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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
733 am EDT Thu Aug 22 2019

Synopsis...
another very warm and humid day as a cold front approaches from
the Great Lakes. A period of showers and scattered thunderstorms
are possible this afternoon and evening as that cold front
briefly stall near the South Coast. Showers and thunder may
linger through the night as a wave of low pressure moves along
the front. High pressure then moves across Canada into the
Maritimes, with steady onshore winds bringing cool temperatures
this weekend into early next week. There may also be a chance
for a few showers Sunday through Tuesday.

&&

Near term /until 6 PM this evening/...

710 am update...

Noting bands of lower clouds lingering across S coastal areas
and up the CT valley as seen through the high thin clouds across
the region as seen on sunrise GOES-east visible satellite
imagery. Should see the low clouds and patchy fog burn off over
the next couple of hours or so, but may take a bit longer along
the S coast with the S-SW winds in place there.

With the remnants of the front hanging along the S coast, might
see a few stray showers develop in the SW flow through midday.
Otherwise, dewpoints will remain in the mid 60s to around 70, it
will remain warm and very humid.

Have updated near term to bring conditions current and
incorporated into the remainder of the morning forecast.

Previous discussion...

Remainder of today...

Closed upper low from James Bay sweeps southeast today and
Friday. This shifts the northern jet over New England and places
southern New England in the right entrance region of the jet by
late in the day and at night. A surface cold front sits over
the eastern Great Lakes with high humidity on this side of the
front and much drier air behind the front over Ontario.

Precipitable water values diminish a little this morning over southern New
England, but remain at 1.5 inches or higher through the day and
increase along the South Coast to 2 inches late in the day.
Cross sections back this up with moisture below 700 mb the first
half of the day, then developing a deep column of moisture
after 18z that lingers into the evening.

Instability remains in place, although not at The Levels seen
yesterday. The most favorable indices are the SBCAPE at 500-1000
j/kg and Li values between -2 and -4. Helicity between 0-3km is
low through early afternoon, then climbs to 100-200 by late in
the day.

Moisture, instability, and low-level convergence along/ahead of
a cold front...plus increasing upper divergence with the
approaching upper jet. This favors development of
showers/thunderstorms this afternoon and evening. Favored
location would be CT-RI-eastern MA, but all areas could see
precipitation develop.

Mixing reaches to around or a little above 850 mb today,
tapping temperatures of 15-17c. This supports Max sfc temps of
85 to 90.

&&

Short term /6 PM this evening through 6 PM Friday/...

Tonight...

The right entrance region of the upper jet will be overhead and
the sfc cold front aligned with the flow ahead of the trough
axis. Expect the front to stall for a short time tonight, and a
low pressure wave may develop along the front. The wave would
pass south of our area, but maintain a chance of
showers/scattered tstms during the night. Best chance would be
in CT-RI-se mass.

Precipitable water remains high through the night and the moisture column
remains deep. Look for continued chance of local downpours
during this time.

We will be north/west of the cold front during the night, so
expect signs of an airmass change with dew points 55 northwest
to 65 southeast. Expect min temps in the upper 50s and 60s.

Friday...

Upper trough sweeps down from Canada. This brings small cold
advection aloft, as well as drier air at all levels.

Main question for Friday is...will it shower or stay dry. The
GFS dries out the airmass and precludes any precipitation. The
European model (ecmwf) shows showers moving up from the southwest, likely in
response to the upper trough and jet entrance region.

For now we will show a few showers lingering along the South
Coast early, then moving out to sea. Otherwise dry weather.
Drying of the surface air will continue with dew points falling
into the 50s to around 60. Mixing reaches above 850 mb with
temperatures supporting Max sfc temps in the mid 70s to around
80.

&&

Long term /Friday night through Wednesday/...

Highlights...

* expect mild temperatures and dry conditions Saturday
* cooler than normal temperatures expected Saturday night
through Monday, with spotty light rain or showers at times
* temperatures moderate by mid week, with another chance for
showers Tuesday night and Wednesday

Details...

Friday night and Saturday...

Expect dry conditions as large high pressure slowly builds east
across Ontario and Quebec. Expect a general northerly wind
Friday night and early Saturday which will gradually shift to NE
during the day. After mostly sunny skies across the region into
Saturday afternoon, will begin to see some clouds develop later
in the day. Some spotty light rain may develop across NE mass
around 21z or so.

Temps will top off in the 70s, mildest across interior east mass
into Rhode Island as well as the lower CT valley.

Saturday night through Monday...

00z model suite has come into somewhat better agreement, though
some timing and track differences with the development of spotty
light rain and/or showers.

As the large high anchors across northern New England into
eastern Canada, winds become a steady east-NE during most of this
timeframe. Noting an increasing pres gradient on the southern
extent of the high, which could mean gusty winds across coastal
areas during the daytime hours Sunday and Monday. Gusts may
reach up to around 25 mph there. This will bring in cool
temperatures both Sunday and Monday with highs in the lower-mid
70s, though it may not even break 70 along the East Coast.

With the onshore flow, will see a steady fetch of low level
moisture working westward. This could bring patchy light rain
and/or showers, with the best chances across the coastal plain.
Have carried chance pops through late Monday, but can't rule out
isolated showers further inland with the easterly flow.

Tuesday and Wednesday...

Quite a bit of model solution spread amongst the 00z members.
Have used a non-Canadian blend for this portion of the forecast.

Temperatures moderate during this timeframe as the high begins
to shift east as winds shift to S-SW. However, another front will
approach later Tuesday night and Wednesday, bringing a chance
for showers.

&&

Aviation /11z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through Friday/...moderate confidence.

Today...areas of MVFR/LIFR through around 14z, possibly a bit
longer across S coastal terminals. Otherwise VFR. Scattered
showers/thunderstorms possible by the afternoon. Local downpours
may reduce vsbys to IFR and ceilings to MVFR as any
shower/tstms go past. Strong thunderstorm winds are possible,
especially across CT-RI-eastern Massachusetts.

Tonight...
VFR with brief MVFR/IFR possible in scattered showers/tstms.

Friday...
VFR. Scattered showers along the South Coast in the morning.

Kbos terminal...moderate confidence in taf due to
timing/location of afternoon/evening showers/thunderstorms.

Kbdl terminal...moderate confidence in taf due to
timing/location of afternoon/evening showers/thunderstorms.

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

Friday night through saturday: VFR.

Saturday night: IFR. Breezy.

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

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

Monday: breezy.

&&

Marine...
forecaster confidence levels...

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

Short term /through Friday/...high confidence.

Today and tonight...

Winds have diminished below 25 kt, but seas on the southern
outer waters remain at 5 feet. We will end the Small Craft Advisory on Rhode Island and
Block Island Sound, but maintain the headline through the day on
the southern and southeast outer waters.

Vsbys may also be reduced at times in fog this morning. A few
showers and scattered thunderstorms are possible moving off the
land during the afternoon and evening. These storms could
produce strong wind gusts and create poor visibility in
downpours.

Friday...

Winds gust to 20 kt with seas less than 5 feet. Leftover
showers/thunder in the morning on the southern waters, but these
should move out to sea.

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

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

Saturday: winds less than 25 kt.

Saturday night: winds less than 25 kt. Seas locally approaching
5 ft. Visibility 1 nm or less.

Sunday: low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. Slight chance of
rain. Local visibility 1 nm or less.

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

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

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 8 PM EDT this evening for
anz254>256.

&&

$$
Synopsis...wtb/evt

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