Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Taunton Massachusetts
1229 PM 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/...
1230 PM update...
Not much change to the previous discussion. Noting a reflectivity
picking up on some developing snow showers along and north of the
Mass-Pike. Feel forecasted pop trends for slight chance to chance
of snow-squalls continues to apply. Light accums. Slight
moderation of forecasted temperatures by a degree or two...but highs
remain relatively unchanged.
Per 12z Chatham sounding...boundary layer to become well-mixed up
to 850 mb allowing for the mix-down of faster momentum and drier air
to the surface. Already westerly gusts being reported around 25
miles per hour with gales over the adjacent waters. Winds just above the
surface are reporting sustained around 40 miles per hour. Will continue with
headlines as necessary. Gusts across the interior should remain
below Wind Advisory thresholds...may need to considering gales for
the inner waters along the shores.
Near-term guidance exhibiting modest low-level convergence and
decent moisture within an unstable profile ahead of the Arctic
front presently pushing south over northern New England. Have thus
placed slight chance to chance probability of precipitation with the possibility of snow
squalls developing ahead of the front /hrrr would strongly suggest
such outcomes/. Low visibilities being reported upstream will
certainly be plausible with any snow activity. Light accumulations
are even possible.
Very cold air in place...will continue with high temperature
forecast with values around the middle- to upper-20s.
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 /17z Friday through Tuesday/...
Forecaster confidence levels...
Low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.
Near term /through Sunday morning/...
* high confidence through tonight
* Saturday into Sunday morning...high confidence on trends yet
moderate confidence on timing
VFR. Gusty west winds around 25-30 kts. Broken ceilings 5-10 kft. Chance
-sn ahead of an Arctic front this afternoon. Possibility for
terminals along and north of Mass-Pike of seeing -sn with IFR
impacts. Accums light. Moderate confidence for snow.
Conditions deteriorating into Saturday as ceilings and visibilities lower
with the approach of snow. IFR-vlifr. While a mix-in with rain
along the immediate South-Coast is possible throughout the
upcoming storm event...a sleet-freezing rain mix will be possible
around early- Sunday morning for terminals across the coastal
plain. Precipitation comes to an end around Sunday morning. Northerly
winds across the interior with blustery east winds along the
shores and adjacent coastal plains.
Kbos taf...high confidence in taf.
Kbdl taf...high confidence in taf.
Outlook...Sunday through Tuesday...
Sunday...snow moving out...conditions improving.
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.
1030 am update...
Westerly winds reaching gale force with rough seas 5 to 9 feet on
the outer waters. May need to put up headlines for the near-shore
waters for gales. Arctic front will push across the waters towards
evening behind which winds will back more northwesterly and
diminish into this evening. Seas will begin to lessen.
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.
tides/coastal flooding...weather forecast office box staff