Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
727 am EST sun Feb 1 2015

Synopsis...
a major winter storm will affect the region very later tonight
into Monday evening with heavy snow for many locations. Bitterly
cold wind chills follow later Monday night into Tuesday morning.
Moderating temperatures Wednesday will be accompanied by some light snow
then very cold conditions will return toward the end of the week.
A coastal storm will pass well south of New England late in the
week but will have to be monitored.

&&

Near term /until 6 PM this evening/...
715 am update...
noting winds slackening across the interior over the last few
hours as they have backed to W-SW. Looks like the Arctic front
lying across north Vermont/north central New Hampshire into central ME at 12z as winds north
of it have shifted to northwest. Also noting a thin band of light snow
showers that has developed from near kalb-kpsf-kbdl moving southeast
ahead of the cold front. Area of low and middle level clouds has also
moved into west Massachusetts/west CT as well with the snow showers...but leading
edge of main area of clouds ahead of the approaching low across east
PA/central New Jersey.

Where winds have dropped to calm noting temperatures below zero...as low
as -9 at kore and -4 at kowd...though milder where the clouds have
moved in as well as light SW wind flow mainly across the CT
valley.

Have updated near term forecast to bring conditions current.

Previous discussion...
Arctic front will drop south across southern New England this afternoon and
reaching the South Coast toward evening. Models indicate some low
level moisture today which may lead to some lower clouds...
otherwise sunshine for most areas will give way to increasing
middle/high clouds during the day. Maximum temperatures will reach 25 to 30
degrees but fall into the teens toward evening in far north zones as
Arctic air drains south. Gusty winds on the cape and islands will
diminish...otherwise generally light winds expected today.

&&

Short term /6 PM this evening through 6 PM Monday/...
highlights...

* major winter storm very late tonight into Monday evening
* heavy snowfall expected for much of the region
* biggest uncertainty is snowfall amts across Rhode Island and southeast Massachusetts due to
ptype issues

Overview...

Models are in good agreement on a progressive but rather potent
middle level trough which moves east into southern New England Monday and becomes
negatively tilted. The surface low tracks east from the Ohio Valley and
appears to jump/redevelop along the middle Atlantic coast...tracking near
or just S of Long Island to ack during Monday. With Arctic high
in place to the north...plenty of low level cold air in place but
warming aloft will make for a tricky snowfall forecast near the S
coast. Good model agreement on quantitative precipitation forecast with 0.75-1.00" with higher
amounts across southeast new eng...but there are some differences with
the thermal profile. NAM is the coldest but likely a result of it
being faster with the surface low. GFS has trended a bit colder while
European model (ecmwf) a bit warmer and only minor differences exist aloft. Given
strong SW flow aloft all models show warming in the 850-700 mb
layer moving north along the southern New England coast during Monday. The uncertainty
for the snowfall forecast is determining how far north the mix line
gets.

System remains an open wave at 700 mb as it crosses central new
eng late Monday so not looking at typical banding signal associated with
strong middle level frontogenesis. This will be a classic strong
overrunning event as milder air and moisture overruns very strong
low level baroclinic zone across southern New England with Arctic to the north.

1) timing and ptype

Snow develops in the CT valley by midnight then quickly
overspreads the rest of southern New England through daybreak. The snow will
quickly become heavy late tonight into Monday morning and will
have a significant impact on the morning commute. Good snow
growth around 12z and strong middle level Omega will result in a
good front end thump with snowfall rates of 1-2 inches per hour
across the region for several hours in the morning.

The uncertainty increases closer to the S coast middle/late morning
into the afternoon as warmer air aloft surges northward. We
blended the models which suggest a mix or change to sleet briefly
getting to Massachusetts/north Rhode Island border by midday before collapsing southeast during
the afternoon. Further S...deeper warm layer will likely result in
a period of freezing rain along the South Coast and possibly some
rain as the boundary layer warms. A change to plain rain is most
likely over the islands. Any rain or mixed precipitation will change back
to snow middle/late afternoon but not until evening for the
Outer/Cape islands.

This is still a very tricky ptype forecast due to the strong
low/middle level temperature gradient so ptype will be very sensitive to
just a minor shift in the surface and 850 mb low track.

Snowfall in the afternoon will not be as heavy as the morning as
best isentropic lift weakens and snow growth not quite as
favorable...but still some decent snowfall especially in the
north.

2) snowfall accumulations and headlines

We have high confidence in heavy snowfall for much of the region
with the heaviest snow likely along and north of the Mass Pike
where definitely no mixing issues. Expect snow accumulate around a
foot for this region...with lesser amounts around 6 to 10 inches
further S across Rhode Island and southeast Massachusetts due to mixing issues. We have enough
confidence for up to 6 inches near the S coast to upgrade to a
warning. In addition the potential exists for up to a tenth of an
inch of ice accretion near the coast. Amounts may fall short of 6
inches on Cape Cod...but we included them in the warning due to
combination of snow and some ice.

For the islands...we will go with a Winter Weather Advisory for
accumulate 2 to 5 inches...least on Nantucket.

3) strong winds and blowing and drifting snow

Northeast wind gusts of 30 to 40 miles per hour expected along the eastern
mass coast Monday as storm intensifies and pressure gradient
increases. This will result in poor visibility along with blowing
and drifting snow. Its possible that a few locations along the
coast could see near blizzard conditions for a very short
time...but storm doesn/T seem to warrant those type of headlines.

4) temperatures

A very strong coastal front will set up during Monday with temperatures
in the teens north of I-84 to I-90 corridor and west of I-495
with even some single numbers far north/northwest Massachusetts...while temperatures will reach
the lower to middle 30s southeast Massachusetts. A flash freeze is likely across southeast
coastal Massachusetts and cape/islands in the evening as the coastal front
moves through and temperatures plummet.

&&

Long term /Monday night through Saturday/...
highlights...

* cold temperatures will continue through most of this week into
the weekend
* may see a brief break of the cold temperatures around middle week as high
pressure moves S of the region
* clipper low pressure will pass across northern New England Wednesday
night/Thursday with a cold front late Thursday

Overview...

00z model suite and ensembles continue with similar upper level
pattern across North America that has been in place for over a
week at least. Noting 500 mb ridge building along the Pacific coast
while persistent long wave troughing remains across the east with
vortex centered around Hudson Bay early this week. Does look like
this will tend to flatten out around middle week before another short
wave works out of central Canada along with what appears to be an
clipper type surface low. As this system works toward New England
around Wednesday night or Thursday...do note some model spread in digging the
associated short wave. Also...as the clipper heads toward the
Maritimes late this week it will deepen...which may slow the
passing cold front down offshore as an ocean low appears to form
down the coast. Some agreement with this feature on the 00z
Canadian ggem and European model (ecmwf)...but it does look to remain offshore at
this point.

Used a blend of available guidance for this morning/S forecast...
which showed good continuity with the previous forecast.

Details...

Monday night...leftover snow will end across the CT valley early
Monday night...then back edge will shift east overnight. Some light
precipitation may linger until daybreak across east coastal areas. Very cold
and blustery conditions expected. Will likely need wind chill
headlines across the interior.

Overnight lows may not be quite as cold as previously forecast
due to lingering clouds...but will still see some locations drop
below zero after midnight.

Tuesday-Tuesday night...gusty northwest winds will linger through around
midday or so across the region behind the strong low pressure over the
Gulf of St Lawrence. NE-SW oriented high pressure will build east during
the day with dry conditions. Highs will only be in the teens for
most areas...except 20-25 along the immediate S coast...Cape Cod
and the islands.

High pressure ridge crests across the region Tuesday night as light winds
actually back to SW. With a new and deep snowpack though...lows
will still fall back to the single digits except the teens along
the S coast. May start to see some clouds increase after midnight
across western areas.

Wednesday-Thursday...clipper low moves quickly southeast toward the
region. With light SW flow in place...temperatures may actually rebound
to the upper 20s to middle 30s on Wednesday. However...leading edge of
precipitation may bring scattered snow showers mainly across north and west Massachusetts during
the day into Wednesday night as the low passes to the north. Current timing
of trailing cold front suggests passage during Thursday with best shot
for light snow showers. Not much quantitative precipitation forecast with this feature at this
Point. May see some light snow accumulations during Thursday as another
weak short wave moves along the front. North winds may become blustery
late Thursday/Thursday night along the coast as colder air returns.

Friday...looks to be a generally dry but cold day. Might see some
leftover snow showers lingering across Cape Cod and the islands.

&&

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

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

Short term.../through Monday/...

Today...high confidence. Mainly VFR. Noting MVFR ceilings across middle
and lower CT valley with patchy -shsn at 11z...which should
improve by late morning. May see patchy MVFR ceilings across north CT/Rhode Island
during the afternoon.

Tonight into Monday...high confidence in trends but lower
confidence in timing.

VFR this evening deteriorates to IFR-LIFR ceilings/visibilities in moderate
to heavy snow after midnight to daybreak from west to east.
IFR/LIFR in moderate/heavy snow continues through the day Monday. Ptype
may transition to a period of sleet/freezing rain around midday
from pvd-tan-pym south and changing to rain over the Outer
Cape/islands. NE wind gusts to 30-35 knots developing across east
coastal Massachusetts Monday...with 25 knots across Rhode Island and remainder of east Massachusetts.

Kbos taf...high confidence. Lower confidence in exact timing of
IFR conditions in snow.

Kbdl taf...high confidence. Lower confidence in exact timing of
IFR conditions in snow.

Outlook /Monday night through Thursday/...

Monday night...moderate confidence in trends...lower in timing.
Expect MVFR-IFR conditions early...then should improve from west-east
during the night as precipitation exits. Lower conditions may linger
across along East Coast through most of the night. Expect north-northwest winds
gusting to 25-35 knots...highest along the coast.

Tuesday...moderate confidence. Mainly VFR on Tuesday. Northwest wind
gusts to 25-30 knots early along the coast.

Wednesday...moderate confidence. Could see some MVFR ceilings in scattered
snow showers across north central and west Massachusetts.

Thursday...moderate confidence. Mainly VFR. May see local MVFR
ceilings/visibilities in scattered snow showers. Northwest wind gusts to around 25 knots
developing along the coast late in the day.

&&

Marine...
forecaster confidence levels...

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

Short term /through Monday/...high confidence.

Today...winds and seas gradually diminish through the day but
expect Small Craft Advisory waves to remain through the period. Light freezing
spray will persist.

Tonight into Monday...moderate to high confidence.

Winds/seas quickly ramp up during Monday as low pressure
intensifies near the South Coast. Gale watches continue. The main
focus for gusts to 40 knots Monday will be across eastern Massachusetts coastal
waters...especially north of Cape Cod...with lighter winds over
the south coastal water. Seas build to 15+ feet across our eastern
waters by late Monday afternoon/evening.

Outlook /Monday night through Thursday/...moderate to high
confidence.

Monday night...expect gale force northwest wind gusts. Heavy freezing
spray watch continues. Seas build up to 9-14 feet on the outer
waters.

Tuesday...northwest gales likely along with freezing spray...possibly
moderate to heavy. Mainly dry conditions with good visibility except
far offshore where snow showers will be common.

Wednesday...winds ease and become southwest as clipper low enters
the Great Lakes and New York state. Not as cold so freezing spray
unlikely. Visibilities may briefly lower in a period of light snow.

Thursday...cold front passes with strong small craft conditions.
Chance of light to locally moderate freezing spray late.

&&

Tides/coastal flooding...
*** low risk for minor coastal flooding Monday night eastern Massachusetts
coastline ***

The first tide cycle Monday morning should not pose any risks as
winds will not be strong enough for a long enough period of time
to produce the wave action or surge necessary for coastal
flooding. A surge of 1 foot is possible but seas will be well
under 10 feet.

As for the Monday evening high tide cycle...astro tides are about
1 feet lower but waves will be building to 15 to 20 feet. We expect
about a 1.5 to 2.5 feet storm surge but this may not overlap the
evening high tide and may occur closer to low tide with surge
values decreasing toward the time of high tide as winds turn north.
If this timing plays out...the probability for any coastal
flooding will be low. However...we are talking about only 6 hours
and if the maximum surge is delayed and overlaps the high tide...there
will be a higher risk of minor coastal flooding with a low risk of
isolated moderate flooding...mainly from Cape Cod to Cape Ann with
perhaps the highest risk from Hull/Cohasset/Scituate/
Marshfield/Plymouth to Sandwich Harbor. Again...this will depend
om the timing of the maximum surge occurring close to high tide which
does not appear likely at this time.

Winds...seas and storm surge will be considerably less than the
previous coastal storm but there could still be some impact
especially for those communities such as Marshfield Massachusetts that were
hit hard earlier this week where coastal structures such as
seawalls were damaged.

&&

Climate...
with extremely cold air moving in behind the departing low Monday
night...temperatures may fall back close to or surpassing record
levels early Tuesday morning.

Here are the record lows for Tuesday 2/3...

Boston -5 set in 1881
Providence -6 set in 1961
Worcester -7 set in 1931, 1955 and 1971
Windsor Locks -3 set in 1955
Blue Hill observation -7 set in 1955

&&

Box watches/warnings/advisories...
CT...Winter Storm Warning from 9 PM this evening to 1 am EST
Tuesday for ctz002>004.
Massachusetts...Winter Weather Advisory from 9 PM this evening to 1 am EST
Tuesday for maz023-024.
Winter Storm Warning from 9 PM this evening to 1 am EST
Tuesday for maz002>022-026.
Rhode Island...Winter Weather Advisory from 9 PM this evening to 1 am EST
Tuesday for riz008.
Winter Storm Warning from 9 PM this evening to 1 am EST
Tuesday for riz001>007.
Marine...gale watch from Monday morning through Tuesday morning for
anz231>235-237.
Small Craft Advisory until noon EST today for anz231-232-251.
Heavy freezing spray watch from Monday evening through Tuesday
morning for anz231>235-237-250-251-254>256.
Freezing spray advisory until 8 am EST this morning for anz231-
232-250-251-254-255.
Small Craft Advisory until 7 am EST this morning for
anz233>235-237.
Gale watch from Monday morning through late Monday night for
anz230-236.
Gale watch from late tonight through Tuesday morning for
anz250-251-254>256.
Small Craft Advisory until 6 PM EST this evening for anz250-
254-255.
Small Craft Advisory until 10 am EST this morning for anz256.

&&

$$
Synopsis...kjc/evt
near term...kjc/evt
short term...kjc
long term...evt
aviation...kjc/evt
marine...kjc/evt
tides/coastal flooding...kjc
climate...staff

National Weather Service Glossary of Abbreviations