Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Taunton Massachusetts
713 am EST Friday Dec 13 2013
mainly dry but frigid weather continues across southern New England
today and tonight. A coastal storm will likely bring heavy snow to
much of the region beginning later Saturday into early Sunday. Cold
weather continues into early next week along with a chance of snow
showers Monday night through Tuesday night.
Near term /until 6 PM this evening/...
7 am update...
Not much change from previous forecast. Cold temperatures in place this
morning across the entire region with a second Arctic front poised
across northern New York state into northern New England...which will
move south across our area this afternoon and early evening. Not
much warming ahead of this front today some middle level cloudiness
streaming across the region limiting sunshine. Front will be
mainly dry with just a wind shift and a reinforcing surge of
Arctic air. However all model guidance especially the mesoscale model
hrrr and rap13 suggest some lake effect moisture may get entrained
along and ahead of the frontal boundary yielding some snow
squalls. Low level lapse rates and moisture profiles suggest
greatest risk for any snow squalls would be north of the Massachusetts Pike
into southwest New Hampshire and west of i495 in northeast Massachusetts.
West-northwest winds will increase later this morning as blyr depth
increases. Earlier discussion below.
Light snow showers will push offshore out of Nantucket in the next
hour or two. Snow showers were reported in Nantucket...Marthas
Vineyard earlier this morning. Ocean effect clouds with the Arctic
airmass in place will keep going through middle morning...then should
push offshore as somewhat milder air moves in ahead of next Arctic
Some gusty west-northwest winds on the Outer Cape through middle morning with
low level jet moving across /h925 winds to 35 knots and 850 mb to 40
kt/. Excellent mixing in place allowing for this jet to mix down
through especially over the relatively milder waters. Also expect
gusts up to 25 to 30 knots inland as well during the morning into
early afternoon hours...highest across the higher terrain.
Arctic front approaching the northern New England border early
this morning will shift S across the region this afternoon...
bringing another surge of frigid air to the region. Will see some
clouds and possibly some snow showers along this front...which may
also bring some moisture from Lake Ontario along it similar to
what happened with the last front on Wednesday. Short range models
do indicate moisture drying out as the front moves through so have
not mentioned any organized areas at this point.
Temperatures will remain cold with highs in the 20s to lower 30s.
Short term /6 PM this evening through 6 PM Saturday/...
tonight...large high pressure builds crosses central Canada into
Quebec through the night. 850 mb temperatures drop to -11c to -17c by early
Saturday morning from S-N...so temperatures will plummet to zero to 5
above across S New Hampshire ranging to 15-20 along the immediate S coast.
The high will shift into north New Hampshire/west ME late at night...orienting northwest to
southeast into the Gulf of Maine. The Arctic front that passes through
today will stall over S New Jersey while low pressure develops across the
Southern Plains to central Mississippi Valley. Upper level
troughing develops with this low...allowing 500 mb winds to back to
west-SW overnight. This will send middle and high level clouds toward
the region mainly after midnight. Models are trying to bring some
snow into southern areas toward daybreak. Have kept slight chance
to low chance probability of precipitation going for now.
Saturday...low pressure will continue to form as it crosses the middle
Appalachians during the day...then redeveloping off the middle Atlantic
coast. The high over northern New England slowly shifts NE into
the Maritimes. This is a set up for moisture to push into the
region on east winds. With the very cold dome of air over the
region...expect snow to break out...with the best shot during the
afternoon as the lower levels moisten up.
Will see some accumulations by Saturday evening...but looks like
it should be around an inch or so across eastern and central areas
possibly up to 3 inches over the east slopes of the Berkshires by
evening as better influx of moisture works in with the system.
Daytime highs will range from the upper teens over S New Hampshire to the
lower 30s along the S coast...possibly reaching the middle 30s by
late in the day as east winds freshen across the islands.
Long term /Saturday night through Thursday/...
*** heavy snow appearing more likely Sat night into early sun ***
00z models including the gefs/European model (ecmwf) and 12z/12 ecens continue to
remain in good agreement on a moist southern stream short wave
ejecting northeast toward the region Sat...with its surface
reflection /a coastal low/ passing in the vicinity of Nantucket Sun
morning. Model quantitative precipitation forecast is also in very good agreement /especially at
this time range/ with a moderate to high probability of up to 1 inch
quantitative precipitation forecast across CT/Rhode Island and central-eastern Massachusetts into southeast New Hampshire. The
uncertainty lies in the track of the 850 mb low and resultant thermal
fields. The NAM is on the southern most edge of the guidance
envelope with its 850 mb circulation tracking along the southern New
England coast. This cuts off the northward progression of the middle
level tongue of warm air...yielding a colder/snowier solution with
heavy snow all the way to the South Coast. Meanwhile the 00z GFS is
on the other extreme...warmest of the 00z guidance with mlvl warm tongue
into Massachusetts/New Hampshire border 12z sun. This would support sleet mixing with snow
to the Massachusetts/New Hampshire border. The 00z European model (ecmwf) offers a nice compromise
splitting the difference...although slightly closer to the GFS than
the NAM. However the 21z sref provides some support to the colder
NAM. Perhaps the higher resolution of the NAM is on to
something...as the low level baroclinic zone may settle farther
southward given polar airmass over the region along with Arctic high
remaining over the Maritimes.
Given the time range here and uncertainty...still 48-60hrs away
think a model blend is still our friend. So we will follow a model
consensus with a solution closer to the 00z European model (ecmwf) and 21z sref. This
supports a moderate to high risk of 6+ inches of snowfall before any
possible changeover to northern CT/northern Rhode Island and extending
northeast into the greater Boston area. This also includes western-
central Massachusetts into southern New Hampshire. Thus will issue a Winter Storm Watch
for this area. Farther south across southern half of Rhode Island into
southeast Massachusetts...there is a low risk of 6+ inches before a changeover
but confidence is not high enough at this time range to hoist a
Winter Storm Watch.
Timing...good model agreement that snow will overspread the region
Sat afternoon from southwest to northeast. Also very good model agreement
that the steady/heavier snow will hold off until Sat night into
early Sun morning.
Quantitative precipitation forecast/ptype/snowfall potential...
Quantitative precipitation forecast...00z GFS/NAM and European model (ecmwf) in good agreement that storm total quantitative precipitation forecast
up to 1.25 inches is possible across CT/RI/central-eastern Massachusetts into
southeast New Hampshire. 00z gefs as well with probs of 60-70% for quantitative precipitation forecast at least
Ptype...still too early to nail down the exact transition to mixed
precipitation. However a model blend brings a wintry mix as far north and
briefly into northern CT-RI-southern Worcester County and into the
greater Boston area between 09z-12z. A coastal front may develop
across eastern Massachusetts into Rhode Island as Arctic high pressure over southeast
Quebec into New Brunswick wedges cold air down the eastern New
England coastline. This combined with relatively early season warm
ocean temperatures will enhance the low level baroclinic zone along the
coast. This in addition to pressure fall center tracking along or near
the South Coast of New England will facilitate a coastal front
with more of a northerly surface wind component across northeast
Massachusetts into northern Rhode Island. This will help delay low level warming across
Snowfall potential...as mentioned above its still too early to nail
down specifics regarding the exact location of the transition line
from snow/sleet/freezing rain to all rain. However a model blend
supports all snow across northwest Massachusetts into southern New Hampshire. Quantitative precipitation forecast
approaches an inch over this area...so 8-12 inches is possible.
Farther south across northern CT-RI-greater Boston area a brief
change to sleet and freezing rain is possible late Sat night into
Sun morning. However the bulk of the quantitative precipitation forecast has fallen by that time
yielding a moderate risk of 5-9 inches across this area. As a result
will issue a Winter Storm Watch to highlight this potential as well
as a period of sleet and freezing rain possible. Farther south
across southeast Massachusetts into southern Rhode Island the probability increases for
snow to change to a wintry mix sooner...yielding preliminary snow
totals of 2-5 inches.
The theme is for below normal temperatures to continue as long wave trough
and its associated below normal heights persist over the northeast.
Although not as bitterly cold as the current Arctic airmass over the
region. Next short wave rotates through the long wave trough from
late Monday night into Tuesday night. Weak surface reflection moves across
southern New England during this time before bombing out over the
Maritimes. Thus looks like a snow shower event for southern New
England Monday night through Tuesday night.
Aviation /12z Friday through Tuesday/...
forecaster confidence levels...
Low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.
Near term /through Saturday/...high confidence through
tonight...then moderate confidence Saturday.
7 am update...
Today...VFR. Low probability of MVFR ceilings across the Outer Cape and
Nantucket with ceilings bkn035-045. Also north of the Massachusetts Pike and
west of i495 in northeast Massachusetts into southwest New Hampshire where there is a
low risk of scattered snow squalls along and ahead of Arctic
Tonight...VFR. Will see increasing clouds from the SW through the
Saturday...VFR to start...then conditions lowering to MVFR-IFR
ceilings/visibilities in developing light snow during Sat afternoon from light snow shower
Kbos taf...high confidence in taf.
Kbdl taf...high confidence in taf.
Outlook...Saturday night through Tuesday...
Moderate to high confidence.
Sat night/sun...IFR-LIFR likely in periods of heavy snow with a
wintry mix of sleet and freezing rain coming onshore to South Coast
late Sat night and may lift northward to bdl-orh-bos by 12z sun.
Preliminary snow accumulations 5-10 inches bdl-orh-mht...4-8in pvd-
bos and 2-4in fmh/hya/ack. Considerable blowing and drifting
snow. Snow tapers off Sun afternoon with improving conditions.
Monday and Tuesday...low to moderate risk of MVFR in snow showers.
forecaster confidence levels...
Low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.
Near term /through Saturday/...high confidence.
Today...noting west-northwest wind gusts up to 35 knots on the southern waters
so have issued gales through the day there. Elsewhere...have
continued small craft advisories. Seas up to 5-9 feet on the
southern outer waters. Winds and seas will begin to diminish this
afternoon from W-E.
Tonight...leftover gales and small crafts will end as winds and
seas diminish by around midnight. Winds shift to north after midnight.
Winds and seas may remain at or above small craft criteria through
most of the night on the eastern outer waters.
Saturday...NE winds and seas should remain below small craft
levels for most of the waters. May see some gusts up to 25 knots on
the southern outer waters during the afternoon.
Outlook...Saturday night through Tuesday...moderate confidence.
Sat night and sun...high risk of NE gales as coastal low tracks near
Nantucket 12z/7am Sunday. Low visibility in heavy snow changing to
sleet/freezing rain and all rain. Seas build 15-20 feet across the
eastern Massachusetts waters early sun. Visibility improves Sun afternoon as precipitation tapers
Mon/Tue...another surge of cold air on northwest gales likely Monday then
easing Tuesday as high pressure builds over the NE.
moderate risk of minor coastal flooding for the eastern Massachusetts coastline
during the Sun morning high tide cycle. Surge guidance offers a 1.5
feet surge but could see values closer to 2 feet given known low bias on
strong NE winds. A 2 feet surge will bring storm tides about 1 feet
below flood stage for most locations. However large wave action just
offshore on the order of 15-20 feet may be sufficient for minor
coastal flooding. No coastal Flood Watch as moderate to major
flooding is not expected. At this time a very low risk of isolated
CT...Winter Storm Watch from Saturday afternoon through Sunday
morning for ctz002>004.
Massachusetts...Winter Storm Watch from Saturday afternoon through Sunday
morning for maz002>018-026.
New Hampshire...Winter Storm Watch from Saturday afternoon through Sunday
morning for nhz011-012-015.
Rhode Island...Winter Storm Watch from Saturday afternoon through Sunday
morning for riz001>003.
Marine...Small Craft Advisory until midnight EST tonight for anz231-232.
Small Craft Advisory until 10 PM EST this evening for anz233-
Small Craft Advisory until 5 PM EST this afternoon for anz230.
Small Craft Advisory until 8 PM EST this evening for anz236.
Small Craft Advisory until 11 PM EST this evening for anz235-
Gale Warning until 9 PM EST this evening for anz250-254-255.
Gale Warning until 5 PM EST this afternoon for anz256.