Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 272359
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
759 PM EDT Tue Jun 27 2017

.SYNOPSIS...
Upper shortwave and associated jet stream will maintain
showers and thunderstorms through this evening. A few of the
storms may contain gusty winds and small hail. High pressure
builds fair and dry weather for Wednesday. A warming trend
begins for the latter half of the week. Expect summer heat and
humidity from Friday through the weekend. A warm front passes
Thursday night, bringing scattered showers and thunderstorms.
More showers and thunderstorms are possible from Friday through
the weekend. A cold front tries to shift toward the region early
next week.

&&

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

740 PM Update...
Last line of showers/thunderstorms moving across NE CT/Central
Mass, including severe storms in NE CT/central and NW Mass.
These storms should weaken by around 02Z. Have kept enhanced
wording through 02Z in the zones for strong winds and small
hail.

Noting a weak wave formed on the trough as it slowly crossed E
NY state by around 18Z and slowly shifted E through this
evening. As it hit the CT valley, with some weak surface
convergence in place at around 22Z-23Z as seen on surface obs,
convection re-fired across N central CT along one line, and
another area that shifted out of western Franklin and western
Hampshire counties before causing damage in eastern Franklin
county in the Orange area.

Have updated near term to keep showers/tstms going through
02Z-03Z then should weaken. Also updated remainder of forecast
to bring conditions current.

Previous Discussion...
Once the convection dies down, expect dry weather the remainder
of the night. Light winds and mostly clear skies will allow fog
to form in some spots, especially parts of the CT Valley.

Dew points in the upper 40s and 50s should leave room for temps
to cool down into the 50s most places by early morning.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
The upper trough is overhead along with its cold pool. So
continued support for lift. But the airmass will be much drier
with a moist layer limited to the 750-850 mb layer. Expect
diurnal clouds but precip is unlikely. Temps aloft will be
equivalent to 8-10C at 850 mb, supporting max temps 75-80.

High pressure maintains mostly clear skies and light wind should
allow overnight temps to reach the 50s, with lower 60s in the
urban areas.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights...

* Scattered showers/thunderstorms from Thursday night through
  Sunday night
* Very warm, humid conditions return Friday, lasting through
  the weekend

Overview...

12Z medium range models and ensembles continue same theme for
summer conditions. Active northern stream in place with a broad
mid level fast flow that looks to continue through the upcoming
weekend at least.

H5 short wave/cutoff low moves out of Saskatchewan and Manitoba
on Thursday, which will push across southern Canada into the
northern Great Lakes through this weekend, then into Quebec
Monday. Weak short waves will move along in the general W-SW
steering flow around the base of the cutoff low through the
broad trough to its S and SW. Will see rounds of scattered
showers/thunderstorms as each short wave moves across. Very
sultry airmass in place thanks to Bermuda high sitting off the
Carolina coast. Will see dewpts rising to the mid-upper 60s late
this week through the weekend. The offshore high and western
Atlantic mid level ridging will cause an approaching cold front
to stall west of the region.

Question will be when the H5 trough will move across the region
early next week. Model solution spread leading to lower
confidence on timing of cold front that may approach around
Monday or the 4th of July.

Details...

Thursday-Thursday night...
High pressure off the mid Atlantic/SE U.S. coast will keep dry
conditions across the region through about midday Thursday. A
warm front will start to approach, with some showers that may
develop across N and W Mass, mainly N of the Mass Pike around or
after 21Z. Will see some more instability start to work into W
Mass/N central CT Thu night as the warm front moves across, so
have included chance thunder from W-E.

Highs Thursday will mainly be in the lower-mid 80s away from
the S coast, only in the 70s along the S coast. Overnight lows
will be in the mid-upper 60s.

Friday through Sunday...
With the ridge in place off the coast, continued SW flow
through this period. Typical summertime airmass in place, with
dewpts up to the mid and upper 60s with a few spots that may
touch 70, along with H85 temps rising to +16C to +18C Fri-Sat.
With the soupy airmass, will likely see nighttime fog develop
mainly around or after midnight as temps fall back close to the
higher dewpts.

Will see scattered showers and thunderstorms from the midday
through evening hours each day, especially across the interior.
Looks like best shot for convection will occur Friday
afternoon/evening, then again Saturday especially over western
areas. The ridge offshore may allow for somewhat drier air and
subsidence to move into coastal areas on Saturday, so could be
drier during the daylight hours. Approaching cold front on
Sunday will trigger another round of convection as well.

Will likely see some brief heavy downpours in some activity
thanks to PWATs increasing to 1.5 to 1.8 inches, especially
Friday afternoon/evening and again Sat night-Sunday.

Some question as to whether some stronger storms will develop
during this timeframe. Will continue to monitor this aspect.

One other issue is possible heat headlines on Fri and Sat, due
to combination of high temps (lower 90s) and dewpts (up to 70
degs). Could reach high indices close to 95, which would trigger
possible advisories. Current forecast suggests lower 90s. Will
monitor this as well.

Monday-Tuesday...
Low confidence on this portion of the forecast due to model
solution spread, mainly with frontal passage. Some question on
frontal passage late Sun night or Mon morning which will wash
out rather quickly as winds shift back to SW during Mon
afternoon. Another front may approach during Tue which may
trigger more convection.

&&

.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

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

Tonight...

Updated 750 PM...

Areas of MVFR CIGS/VSBYS in sct showers/tstms with hail and
strong wind gusts possible through 02Z-03Z. Expect conditions to
improve to VFR around or after 03Z. Areas of fog possible after
midnight with the best chance in the CT River Valley north of
Springfield.

Wednesday...

VFR and drier weather. Showers/tstms will fire again north of
Mass, but all quiet in Southern New England. West-northwest
winds.

Wednesday night...

VFR. Light winds and clearing skies. Patchy fog possible late at
night.

KBOS Terminal...High confidence in TAF. Showers and T-storms
likely this evening. A few of the stronger storms may contain
small hail and gusty winds.

KBDL Terminal...High confidence in TAF. Showers and T-storms
diminish this evening.

Outlook /Thursday through Sunday/...

Thursday...High confidence.
Mainly VFR. Dry through midday, then scattered afternoon/
evening showers across N and W Mass mainly N of the Mass Pike.
Showers/ thunderstorms possible entire region Thu night. Gusty
SW winds to 20- 30 kt developing along the S coast late Thu/Thu
night.

Friday through Sunday...Moderate confidence.
Patchy early morning fog each day with local MVFR-IFR
conditions. Otherwise mainly VFR. Local MVFR-IFR in possible
strong thunderstorms, mainly during the midday through evening
hours.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Tonight...

Updated 750 PM...

Showers and thunderstorms over land will move over parts of the
waters this evening. Best chance will be along the Massachusetts
North Shore.

Have ended the small craft for the nearshore waters as S-SW
winds have diminished. Expect gusts up to 20 kt through around
04Z or so. Seas generally 3 feet or less.

Wednesday and Wednesday night...

Winds become WNW at modest speeds. Seas 3 feet or less.

Outlook /Thursday through Sunday/...

Thursday...Moderate confidence.
Small craft advisories will likely be needed. SW winds will
increase, with gusts to 25-30 kt highest across the southern
outer waters. Seas build to 6 to 8 ft over southern waters Thu
night.

Friday...Moderate confidence.
SW winds continue, gusting to around 25 kt, diminishing after
midnight Fri night. Seas up to 5 to 7 ft. Scattered
thunderstorms, some with reduced visibilities and strong gusty
winds. Patchy late night fog.

Saturday-Sunday...Moderate confidence.
SW winds continue, though diminishing. Seas around 5 ft on the
outer waters Sat, then slowly subside. Patchy late night fog
each night with reduced visibilities. Chance of
showers/thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...WTB/EVT
NEAR TERM...WTB/EVT
SHORT TERM...WTB
LONG TERM...EVT
AVIATION...WTB/EVT
MARINE...WTB/EVT


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