Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
717 am EST Thursday Nov 27 2014

strong low pressure over Nova Scotia will continue moving away
from New England today. However scattered light snow showers or
flurries will develop later this morning and afternoon across the
region. A trough of low pressure may produce accumulating snow
across eastern Massachusetts including the Boston area late
tonight into Friday. Elsewhere just light snow or flurries are
expected. Dry but cold weather follows Friday night into Saturday.
Another period of light snow or flurries is possible Saturday
night. Dry and milder weather arrives Sunday and into early next


Near term /until 6 PM this evening/...

7 am update...

Chilly morning in progress across the region with temperatures
mainly in the u20s and l30s. Gusty winds over Cape Cod and the
islands providing a bit of a wind chill...however winds will
diminish later this morning.

Clouds eroding early this morning as short wave ridging moves
across the area. However a short wave trough over Ohio Valley into
PA will move across southern New England this afternoon and
evening yielding light snow showers or flurries. Previous forecast
captures these details so only changes with this update is to
hourly temperatures/dew points and probability of precipitation to better reflect current trends.
Earlier discussion below.


Leftover light snow from the exiting low pressure heading NE out of
the Gulf of Maine continues across S New Hampshire/north central and east Massachusetts while
a mix of light rain and snow reported on Cape Cod and the islands
at 09z. NE regional 88d radar showing the light precipitation moving into
northern New England...though a few spots hanging in across east Massachusetts.
Should see precipitation end over the next few hours.

For today...expect a brief break in the action as weak high pressure
ridge builds across by around midday. May see some brightening of
the sky as lower clouds move out. However...500 mb long wave trough
remains west of the region and will begin to shift east this afternoon.
Weak surface reflection of an 500 mb short wave moving around the base of
the long wave trough across western NC/Virginia will rotate off the
coast this afternoon then push NE. As it moves across the
Atlantic...will pick up some moisture along with the associated lift
and bring them into the region. By sunset...noting the trough is
showing some good cyclonic spin as it approaches New England with
negative tilt troughing on the backside.

Have mentioned slight chance to low chance probability of precipitation for this afternoon
but not a lot of quantitative precipitation forecast associated with this so will see light rain
and/or snow showers. Colder air continues to filter across...with
850 mb temperatures dropping to -5c to -8c by the end of the day. Expect
highs only in the middle 30s to lower 40s.


Short term /6 PM this evening through 6 am Friday/...
negative tilt troughing shifts east during the night as long wave trough
approaches...keeping low level clouds and moisture across the
region. This trough will begin to set up across central and
eastern looks like decent shot for precipitation though
remains light across western areas. Depending upon where this
trough sets could bring in some more low level moisture
off the ocean. 00z and 06z NAM and GFS both showing this rather
well. May see just enough low level convergence to bring round of
light rain and/or snow especially across east MA/RI. Big question
will be how much moisture will work in. 06z GFS showing about 1/4
inch quantitative precipitation forecast so...depending upon how much energy gets going due to the
cold air working over the above normal coastal waters...could see
a period of decent snowfall mainly after midnight in the area of

This situation will be monitored very closely. More details on
this in the long term section.

Cold air continues to filter in...with lows dropping to the
20s...though holding between 30 and 35 on the Outer Cape and
Nantucket closer to the milder waters.


Long term /Friday through Wednesday/...
updated 430 am

*** accumulating snow possible late tonight into early Friday afternoon
across eastern Massachusetts from Cape Ann to Boston to Cape Cod ***


Interesting setup for possible norlun type trough along the eastern
Massachusetts coastline from Cape Ann southward through the city of Boston...
Plymouth County to Cape Cod tonight into early Friday afternoon. Robust
northern stream trough moves across the area tonight and Friday...
inducing an inverted trough from offshore low northwestward back
into eastern New England. The middle level trough will provide modest
synoptic scale lift and will combine with mesoscale frontal scale
forcing from the inverted trough to yield a narrow band of
modest/appreciable quantitative precipitation forecast. Both the NAM /00z and 06z/ and GFS have an
axis of quantitative precipitation forecast up to 0.25 inches from Cape Ann southward into Boston
and through coastal Plymouth County. Embedded in this area of quantitative precipitation forecast is
a bullseye of 0.50 to 0.60 inches over the Outer Cape.

Above average confidence in this quantitative precipitation forecast as low level streamline
analysis indicates strong convergence vicinity of the inverted
trough along with steep lapse rates and nearly saturated within and
above the area of ascent. Thus the forcing should have a strong
response as instability will be realized. In addition BUFKIT time
sections from the NAM and GFS both suggest at least some of the lift
will occur in the dendritic snow growth region which may provide a
bit of boost to snow to liquid ratios...say 12-1 to 15-1.

Ptype will be snow as temperatures aloft are cold fairly cold...-8c at 850
mb by 12z Friday. However across the Outer Cape /east of hya/...
despite a north-northeast wind backing to north-northwest this surface wind trajectory will
be crossing over water temperatures of 45-50. BUFKIT soundings indicate
this blyr warmth...which may partially erode via moderate to heavy
precipitation /dynamical and diabatic cooling/. However snow may struggle
to accumulate on paved surfaces here.

So with all of this in mind there is a moderate risk /30-60 percent/
of accumulating snow across eastern Massachusetts late tonight into early Friday
afternoon. Given model quantitative precipitation forecast early indications are for a moderate risk
of 2-4 inches of snow...except less on the Outer Cape given marginal
surface temperatures. Low risk /less than 30 percent/ of up to 6 inches or
so if inverted trough lingers long enough over eastern Massachusetts. However
these inverted/norlun troughs are very difficult to pinpoint
especially 30-42 hours in advance. For example if the inverted trough
shifts 25 miles or so farther east most of the snow will remain
offshore. Thus big bust potential. Forecast confidence not high
enough to issue a Winter Storm Watch and still too much uncertainty
for an advisory. Thus to heighten public awareness and convey the
uncertainty will issue a Special Weather Statement. It won't take
much for this event to have some impact given the number of Holiday
travelers/shoppers on the roads tonight and Friday morning. So stay
tuned to later forecasts and discussions.

This weekend...

Quiet and cold early Sat then increasing clouds with a chance of
light snow or flurries late in the day or at night as weak warm air advection
pattern sets up.

By Sunday warm front lifts north of the area with milder
temperatures...u40s to l50s.

Next week...

Mild to start the week with highs 50-55 Monday ahead of an
approaching cold front. Chance of showers late Monday/Monday night ahead
of polar front. Much colder Tuesday with 1040 mb high over Quebec. Next
chance of precipitation middle of next week as warm air advection pattern sets up. Could
be some mixed precipitation as cold airmass may be slow to


Aviation /12z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through tonight/...moderate confidence.

7 am update...

No major changes from 06z tafs. Ocean effect snow showers over
mass Bay and Cape Cod Bay will diminish by midday. Then light snow
showers or flurries will develop over much of southern New England
this afternoon and evening. VFR throughout this time. Earlier
discussion below.


Today...leftover MVFR-IFR ceilings/visibilities across central/north Massachusetts into S New Hampshire
should improve to VFR by middle morning. May see MVFR redevelop
across north CT/RI/se Massachusetts during the afternoon along the East Coast along
with more clouds moving NE into north CT/RI/S coastal Massachusetts.

Tonight...MVFR ceilings move across the region. Precipitation develops across
east Massachusetts into Rhode Island...possibly as far west as inverted
trough take shape near the coast. Areas of MVFR-IFR visibilities develop
in rain and/or snow. Visibilities should remain mainly VFR across central
and western areas...though patchy fog may form late at night.

Kbos terminal...high confidence today...lower confidence tonight in
exact timing as precipitation develops.

Kbdl terminal...high confidence.

Outlook...Friday through Monday...moderate confidence.

Updated 430 am

Friday...MVFR or IFR across eastern Massachusetts in snow with improving trend
during the afternoon and especially at night. VFR elsewhere.

Saturday-Sunday...VFR with potential MVFR late Sat/Sat night in a
period of light snow or flurries. Also potential for low level wind
shear Sunday.

Monday...cold front moves across New England with scattered showers.
Brief MVFR ceilings/visibilities in any showers. Continued southwest winds
ahead of the front with 35-40 knots at 2000 feet above ground level and potential
low level wind shear.


forecaster confidence levels...

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

Short term /through tonight/...moderate to high confidence.

Today...still a few spots with gale force wind gusts during the
pre dawn hours...but will diminish quickly as the low moves into
Nova Scotia and pressure gradient weakens. Expect light winds by late
morning but seas will remain high. Have converted gales to small
crafts except on Narragansett Bay where seas are lower though
could be a bit tough near the mouth of the Bay this morning.

Tonight...leftover small craft seas should subside...though may
linger on the outer waters. Visibilities will lower in areas of snow
and/or rain with some patchy fog.

Outlook...Friday through Monday...moderate confidence.

Updated 430 am

Friday...low visibility possible in snow across eastern Massachusetts waters. Snow
near shore with rain offshore. Visibility improving in the afternoon and
especially at night.

Saturday-Sunday...southwest winds gusting to 20 to 25 knots
Saturday and around 25 knots Sunday. Seas will be less than 5 feet
Saturday and then build to 5 to 6 feet on the exposed waters
Sunday. A Small Craft Advisory may be needed Sunday.

Monday...southwest winds gusting 20 to 25 knots until the cold front
moves through. Winds then shift to west-northwest behind the front
with similar gusts. Seas on the exposed waters will be 5 to 8 feet.
A Small Craft Advisory may be needed.


Box watches/warnings/advisories...
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory until 3 PM EST this afternoon for anz231-
Small Craft Advisory for hazardous seas until noon EST today
for anz233-234.
Small Craft Advisory until 10 am EST this morning for anz230.
Small Craft Advisory until 9 PM EST this evening for anz235-
Small Craft Advisory until midnight EST tonight for anz250-251.
Small Craft Advisory until 6 am EST Friday for anz254>256.


near term...nocera/evt
short term...evt
long term...nocera

National Weather Service Glossary of Abbreviations