Special Weather Statement, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
255 PM EDT Tue Jul 14 2020

.SYNOPSIS...
A few severe thunderstorms are possible through this evening,
with the main threats being damaging wind gusts, hail, and flash
flooding. High pressure will build over the region Wednesday and
Thursday with dry and cooler weather, but clouds will linger. A
warm front will bring scattered showers and storms Friday,
before warmer and more humid weather returns this weekend. A
cold front should bring more showers and storms early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
3 PM Update:

Radar indicates storms are starting to focus across RI and SE MA
with a few more storms developing across NE MA. Seeing a lot of
storm blowoff on satellite across much of eastern MA which may
inhibit further storm development aside from RI, despite
presence of 1000-1500 J/kg of mixed layer CAPE, 20-30kt of
effective shear and mid level lapse rates near 6.5C/km. Need to
keep a close eye on any storms that develop in SE MA for
greatest severe potential, as that is the area where all 3
conditions overlap, but presence of higher cloud cover may put a
lid on that.

Storms should gradually diminish this evening but cold pool
aloft may help storms maintain themselves past sunset. High
pressure builds north of region tonight which will result in NE
flow and bring in plenty of low level moisture, enough to
maintain cloudy skies overnight. Somewhat drier air (lower
dewpoints) should prevent much in way of fog formation.

Lows will fall back into upper 50s to mid 60s by daybreak.

&&

.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
Continued NE flow will bring drier and cooler weather with highs
in 70s to around 80 and overnight lows in 50s to lower 60s.
Dewpoints are expected to drop into 50s and lower 60s so we will
lose tropical humidity that we've dealt with past few days.

That said, model forecast soundings show plenty of low level
moisture trapped beneath subsidence inversion. That means
cloudiness will be tough to erode, resulting in cloudy skies
with some breaks of sun, especially across CT and western MA.
Additionally, there may be a few sprinkles near Berkshires due
to upslope flow.

&&

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

 * Showers/storms Friday
 * Turning warmer/more humid over weekend
 * Showers/storms again Monday-Tuesday

On larger scale, northern stream begins to retreat over weekend
as we come more under influence of broad upper ridge building
over southern 2/3 of country. Models suggest we remain on
northern periphery of ridge, which may very well keep more
oppressive heat/humidity to our south over weekend and early
next week, but odds favor summertime heat and humidity returning
to SNE over weekend and early next week.

Pattern suggests a few chances of showers/storms - first Friday
as warm front lifts through region, and second in Monday/Tuesday
timeframe as pre-frontal trough is expected to cross area in
advance of cold front which should still hang back across
eastern Great Lakes.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

18z TAF Update: Moderate confidence.

Scattered showers/storms will bring local MVFR/IFR conditions.
Focus should shift from CT Valley to central/eastern MA and RI
later this afternoon. Meanwhile, IFR ceilings near Cape Cod and
Islands should lift by mid afternoon, resulting in VFR
conditions.

MVFR/IFR ceilings prevail tonight and will be slow to erode
Wed but expect gradual improvement to MVFR and lower-end VFR
ceilings Wed afternoon and Wed night.

KBOS Terminal...Moderate confidence in TAF, mainly due to
uncertainty with location of TS. Main time frame should be
19Z-23Z but per coordination with CWSU ZBW, leaving out mention
due to low probability.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday through Sunday/...

Wednesday Night through Thursday: Mainly VFR, with local MVFR
possible.

Thursday Night: VFR. Breezy. Slight chance SHRA.

Friday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, chance TSRA.

Friday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA, slight chance TSRA.

Saturday and Sunday: VFR.

&&

.MARINE...
Small Craft Advisories will be allowed to expire this evening on
open south coastal waters where seas will subside, but issued
another one for eastern MA waters tonight and Wed as increasing
NE winds (below 25kt) bring building seas of up to 6 ft as high
pressure builds north of region. NE winds diminish Wed night,
allowing seas to subside once again.

Scattered storms will affect south coastal waters through this
evening, some of which may produce hail and 30-40kt wind gusts,
especially near shore. Dry weather expected Wed into Thu.

Outlook /Thursday through Sunday/...

Thursday through Thursday Night: Winds less than 25 kt.

Friday: Winds less than 25 kt. Chance of thunderstorms, slight
chance of rain showers.

Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, chance of thunderstorms.

Saturday and Sunday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Surf Advisory until 8 PM EDT this evening for MAZ023-024.
RI...High Surf Advisory until 8 PM EDT this evening for RIZ008.
MARINE...Small Craft Advisory from 9 PM this evening to 6 PM EDT
     Wednesday for ANZ250-254.
     Small Craft Advisory until 8 PM EDT this evening for ANZ255-
     256.

&&

JWD

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Special Weather Statement
National Weather Service Boston/Norton MA
421 PM EDT Tue Jul 14 2020

MAZ006-007-142100-
Eastern Essex MA-Western Essex MA-
421 PM EDT Tue Jul 14 2020

...LINE OF STRONG THUNDERSTORMS APPROACHING ESSEX COUNTY...

At 420 PM EDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Brentwood to Haverhill to Boxford.
Movement was east at 30 mph.

Penny size hail and winds in excess of 40 mph will accompany these
storms. Torrential rain can also lead to localized urban poor
drainage flooding.

Locations impacted include...
Lawrence, Haverhill, Methuen, Beverly, Andover, Gloucester, North
Andover, Danvers, Newburyport, Amesbury, Ipswich, Middleton,
Salisbury, Georgetown, Boxford, Hamilton, Rockport, Newbury,
Groveland and Merrimac.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Get indoors when these storms approach. Winds may be strong enough to
produce minor damage, such as a few downed branches.

Do not drive through flooded roads or underpasses. Avoid low lying
areas near small streams.

Frequent lightning was occurring. Lightning can strike far away from
the storm. Go inside a building or vehicle. Wait at least 30 minutes
until after the storm has passed to resume outdoor activities.

&&

LAT...LON 4287 7082 4282 7085 4270 7077 4270 7078
      4277 7082 4270 7083 4267 7073 4264 7076
      4269 7063 4264 7059 4258 7066 4259 7066
      4260 7065 4255 7080 4264 7112 4281 7116
      4281 7107 4286 7103 4289 7093
TIME...MOT...LOC 2020Z 284DEG 24KT 4292 7112 4280 7109 4268 7104

Chai

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