Small Craft Advisory 97, NWS Boston/Norton, MA

Area Forecast Discussion
National Weather Service Boston/Norton MA
708 PM EDT Wed Jul 17 2019

.SYNOPSIS...

The threat for thunderstorms and localized flash flooding this
evening was now confined to mainly Rhode Island and southeast
Massachusetts. This activity will push south through the
evening and we may see a lull in the activity. Additional
showers are expected overnight into Thursday, which may also
result in localized flooding. Hot and humid Friday through
Sunday with heat- related headlines likely but will be watching
for possible surprise thunderstorm activity. A sweeping cold
front Monday with a scattered line of showers and thunderstorm
Monday behind which relief in the form of seasonable
temperatures, lower humidity is on tap into late- July.

&&

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

7 pm update...

We continue to trim the Flash Flood/Severe Thunderstorm watches
further south with the cold front. The threat for any isolated
severe/localized flooding will be from near Providence to
Plymouth, points south and east. However, this risk will be
isolated given that the outflow has surged southeast of the main
area of convection.

Otherwise...just some scattered showers the rest of the night.
Areas of dense fog will be a concern for the islands and perhaps
further northwest onto the south coast. Low temps will mainly
range from around 70 to the lower 70s.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH 6 PM THURSDAY/...

* Additional showers with localized heavy rainfall will be a
  concern on Thursday mainly south of the MA Turnpike.

After a lull in the activity later tonight, showers should
increase in coverage toward daybreak Thursday. Showers will
continue at times Thursday with the focus south of the MA
turnpike. A few embedded thunderstorms are also possible. We
can not rule out localized flooding mainly south of the MA
turnpike, in the vicinity of a stalled boundary and higher
Pwats. Confidence is rather low though on what exactly will
transpire, but if any flooding occurs it probably will be rather
localized. High temperatures on Thursday will be held mainly in
the 70s with clouds and a northeast flow.

&&

.LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/...

*/ Highlights ...

 - Hot and humid Friday thru Sunday, likely heat-related headlines
 - Sweeping cold front with possible showers, thunderstorms Monday
 - Relief with lower humidity, comfortable conditions after Monday

*/ Overview ...

Mid-July heat, late-July relief. S U.S. heat bubble brought about by
Gulf of AK low (+EPO) and SW CONUS monsoon high. Contributions from
the two, subsequent heat pump and higher heights per H85 temperature
anomalies around +5C, for example. Near-flat flow across the N CONUS
and S Canada, will be watching closely heat / humidity / instability
preceding any ejecting N Pacific mid-level impulses, whether there's
any shower and/or thunderstorm activity, plus Barry's remnants. But
into late-July a possible H5 pattern retrograde. Shift to a -EPO as
the 4-corners monsoonal H5 high builds. Deeper E CONUS H5 troughing
with prevailing NW flow aloft points towards relief of cooler, drier
Canadian air being ushered S. CPC 8-14 day outlook hints at possible
below-average surface temperatures for the Great Lakes into the NE
CONUS. Break down the details below.

*/ Details ...

Friday through Sunday ...

Hot and humid. H85 temperatures around +20-22C, H7 around +10-12C.
But concern. Steep mid-level lapse rates around Saturday, a lot of
low-level instability. Shear vectors and orientation that suggest
that if something were to become organized over Michigan within the
region of steep mid level lapse rates and advect that perhaps we
could see MCS / derecho activity Saturday and Sunday. Initial
thinking, but nothing set in stone. Initial indications per 17.12z
NAM and 17.0z EC, but not much indication in SREF / GFS. It would
subsequently impact 2m temperature with cloud cover altering the
heat and humidity forecast. Typically when we get the heat and
humidity where highs alone approach triple digits do we see via
climatology MCS / derecho events. Continued forecast of mid to
upper 90 highs, some locations at or just slightly above 100
with dewpoints in the low 70s as boundary layer moisture is
trapped beneath the cap, fueled by S flow, yielding heat indices
around the triple digits, for some locations at or above 110F.

Beginning Monday ...

Relief. Pattern change discussed in OVERVIEW, a series of sweeping
cold fronts will prevail beneath preferred H5 troughing across NE N
America. Usherance of cooler, drier air S out of Canada along the
leading edge of which a line of scattered shower and thunderstorm
activity is to be expected, Monday into Monday night especially.
Indications of a decent plume of continental-tropical moisture that
precedes the cold front. Timing crucial as early indications suggest
modest uni-directional deep-layer SW shear, accompanying instability
plume nudging into S New England. Low confidence forecast, something
to closely monitor. Seasonable temperatures around the low to mid
80s with much lower humidity, comfortable conditions after the cold
front sweeps through.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...Moderate confidence.

Tonight...The bulk of the showers and thunderstorms were mainly
along and southeast of a Providence to Plymouth line and will
push south of the region over the next few hours. Otherwise,
much of the overnight hours will feature just a few showers .

Outside the activity, VFR conditions should prevail much of
tonight. However low clouds and fog across the Cape/Islands will
begin to expand northwest toward daybreak.

Thursday...MVFR to IFR conditions expected in rounds
of showers and perhaps a few embedded thunderstorms. The
heaviest rainfall may setup near and south of the MA
Turnpike...which may result in brief LIFR conditions. LIFR
conditions also possible in fog across the southeast New England
coast.

KBOS Terminal...Moderate confidence in TAF.

KBDL Terminal...Moderate confidence in TAF.

Outlook /Thursday Night through Monday/...High confidence.

Thursday Night: Mainly VFR, with areas IFR possible. Chance
SHRA, slight chance TSRA.

Friday: Mainly VFR, with local IFR possible.

Friday Night: VFR.

Saturday through Sunday Night: VFR. Slight chance SHRA,
isolated TSRA.

Monday: Mainly VFR, with local MVFR possible. Chance SHRA,
slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

Tonight and Thursday...Main concern this evening in
thunderstorms with locally strong wind gusts. Showers and
isolated t-storms will continue at times overnight into Thursday
along with areas of fog. Also, will have to watch for a period
of northeast wind gusts of 20 to 25 knots across the northern
waters behind the remnants of Barry. We may need to hoist a
small craft advisory.

Outlook /Thursday Night through Monday/...High confidence.

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

Friday: Winds less than 25 kt. Seas locally approaching 5 ft.

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.

Saturday: Winds less than 25 kt. Seas locally approaching 5 ft.
Slight chance of rain showers.

Saturday Night through Sunday: Winds less than 25 kt. Seas
locally approaching 5 ft. Slight chance of rain showers,
isolated thunderstorms.

Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers, isolated thunderstorms.

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers, slight chance of thunderstorms.

&&

.CLIMATE...
Record highs for Wednesday July 17 are as follows:

BOS: 98F in 1977 & 1999
BDL: 97F in 1999
PVD: 97F in 1977 & 1999
ORH: 93F in 1900

Record highest min temps for Saturday July 20:

BOS: 75 in 2016
BDL: 73 in 1975
PVD: 77 in 1983
ORH: 72 in 2013

Record highs for Saturday July 20:

BOS: 99 in 1991
BDL: 100 in 1991
PVD: 101 in 1991
ORH: 92 in 1901

Record highest min temps for Sunday July 21:

BOS: 81 in 1991
BDL: 74 in 1977
PVD: 77 in 1980
ORH: 73 in 1991

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flash Flood Watch until 11 PM EDT this evening for CTZ002>004.
MA...Flash Flood Watch until 11 PM EDT this evening for MAZ012-013-
     016>022.
RI...Flash Flood Watch until 11 PM EDT this evening for RIZ001>007.
MARINE...None.

&&

SYNOPSIS...Frank/Sipprell
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...Sipprell
AVIATION...Frank/Sipprell
MARINE...Frank/Sipprell
CLIMATE...Staff

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