Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 251110
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
710 AM EDT Sun Jun 25 2017

.SYNOPSIS...
Daytime scattered showers and possible thunderstorms will be the
story for today and again Monday. Another upper level disturbance
may bring a few showers or an isolated thunderstorm Tuesday. Then
a warming trend begins for the latter half of the week with a
return to summer heat and humidity by Friday. A warm front may
bring some showers Thursday with more showers and thunderstorms
possible Friday and Saturday. A backdoor front may drop south
into the region Saturday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
7 AM update...

Sunshine to start the morning will mix with afternoon diurnal
clouds. Rather robust shortwave across MI tracks east to the St
Lawrence Valley late today with modest mid level cooling across
western New Eng. Steepening low level lapse rates and
moistening in the 850-700 mb layer may trigger a few afternoon
showers in the interior, especially northern and western MA.
Also can`t rule out an isold t-storm but soundings suggest this
is a low probability. Better chance will be across northern NY
and VT closest to the core of the cold pool aloft. Minor
adjustments to PoPs, otherwise forecast unchanged.

Previous discussion...

Scattered showers and possible thunderstorms with breezy SW winds.
Cyclonic flow and attendant elongating vortmax energy through the
mid-latitude long-wave trough pattern yields ascent upon available
moisture. Within an environment of favorable lapse rates and deep
boundary layer mixing, the better of which is N/W beneath a more
favorable H5 cold pool, diurnal forcing yields scattered to broken
cumulus along with the wet-weather activity. Emphasis along the high
terrain given the low-level wind profile and subsequent convergent
forcing. With the deep boundary layer mixing, instability potential
up to 500 j/kg fuels possible thunderstorms, while allowing the mix-
down of faster momentum and drier air. Likely will over-achieve on
highs and thus leaned warmest of guidance, around the low-mid 80s.
Dewpoints likely to drop off into the upper 40s to low 50s across
the interior netting less humid, more comfortable conditions. Will
be breezy out of the SW with gusts up to 20 mph possible. Some hints
per high-res guidance of the possibility of an immediate onshore sea-
breeze, but feel the SW flow will be strong enough to keep it off-
shore.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
Tonight...

Turning quiet. Diurnal forcing with daytime heating concludes and so
does cloud and shower activity. Should see the two dissipate towards
midnight. Clearing out and winds becoming light out of the W, there
is an opportunity for radiational cooling. Some question though as
to whether clouds linger and act as a blanket. For now will lean
towards cooler guidance with lows around the mid 50s given just how
dry it will become earlier in the day.

Monday and Monday night...

Showers and thunderstorms possible again. Pretty much a repeat of
today and tonight (see the discussion above). However, wet-weather
activity may be delayed on the point that favorable ascent may be
delayed till the later-half of the day. Acting upon a more favorable
environment of steep lapse rates across S New England as the H5 cold
pool pushes S/E. Another day of over- achieving highs with low humidity
dewpoints. Wet-weather activity lingers into the evening hours from
earlier in the day, gradually dissipating as conditions clear and
winds become light. Leaning the warmest guidance during the day,
coolest guidance at night given synoptic conditions.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
Highlights...

* A few showers/isold t-storm possible Tue/Wed
* Warming trend toward end of the week with return to summer
  heat/humidity by Fri
* A few showers possible Thu with sct showers/t-storms Fri/Sat
* Backdoor front possible Sat but low confidence

Overview...

Mid level trof will begin to exit the northeast Wed, followed by
building heights as subtropical ridge builds over the western
Atlantic. Polar jet will be in the vicinity of New Eng late in the
week into the weekend which will leave us susceptible to weak
shortwave passages and bouts of showers and t-storms. Temps starting
out slightly below normal Tue then warming through the end of the
week. Will have to watch a backdoor front for next Sat.

Details...

Tuesday and Wednesday...

Fairly robust shortwave lifts NE from the Gt Lakes Tue with mid
level trof moving into New Eng by Tue evening before exiting on Wed.
Increasing dynamic forcing and 500 mb temps cooling to -18 to -20C
will bring sct showers and isold t-storms to SNE Tue into Tue
evening with best chance in the interior. ECMWF is showing a more
amplified trof with a second shortwave rotating around and inducing
a coastal low south of New Eng Tue night with area of rain for SE
New Eng. This does not have support of other global models so we
have discounted for now but something to watch in later forecasts.
On Wed, mid level trof begins to exit the region but 500 mb temps
around -21C through 18z before warming toward evening. Moisture is
limited but can`t rule out a few showers given the rather cold temps
aloft. Temps mid/upper 70s Tue and a few degrees warmer on Wed.

Thursday...

Increasing warm advection pattern as warm front approaches from the
SW will lead to increasing clouds. GFS and ECMWF differ on the
positioning of the warm front which impacts sensible weather and
temps. GFS lifts the front to the north with main focus for
showers/t-storms to the north and west. ECMWF holds the front to the
south with wetter and much cooler conditions. A low confidence
forecast this far out so minimal changes were made to PoPs and temps
and will have chc pops in the interior. Higher theta-e air and
better instability remains to the west where best t-storm chances
are. Gusty SW winds developing, especially in the coastal plain as
low level jet develops. The risk for t-storms will increase Thu
night as higher theta-e air moves in.

Friday...

Warm sector airmass with low level theta-e ridge axis across the
region and decent instability so expect sct showers and t-storms
developing. Difficult to pinpoint shear profiles this far out but it
does appear the region will be on the southern edge of stronger deep
layer shear so severe weather is possible. Temps should reach well
into the 80s to near 90 degrees but cloud cover could have some
impact. Very humid with dewpoints near 70.

Saturday...

Tricky forecast as we may be dealing with a backdoor front close to
New Eng. Impossible to know where this front will be but there is
potential for a sharp temp gradient across the region depending on
where the front is located. Very warm/humid south of the front with
temps well into the 80s but much cooler to the north of the boundary
with 60s possible. It will all depend on where the front is.
Continued risk for showers/t-storms, mainly south and west of the
boundary.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...High confidence.

Today...
VFR. SKC initially becoming SCT-BKN across the interior towards
midday and continuing through afternoon. SCT SHRA mainly N/W of
BDL-SFZ-BOS with a lower risk of TSRA. SW winds becoming breezy
with gusts up to 20 kts. Low risk MVFR with -SHRA, higher
confidence over high terrain terminals.

Tonight...
VFR. SCT-BKN CIGs dissipating along with -SHRA chances. W/SW
winds diminishing.

Monday into Monday night...
VFR. Repeat of Sunday into Sunday night. Mostly SKC initially
with SCT-BKN CIGs developing midday and continuing through
afternoon with the threat of -SHRA and a lower threat of TSRA
mainly N/W of BDL-SFZ-BOS. However mid to high level CIGs will
be on the increase from the W late Monday night into Tuesday morn.

KBOS Terminal...
12z update...A weak seabreeze may develop east of BOS around
midday but there should be enough SW flow to keep it just
offshore.

For both Sunday and Monday, will keep most of the -SHRA
activity and lower risk of TSRA N/W. Blustery SW winds during
the daylight hours with gusts up around 20 kts.

KBDL Terminal...Similar thinking as described above for KBOS.

Outlook /Tuesday through Thursday/...

Tuesday and Wednesday...High confidence.
Mainly VFR. A few showers and isold t-storm possible Tue and still
can`t rule out a shower on Wed.

Thursday...Moderate confidence.
Mainly VFR with cigs lowering from NW to SE. Afternoon showers
possible in the interior with showers/t-storms possible all SNE Thu
night. Gusty SW winds developing, especially coastal plain.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...High confidence.

General trend of light winds out of the W during the overnight
periods while becoming blustery out of the SW through the day
with potential for gusts up around 20 kts. This as frontal waves
emerge from the W associated with a broad area of low pressure
over the Great Lakes while high pressure remains dominant over
the NW Atlantic.

Rough seas and swell linger over the S/SE waters through Sunday
around 5 to 6 feet. SMALL CRAFT FOR HAZARDOUS SEAS accordingly.

Outlook /Tuesday through Thursday/...

Tuesday and Wednesday...High confidence. Quiet boating weather with
winds and seas below SCA. Gusts to 20 kt possible nearshore waters.

Thursday...Moderate confidence. Increasing SW winds with gusts to
25+ kt likely, especially over nearshore waters. Building seas may
reach 8 ft over southern waters Thu night. Chance of showers and
thunderstorms Thursday night.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 2 PM EDT this
     afternoon for ANZ254>256.
     Small Craft Advisory for hazardous seas until 8 AM EDT this
     morning for ANZ235-237.

&&

$$
SYNOPSIS...KJC/Sipprell
NEAR TERM...KJC/Sipprell
SHORT TERM...Sipprell
LONG TERM...KJC
AVIATION...KJC/Sipprell
MARINE...KJC/Sipprell


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