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Area forecast discussion
National Weather Service Taunton Massachusetts
402 PM EDT sun Mar 26 2017

Synopsis...

High pressure over marine will retreat into the Maritimes but
provide cool weather across southern New England tonight into
Monday. A warm front will approach the region during this time and
result in periods of rain with areas of freezing rain across the
high terrain. Wet weather continues Tue and Tue night as low
pressure tracks over or near the region. Although Tue should be
milder than Monday. High pressure brings dry but chilly weather Wed
and Thu. More unsettled weather is possible late Fri into Sat.

&&

Near term /until 6 am Monday morning/...

*/ highlights...

* Freezing Rain Advisory through 8 am Monday for the
expectation of light icing on untreated surfaces in north/west
portions of MA, especially across the higher terrain of the
Berkshires and Worcester Hills.

* Special Weather Statement issued to address the freezing rain
in the forecast however lower confidence of icing on
untreated surfaces.

*/ Discussion...

Into evening...

Most of southern New England has warmed above freezing as another
band of precipitation approaches from the SW invoked by mid-level
forcing and an increasing warm-moist airmass encroaching into the
region. Increasing pops accordingly for short wave Massachusetts and CT behind the
present lull in the weather. Temperatures holding steady with most
locations seeing an east flow aside from the CT River Valley still
observing some north funneling of winds.

Tonight...

Main focus of freezing rain impacts is mainly in Franklin / northern
Worcester County of Massachusetts, especially along the high terrain
and with impact focus along the Route 2 and northern portion of the
I-91 corridor, more importantly during early Monday morning commute.

Will keep with the Freezing Rain Advisory giving greater exposure
and awareness to the general public. Special Weather Statement else-
where where freezing rain is forecast highlighting the lesser
confidence of icing on untreated surfaces.

Seemingly such a marginal icing event. Absence of Arctic air as well
as one of considerable depth at the surface. The surface high setup
more North/East rather than north/northwest thus less north component of ageostrophic /
isallobaric flow, more east onshore component off warmer ocean waters.
While it's late March, significant icing events are more favorable
when temperatures are forecast in the 20s or below. Also noteworthy
cips analogs are fairly quiet, not showing much of a strong signal.
The sref probabilities mainly confined to the high terrain of the
Berkshires and Worcester Hills.

Still a tough forecast given the synoptic setup especially with 2m
surface temperatures. The warm front will be slow to creep North/East and
given some uncertainty on timing, a forecast off by 1 to 2 degrees
can make some difference. Could even hang for a period up against
the North/East colder airmass, north funneling flow. Some inherent forecast
challenges.

So going into it, precipitation will overspread the area overnight
per dominant low to mid level lift and forcing, more so against high
terrain. While considering ice to be present within the column, the
depth and magnitude of the warm layer around 850 mb above the shallow
surface cold airmass favors any precipitation falling into a region
of sub-freezing temperatures to be mainly in the form of freezing
rain. Gradual change over to rain with a near-steady to non-diurnal
temperature trend forecast, more than likely a combination of 1.)
Falling liquid through an increasing warm- layer, 2.) Latent heat
release of liquid freezing as ice on untreated surfaces, especially
those elevated and exposed, and 3.) Onshore east flow off the warmer
waters ushering into the interior.

Freezing rain impacts erode north/west towards Monday morning. Majority of
the threat focused across the north/west high terrain. Particular focus to
the north/west Massachusetts Route 2 and north Massachusetts I-91 corridor. Ice accretions mostly
that of a glaze but the potential is there for up to a tenth of an
inch. If having to travel this area tonight into the early morning
hours of Monday, please be aware of the potential hazards for the
area within the Freezing Rain Advisory.

&&

Short term /6 am Monday morning through Monday night/...

Monday...

Mainly rain with, however, some lingering pockets of freezing rain
during the morning for north/west MA, especially along the high terrain.
Warm front lifts across the area with associated forcing mechanisms
collocated with a +2 Standard deviation precipitable water airmass.
Yet notable the stronger qg-forcing and perhaps convergent focus is
across upstate New York into north New England. Thus some spread in the model
forecast outcomes of more moderate rainfall.

With little to no impact impacted with model forecast rainfall, will
go with a consensus blend of the guidance. It'll be a cold rain for
some, especially those North/East still within a lingering airmass just
above freezing, perhaps for a long period of time depending on the
evolution North/East of the surface warm front, if it becomes hung up,
which in turn will have implications on the surface wind forecast.

Some challenges to be sure but think we'll be okay otherwise. Will
see temperatures warm into the 40s, lower 50s for locations short wave. East
winds along the warm front becoming more S and a bit more breezy
behind. Majority of the rain focus around the midday hours coming to
an end into the evening.

Monday night...

Cloudy, dreary night. Lot of low level moisture given the recent
rains plus the continued S flow off the waters, trapped beneath the
continued Stout warm layer inversion immediately aloft. Lots of low
clouds, perhaps even some drizzle can be included. Given the over-
cast deck, don't expect temperatures to drop much overnight. Will
remain mild, in thinking, with lows down around the 40s.

&&

Long term /Tuesday through Sunday/...

Highlights...

* active weather pattern with precip likely Tue/Tue ngt & Fri/Sat

* dry but cool and blustery weather Wed and Thu

Pattern overview...

Deterministic and ensemble guidance both support a split flow regime
across the Continental U.S. This period. Thus active weather pattern into New
England this week and possibly into next weekend with a parade of
moisture rich southern stream systems. Given it's still late March,
northern stream still showing appreciable amplitude so have to watch
for the potential of precip type issues late Fri into Sat.

Details...

Tuesday/Tue night...moderate forecast confidence.

Southern stream system currently located over the Southern Plains
will move across New England this period. All model guidance agrees
that southern New England begins in the warm sector Tue so
definitely a milder day than Monday. A model blend offers highs Tue
between 55-60...cooler South Coast. Could be warmer if clouds and
precip are delayed until late in the day or at night. In addition,
dew pts climbing to near 50 will help remove any coolness to the
airmass. By Tue evening height falls shift frontal boundary south
with low pres tracking over or just south of New England. Thus much
cooler late Tue/Tue night as frontal wave passes and north-northeast
flow overspreads the region.

Gefs has pwats climbing to about +2 Standard deviations above climo.
This combined with surface convergence from frontal boundary and
surface wave could yield a period of moderate to perhaps heavy rain
at times. As previous forecaster mentioned could see isolated
thunder given weak elevated instability.

Wednesday and Thursday...moderate to high forecast confidence

Dry but cool weather as northern stream mid level closed low tracks
from eastern Quebec thru Maine and then into the northern Atlantic.
Wed will get off to a somewhat mild start with lows only in the
upper 30s and lower 40s. However cold air advection overspreads the area by
afternoon via gusty northerly winds. Clouds may be stubborn on Wed
over the cape and islands given northerly flow across the waters. In
fact could have some ocean effect rain/snow showers over the Outer
Cape! A few degs cooler Thu given less blyr mixing with high pres
overhead. However will be fairly pleasant given light winds and lots
of sunshine.

Friday/Saturday/Sunday...low confidence given time range and
amplitude and interaction of northern and southern streams

Deterministic and ensembles agree next southern stream system may
impact our region in the late Fri-Sat time frame. Northern stream
showing some amplitude downstream across the Maritimes and this may
result in cold confluent flow over or just to the north of the
region. Thus a chilly rain with potential for wet snow/sleet at
times on the northern portion of the precip shield. All guidance
suggest system should be or moving offshore by Sunday, thus drying
trend possible second half of the weekend.

&&

Aviation /20z Sunday through Friday/...

Forecaster confidence levels...

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

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

Tonight...
MVFR lowering to IFR-LIFR. East winds overall but remaining north within
the CT River Valley. Rain / freezing rain coverage increasing across terminals.
Freezing rain more likely for interior areas (e central Massachusetts across to west MA,
northwest RI, north ct) with the highest risk across the high terrain of the
Berkshires and Worcester Hills. Mainly trace accumulations but as
high as a tenth of an inch. Main impacts after midnight into Monday
morning, freezing rain threat eroding to the north as well towards Monday morning.

Monday...
freezing rain risk diminishes, lingering over north/west MA, especially the high terrain
for the first few hours. Otherwise -ra/rain with MVFR-LIFR cigs with
light east winds initially turning southeast and increasing.

Monday night...
rain diminishing however IFR-LIFR cigs remain. Could see some dz in
addition. Light winds.

Kbos taf...not expecting any freezing rain impacts at the terminal with east onshore
flow. Lowering MVFR down to LIFR after midnight Monday into Monday
morning.

Kbdl taf...freezing rain threat around midnight Monday, yet expect spotty impacts.
Not expecting any ice accretion, at most a trace as a worse case.

Outlook /Tuesday through Friday/...

Tuesday...moderate confidence. MVFR conditions likely in areas of
rain and fog with a trend lowering to IFR as frontal boundary and
surface low approach. Low risk for thunder.

Wednesday into Thursday...high confidence. Dry and VFR weather
likely but MVFR in scattered rain/snow showers possible over the
cape and islands Wed/Wed night. Gusty northerly winds Wednesday
into Thu.

Friday...moderate confidence. VFR and dry to start but likely
lowering to MVFR or possibly IFR Fri night in rain. Low risk of rain
mixing with snow/sleet interior Fri night.

&&

Marine...

Forecaster confidence levels...

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

Short term /through Monday night/...

Warm front will lift across the waters tonight into Monday along
and behind which east flow may gust as high as 25 kts and influence
seas to 5-6 feet. Greater confidence for the outer waters than
the inner waters, so opted to issue scas accordingly, will let
the night crew evaluate whether it needs to be expanded. A weak
frontal boundary sweeps the waters Monday night and winds turn
westerly but remain light. This will allow seas to subside and
scas to conclude.

Outlook /Tuesday through Friday/...

Tuesday...moderate confidence. Light southerly winds with frontal
boundary north of the region. However winds becoming northeast late
Tue night as front slips south and wave of low pres tracks along the
front south of New England. Vsby reduced at times in rain and fog.

Wednesday into Thursday...high confidence. Gusty north winds near
Wednesday into early Thu then diminishing as high pressure builds
into the area later Thu.

Friday...high confidence. Quiet weather with high pressure over the
area early Fri. However increasing east-southeast winds Fri night as
low pres approaches from the west. Also vsby lowering in rain and
fog Fri night.

&&

Box watches/warnings/advisories...

CT...none.
Massachusetts...Freezing Rain Advisory from 8 PM this evening to 8 am EDT
Monday for maz002>004-008-009-012-026.
Rhode Island...none.
Marine...Small Craft Advisory from 2 am Monday to 2 am EDT Tuesday for
anz250-254.
Small Craft Advisory until 11 PM EDT Monday for anz255-256.

&&

$$

Synopsis...nocera/sipprell

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