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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
354 PM EDT Fri Aug 23 2019

Synopsis...
large high pressure over the Great Lakes will continue to
transport drier and less humid air into New England and
eventually a fall-like airmass this weekend. Expect mild
afternoons and cool nights with dry weather Saturday followed by
possibly a few showers Sunday especially eastern Massachusetts into RI, but
a washout is not expected. High pressure settles over the
Canadian Maritimes early next week supporting dry cool weather
here along with breezy conditions across Cape Cod and the
islands. Meanwhile we'll be watching a potential tropical system
tracking from the Bahamas to well south of New England early to
middle of next week.

&&

Near term /until 6 am Saturday morning/...

350 PM update...

1026 mb high over the Great Lakes continues to advect a dry
airmass (dew pt down to 49 at bos) across the region. Very
pleasant 330 PM conditions with temps in the 70s (81 at boston)
and dew pts in the 50s.

However a potent northern stream short wave over the eastern
Great Lakes providing SW flow aloft across southern New England
and streaming warm advection cloudiness over the area. In fact
this short wave fairly potent and trying to develop a baroclinic
Leaf on satellite over eastern PA to southeast New York. However with
frontal boundary well south of New England per offshore buoys
all reporting northerly winds, next frontal wave tracks well
south of New England with too much dry air over CT/RI/MA for any
risk of rain. Thus dry weather prevails this evening and
overnight.



Fairly light north-northwest winds shift to north-northeast after midnight. Very
comfortable tonight with lows in the 50s regionwide, except low
to mid 60s along the coast including city of Boston and possibly
Providence too. But with dew pts in the 50s very comfortable.

&&

Short term /6 am Saturday morning through Saturday night/...

350 PM update ...

Saturday ...

1025+ mb high over Ontario advects eastward into Quebec,
providing a dry north-northeast flow into southern New England. Expect highs
in the 70s and dew pts in the upper 40s to lower 50s. Definitely
a touch of early fall in the air. Cross sections reveal some
cirrus and diurnal cumulus/scu so expect a mix of sun and clouds
Saturday. Overall a real nice day by late Aug standards.

Saturday night ...

Northeast pressure gradient increases as frontal wave develops
well south of New England but combines with 1030 mb high (yeah a
sign of early fall) over the Maritimes. Thus becoming breezy and
with a surge of wind and moisture off the Atlantic expecting
scattered showers to come onshore especially toward Sunday
morning and over eastern Massachusetts into Rhode Island. Remainder of the region
should remain dry until Sunday.

&&

Long term /Sunday through Friday/...

350 PM update ...

Highlights...

* expect much cooler than normal temperatures Sunday and Monday
especially along the East Coast
* onshore winds will bring spotty light rain or showers at times
into mid week
* temperatures return to near normal levels around mid week
* another chance for showers late Wednesday into Thursday

Details...

Sunday through Tuesday...

This period of the forecast will feature mostly dry conditions with
cooler than average temperatures throughout and comfortable
humidity. Sunday we see the high over northern New England shift
east a bit, bringing northerly winds around to the northeast. While
most of the column is very dry, below 850mb moisture advection off
the ocean will bring some low level cloud cover and light showers or
drizzle. The only locations that should deal with this would be
eastern Massachusetts into Rhode Island with clearer skies to further west. Not a washout
by any means, but could see scattered drizzle. A tightening pressure
gradient on the south end of the high will bring increasingly gusty
northeast winds, especially on the southern waters and along the
South Coast where gusts will reach 25-30 mph. Those on the water
will likely see increased seas as well. Temperatures are quite a bit
below normal given 850 mb temps of 8c and onshore flow...those along
the East Coast may not make it out of the 60s.

Monday and Tuesday the inverted ridge of high pressure lingers over
southern New England helping keep things dry while the mid level low moves out
replaced by weak ridging. Mid level height rises and some drying at
the low levels should keep US mostly precip free, but with lingering
low/mid level clouds each day. Onshore east/NE flow continues
throughout keeping the immediate East Coast cooler. Highs remain in
the low 70s, 5-10 degrees below average for late August. Comfortable
humidity sticks around too, with dewpoints in the 50s to low 60s. As
the high moves away the gradient relaxes and the gusty winds along
the South Coast should subside through the day Monday.

Wednesday through Friday...

High degree of uncertainty with how mid-late week plays out,
dependent on the trajectory of two low pressure systems, including
one potential tropical system currently in the early stages of
development off the coast of Florida which may pass southern New England somewhere
off the coast around mid week. At this point confidence is
increasing that we'll see temperatures moderating back toward normal
by Wed/thur into the 70s and 80s as a warm frontal boundary moves
north. This, associated with a large low pressure system moving into
the Hudson Bay area. Ahead of its cold front warm moist air is
pulled up over the East Coast Wednesday/Thursday bringing rain and
thunderstorm chances back to the forecast late Wednesday and
Thursday. At this point the system off of Florida looks to stay well
offshore of sne, phasing with the passing trough over the Maritimes
by late week. This should limit any impacts in southern New England
to potentially some higher surf and rough seas, but given that this
is still 5-6 days out, much too soon to get into any
details...something we will be monitoring over the next week.

&&

Aviation /20z Friday through Wednesday/...
forecaster confidence levels...

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

Short term /through Saturday night/...moderate confidence.

18z update ...

Tonight ... VFR and dry weather with light north-northwest winds.

Saturday ... VFR with north-northwest winds becoming north-northeast. Dry weather
continues.

Saturday night ... VFR to start but becoming MVFR from east to
west as north-northeast onshore flow increases up to 20 kt across eastern
Massachusetts.

Kbos terminal...high confidence in taf

Kbdl terminal...high confidence in taf.

Outlook /Sunday through Wednesday/...moderate to high confidence.

Sunday: mainly VFR, with local MVFR possible. Breezy. Slight
chance rain showers.

Sunday night: mainly VFR, with local MVFR possible. Breezy.

Monday: VFR. Breezy.

Monday night: VFR.

Tuesday: VFR. Breezy.

Tuesday night: mainly VFR, with local MVFR possible. Breezy.
Slight chance rain showers.

Wednesday: mainly VFR, with local MVFR possible. Slight chance
shra, isolated thunderstorms and rain.

&&

Marine...
forecaster confidence levels...

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

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

350 PM update ...

Tonight ...

Large high pressure moves from the northern Great Lakes into
Ontario providing dry weather across the southern New England
waters along with a modest north-northwest wind.

Saturday ...

Large high pressure moves from Ontario to Quebec with modest north-northeast
winds but dry weather continues.

Saturday night ...

A series of weak waves of low pres develop off the mid Atlc
coast. This combines with strong 1030+ mb high pres over the
Maritimes to yield NE 20-30 kt across the southern New England
waters. Strongest winds likely south of Cape Cod and the
islands. Scattered showers may lower vsby at times.

Outlook /Sunday through Wednesday/...moderate confidence.

Sunday: low risk for Small Craft Advisory winds with gusts up
to 30 kt. Seas up to 5 ft. Slight chance of rain showers.

Sunday night: low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft.

Monday: low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Slight chance of rain showers.

Monday night through Tuesday night: winds less than 25 kt. Seas
up to 5 ft. Slight chance of rain showers.

Wednesday: winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers, isolated thunderstorms.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...nocera/bw

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