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Area forecast discussion
National Weather Service Taunton Massachusetts
700 PM EST sun Jan 21 2018

Synopsis...
warm air aloft moves north tonight and brings clouds to southern
New England. Some light precipitation is possible Monday. Most
of it will be rain, but some light icing is possible in the
interior. A fast moving storm system will bring a period of
heavy rain to the region Tuesday along with strong winds on the
coast. Blustery, dry and colder weather follows Wednesday
through Friday, before above normal temperatures likely return
next weekend.

&&

Near term /until 6 am Monday morning/...
7pm update...
overall, the forecast remains on track. However, will slow the
progression and lower the overall pops overnight as the column
moistens in the lower lvls slowly. Mainly a drizzle risk given
low relative humidity in the ice regime aloft. The key will be temperatures
which may hug close to possibly even above freezing. Special
statements may be needed if drizzle is observed with temps near
0c, but this is likely to be a very localized risk, hence the
low pops.

Previous discussion...

High pressure is situated to provide dry weather much of the
night. Low level flow remains light most of this time, but shows
signs of overrunning after midnight. Meanwhile, satellite images
and sfc observations show a cloud deck over eastern PA edging
northeast. Between this and the favorable development of lift
overnight, expect increasing clouds through the night.

Can not rule out some light precip late, especially in western
Massachusetts and CT, but the chance before 12z is rather low so we used
slight chance pops. If any precipitation were to form late
tonight, temperature profiles suggest rain in the south and a
snow/sleet/freezing rain mix north.

Clouds will limit the overnight cooling, and cold air damming
signal does not show much until late night. We maintain a range
of upper 20s north to mid 30s South Coast.

&&

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

High pressure over Quebec Monday and increasing cold air
damming into New England. The damming seems focused on northern
New England, but could further expand into our area as well
during Monday morning. Our temperature profiles support rain
most places, but a rain/snow mix north of the Mass Pike.

The GFS and European model (ecmwf) bring a front south into southern New England
or just to our south. This shows increasing lift potential, thus
increasing pops. We show chance pops all areas Monday morning
and likely pops in the afternoon in western and central mass and
adjoining CT. An east wind should be enough to keep ptype as
rain, but it may be marginal in western mass/northern Worcester
County.

Even with better lift, the focus of southerly low level winds
and moisture remains to our west across New York state through
the day. So we continue to expect quantitative precipitation forecast amounts up to 0.1 inches.

Monday night...

Evening cooling and the stalled front to our south may allow a
brief period of freezing rain in central and western mass,
especially along and north of the Route 2 corridor. The front
moves north during the night with winds shifting from the south.
This will bring slowly rising temperatures overnight ending the
icing risk.

South-southwest winds 35-40 knots move up over the region during
the late night. This transports 1 inch precipitable water values into the area.
Expect increasing chance of rain toward morning with
categorical pops...75 pct or higher...over western mass and
western CT.

&&

Long term /Tuesday through Sunday/...
highlights...

* heavy rain Tue with a period of strong winds southeast new eng
* some minor flooding issues are possible
* dry/blustery and turning colder Wed and especially Thu/Fri

Overview and model preferences...
the medium range will feature gradual breakdown of strong
positive height anomalies as ridging shifts offshore, allowing
an opening cutoff to phase with the northern stream. This
transition, and the fact that the cutoff will have tapped
subtropical moisture will feature a widespread rainfall event
Tue, until the drier cp air associated with the north stream filters
in mid week. This colder airmass is not significantly cold, as
low and mid lvl temps anomalies are barely 1 full Standard
deviation below normal, but colder wx can be expected to follow
the rainfall. As a pacnw wave deepens in the Lee of The Rockies
late week, this will force yet another rise in height anomalies
with strong ridging into the weekend, which would favor another
moderation in temperatures and suggests that any precip
associated with low pres attendant to the wave moving east of The
Rockies is likely to remain warm at least initially. Overall,
both ensembles and deterministic guidance are in fairly good
agreement with these synoptic scale features, so will use a
consensus guidance blend as the baseline for this forecast.

Details...

Tue...
low pres, moving out of the Great Lakes will initially
slide along the Canadian border into north Canada. The fact that
this system is continuing to deepen on Tue suggests the lack of
an occlusion processes yet and favors a more progressive
passage. Several factors point to periods of moderate and
occasional heavy rain as the cold front slides through mid day
Wed into early Wed evening. With the influence of subtropical
moisture allowing pwats to reach near 1.00 inches (almost 3 std
deviations above normal) combined with deep warm rain processes,
conditionally unstable profiles supporting some modest elevated
cape and low level jet approaching +60 kt all should act in accordance.
Therefore, expecting widespread rain with periods of rainfall
approaching 0.5-1.00in/hr on occasion, especially as any
convective elements develop. These rainfall rates do suggest a
lingering risk for urban and poor drainage flooding, as well as
some flashier river flooding especially where any lingering ice
is involved. Area hydrographs, particularly in northwest Massachusetts continue to
show the influence of ice. This may require the hoisting of a
Flood Watch in the future. Final quantitative precipitation forecast totals generally range
between 0.75 and 1.50 inches, although a few spots may approach
2.00 inches before all is finished. Low risk for occasional
tsra, especially along the low level jet core across southeast Massachusetts/RI, but there
is a low risk across most of S New England.

Aside from the heavy rain/flood risk. The core of a 60+ kt
(h92) low level jet will slide across southeast Massachusetts/Rhode Island during the afternoon and
early evening hours. Conditionally unstable soundings support
some mixing of this momentum on its own, but as previous
forecaster noted, the addition of warmer sfc temps (if they
reach near 60f) will enhance the lower level mixing. Also,
convection will also enhance this mixing. Therefore, will
continue to yield about 60-70 percent of this momentum to the
surface in this area, which is likely require wind advisories if
realized. Near shore locations may be slightly inverted thanks
to cooler SSTs, but these could be overcome by the mechanical
and frictional forces mentioned above.

Mild, thanks to warm h92 temps in place into the overnight
hours. Expecting highs mainly in the 50s across most of S New
England.

Wed through Fri...
cooling trend expected as lower height anomalies associated
with a gradually opening trof settles across New England. This
will also be associated with high pres and generally dry
conditions thanks to strong subsidence inversion, especially as
the high crests above 1035hpa. Wed, the delay in cold advection
should keep highs near seasonal normals, mainly the upper 20s to
mid 30s, although a few spots mainly across east Massachusetts/Rhode Island could reach
around 40f thanks to some downsloping. Otherwise, the cooler
days are expected on Thu and Fri as h92 temps settle to -10c and
-7c on average respectively. This will keep highs in the mid
20s to low 30s each day. The best chance for radiational cooling
appears to be Thu night into Fri, thanks to a slackening in the
sfc pres gradient which will be too strong Wed night. This will
likely allow mins to fall into the single digits and teens.

Next weekend...
gradual transition as high pres and associated ridging move E,
allowing for another deepening trof to impact the region.
Initially, the warming signal with rising heights for Sat favors
mainly a warm solution for low pres passage on sun, but given
the possibility of some entrenched lower level cold air in the
northwest at precip onset, as well as uncertainty with the north stream
influence does hint at the possibility of a colder solution.
Will lean more heavily on ensemble means, which, for the most
part yields a warmer (rain) solution, but this will have to be
watched as all of the players become better sampled.

&&

Aviation /23z 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/...

Overnight...high confidence.
VFR with light and vrb winds through 06z...after which gradual
reduction to MVFR ceilings expected from west-E, along with isolated
rain showers even light fzdz across the interior, but these will be few
and far between.

Monday...moderate confidence.

MVFR cigs with areas of IFR cigs developing in the afternoon.
Increasing chance of light rain through the day. Generally MVFR
vsbys in rain and some mixed rain/snow. Light wind becomes
light east-northeast.

Monday night...moderate confidence.

MVFR conditions with areas of IFR. Rain most places with a
chance of freezing rain early in northern areas of
Massachusetts such as vicinity of fit and Oregon. Winds turn from
the southeast and south toward morning.

Kbos terminal...moderate to high confidence in taf.

Kbdl terminal...moderate to high confidence in taf.

Outlook /Tuesday through Friday/...

Tuesday: mainly MVFR, with local IFR possible. Strong winds
with gusts to 40 kt. Ra, slight chance thunderstorms and rain.

Tuesday night: mainly VFR, with local MVFR possible. Strong
winds with local gusts to 40 kt.

Wednesday through friday: VFR. Breezy.

&&

Marine...
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 and Monday... light winds shift from the south late at
night and from the southeast Monday. Speeds remain below 15
knots. Seas 2 feet or less. Vsby lowers to 3-5 miles in rain
during Monday afternoon.

Monday night...

Southeast to south wind increasing, but remaining below 25
knots. Seas building to 4 feet. Rain with vsbys 3-5 miles.

Outlook /Tuesday through Friday/...

Tuesday: moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 11 ft. Rain, slight chance of
thunderstorms.

Tuesday night: moderate risk for gale force winds with gusts up
to 40 kt. Rough seas up to 10 ft. Chance of rain showers.

Wednesday: moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 10 ft.

Wednesday night: low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas.

Thursday: moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas.

Thursday night: moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Areas of rough seas.

Friday: low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas.

&&

Hydrology...
a Flood Warning remains in effect for the Connecticut River at
Middle Haddam, where ice jams are causing river fluctuations.
No significant change from earlier forecasts.

For details on specific area rivers, including observed and
forecast river stages, please visit the advanced hydrologic
prediction service (ahps) graphs on our website.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...gale watch from Tuesday morning through late Tuesday night for
anz232>235-237-250-254>256.
Gale watch from Tuesday morning through Tuesday evening for
anz230-236.
Gale watch from Tuesday morning through late Tuesday night for
anz231-251.

&&

$$
Synopsis...wtb/doody
near term...wtb/doody

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