middle 40s well inland to the
middle 50s at the coast...but will not top off until close to
midnight Wednesday night...which will help support the instability.
Models continue to signal fast movement of the surface system
late Wednesday night...with dry slot working across the region after
midnight with dry conditions by dawn on Christmas morning. Winds
will quickly shift around to west-SW and will become very gusty with
the low level jet in place and colder air wrapping in.
May have some coastal flooding issues with highest astronomical
tides of the month occur on Wednesday and Thursday...up to 11.7 feet and 11.5
feet respectively for Boston. Any easterly component will push the
tide above flood...so coastal flooding is possible as well as
beach erosion on the Wednesday tide along east facing beaches. The south
coastline will also have to be watch closely on Thursday as winds shift
to west- SW...though east coastal areas could see problems with
continued east swells. As mentioned above...strong southerly jet
develops Wednesday into Thursday and...with astronomical high tides...could
also see minor to moderate coastal flooding. Another issue is the
potential pressure falls if this system strengthens even
more...it could add another foot onto the surge.
Once upper low and surface system moves into southern Canada...
expect a rather fast upper west flow in place. May see some leftover
snow showers across the east slopes of the Berkshires. Gusty west-SW
winds continue as temperatures fall back to more seasonable levels.
Aviation /06z Saturday through Wednesday/...
Forecaster confidence levels...
Low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.
Short term /through Sunday/...
105 am update...
Today...moderate confidence. MVFR ceilings will continue to overspread
eastern New England this morning and eventually into the interior
this afternoon. Scattered precipitation will overspread
southeast Massachusetts/cape/islands this afternoon. Ptype mainly snow showers
northwest of the southeast Plymouth County coast and rain showers
southeast of that region. Localized IFR conditions possible just
inland from the southeast New England coast...where ptype is snow.
Tonight and Sunday...moderate confidence. MVFR conditions dominate
in lower clouds and scattered precipitation. Localized IFR
conditions possible with the best chance in snow showers.
Precipitation affects mainly eastern New England but still some
scattered activity across western New England. Ptype mainly snow
showers northwest of the southeast Plymouth County coast and rain
showers southeast of that region.
Kbos terminal...moderate confidence in taf. Snow showers may arrive
towards 00z this evening.
Kbdl terminal...moderate confidence in taf.
Outlook...Sunday night through Wednesday...
Monday...moderate confidence. Mainly VFR.
Monday night-Tuesday...moderate confidence. Developing rain or
mixed precipitation from S-north Monday night with MVFR conditions. May see
-fzra/-pl across central and western areas around or after
midnight into Tuesday morning then should change to rain as temperatures
rise. East-southeast winds gusting to 20-25 knots along S coast Tuesday.
Tuesday night-Wednesday...moderate confidence. MVFR-IFR
conditions in rain...heavy at times during Wednesday. Isolated thunder
possible Wednesday afternoon/evening. East winds shifting to southeast-S late Tuesday
night/Wednesday with gusts up to 25-30 knots...highest across Cape Cod and
forecaster confidence levels...
Low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.
Short term /through Saturday night/...high confidence.
High pressure building east to Maine and the Maritimes will keep
winds/seas below Small Craft Advisory thresholds through Saturday night...with wind
speeds less than 20 knots. Northerly winds tonight becoming NE
Sat/Sat night. Gusts up to 20 knots possible Sat night across NE Massachusetts
Outlook...Sunday through Wednesday...moderate to high confidence.
Sunday through Monday night...winds and seas below small craft
criteria. East winds gusting up to 20 knots Monday night.
Tuesday-Wednesday...east winds increase...gusting to 30 knots Tuesday then
shift to southeast gusting to 30-35 knots Wednesday...highest on the outer
waters. Seas build up to 7-11 feet by Wednesday. Gales on the outer waters