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Area forecast discussion 
National Weather Service Taunton Massachusetts
532 PM EST Wednesday Dec 17 2014

Synopsis...
scattered rain or snow showers are possible tonight as an upper
level disturbance crosses New England. High pressure will slowly
pass across northern New England late this week...bringing dry and
seasonable conditions. Low pressure will pass well south of the
region...bringing a period of scattered rain and/or snow showers this
weekend into early next week. Stronger low pressure well west of
New England looks to bring rain and gusty onshore winds the middle
of next week.

&&

Near term /until 6 am Thursday morning/...
225 PM update...

Seeing a few breaks in cloud cover across southern New England
this afternoon...but expect these to fill back in as we approach
sunset due to leftover moisture and presence of upper level trough
over region.

Still looking at dry weather through at least this evening per
high-res models...before scattered rain/snow showers develop early
tonight in response to approaching short wave. Most of this
activity should be focused north of Mass Pike...but could see a
few rain or snow showers into parts of north CT...Rhode Island and southeast Massachusetts as
well. Not looking overly impressive for any snow squalls per btv
4km WRF and other high-res models. Despite presence of moisture
and upper trough moving through New England...lacking good
instability and lapse rates. So our thinking is we will see
dusting to perhaps an inch of accumulation of snow at
most...mainly across higher terrain.

Lows tonight will drop back into 30s...with freezing line probably
staying just west of I-495 corridor in east Massachusetts by daybreak Thursday.

&&

Short term /6 am Thursday morning through Thursday night/...
expect to see few lingering snow showers Thursday morning across NE Massachusetts
as system departs. Otherwise dry weather will prevail Thursday and Thursday
night.

Gusty west/northwest winds /30-35 miles per hour/ anticipated as a result of low
pressure deepening over Maritimes. This should help to scour out
cloudiness a bit...but do not expect total clearing as upper
trough still hangs back from New England to Great Lakes...even
through Thursday night.

Stayed close to blend of model guidance which results in highs
from upper 30s northwest to middle 40s southeast Thursday...and lows mainly in 20s Thursday
night.

&&

Long term /Friday through Wednesday/...
highlights...

* dry weather and near or slightly below temperatures for Friday and Sat
* low pressure will pass S of New England this weekend...but will
still see scattered light rain and/or snow showers
* stronger low pressure developing well west of the region looks to
bring periods of heavy rain and gusty winds by Christmas evening

Overview and model preferences...
12z medium range model suite was in generally good agreement...
though timing and track issues did come into play for weak low
pressure passing well S of the region Sun night/Mon...then with the
strengthening low pressure center across the Great Lakes associated
with a long wave negative tilt trough which will wrap in moisture by
around Christmas evening day.

Middle and upper level steering currents remain in a split flow
across North America early in this forecast cycle. Models in very
good agreement that this pattern evolves into an amplified
trough/ridge set up...with long wave trough digging through the
central U.S. To the western Gulf of Mexico coast by early next
week. 12z European model (ecmwf) and GFS both continue to evolve cutoff 500 mb low
pressure...which will cause the long wave trough to become negatively
tilted across the southeast U.S. While both models did signal this
evolution...timing is in question...especially with the
development of low pressure and a slug of heavy rainfall moving up
the eastern Seaboard with the possibility of some mixed precipitation to
start well inland around Tuesday that will change to rain with
gusty east to southeast winds bringing milder air by Christmas evening
day.

One thing to note on this pattern evolution...considering we are
talking about a system about a week away...the 12z GFS/ec op runs
were rather close with the strength of the parent low over the
Ohio Valley moving into the Great Lakes early next week.
However...there are timing and track issues for the late Monday-Wednesday
timeframe of the low and short wave wrapping around it which looks
to affect the region...which lends to somewhat lowered confidence.
Do feel there is a good shot for precipitation...it is a matter of the
onset and ending of this precipitation. So...leaned toward the wpc
guidance as well as blend of the GFS/ec ensembles along with a bit
of the GFS/ec op runs.

Details...

Friday and Saturday...high confidence.
As upper low moves into the Maritimes early Friday...upper level
ridge and bubble cutoff high over central Canada will shift across
Quebec during this timeframe. Associated surface high will slowly
shift east as well...bringing mainly dry conditions Friday into early
Saturday. Expect temperatures to run close to or slightly below seasonal
norms.

During middle to late Saturday...as the axis of the high passes over
southern Quebec...winds shift to E-NE. All medium range models
bring light precipitation toward the coast by Saturday evening and...with
marginal temperatures away from the coast...have included slight chance
to low chance probability of precipitation for rain and/or snow showers with the best shot
right along the coast. May also see patchy drizzle and fog
develop.

Low pressure will approach the middle Atlantic coast late Sat night...so
appears some moisture from this system will work north into CT/S Rhode Island
during the pre-dawn hours...so could see some scattered rain/snow
showers there.

Sunday and Monday...low to moderate confidence.
This was a tough portion of the forecast as pattern begins to
undergo change to the west. A rather flat short wave works off the
middle Atlantic coast with associated surface reflection. This system out
of the southern stream does not look to phase up with anything in
the northern stream. However...will still see periods of scattered rain
and/or snow showers with the continued onshore flow as this
southern low slowly shifts offshore while the high to the north slowly
moves east as the overall steering pattern slows down and becomes
more amplified with the digging long wave trough over the central
U.S.

With the onshore winds...expect any mixed rain/snow to change to
rain across eastern Massachusetts/RI/portions of north CT...while scattered light snow
showers continue inland. Not much in the way of quantitative precipitation forecast...so not
expecting too much snow accumulations mainly across the higher
inland terrain. Expect highs both days in the middle-upper 30s well
inland to the middle 40s along the coast.

Tuesday and Wednesday...low confidence...mainly for timing.
Another low that develops across the eastern Gulf of Mexico will
pass well S of the region during Tuesday...but will keep easterly
winds in place. This will also keep the chance for light precipitation
across the region. Noting some placement issues for the axis of
this moisture and how far north it will shift from the second passing
low. With continued onshore flow...temperatures will be a little bit
milder with highs from the upper 30s over the highest inland
terrain to the middle and upper 40s along the S coast.

In the meantime...the upper level pattern will take shape by
Tuesday across the central u... long wave trough digging to
the Gulf of Mexico. 500 mb cutoff low forms across the Upper Middle west
/though placement issues in play/...while surface low pressure forms
across the Mississippi Valley. The upper trough becomes negatively
tilted by 12z Wednesday...which will allow the surface system to deepen
as it rotates across western PA/central Virginia. A slug of quantitative precipitation forecast...tapped
from the tropics...will shift up the eastern Seaboard during Wednesday.
Noted precipitable waters on order of 1.3 to 1.4 inches...which is about +2 South Dakota
above seasonal normals for middle to late December. Will also see
east-southeast winds increase with strong low level jet working up the coast
/h85 winds briefly increase to 60-70 knots as jet crosses during
Wed/.

Have pretty good confidence in precipitation moving in during Wednesday
into Christmas evening as models continue to signal this...but big
question will be the onset and ending of this slug of precipitation.
Also...since temperatures will be chilly to start well inland...will see
period of mixed precipitation there to start before changing over to
rain. Have carried likely probability of precipitation for this timeframe...though still
questions on exact timing. Temperatures will rise with the increasing
onshore flow... likely to as much as 10 degrees above seasonal
norms.

&&

Aviation /00z Thursday through Monday/...
forecaster confidence levels...

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

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

MVFR ceilings will prevail through this evening. Ceilings lift to VFR
across most of area tonight with increasing west/northwest winds...gusting
to 25kt especially near coast. Also expect scattered rain/snow
showers...mainly across northern Massachusetts with brief MVFR cigs/vsbys.

VFR ceilings dominate Thursday after leftover MVFR ceilings during morning...
perhaps scattered rain/snow showers across NE Massachusetts. Northwest winds gust as
high as 25-30kt.

Kbos terminal...moderate confidence. Low-end MVFR ceilings through
evening push. Could see ceilings improve to ovc025 as best case...but
not likely at this time.

Kbdl terminal...high confidence.

Outlook...Friday through Monday...

Friday...high confidence. VFR.

Saturday...low to moderate confidence. VFR to start...then
developing MVFR conditions especially across the coast mainly Sat
afternoon/night.

Sunday and Monday...low confidence. Low end VFR to MVFR ceilings. Patchy
MVFR to local IFR visibilities in scattered rain and/or snow showers.

&&

Marine...
forecaster confidence levels...

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

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

Small Craft Advisory remains posted for all waters through Thursday...and for outer
waters through Thursday night. Low pressure deepening over Maritimes
will result in strong west/northwest winds across waters...solidly 25-30kt
gusts with outside chance of some 35kt gusts on outer waters. However
we are not confident enough to upgrade to gale warnings at this
time. Peak gusts should occur late tonight through Thursday afternoon.
Winds gradually subside Thursday night but seas will take a bit longer
to diminish.

May see scattered rain/snow showers on east Massachusetts waters late tonight
into Thursday morning...especially near Cape Ann and Merrimack River.

Outlook...Friday through Monday...

Moderate to high confidence through Sunday...then low confidence
Monday.

Friday...northwest wind gusting up to 20 knots. Seas start off around 5 feet
across the outer waters early Friday...but should subside by Friday
evening.

Saturday and Sunday...winds and seas below small craft criteria.
Wind shift to east-NE during sun and seas build up to 4 feet on the outer
waters.

Monday...expect NE winds gusting up to 20 knots. Low probability of 25 knots
gusts as well as 5 feet seas.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 7 PM EST Thursday for anz232>235-
237.
Small Craft Advisory until 7 PM EST Thursday for anz230-236.
Small Craft Advisory until 6 am EST Friday for anz231-250-251-
254>256.

&&

$$
Synopsis...evt/jwd
near term...jwd
short term...jwd
long term...evt
aviation...evt/jwd
marine...evt/jwd

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