Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
1125 PM EST Thursday Nov 27 2014

Synopsis...
a weak shortwave moving up the coast may produce accumulating
snow across eastern Massachusetts including the Boston area. High pressure
moves over New England Friday and shifts offshore over the
weekend. A warm front moves across the region late Saturday night
and early Sunday. A cold front then swings through New England
Monday. Another warm front/cold front combination will move
through the region Wednesday and Wednesday night.

&&

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

11 PM update...
precipitation from first short wave moving NE...but noting some mix of
short wave energy with some ocean effect snow moving toward the
coast at 03z. Steadiest precipitation noted across Rhode Island/east Massachusetts...though has
been weakening especially across central and western areas over
the last couple of hours. Where it has been mainly snow it has
been light. Little if any accumulation reported to US so far
though have received one report of an inch of snow in West Warwick
Rhode Island at 940 PM. Only a trace here at kbox so far.

Have updated forecast to reflect decreasing trends well inland but
did increase probability of precipitation a bit along the coast mainly where the advisory
is in effect...for now.

&&

Short term /6 am Friday morning through 6 PM Friday/...
*** accumulating snow possible late tonight into early Friday afternoon
across eastern Massachusetts from Cape Ann to Boston to Cape Cod ***

Low pressure off of North Carolina will move northeastward...passing
well outside the 40n/70w benchmark. However...an inverted trough
will develop as the shortwave crosses southern New England. These
two features will provide enough lift and instability along the
East Coast of New England to produce a narrow band of rain/snow
showers. The main question is just where does this trough set up
and how close to the coast does it get.

These features leave a lot of uncertainty as a shift of 10 to 20
miles to the east will leave most areas without snow. However...
models have consistently been forecasting up to half an inch of quantitative precipitation forecast
along the East Coast of Massachusetts to just off the coast for the
last three model runs. That lends some level of certainty to the
potential for a quick few inches of snow. In addition...steep low
level lapse rates and total totals over 50 both indicate plenty of
instability that will be realized.

At this point...generally have snowfall totals just under advisory
criteria /3 inches/. However...there are a few pockets of around 3
inches right around the Boston area. Due to the potential for a
change in amounts if highest quantitative precipitation forecast ends up over land rather than over
the ocean...am opting to issue a Winter Weather Advisory along the
immediate East Coast from Cape Ann south through coastal Plymouth
County. Expect snow to begin shortly after midnight and continue
through the morning hours on Friday. While there is quite a bit of
uncertainty with these systems...given the number of shoppers and
travelers who may be out and about Friday morning...felt an advisory
would be a good idea.

&&

Long term /Friday night through Thursday/...
big picture... zonal flow this weekend with 500 mb heights building
over New England while a shortwave trough drops through western
Canada into the Pacific northwest. The shortwave then races east to
cross New England Monday night. Another shortwave races east
across the northern tier during the week. There are differences
between the European model (ecmwf) and GFS regarding the amplitude and speed of
this shortwave with the GFS faster/less amplified.

We favor a model blend for most of the long term. Movement of
shortwaves through zonal flow leads to questions on timing so
confidence is at best moderate. Late in the long term we tend closer
toward the European model (ecmwf). As noted by HPC...the greater amplitude trough
shown by the European model (ecmwf) is more reasonable due to deep low pressure in
the Bering Sea and resulting building of a ridge along the Pacific
coast.

The dailies...

Friday night-Saturday... high pressure builds over New England with
dry chilly weather. Cross sections show increasing middle and high
level moisture Saturday as the high shifts east and warmer air
starts moving in over the top of the colder air. So we expect mostly
clear skies Friday night and increasing sky cover on Saturday. We
stayed close to a composite of guidance for temperatures both periods.

Saturday night through Monday... a warm front approaches Saturday
night. This will maintain clouds over the region. Low level profiles
look rather dry and lift is weak. So any precipitation should be limited. We
showed slight chance probability of precipitation mainly along and north of the Mass Pike
Saturday night and in southern New Hampshire Sunday morning. The warm front lifts
north on Sunday allowing for milder temperatures and dewpoints as well as a
generally dry day.

The models show differences in timing the passage of the cold front.
This is not a surprise as we will be in a zonal flow. The GFS is
fastest with passage Monday morning. The European model (ecmwf) is slower with
passage Monday evening. We used a blend of this data with chance
probability of precipitation in the west in the morning...slight chance all areas afternoon...and
chance all areas early Monday night. At best moderate confidence in
this timing.

Tuesday... high pressure builds over the region with colder air.
Mixing will be limited to 925 or 950 mb...and temperatures at that level
support maximum temperatures in the upper 20s and 30s.

Wednesday-Thursday... shortwave moves out of the Pacific northwest
and pushes a cold front across the northern tier of states Tuesday-
Wednesday. This will draw a southwest low level jet of 40-45 knots
leading the cold front up to our area by Wednesday.

This jet will supply moisture and generate sufficient lift for
clouds and a chance of precipitation. Confidence in timing is low but as
noted above we favored the timing shown by the European model (ecmwf) for onset of
clouds and any precipitation. Low level temperatures will determine precipitation
type...but trend should be from snow to rain early Wednesday and
Wednesday morning. Lingering cold air at the surface may support a
period of sleet or freezing rain during the transition during the
morning...then temperatures should be warm enough for precipitation to fall as rain
any other time of the day. Timing of the cold front per the European model (ecmwf)
would be late Wednesday or early Wednesday night.

High pressure brings drier weather on Thursday.

&&

Aviation /04z Friday through Tuesday/...
forecaster confidence levels...

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

Short term /through Friday/...moderate confidence.

Tonight...MVFR ceilings remain over southern New England through the
night. One area of light rain/snow showers moves northeast across
CT/RI/southeast mass through midnight. Reported visibilities in the
showers have been 3-5 miles or higher. Could be brief IFR
ceilings/visibilities but mainly expect 1200-1700 foot ceilings and 3-5 mile
visibilities through 11 PM.

Second area of precipitation develops across east Massachusetts into Rhode Island...possibly as
far west as korh-kijd after midnight...as inverted trough takes
shape near the coast. Areas of MVFR-IFR visibilities develop in rain
and/or snow. Visibilities should remain mainly VFR across the CT valley.

Friday...snow/rain showers will continue through middle-morning along
the East Coast of mass...possibly as far inland as orh-ijd. Precipitation
comes to an end late morning...soonest west and latest east.
Conditions will improve to VFR Friday afternoon.

Kbos terminal...medium confidence in timing of rain/snow tonight
and Friday morning. High confidence in improving conditions
Friday.

Kbdl terminal...high confidence.

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

Friday night and Saturday...VFR. Increasing sky cover Saturday.
Northwest winds become southwest but remain light.

Saturday night-Sunday...lowering clouds Saturday night with
potential for MVFR ceilings/visibilities in widely scattered precipitation late at night
and Sunday morning. Low level winds...around 2000 feet...will
increase overnight and Sunday to 40-45 knots and may create low
level wind shear.

Monday...VFR with scattered showers as a cold front moves through.
Southwest winds ahead of the cold front will switch to northwest
after the front moves through.

Tuesday...VFR with north to northeast winds.

&&

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.

Overnight...still noting seas up to 7 feet on the eastern waters at
02z...but appear to be subsiding. Have kept small crafts going.
Otherwise continued trends from previous forecast with seas
subsiding early...then beginning to build again late as low pressure
passes S of the waters. Will also see north-northwest winds that may become
NE along the coast but remain light though gusts increase toward
daybreak.

Friday...winds increase...gusting to near 25 kts. Have small
craft advisories up for most of the coastal waters for either
seas...gusty winds or both.

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

Friday night...diminishing winds and seas. Lingering 5 foot seas on
the outer waters before midnight may require a Small Craft Advisory.

Saturday...winds and seas should remain below small craft thresholds.

Sunday...southwest winds increase to 25 knots and seas build to 5-7
feet. A Small Craft Advisory may be needed on most waters.

Monday...a cold front moves through during the afternoon or evening.
Southwest winds shift to northwest later in the day...with speeds 20
to 25 knots. Seas diminish a little but remain at or above 5 feet on
the exposed waters. A Small Craft Advisory may still be needed.

Tuesday...north winds become northeast with gusts around 25 knots.
Seas linger at 5 to 6 feet on the outer waters. A Small Craft
Advisory may be needed.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...Winter Weather Advisory until noon EST Friday for maz007-015-
016-019.
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory until 6 PM EST Friday for anz235-237-250-
251-254>256.

&&

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

National Weather Service Glossary of Abbreviations