Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
410 PM EST Tuesday Nov 25 2014

Synopsis...
a strong noreaster will move close to the 40n/70w benchmark late
tonight into tomorrow. This winter storm will impact the region
Wednesday into Wednesday night with snow...wind and coastal issues.
Conditions will improve for Thanksgiving day. High pressure
builds over New England Friday and shifts offshore over the
weekend. A cold front swings through New England late Sunday night
or Monday.

&&

Near term /until 4 am Wednesday morning/...

400 PM update...

Dry air has moved across the region this afternoon. Otherwise dry
weather will mostly prevail. A few radar echoes are skirting close
to the islands...but believe that the middle-levels are dry enough that
virga may fall out. Otherwise high temperatures in the low 60s will begin
to drop this evening. Clouds cover continues to be persisent and
expect this to continue through the overnight.

Tonight...

Cold front will move offshore tonight as confluent flow aloft
approaches the region. This is due to the development of the low
pressure system that will begin to take shape the Carolina coast.
Expect a good portion of the region to remain dry for the first half
of the night...until precipitation will Blossom from south to north near
09z.

Not a lot of cool air aloft so overnight temperatures will remain mild as
cloud cover will limit diurnal radiation. Low overnight will drop in
the middle 30s to low 40s. However once precipitation moves into the
region...and depending on intensity temperatures may wet bulb and drop at
or below freezing.

Have noticed that this system is speeding up so the morning commute
may be impact for CT and Rhode Island tomorrow morning.

&&

Short term /4 am Wednesday morning through Wednesday night/...

Tomorrow into tomorrow night...

*** early season winter storm to impact much of the region Wednesday/Wednesday
night ***

Impacts...

* heavy snow across much of the interior impacting Wednesday travel
* gusty winds across coastal Plymouth Colorado and the islands
* strong gale force winds across the waters
* minor beach erosion and splashover for the Thursday am high tide

Synoptic overview...

Upper level trough will continue to dig into the Southern Plains.
This has allowed a good plume of moisture to move up the eastern
Seaboard...tapping in tropical moisture from both the Gulf as well
as the Bahamas. From this shortwave...a weak wave will develop and
generate a surface low off the Carolina coast. This low will
strengthen as it moves up the coast and develop an early season
Nor'easter for southern New England. This is a very quick system due
to the lack of blocking upstream. Surface low is still tracking near
the 40n/70w benchmark...with 850 mb circulation moving across Cape Cod
and 700 mb low developing and tracking over bdl-mht line. This system
will quickly move out of the region by Wednesday night and precipitation should
be winding down by day break Thursday.

Guidance...

Good general agreement among the models for the 12z suite. Biggest
thing is that the system is moving a few hours faster then
originally thought. It should be noted that while the models are in
good agreement with respect to the larger-scale features including
the surface low and upper shortwave trough...differences in the
details remain. Appears that there is a 700 mb warm-air
intrusion/dry slot impending up Narragansett Bay and up into Norfolk
Colorado. Noted that the NAM/CMC where the colder solutions compared to
the GFS/ec/sref and UKMET which were slightly warmer in their temperatures
aloft. Because of this good clustering trended the forecast with the
GFS/ec/wpc in terms of thickness and quantitative precipitation forecast.

Ptype and snowfall...

Trickiest part of the forecast was determining snowfall accums and p-
types. Biggest reason is the 700 mb warm layer moving into the
region during the late afternoon hours. Depending on how far
northward this warm layer moves into southern New England could play
issues on the total snow accums. Right now appears that this warm
layer will move into well into Rhode Island and Norfolk Colorado and points south
and east keeping Boston right on the fringe of sub-zero thermal
profile. And with 925 mb temperatures below 0c west of the canal believe
sleet/wintry mix will be introduced. However the 18z NAM came in a
lot warmer at the 700mb level...if this trend continues through the
remainder of the guidance this snow totals will be to high. Lastly a
coastal front will also set up somewhere along the I-95
corridor...so will need to monitor where it decides to develop.

At the onset...because of boundary layer temperature issues believe rain
will start over a good portion of the region with rain/snow mix
across higher terrain...near day break. As precipitation overspreads the
region and thermal fields aloft tighten...expect a good banding
signature to develop...near bdl to orh to bed at around 15/18z.
Precipitation intensity associated with this band should be sufficient for
a changeover to all snow along with snow rates of 1-2" per hour.
During the late morning hours believe that a coastal front will set
up and keep most of the coastal region in rain rather than snow.
However as the surface winds back to a north-northeast direction as well expect
more low level cooling which will keep precipitation to all snow for a
majority of the region...expect along the 95 corridor where a wintry
mix of snow/sleet/rain is possible. The frontal band will also
switch to a more north-northeast association as the thermal fields get backed
from the middle-level warm air moving into the region. This will push
the heavier snowfall over the Berks and southern New Hampshire. Finally as the
storm exits out...a more northerly flow will allow for all precipitation to
switch to snow and allow for a quick couple of inches of
accumulations.

Overall have expanded the current winter weather warnings to all of
southern New Hampshire...and most of mass minus the I-95 corridor and northern
CT. Decided to go with advisory for portions of Rhode Island up to coastal
Essex including the cities of Providence and Boston. Although
amounts are under advisory for the metropolitan regions...because of the
high impact this event can have on the pre-Thanksgiving travel felt
the advisory was warranted. Also with a slight shift in the guidance
or if temperatures drop just a degree...accumulations could add up. For now
believe that an inch or two of snow is possible but another couple
of inches of sleet cannot be ruled out.

Winds...

A strong low level northeast jet will move across the South Coast of
Rhode Island and Massachusetts including Cape Cod and the islands Wednesday afternoon into the
overnight hours. Models projecting 45-50 knots jet at 925 mb. This
coupled with strong pressure falls and relatively warm ocean waters
providing a well mixed blyr will yield a moderate risk of 50 miles per hour
wind gusts from south coastal Plymouth County/Cape Cod and the
islands. Timing of strongest winds appear 4 PM Wednesday until 11 am Thursday.
Because the jet dynamic does not seem as strong as previous runs
have dropped the high wind watch to a strong Wind Advisory for this
area. If the coastal system does deepen more then forecasted...and
the low level jet does strengthen then a High Wind Warning may be needed.

Coastal flooding...

Forecast tides of 10 feet are expected at Boston shortly after
midnight Thursday. This system is very quick so the potential for a
long duration easterly fetch to build the seas is low. Even the
duration of onshore winds are short plus with pressures beginning to
rise toward high tide not expecting much if any coastal flood issues
including erosion. Worse cast...could see a surge near 1.5 feet
which may cause some splashover and minor beach erosion for typical
prone areas.

&&

Long term /Thursday through Tuesday/...
big picture...
southern stream trough moves up the coast across New England
Thursday. Northern stream shortwave then moves across the region
Friday. The flow then transitions to zonal over the weekend.

The long range models show good agreement on 500 mb heights through
Monday and then small divergence Tuesday. The GFS is faster/farther
east with a clipper system crossing Canada over the weekend. But the
models come more in line with the trailing cold front in New England
Sunday night. This maintains at least moderate confidence in the
forecast through the weekend with diminishing confidence by
next Tuesday/day seven.

The dailies...

Thanksgiving day... southern stream upper trough trails the
Wednesday storm...and moves over southern New England during the
day. Model cross sections from the GFS and European model (ecmwf) show a deep supply
of high-relative humidity air and some weak lift as the trough moves across the
region. We will expect mostly clouds as well as a chance of precipitation.
Temperature profiles suggest scattered snow showers inland and
scattered rain showers in Rhode Island/southeast mass. Temperature profiles
support maximum temperatures in the 30s...except lower 40s cape and islands.

We stayed close to guidance Thursday night with min temperatures in the
20s except lower 30s cape and islands. Any wet surfaces from the
daytime will refreeze in most locations resulting in tricky Post-
meal travel on any untreated surfaces.

Friday... northern stream shortwave races across New England in the
morning with lingering airmass moisture but weak lift if any. This
could hold the clouds in a little longer Friday morning. But it
moves out quickly and surface high pressure builds in. So the trend
will be for clearing skies. Mixing depth with the sunshine is
expected to be rather shallow...around 950 mb or maybe a little
higher. This will support maximum temperatures in the 30s and winds gusting to
15-20 knots.

High pressure moves overhead Friday night. This will allow temperatures to
initially fall with mostly clear skies and light winds. But moisture
fields suggest some middle and high clouds overnight...and possibly low
clouds moving up from the ocean toward Cape Cod and the islands.
Forecast temperatures are close to guidance with 14-23 except 24-30
along the South Coast.

Saturday-Sunday...continued fair weather over the weekend as a
warm front sweeps north across New England and high pressure
builds offshore. The combination suggests a warming trend over the
weekend with a 12c increased in temperatures aloft between Saturday and
Sunday. The warm front will bring variable sky cover...as noted by
relative humidity fields aloft in the models. Southwest winds at 925 mb increase
to 30 knots Saturday afternoon and 30-40 knots Saturday night and
Sunday. Expect some gustiness to our surface winds each day. But
low-level mixing will be less than ideal each day with better
mixing Saturday and less Sunday.

Monday-Tuesday...shortwave in the zonal flow will push a cold front
through New England either late Sunday night or Monday. Timing of
the shortwave in a zonal flow this far out is in question but that/S
our best shot at this time. Limited moisture aloft together with
favorable jet dynamics from a 125 knot jet...so we have chance probability of precipitation
for rain/snow showers. High pressure then builds dry weather for
the rest of Monday and Tuesday.

&&

Aviation /21z Tuesday through Sunday/...
forecaster confidence levels...

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

Short term /through tomorrow night/...moderate confidence.

Today...VFR. SW wind gusts up to 20-25 knots through midday along S
coast.

Tonight...expect VFR conditions up to 09z then conditions will
begin to deteriorate from VFR to IFR.

Tomorrow and tomorrow night...coastal storm anticipated.
Significant accumulation snows over interior terminals immediately
west of the I-95 corridor. +Sn possible especially bdl-orh and
possibly for a time at bos around 18z Thursday. Strong NE winds up to
45 knots at hya and ack around 00z Thursday.

Kbos terminal...moderate confidence in taf.

Kbdl terminal...moderate confidence in taf.

Outlook...Thursday through Sunday...moderate confidence

Thursday...MVFR with scattered rain/snow showers. Diminishing
northwest winds.

Friday...VFR with northwest winds gusting 15-20 knots.

Saturday-Sunday...increasing sky cover but mainly VFR. Low potential
for MVFR ceilings moving north off the ocean into Cape Cod and islands.
Increasing winds at 2000 feet...reaching southwest 35-40
knots...with lighter southwest winds at surface.

&&

Marine...
forecaster confidence levels...

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

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

Tonight...the lull before the storm. SW winds diminish as they
veer during the night. Seas remain near 5 feet.

Wednesday into Wednesday night...

Nor'easter anticipated to pass around the 40n/70w benchmark 00z Thursday.
High probability of NE gales with a low risk of storm force gusts 21z
Wednesday to 06z Thursday. Heavy rain lending to reduced visibilities at
times. Gale Warning is out with high end gusts anticipate. Cant
rule out a few storm force gusts.

Outlook...Thursday through Sunday...moderate confidence.

Thursday... northwest winds gusting to 25 knots in the morning but
diminishing. Seas start the day 5 to 11 feet on the exposed waters
and subside to 5 to 6 feet by the end of the day. Small Craft
Advisory will be needed on most waters.

Friday... lingering 5 foot seas on the outer waters. Northwest winds
will gust 20 to 25 knots. A Small Craft Advisory may be needed on
some of the waters.

Saturday-Sunday... southwest winds gusting to 25 knots and seas
building to 5 to 6 feet on the exposed waters. A Small Craft
Advisory may be needed.

&&

Box watches/warnings/advisories...
CT...Winter Storm Warning from 7 am Wednesday to 7 am EST Thursday
for ctz002>004.
Massachusetts...Winter Storm Warning from 7 am Wednesday to 7 am EST Thursday
for maz002>006-008>012-026.
Wind Advisory from 2 PM Wednesday to 3 am EST Thursday for
maz019-022>024.
Winter Weather Advisory from 7 am Wednesday to 7 am EST
Thursday for maz007-013>016.
New Hampshire...Winter Storm Warning from 7 am Wednesday to 7 am EST Thursday
for nhz011-012-015.
Rhode Island...Wind Advisory from 2 PM Wednesday to 3 am EST Thursday for
riz008.
Winter Weather Advisory from 7 am Wednesday to 7 am EST
Thursday for riz001>003.
Marine...Gale Warning from 3 PM Wednesday to 5 am EST Thursday for
anz230>237.
Small Craft Advisory until 7 PM EST this evening for anz235-
237-255-256.
Gale Warning from 3 PM Wednesday to 7 am EST Thursday for
anz250-251-254>256.

&&

$$
Synopsis...wtb/dunten
near term...dunten
short term...dunten
long term...wtb
aviation...wtb/dunten
marine...wtb/dunten

National Weather Service Glossary of Abbreviations