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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
1047 PM EDT Wed Sep 18 2019

Synopsis...
high pressure builds over New England tonight and lingers over
the region through early next week. Below-normal temperatures
late this week will trend to above-normal temperatures over the
weekend and early next week. A cold front brings the next chance
of showers early next week. Distant Hurricane Humberto will
bring high surf and dangerous rip currents late this week, which
should peak on Friday.

&&

Near term /until 6 am Thursday morning/...
1045 PM update...

Leftover ocean-effect clouds across the cape and the islands
still expected to diminish soon, leading to all clear skies for
much of the night. Still have some northeast breezes along the
coasts, but strong radiational cooling still looks on track
across the coastal plain and into interior Massachusetts/CT and Rhode Island tonight.
Adjusted temperatures to reflect observed trends.

Frost advisories for the Pioneer Valley into northern Worcester
County remain in effect. May still be a few isolated patches of
frost further southeastward across parts of Middlesex and
Norfolk counties, though anticipated isolated/limited coverage
doesn't warrant expansion of frost advisories. Will need to
monitor the development of any fog and stratus within the
valley, which may prohibit frost formation.

Previous discussion follows...

Clouds diminish this evening with clear skies most of the night.
High pressure moves over the region tonight, bringing light wind
to the interior and resulting radiational cooling. Northeast
winds linger along the cape and islands with gusts to 25 kt
possible.

Observed dew points at 3 PM were 37 to 45. Radiational cooling
may push these values a couple of degrees lower. Our min temp
forecasts range from the mid 30s to around 40 inland and 40 to
50 along the coast.

The frost advisory for tonight will be expanded to include
northern Worcester co and western Hampden co. The frost period
will be after midnight until after sunrise.

Swell from Humberto continues to arrive and increase. We will
continue the rip current risk statement through midnight, at
which time we will move to a high surf advisory for the South
Coast.

&&

Short term /6 am Thursday morning through Thursday night/...
high pressure remains in place Thursday and Thursday night.
Expect mostly clear skies and light wind through the period.
Mixing is forecast to 925 mb, where temperatures of 11-13c
support Max temps of mid 60s to around 70.

East-southeast swell to 7 feet from Humberto remains along our
South Coast. This will continue to generate high rip current
risk as well as high surf. Will continue the high surf advisory
through the day. The swell expands up the East Coast of mass
Thursday night, at which time the advisory will also expand up
the South Shore and North Shore.

&&

Long term /Friday through Wednesday/...
highlights...

* high surf and building seas from Humberto on coastal waters
Friday, highest east. Subsiding trend late Friday.

* Dry pattern through the weekend with above- to significantly above
normal temperatures.

* Cold front early next week offers next chances for rain.

Details...

Amplified mid and upper level ridge aloft to build and crest across
much of New England through the weekend, with surface ridge to
remain established across the region. Plenty of sun with sizable
diurnal temperature ranges should be expected Friday through the
weekend. 850 mb temperatures increasing to the low to mid teens
celsius range supports high temperatures into the low to mid 80s
Friday thru Sunday across the interior into the coastal plain, into
the mid 70s along the coasts. These are some 10-15 degrees warmer
than late-September climatology. Lows mainly in the 50s. Dewpoints
will also be slowly on the rise each afternoon, albeit still at
comfortable levels humidity-wise. Overall, shaping up to be an
outstanding close to the week into the weekend weather-wise.

With the above said however, with the potential for many to head to
the beaches/coasts to take advantage of the nice stretch of weather
ahead...concerns continue for Friday regarding the threat for high
surf and rip currents in the coastal zone, and for building seas in
the outer waters. While Humberto will pass well south of Nantucket
late Thursday into early Friday, swell from Humberto's circulation
will cause waves to rise considerably on the outer waters (to around
8-10 feet) and contribute to high surf and a continued threat of rip
currents. Especially as we're now progressing into the off-season,
beachgoers should watch the building surf from a safe distance with
swimming not recommended. Seas will eventually subside as we move
into late Friday into Friday night.

Monday into Tuesday...

Cold front over the lower Great Lakes looks to progress across the
region on Monday. Still looks to be some question over timing the
front's eastward advance, and if the moisture recovery ahead of the
frontal passage would be sufficient to help generate enough
instability for thunderstorms. To the latter point, the GFS has an
axis of pwats over 1.3" - 1.7" ahead of the front, which would
suggest moisture levels may not necessarily be lacking and could
support brief downpours. Doesn't appear to be enough instability for
thunder though given model soundings revealing limited if any
instability, so will leave as showers for now. Front may not fully
clear the region until late Monday night, particularly if the European model (ecmwf)
is correct.

Tuesday - wednesday:

May still have some lingering light showers towards the cape and
islands early Tuesday, but ridging looks to build in from Quebec
allowing for cooler and drier air to filter in from the northwest.
By Tuesday night, 850 mb temperatures look to fall back into the
single digits which should bring temperatures more in line with
seasonal climatology - low 70s and lows in the 40s to lower 50s.

&&

Aviation /03z 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 night/...

VFR conditions through the 00z taf period. Northeast winds will
remain the strongest along the coast and towards the cape
tonight around 6-9 kts, and especially near Nantucket where
gusts to 25 kts may continue. Winds become light north/northeast
across the interior tafs.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook /Friday through Monday/...

VFR.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday night/...high confidence.

High pressure centered over New England. Northeast winds
continue across the southern and southeast waters with gusts of
20 to 25 kt. Swell from Hurricane Humberto is building offshore
with an east-southeast swell up to 7 feet forecast in the
southern waters tonight and Thursday, then spreading to the
eastern waters Thursday night.

Outlook /Friday through Monday/...moderate confidence.

Friday: winds less than 25 kt. Rough seas up to 12 ft.

Friday night: winds less than 25 kt. Rough seas up to 10 ft.

Saturday: winds less than 25 kt. Seas up to 5 ft.

Saturday night: winds less than 25 kt. Seas locally approaching
5 ft.

Sunday: winds less than 25 kt.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...high rip current risk until midnight EDT tonight for
maz022>024.
High surf advisory until 6 PM EDT Friday for maz020-022>024.
High surf advisory from midnight Thursday night to 6 PM EDT
Friday for maz007-019.
Frost advisory from 3 am to 8 am EDT Thursday for maz003-004-
010-011.
Rhode Island...high rip current risk until midnight EDT tonight for riz008.
High surf advisory until 6 PM EDT Friday for riz006>008.
Marine...Small Craft Advisory until 3 am EDT Thursday for anz231>234.
Small Craft Advisory until 6 PM EDT Thursday for anz235-237.
Small Craft Advisory until 2 am EDT Saturday for anz250-
254>256.
Small Craft Advisory until 2 am EDT Thursday for anz251.

&&

$$
Synopsis...wtb/loconto
near term...wtb/Belk/loconto

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