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Area forecast discussion
National Weather Service Taunton Massachusetts
1234 am EST Tue Feb 28 2017

Synopsis...
high pressure centered S of New England today moves out to sea
Tuesday. This brings back a mild flow of air with above normal
temperatures. A couple of weather systems will bring periods of
wet weather later Tuesday through Wednesday night. Colder air
returns late this week into next weekend.

&&

Near term /until 6 am this morning/...
aside from normal adjustments, mainly to time the increasing
dwpts through the overnight hours and the hourly/min temperature
response (mainly warmer than previous forecast), only minor
changes will be made with this evening's forecast update.
Forecast on track otherwise.

Previous discussion...

A low risk of spotty light showers, otherwise increasing,
thickening clouds. Light S flow and slightly milder with lows
remaining in and around the low to mid 30s. Leaning towards
warmer guidance with the blanket of clouds forecast and S
onshore flow with slightly rising surface dewpoints. Not
thinking any issues with fog, still dry at the surface.

As to the spotty shower chances, rich S moisture encroaching
round the low to mid level ridging pattern, undergoing
isentropic ascent, albeit weak. The atmospheric column hardly
saturates but undergoes top-down moistening gradually. Perhaps
support per mid-level forcing via ascent associated with lead
impulse energy through the SW to NE gradient flow out ahead of
deeper troughing across the west Continental U.S. And Lee cyclogenesis of The
Rockies. Also additional support up against high terrain. Can't
rule out a spot shower but am somewhat doubtful given lingering
dry air, plus supporting ensemble signals of little of any way
of outcomes. Going with slight chance pops focused over the northwest
high terrain and S/east over the waters off the Massachusetts/Rhode Island coast with
all other areas remaining dry.

&&

Short term /6 am this morning through 6 PM Wednesday/...
Tuesday...

Increasing wet weather chances, mostly late. Outcomes look
mainly light with focus across the entire region however
greater emphasis over southeast and west portions of S New England. The
details below spell out the thinking, otherwise increasing S/SW
surface winds and a mild day on tap with usherance of warmer air
north. Looking at highs around the low to mid 50s. Holding away
from warmest of guidance considering forecast cloud cover.
However with any sunshine we could quickly warm into the upper
50s to low 60s, something to watch.

As to wet weather chances, continued column moistening with S
flow ushering warm high Theta-east air within h925-7 layer.
Moisture not the issue, rather available lift and forcing.
Several synoptic features Worth noting: 1.) Isentropic upslope
and lift along an encroaching warm front, 2.) Increasing h925-85
low level jet convergence however stronger focus is across the
Ohio River valley / east Great Lakes region with ensemble signals
suggesting around +2 Standard deviations in precipitable water
and low level winds, 3.) Supporting lapse rate environments,
especially looking at h925-7, and 3.) Potential ascent
associated with continued lead impulse energy within the mid-
levels originating from deeper troughing into the central Continental U.S..



Altogether looking a low level, synoptic scale lifting
environment over southeast areas of S New England with some weak
instability noted, while the deeper plume and favorable dynamics
encroach into west New England late with the accompanying warm
front. Will focus chance pops in these locales respectively with
lower chance pops elsewhere. Appears interior central areas of
New England will remain dry and end up with a rather quiet day.
Bonus if any sunshine.

Tuesday night...

Light to moderate rain likely. Deeper moisture, low level
warming and convergent winds, along with steepening lapse
rates, also the concern of some embedded thunderstorms. The
increasing gradient flow along with the onset of rain, dewpoints
on the rise which will keep conditions mild, and over slightly
colder ground temperatures, likely to be contending with a mix
of low clouds and fog. Lows around the mid to upper 40s
occurring around midnight and then warming thereafter with the
good S flow ongoing.

As to the rain and thunderstorm threat, deep moist plume of
higher Theta-east air +2-3 Standard deviations, with respect to
precipitable waters, pushing into the NE Continental U.S. Along with the
lifting warm front. Collocated isentropic upslope and convergent
forcing of h925-85 low level jet, +1-2 Standard deviations,
precipitable waters pushing at or above 1 inch along with
pockets of 850 mb dewpoints at or exceeding 10c. Rich, robust,
moist sub-tropical environment, atypical for this time of year,
within a conditionally unstable profile above boundary layer
decoupling / shallow surface inversion. Still potential for lead
shortwave energy along with increasing diffluence aloft to aid
with ascent. Altogether, weak instability and shear is present,
yet however the stronger, deep layer forcing remains well to the
west.

Synoptically, there is concern with the weak instability and
modest shear. With freezing levels around 7 kft, there's
enough forecast guidance to suggest a possible hail / damaging
wind threat. However am considering it a low risk for the time-
being, yet not ignoring. More focused on the low level jet
convergence of anomalous, robust, sub-tropical moisture.
Thinking widespread rain with embedded heavy showers and
thunderstorms. Quick moving, so think flooding impact is low but
something to still think about.

So, reiterating, widespread rain, embedded heavier showers and
the chance of thunderstorms. Will incorporate areas of dense
fog although thinking a mix with low clouds. No specific wording
within the weather grids of any severe weather elements.

Will continue to evaluate convective threats, especially with
later forecast guidance. The whole month of February has been
anything but typical, can't have blinders on with this event,
rather have to expect the unexpected.

&&

Long term /Wednesday night through Monday/...
highlights...

* above average temperatures for Wednesday
* showers and thunderstorms likely Wednesday into Thursday
* more seasonable conditions return on Friday

Overview...

27/12z guidance remain in rather good overall agreement through
this week. Confidence in the details diminishes this weekend
into early next week. Two distinct camps, with the international
models more progressive with a trough moving east from the West
Coast late next weekend than the GFS and its ensembles. Prefer a
consensus approach for this portion of the forecast for now.

Details...

Wednesday into Thursday...moderate confidence.

This should be the portion of the forecast with the most
significant weather. A low pressure should move from the Great
Lakes through the Saint Lawrence River valley. Expecting its
cold front to move offshore sometime late Wednesday night into
Thursday morning. However, cyclonic curvature and cold air
advection persist into Thursday.

Above normal temperatures Wednesday. This will, in part, fuel
the risk for more thunderstorms Wednesday into Wednesday night.
Not a lot of buoyancy to work with, about 500-1000 j/kg of cape.
However, there should be a large amount of low level shear and
helicity. That could maximize the use of whatever energy does
develop. Can't completely rule out a few strong storms at this
time, but it is 3 days away, and a lot of things can change.

While the entire period don't be wet, as dry air intrudes in
the mid levels at times, it still appears that moderate to heavy
rain could fall at times, especially within any thunderstorms.
Main threat for these storms will be heavy rain and strong gusty
winds.

Friday and beyond...moderate confidence.

Northwest winds dominate Friday through Saturday, meaning a
return to more seasonable temperatures. Models continue to
indicate the potential for a clipper system on Friday/Saturday.
The GFS remains slightly more robust then other guidance with
this clipper, so leaned towards an ensemble approach. Good
mixing and cold air over the ocean may result in ocean effect
snow showers over portions of this weekend, before another
clipper system comes through towards the end of this weekend.

&&

Aviation /06z Tuesday through Saturday/...
forecaster confidence levels...

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

21z update...

Short term /through Wednesday/...

Tuesday...moderate confidence.
Mainly VFR, although there is a low risk for some MVFR mainly
across the cape/islands late today. Otherwise, isolated shra,
but bulk of these hold off until the overnight hours. Winds
shifting to S-se.

Tonight...moderate confidence.
Mainly IFR with spotty MVFR. Widespread rain. Low risk for thunderstorms and rain.
Areas of fog developing. Increasing winds out of the S with
gusts up to 20 kts. There is a low level wind shear risk for southeast New England
terminals with 50 kt SW winds 2 kft agl.

Wednesday...moderate confidence.
Rainfall tapers to rain showers with breaks during the day Wed. More
widespread rain showers/thunderstorms and rain possible mainly during the late
afternoon/evening hours, especially across west MA/CT. Widespread
MVFR/IFR continues, but a few breaks to VFR during the day
cannot be ruled out.

Kbos taf...high confidence in taf.

Kbdl taf...high confidence in taf.

Outlook /Wednesday night through Saturday/...

Thursday...moderate confidence. VFR conditions from west to
east during the morning. Blustery westerly winds.

Friday and Saturday...moderate confidence. Mainly VFR, with
MVFR possible toward the South Coast in -shsn.

&&

Marine...
forecaster confidence levels...

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

Short term /through Wednesday/...high confidence.

Winds and seas continue to diminish this evening. Canceled some
of the near shore small craft advisories over the past couple of
hours, once observations suggested the risk for frequent 25 kt
gusts had ended. Current timing of small craft advisories for
the outer coastal waters still looks reasonable. Will have to
wait for rough seas to diminish, too.

Quiet through Tuesday. Increasing S winds with gusts up to 25 kt
Tuesday night will also result in building wave action towards
5 to 6 feet by Wednesday morning. Additional Small Craft
Advisory headlines will likely be needed.

Outlook /Wednesday night through Saturday/...

Wednesday into Thursday...moderate confidence. Approaching
system from the west will push a cold front through late
Wednesday night into Thursday morning, increasing both seas and
winds during this period. Heavy rain and a few thunderstorms
are possible, lowering vsbys. There is the potential for
southerly gales across the waters Wed night, with the better
chance for northwesterly gales on Thursday behind a cold front.

Friday...moderate confidence. Gusty west winds continue. Small
craft advisories likely will be needed for some of the coastal
waters.

Saturday...moderate confidence. Possibility of northwest gales
for some of the waters. Rough seas continue across the outer
coastal waters.

&&

Hydrology...
river levels, especially along the mid to lower CT River Valley
in Massachusetts and CT, are currently cresting or beginning to crest as a
result of rains and snowmelt over this past weekend.

There is plenty of snowpack across north New England and subsequent
melt water that can move downriver. In addition, there is a
decent slug of rain forecast across the northeast beginning
Tuesday, and continuing through Wednesday into Wednesday night.
These additional rains combined with snow melt will likely
result in river rises in the near future.

Those with interests along the mainstem rivers should stay aware
of the latest forecasts as conditions will likely change through
the midweek period.

The river gauge at Middle Haddam CT along the CT river has been
fixed by USGS.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...Belk/sipprell
near term...doody/sipprell

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