Special Weather Statement, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Taunton MA
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 W CONUS 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 NW
high terrain and S/E over the waters off the MA/RI 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 SE and W 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
N. 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-E 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
OH River Valley / E 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 CONUS.

Altogether looking a low level, synoptic scale lifting
environment  over SE areas of S New England with some weak
instability noted, while the deeper plume and favorable dynamics
encroach into W 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-E air +2-3 standard deviations, with respect to
precipitable waters, pushing into the NE CONUS 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 H85 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
W.

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 RA. Low risk for TSRA.
Areas of fog developing. Increasing winds out of the S with
gusts up to 20 kts. There is a LLWS risk for SE New England
terminals with 50 kt SW winds 2 kft agl.

Wednesday...Moderate confidence.
Rainfall tapers to SHRA with breaks during the day Wed. More
widespread SHRA/TSRA possible mainly during the late
afternoon/evening hours, especially across W 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 MA 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 N 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.
MA...None.
RI...None.
MARINE...None.

&&

SYNOPSIS...Belk/Sipprell
NEAR TERM...Doody/Sipprell
SHORT TERM...Sipprell
LONG TERM...Belk
AVIATION...Belk/Doody
MARINE...Belk/Sipprell
HYDROLOGY...

Hazardous Weather Outlook
National Weather Service Taunton MA
506 PM EST Mon Feb 27 2017

CTZ002>004-MAZ002>024-026-RIZ001>008-282215-
Hartford CT-Tolland CT-Windham CT-Western Franklin MA-
Eastern Franklin MA-Northern Worcester MA-Central Middlesex MA-
Western Essex MA-Eastern Essex MA-Western Hampshire MA-
Western Hampden MA-Eastern Hampshire MA-Eastern Hampden MA-
Southern Worcester MA-Western Norfolk MA-Southeast Middlesex MA-
Suffolk MA-Eastern Norfolk MA-Northern Bristol MA-
Western Plymouth MA-Eastern Plymouth MA-Southern Bristol MA-
Southern Plymouth MA-Barnstable MA-Dukes MA-Nantucket MA-
Northern Middlesex MA-Northwest Providence RI-
Southeast Providence RI-Western Kent RI-Eastern Kent RI-Bristol RI-
Washington RI-Newport RI-Block Island RI-
506 PM EST Mon Feb 27 2017

This Hazardous Weather Outlook is for northern Connecticut, all of
Massachusetts except Berkshire County, and Rhode Island.

.DAY ONE...Tonight.

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

There is potential for a few strong thunderstorms Wednesday into
Wednesday night.

We will continue to watch the river levels of the lower Connecticut
river. The combination of snowmelt and heavy rain this weekend may
rise the river to above action stage, or even minor flood.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

ANZ230>237-250-251-254>256-282215-
Boston Harbor-Cape Cod Bay-Nantucket Sound-Vineyard Sound-
Buzzards Bay-Rhode Island Sound-Narragansett Bay-Block Island Sound-
Coastal waters east of Ipswich Bay and the Stellwagen Bank National
Marine Sanctuary-Massachusetts Bay and Ipswich Bay-
Coastal waters from Provincetown MA to Chatham MA to Nantucket MA
out 20 nm-
Coastal Waters extending out to 25 nm South of Marthas Vineyard and
Nantucket-
Coastal Waters from Montauk NY to Marthas Vineyard extending out to
20 nm South of Block Island-
506 PM EST Mon Feb 27 2017

This Hazardous Weather Outlook is for Massachusetts and Rhode
Island coastal waters.

.DAY ONE...Tonight.

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

There is a low risk for southerly gales to develop Wednesday.
However, have a higher confidence in west to northwest gales
Thursday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

For the latest updates...please visit our webpage at
www.weather.gov/boston

You can follow us on Facebook at
www.facebook.com/NWSBoston

You can follow us on Twitter at
@NWSBoston

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