Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 250201
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1001 PM EDT Sat Jun 24 2017

.SYNOPSIS...

Weak high pressure brings mostly clear conditions and light winds
tonight, as well as lower humidity. A fast moving disturbance
moves across New York and Northern New England late Sunday,
bringing scattered showers to northern parts of our area. A weak
cool front moves across the area late Tuesday. Then a warming
trend begins for the latter half of the week with a return to
summer heat and humidity by Friday. Thunderstorm chances will
also increase late in the week as a slow-moving cold front moves
across southern New England Friday into Saturday.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...

10 pm update...

Cool, dry, comfortable night ahead. A few showers linger into W
New England associated with mid-level ascent upon lingering
moisture. Light showers / sprinkles noted for W MA and W CT as
we go through the remainder of the evening, dissipating around
midnight.

Otherwise dry air advection continuing to proceed as winds remain
light beneath clear conditions. An opportunity for radiational
cooling, closely evaluating surface dewpoints to get an idea of
how cool it can get. Leaned towards lowest of guidance, lows down
around the mid to upper 50s with spots notoriously known for
radiating out effectively (i.e., Marthas Vineyard) potentially
falling into the low 50s.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...

Broad cyclonic flow across the northern USA, but with a strong
120-knot jet moving through the flow and driving features
quickly through the flow.

Shortwave over the Great Lakes Sunday morning races northeast,
crossing Upstate NY in the afternoon and Northern New England
Sunday night. This shortwave will carry enough cold air at 500
mb to destabilize the airmass across these areas Sunday. The
instability parameters are consistent with this, and favor
thunder to our north and west but not in Southern New England.
Even so, we will be in the right entrance region of the upper
jet, which will favor upper venting and generate some lift. We
will forecast a chance of afternoon/evening showers, primarily
across Western and Central Mass.

The mixed layer is forecast to reach to at least 800 mb and
possibly 750 mb. Winds in layer will be 15-20 knots, so expect
southwest winds gusting to those speeds in the afternoon.
Temperatures at 800 mb will be 7-9C, equiv to 12-14C at 850 mb.
This would suggest max sfc temps in the low to mid 80s.

After the shortwave passes to our north, any showers should
diminish. For most folks, it should be a fair night with temps
in the 50s.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...

Highlights...

* Cooler with a few showers/isold t-storm possible Mon into Wed
* Warming trend toward end of the week with return to summer
  heat/humidity by Fri

Details...

Monday into Tuesday night...

An upper level trough moves eastward from the Great Lakes on
Monday and exits New England Tuesday night. Temperatures cool
aloft Monday, making for somewhat unstable air. The column will
be lacking a lot of moisture, as indicated by low K indices.
The best chances for any showers or an isolated thunderstorm
will be in western and northern MA Monday afternoon/evening.
On Tuesday, with the axis of the cold pool aloft over New
England, there is a better chance of scattered showers and
thunderstorms across the entire region. Total Totals Indices
reach the lower 50s across the northwestern two-thirds of
southern New England. Highs mainly 75 to 80 both days over the
interior with comfortable overnight lows in the 50s except near
60 over Cape Cod.

Wednesday into Thursday morning...

Looks like a dry pattern Wednesday into Thursday morning with
the upper trough having exited and a surface high pressure
ridge building over the Mid-Atlantic states. K Indices will be
very low and surface dewpoints will be in the upper 40s to lower
50s. Highs Wednesday will warm a bit...into the lower 80s.
Minimum relative humidities will approach 30 percent but winds
should be light, preventing fire weather concerns.

Thursday afternoon and Friday...

On Thursday, heat and humidity begins to return around the
western periphery of high pressure near Bermuda and in advance
of an approaching low pressure/cold front system in the eastern
Great Lakes. High temperatures are forecast to reach the mid and
upper 80s. A strong southwesterly low level jet will develop
during the day, with models indicating 40-50 kts at 925 mb.
Mixing should allow wind gusts to reach 25-30 mph at the
surface, especially in eastern MA and RI. Have raised winds
above guidance values.

With south-southeast surface winds and strong southwest to west
winds aloft, forecast 0-3km helicities are above 300 m2/s2
across our region with significant 0-1km values as well. The
setup is there for potential severe weather. However, the day is
dry to start. But both the GFS and ECMWF show a rapid influx of
34-36 K indices into western sections between 18-22Z Thursday
late afternoon. The implication is that convection forms over
the eastern Great Lakes, moves across New York state and into
CT/western MA late in the day. The source of the
convection...the cold front...will remain well to our west so it
should weaken as it heads east Thursday night, but with a high
shear/high helicity environment in place here, we will need to
watch for isolated severe thunderstorm potential.

On Friday, summertime heat and humidity will have peaked with
highs reaching the upper 80s and lower 90s and dewpoints
reaching the upper 60s to near 70 in the CT River Valley by
evening. Depending on model solutions, the cold front will be
either approaching or moving through on Friday afternoon.
There should be strong CAPE and still some 0-3km helicity near
or above 150. Am forecasting 40-50 percent chances of
thunderstorms. The potential does exist for severe storms Friday
into Friday evening.

Saturday...

The Saturday forecast depends on the timing of the
aforementioned cold front. The 12Z model runs have slowed this
front down and consequently, another very warm day is forecast,
with highs in the mid and upper 80s and continuing chances of
showers and thunderstorms. Cooler weather may wait until late in
the weekend, beyond this forecast period.

&&

.AVIATION /02Z 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.

Tonight...
VFR. Mainly SKC. Light W winds. Initial risk of -SHRA over W MA
and W CT towards 3-4z, pushing E as FEW-SCT low-end VFR CIGs,
eroding.

Sunday...
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.

Sunday night...
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...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/...High confidence.

Tuesday...
Mainly VFR. A few diurnally driven showers and perhaps an isold
t-storm northwest sections Monday and across much of the region
Tuesday.

Wednesday...
VFR.

Thursday...
VFR to start, although some patchy fog possible along the south
coast early. Areas of MVFR cigs and vsbys in scattered showers
and t-storms developing in western MA and western CT late in the
afternoon and spreading eastward Thu night.

&&

.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 through Wednesday...High confidence.
Quiet boating weather with winds and seas below SCA. A few
hours of nearshore lower 20 knot wind gusts are possible each
afternoon.

Thursday...Moderate confidence.
Southwest winds will gust to near Small Craft Advisory levels
as our region is in between high pressure over Bermuda and low
pressure over the eastern Great Lakes. Seas will be increasing
to 4 to 6 ft over the outer waters, too. Chance of showers and
thunderstorms Thursday night.

&&

.TIDES/COASTAL FLOODING...

Astronomical tides are rather high through this weekend during the
night time cycles. Boston has a 12.4 ft high tide forecast around
midnight tonight and 12.2 ft just after midnight Sunday night.

While most places had their highest astronomical tide of the
year last month, a few spots along our coast will have it with
this June set of highest tides. Either way, the difference
between the May and June highest tides is small.

While offshore winds are forecast, current conditions suggest a
0.4 surge which will result in minor splashover. Thus will
continue a coastal flood statement for tonight`s high tide.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 2 PM EDT Sunday
     for ANZ254>256.
     Small Craft Advisory for hazardous seas until 8 AM EDT Sunday
     for ANZ235-237.

&&

$$

SYNOPSIS...WTB/GAF
NEAR TERM...WTB/Sipprell/GAF
SHORT TERM...WTB
LONG TERM...GAF
AVIATION...WTB/GAF/Sipprell
MARINE...WTB/GAF/Sipprell
TIDES/COASTAL FLOODING...WFO BOX Staff



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