Area Forecast Discussion
National Weather Service Boston/Norton MA
358 AM EDT Sat Oct 31 2020

.SYNOPSIS...
Large high pressure will build over the region today with
sunshine and cool temperatures. The high moves offshore tonight
and a weak disturbance moving up from the south may bring a
a few showers to southeast New England late tonight and Sunday
morning. Milder temperatures return Sunday with another period
of widespread rainfall late Sunday and Sunday evening ahead of 
a strong cold front. Blustery and cold conditions are expected 
Monday and Tuesday. Turning milder with above normal 
temperatures for the end the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Strong high pres builds over New Eng today with dry conditions
and light winds. Soundings show very dry column through the mid
levels with PWATs down to near 0.1". Expect full sunshine 
today, although some shallow low level moisture below the 
subsidence inversion will result in some strato-cu at times. 
925 mb temps -2 to -3C supports highs in the upper 30s over
interior high terrain and low/mid 40s elsewhere. 

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Tonight...

High pres moves east of New Eng. The evening starts out clear. 
Developing low level southerly flow and increasing low level 
moisture will result in lower clouds developing and 
overspreading SNE from the south overnight. Additionally there
is a weak mid level shortwave which tracks south of New Eng late
tonight which may bring a few showers to the south coast and
Cape/Islands toward daybreak.

Good radiational cooling in the evening will allow temps to fall
sharply after sunset. Lows will likely occur in the evening with
temps dropping into the 20s away from the coast. Then as winds
turn southerly and cloud cover increases expect rising temps
overnight. In fact temps will rise through the 40s late tonight
along the south coast and Cape/Islands and possibly reach 50
toward daybreak. 

Sunday...

Area of deeper moisture moving up from the south and weak low
level convergence assocd with a developing low level jet will 
likely bring a period of showers to Cape/Islands and possibly 
into eastern MA in the morning before showers exit during the 
afternoon. The exact location where this the axis of rainfall 
sets up is not certain and it is possible it could shift 
further west or to the east. 

Meanwhile, deep closed low and amplifying mid level trough moves
across the Gt Lakes during the day. The attendant strong cold 
front will approach New Eng by late in the day. Deepening 
moisture moving in from the west with increasing forcing from 
left exit region of the approaching upper jet will bring 
numerous showers into SNE from the south and west during
mid/late afternoon. It will be a considerably milder day with 
highs ranging from upper 40s in NW MA to mid/upper 50s along 
the south coast. 

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Highlights

* Starting off the week cool, warming up into the 60s by 
  Thu/Fri

* Rain, heavy at times overnight Sunday with some snow on the tail 
  end in the higher elevations

* Chances of snow showers Monday into early Tuesday morning before 
  drying out the rest of the week

Details...

The wettest period of the whole week will kick off the extended 
forecast Sunday night into early Monday morning. A surge of 
subtropical moisture which overspreads the region Sunday will 
encounter the strongest forcing of the day ahead of a potent cold 
front Sunday night. It enters western MA/CT after 8pm and should be 
offshore by sunrise on Monday taking most of the precip with it. One 
fly in the ointment is the potential or lack thereof of snow in the 
highest elevations. At the moment significant snowfall (more than an 
inch or so in the Berkshires) isn't likely. The NAM is the outlier, 
showing a stronger, slower moving low pressure which wraps in colder 
air before the heavy QPF is out, bringing more widespread snowfall 
to the interior overnight. Something to keep an eye on, but we're in 
the far reaches of the NAM's guidance range, so not taking it into 
more consideration until we see some consistency. 

By Monday the bulk of the precip exits as a drier and colder post 
frontal airmass moves in. 850 mb temps drop to at least -8C Mon/Tue, 
potentially even colder on Tuesday. This will keep high temperatures 
in the 30s and 40s, with very cold nights in the 20s Monday and 
Tuesday night (away from the coast). Additionally, given mid level 
cold air advection, the boundary layer will be very well mixed, 
pulling down some gusty winds, 20 to 40 kts on Monday, even higher 
over the ocean, 40-50 kts. As such, it will feel about 10 degrees 
colder than it actually is. 

Precipitation won't be widespread on Monday but snow showers are 
possible beneath a deep cold pool aloft, even a few snow squalls 
given the decent low level lapse rates, mid level trough, and gusty 
winds. In any case, the better chance of additional light snow 
showers comes Monday night and Tuesday morning as a final shortwave 
rounds the base of the broader trough.  

By Tuesday into the late week, high pressure expands into New 
England keeping precipitation out of the forecast. Additionally, 
flow around the high becomes S/SW on Wednesday which will begin a 
period of rising temperatures. At 850 mb temps once again rise above 
+10C. This will allow for highs well into the 60s by Thu/Fri.

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Through 12z...High confidence.
VFR, but patchy ground fog in the CT valley and interior 
eastern MA. 

Today...High confidence. 
VFR with light winds.

Tonight...High confidence.
Low end VFR/MVFR cigs developing from the south and
overspreading the region overnight. A few showers possible
late tonight across southern RI and SE MA. 

Sunday...Moderate confidence.
Mix of VFR/MVFR cigs. A period of showers likely in the morning
over Cape/Islands and possibly eastern MA before exiting in the
afternoon. More showers developing and moving into SNE from the
south and west late in the day with more widespread MVFR cigs.

KBOS TAF...High confidence in the TAF.

KBDL TAF...High confidence in the TAF.

Outlook /Sunday Night through Wednesday/... 

Sunday Night: Mainly VFR, with local IFR possible. Windy with
gusts up to 35 kt. Chance SHRA.

Monday: Mainly VFR, with local MVFR possible. Strong winds with
gusts up to 45 kt. Chance SHSN, slight chance SHRA.

Monday Night: VFR. Strong winds with gusts up to 45 kt. Slight
chance SHSN.

Tuesday: VFR. Strong winds with local gusts up to 40 kt. 

Tuesday Night through Wednesday: VFR. Breezy. 

&&

.MARINE...
Today...Diminishing N winds becoming light southerly late in the
day. SCA for hazardous seas will continue for the outer waters
although seas will be subsiding. 

Tonight and Sunday...Increasing southerly winds with gusts to 
25 kt developing over eastern waters late tonight spreading to 
rest of waters during Sunday. Building seas. Another SCA will 
likely be needed. Vsbys may be reduced in showers and fog at
times late tonight and Sunday. 

Outlook /Sunday Night through Wednesday/...

Sunday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 10 ft. Chance of rain showers. 

Monday: gale force winds with gusts up to 45 kt. Rough seas up
to 13 ft. Chance of rain showers. 

Monday Night: Moderate risk for gale force winds with gusts up
to 45 kt. Rough seas up to 13 ft. Chance of rain showers. 

Tuesday: Low risk for gale force winds with gusts up to 40 kt.
Rough seas up to 11 ft. Slight chance of rain showers. 

Tuesday Night: Winds less than 25 kt. Local rough seas. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 AM EDT this morning for ANZ235-
     237.
     Small Craft Advisory until 8 PM EDT this evening for ANZ250-
     251-254-255.
     Small Craft Advisory until 2 PM EDT this afternoon for ANZ256.

&&