Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 271419

Area Forecast Discussion
National Weather Service Taunton MA
1019 AM EDT Thu Jul 27 2017

High pressure moves offshore today giving way to more humid
conditions which will be accompanied by scattered showers and
isolated thunderstorms which may persist into this evening.
A drying trend likely develops late tonight into Fri as a cold
front moves through the region and then stalls near the south
coast Fri. A coastal storm will push off the mid Atlantic coast
and track south of New England this weekend, which will bring
showers and a few thunderstorms, along with gusty onshore winds
with cool temperatures this weekend especially close to the
south coast. The coastal storm may linger south of New England
into early next week with cool and unsettled conditions



10 am update...

Not much change from previous forecast.

Cloud cover this morning has limited destabilization with temps
at 10 am only in the upper 60s to low 70s. Thus instability is
lacking. In addition, with WNW flow aloft and upper trough
lacking amplitude to back the mid level flow, the true warm
sector airmass will remain well to our southwest across NYC and
points westward. This is evident by model MUcapes never
exceeding 500 j/kg across the region today. Furthermore subtle
height falls not providing much cooling aloft resulting in weak
mid level lapse rates. So while showers and isolated T-storms
are expected this afternoon across the region, updraft strength
will be limited given lack of surface based and instability

Few sprinkles exiting eastern MA from lead short wave earlier
this morning. Secondary/trailing short wave over the eastern
Great Lakes. Ahead of this feature is widespread showers across
NY state and beginning to enter western MA at 10 am. This will
be the area of showers to impact our area this afternoon. Good
moisture advection ahead of this feature with K index climbing
into the mid 30s along with PWATs up to 1.7 inches across NY
state on the SPC mesoanalysis. Thus our greatest concern for any
convection today will be heavy downpours with a low risk of very
localized minor poor drainage/urban flooding. Global and
mesoscale guid including the 00z NCAR ensembles suggest greatest
risk for heavy showers/isolated is across western section of
MA/CT. However given instability is lacking this reduces the
flood threat as well as updraft strength will be limited and not
expecting warm rain processes to contribute.

Otherwise cloud cover limits highs today to the 70s but will
feel a bit more humid than yesterday with dew pts in the upper
50s and low 60s this morning climbing thru the 60s this

Previous discussion follows...

Approaching mid level shortwave from the NW will bring a modest
instability burst into New Eng today as K indices spike into the
low/mid 30s with deepening moisture and PWAT plume exceeding 1.5
inches. Global and Hi-res guidance signaling a cluster of
showers and a few t-storms moving into SNE from the west this
afternoon. Instability is marginal with SBCAPES 500-1000 J/kg
and mid level lapse rates are poor so risk for strong to severe
storms is low, but some of the hi-res guidance indicating
potential for localized heavy rainfall. Greatest threat will be
in the interior north and west of the I-95 corridor where better
instability, but do expect some showers making it to the coastal
plain later this afternoon.

Cloud cover and shower potential will limit heating with highs
mostly in the low to mid 70s and could see temps fall into the
60s in showers over the interior high terrain. Increasing
humidity as dewpoints climb into the low and mid 60s.


Mid level shortwave and attending cold front move into the
region which may help to sustain convection into the evening
before drier air pushes in from the north. Trend will be for
drying conditions overnight as the shortwave exits and K indices
drop off. It will remain humid as low level drier air lags to
the north given slow movement of front across SNE so areas of
low clouds and fog may develop. Mild night with lows in the

Anomalous closed low digs SE across the Great Lakes with
confluent flow across New England likely keeping deeper
moisture to the south with mostly dry conditions. However,
can`t rule out an isold afternoon shower/t-storm as marginal
instability develops, mainly south of the Pike and particularly
across RI and SE MA where stalled cold front may provide a focus
for convection. Expect partly sunny skies developing after any
morning stratus/fog burns off. 850 mb temps 12-14C support highs
reaching into the 80s, except cooler along the coast as sea
breezes develop.



* Risk continues for showers and a few thunderstorms possible Fri
  night and Sat, best chance S coast
* Gusty E-NE winds possible along coastal areas, highest along S
  coast, Cape Cod and the islands Sat
* Dry conditions return early next week, but timing uncertain

Overview and model preferences...
Per latest 00Z upper air obs, it`s somewhat difficult to
believe that what is currently a very innocuous wave along the
Canadian border with North Dakota/Minnesota will be one of the
main components to the weekend forecast. This weak wave,
deepening in response to upstream convection, looks to phase
with a Hudson valley wave rotating into Quebec. What occurs
after the phasing is where most significant disagreement begins.
The N cutoff shifts E allowing the the remnant longwave trof
along the E seaboard to meander. GFS has this longwave cutting
off completely, and hence is the slowest to shift the pattern.
ECMWF/NAM are more progressive with ensembles falling on either
side. For this update, given GFS seems to be an outlier, will
lean more on the more progressive ECMWF solution with this
update. All-in-all with stalled front to the S, a slow frontal
low pres associated with this wave will pivot, mainly SE of the
40/70 benchmark Sat, and slow as the system continues to dig.
This brings about a prolonged period of NE flow off the Gulf of
Maine and how long it last is based on the depth of the longwave
trof. Using the ECMWF timing, will release it from the throes
of this feature by early in the work week, but this remains
somewhat uncertain. Beyond this timing, a period of ridging is
likely, as the longwave trof weakens and the pattern reloads.


Fri night into Sun...
Convection firing along a stalled warm front draped from the
mid Atlantic, although there remains the uncertainty of
convective feedback, will assist in generating a low pres wave
which will traverse the front slowly late Fri night into Sat.
This allows for the development of NE flow, drawing air across
the Gulf of Maine where SSTs rest between the upper 50s and low
60s. The combination of cooling flow and modest increase in
moisture should lead to increased cloud cover through the
period, culminating in temps remaining below normal. PWATs,
closely associated with the low pres hover near 2.00 inches and
ensemble probabilities keep the core of this higher moisture
mainly offshore. Combine this with the influence of high pres
from the NW under anticyclonic flow, and expect a very tight
gradient in QPF and therefore, POPs as well. Will be tightening
the POP/QPF gradient somewhat with this update based on the
ECMWF/NAM blending suggested above. However, along S coastal
areas, some heavy rain is possible given the higher PWATs and
conditionally unstable soundings. Low risk for 1.00+ inches of
rain, but very uncertain on this axis as it could remain
entirely offshore. Overall, a period of potentially
gray/cool/damp weather, with the highest risk for heavy rain
along the S coast.

Temps as much as 5-10 degree below normal except for where
there is a chance for breaks of sun, mainly NW MA. Raw 2m more
supportive of this than MAV/MET data. Otherwise, risk for some
gusty NE winds as LLJ is nearly 5 std deviations above normal,
out of the E. Coastal areas show 40-50 kt E-NE LLJ.

Mon and Tue...
Gradual improvement as the longwave trof slides slowly E
assuming no cutoff forms. The increased anticyclonic curvature
and induction of drier air in the form of subsidence should
allow for more breaks in clouds and a much lower risk in
precipitation as the higher theta-e ridge/PWATs and strongest
LLJ shifts further offshore. Filling low pres wave noted as
well. Moderate NW flow as well, suggest a slight moderation in
temps at least closer to (but still below) normal as the shallow
cold air should give way to better mixing, reaching H85 where
average temps run about +12C yielding low-mid 80s, however,
given the uncertainty, this will be dependent on how quickly the
low pres clears the SE waters.

Wed into late week...
As the longwave trof gives way to resurgence of upstream
ridging, more zonally oriented flow should allow for a return of
drier with temps near to potentially above normal given current
timing. Still uncertain as front begins to parallel the mid-
upper lvl flow, and this setup is strongly dependent on the
timing of the phased wave system this weekend.


Forecaster Confidence Levels...

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

15Z Update...

Not much change from previous TAFs. VFR slowly lowers to
marginal MVFR-VFR this afternoon. Showers over western MA into
NY state overspread CT/RI and all of MA this afternoon. Isolated
thunderstorm possible but main threat is heavy downpours, not
strong winds. Earlier discussion below.


Today...Moderate confidence.
VFR to start but a period of MVFR CIGS possible late this
morning and afternoon accompanied by a period of rainfall. Best
chances for this will be north of the Mass Pike as areas south
should retain their VFR CIGS or will only see MVFR briefly. Then
gradual reduction through MVFR it IFR by late evening first due
to CIGS, then with some fog/vsbys late.

Tonight...Low confidence due to uncertainty with extent of
developing stratus and patchy fog. May see IFR/MVFR conditions
developing. Scattered showers and isold t-storms in the evening
should push offshore to the south overnight.

Friday...Moderate confidence. Areas of MVFR/IFR early,
improving to mainly VFR conditions. Low risk of a few afternoon
showers/t-storms, mainly south of the Mass Pike.

KBOS Terminal...Moderate confidence. A few showers/isold
t-storm possible in the afternoon and evening but VFR should
prevail. MVFR cigs likely developing tonight but low confidence
in cigs lowering to IFR late tonight.

KBDL Terminal...Moderate confidence. A few showers/isold
t-storm possible in the afternoon and evening but VFR should
prevail. MVFR cigs likely developing tonight but low confidence
in cigs lowering to IFR late tonight.

Outlook /Friday Night through Monday/...

Fri night into Sat night...Moderate confidence.
Area of rain/fog and low clouds possible mainly close to the S
coast. This could be coincident with periods of MVFR/IFR
conditions. Meanwhile areas across NW MA and extreme W CT could
stay mainly VFR. Otherwise, winds gusting 25-30 kt possible,
especially across SE MA/Cape Islands and RI out of the NE.

Sun and Mon...Moderate confidence.
Some improvement possible as rain/fog and lower CIGS shift
gradually E of the region, however exact timing uncertain. This
improvement would see more widespread VFR conditions develop
where not already observed. Otherwise, winds still gust 20-25 kt
mainly across SE MA/RI.



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

Short Term /through Friday/...High confidence.

Today...S/SW winds with gusts up to 20 kt and seas below SCA.
Late afternoon shower/t-storm possible.

Tonight...Diminishing SW winds shifting to west overnight.
Light seas. Patchy fog may reduce vsbys. Evening shower/t-storm

Friday...Light winds becoming E/SE in the afternoon with sea
breezes developing nearshore waters. Light seas.

Outlook /Friday Night through Monday/...

Fri night...Moderate confidence.
E-NE winds begin to build, reaching 20-25 kt by the early Sat AM
hours. This also allows seas to build to 3-4 ft. Some rain/fog
possible, mainly during the late evening early morning.

Sat into Sun...Moderate confidence.
Low pres will slowly pass south of the waters, bringing a period
of rain/fog and potential for occasional thunderstorms. NE
winds, could gust to low end gales mainly Sat and Sat night with
seas 7-9 ft offshore. Gale warnings may ultimately be needed, or
at the very least, high end small craft advisories.

Mon...Moderate confidence.
N-NE winds continue, but gusts should generally stay around 25
kt with seas still 5-8 ft. Small craft advisories are likely to
continue even as rainfall/fog improves slowly.




NEAR TERM...KJC/Nocera/Doody
MARINE...KJC/Doody is the U.S. government's official web portal to all federal, state and local government web resources and services.