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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
000
FXUS61 KBOX 141752
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
152 PM EDT Sat Sep 14 2019

.SYNOPSIS...

Line of light to moderate showers sweeping S New England this
afternoon. Expect additional shower activity to persist into
this evening preceding a cold front which will sweep offshore
into Sunday morning. Dry Sunday, pleasant, while a spot shower
is possible overnight into Monday. Thereafter, looking dry into
the week with pleasant weather, mild days and cool nights. By 
late in the week a gradual warming trend develops.

&&

.NEAR TERM /THROUGH TONIGHT/...

2 pm update...

Line of light to moderate showers exceeding expectations. Can
really see the cloud-tops pop in the calculated daytime airmass
satellite product. Pre-frontal trof of shower activity showing 
up in latest WSR-88D radar returns. ThetaE axis collocated with
deep layer moisture all the way through H5 undergoing weak of 
moderate ascent courtesy of a stretched mid- level vortmax. But 
just the initial wave. As surmised earlier, still a fairly low- 
level moist environment beneath falling heights, cyclonic flow, 
some low- level convergent wind tendencies, and orographic 
support, all of which preceding the main cold front over the E 
Great Lakes accompanied by additional stretched mid-level 
vortmax energy, there remains the chance of light to moderate 
showers going into this evening. Keep with chance to likely PoPs
with scattered wording in the forecast grids. Some places have 
already seeing upwards of a two to three tenths, not ruling that
out for other locations. 

Tonight...

Scattered shower activity continuing, sliding S/E overnight. With 
S/SW flow preceding the cold front, warm air and higher surface dew-
point advection, anticipating lower conditions with scattered shower 
activity. Lower clouds, perhaps some patchy fog. With continued weak 
instability aloft within the low to mid level moist profile, can't 
rule out isolated rumbles of thunder with moderate rains. Perhaps 
the better chance along the nose of any low-level convergent winds 
parent with the weak - moderate lift/forcing. Holding with chance to 
likely PoPs with light scattered shower activity. This because model 
consensus indicates W winds around H85 along with strong subsidence 
and drier air behind the cold front. Could see a quick washout of 
activity overnight as a 1020+ surface high builds N. Showers simply 
linger along the S/SE coast towards Sunday morning. Mild especially 
S/SE of Boston - Providence. Lows around the upper 50s to low 60s. 

&&

.SHORT TERM /SUNDAY THROUGH SUNDAY NIGHT/...

2 pm update...

Sunday into Sunday night...

Pleasant and dry Sunday, perhaps the return of light shower activity 
overnight. Washing out with drier air, subsidence building from the 
N with an associated 1020+ high. Surface cold front lingers SE just 
offshore, some spot shower activity for Nantucket especially, will 
go with Sunday morning chance PoPs then drying. Otherwise, pleasant 
and dry, light W winds which should allow near-shore sea-breezes as 
highs warm into the mid to upper 70s (the airmass aloft relatively 
unchanged other than being drier). Mixing up to H9, some draw down 
of faster winds, lower dewpoints especially into the 50s. 

Amplifying H5 trof late into the overnight period with accompanying 
vortmax dropping S, cyclonically rotating through. CAMs a bit more 
robust on outcomes versus global guidance. Given the modest lift and 
forcing along a reinforcing cold front, any continental-moist air-
mass to which consensus of guidance signal in some variation of 
magnitude should squeeze out some light shower activity. Suggestion 
of slight chance PoPs towards Monday morning seems warranted. High 
confidence on cloud cover that should keep overnight lows mild in 
the upper 50s to low 60s. 

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...

4 am update...

Highlights ...

* Very pleasant stretch of weather featuring mainly dry conditions, 
  mild days (70s) and cool nights (50s)
* Core of cool air settles over the area Tue night into Wed night 
  with highs only in the 60s and lows in the 40s inland
* Warming trend late next week with highs near 80 possible 

Temperatures ...

Normal daytime high/low this time of year is 70-75 and 54-59. Much 
of next week will feature temperatures in this range, thus very 
pleasant conditions. However anomalous mid level low over 
Newfoundland mid week will advect a chilly airmass into eastern New 
England. Both deterministic and ensemble guidance only 
supporting highs in the 60s mid week along with lows in the 40s!
Therefore much cooler than normal mid week. Given the negative 
temp anomalies here will blend in some of the cooler guidance to
derive maxes and mins. As mid level low ejects out to sea, OH 
valley ridge builds into New England late next week supporting a
warming trend with highs near 80 possible by Friday. 

Precipitation ...

Other than a very low risk of a brief isolated shower from Sunday 
evening into Monday afternoon/evening from multiple short waves and 
attending cold pools aloft, dry weather prevails much if not all of 
next week.

&&

.AVIATION /18Z SATURDAY THROUGH THURSDAY/...

Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...High confidence.

18z update...

Rest of today...
BKN-OVC low-end VFR with some MVFR mixed in, especially with line
of -RA sweeping E. Expecting initial line to dissipate into the
afternoon hours while additional -SHRA activity blossoms W into
the evening period. S winds around 10 kts across the interior, 
10 to 15 kts with gusts 20 to 25 kts along the coast. 

Tonight...
Still forecasting CIGs to lower, but remain optimistic at widespread
MVFR over S New England. Still need to keep an eye out for IFR-LIFR
CIGs however, remains a threat. -SHRA push SE with S-coastal chances
of RA/+RA/TSRA with subsequent VSBY impacts. Clearing N to S towards
morning, some IFR lingering across the high terrain. S winds shift
W, sustained around 10 kts with gusts around 15 kts. Stronger winds
along S/SE coastal terminals. 

Sunday...
Lower MVFR possible IFR CIGs pushing SE with S-W wind shift. ACK
could see lingering -SHRA through midday. W winds potentially light
enough to allow near-shore sea-breezes during the day. 

Sunday night...
Increasing clouds overnight potentially getting down as low as low-
end VFR especially for the high terrain. Spot -SHRA possible. 
Continued W winds albeit light. 

KBOS Terminal...High confidence in TAF trends. 
Low-end VFR with VCSH through this evening. Remain optimistic
and hold CIGs MVFR 020-030 during the overnight hours. S to W
wind, rather than E, prefer not to go with IFR. Continued breezy
winds this afternoon around 10 to 15 kts. 

KBDL Terminal...High confidence in TAF trends. 
Low-end VFR with VCSH into this evening. Hint at CIGs as low as
020 into the overnight period and then improving into the early
morning hours. Remaining optimistic though there remains a threat
of IFR. 

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

Monday: Mainly VFR, with areas MVFR possible. Slight chance
SHRA.

Monday Night through Tuesday: VFR. 

Tuesday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA.

Wednesday: Mainly VFR, with local MVFR possible.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...High confidence.

2 pm update...

Strong S flow into this evening preceding a cold front will keep
seas heightened around 3 to 5 feet. Lingering E swell, anticipate
some rough seas on the waters with wind-wave combination. Winds
dissipating post cold frontal into Sunday morning, a light W
wind prevailing with near-shore sea-breezes, should allow for
good boating weather the remainder of the day. 

SMALL CRAFT ADVISORIES into this evening. Shower activity moving
into the waters overnight and lingering as late as midday Sunday
more so over the S waters. May be some visibility restrictions
at times. 

Outlook /Monday through Thursday/...Moderate confidence.

Monday: Winds less than 25 kt. Slight chance of rain showers. 

Monday Night: Winds less than 25 kt. 

Tuesday through Wednesday: Winds less than 25 kt. Seas locally
approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ250.
     Small Craft Advisory until 8 PM EDT this evening for ANZ251.
     Small Craft Advisory until 7 PM EDT this evening for ANZ230-
     231.

&&

$$

SYNOPSIS...Nocera/Sipprell
NEAR TERM...Sipprell
SHORT TERM...Sipprell
LONG TERM...Nocera
AVIATION...Nocera/Sipprell
MARINE...Nocera/Sipprell