Area Forecast Discussion
National Weather Service Boston/Norton MA
303 AM EDT Tue Jun 15 2021

.SYNOPSIS...
Rounds of scattered showers and a few thunderstorms continue at
times today. Dry weather with near seasonable temperatures 
returns for Wednesday through Friday. The air turns warmer and 
more humid ahead of a cold front on Saturday with the potential 
for a round of scattered showers & thunderstorms. Dry and 
pleasant weather likely returns Sunday and most of Monday. More
showers arrive ahead of a cold front Monday night.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Some lingering elevated instability this morning. There have 
been a couple of thunderstorms around southern New England, but 
nothing overly potent. Looking at two areas for showers to 
continue through the early morning hours: One along the south 
coast, and the other along a line from about Hartford, to 
Worcester to Lawrence. Not looking at a continuous washout, but 
rather frequent showers with a few rumbles of thunder possible.

The situation changes a bit after sunrise. Our region will be
contending with a combination mid level shortwave and longwave
trough. The main impact of this will be to steepen mid level
lapse rates. Should we get some breaks of sunshine, there is
more potential for generating enough low level instability to 
match up with an occluding front to generate stronger
thunderstorms. At this time, thinking CAPE values max out
between 750-1000 J/kg. With effective shear about 30-40 kt,
there is a low risk for strong to severe thunderstorms this
afternoon, mainly across the eastern half of southern New
England.

Given the humid air, the main threat remains locally heavy
rainfall. However, we might be dealing with some hail or gusty
winds with the stronger storms.

Temperature forecast remains tricky. Thinking clouds will be
present the majority of today, so favored slightly below normal
high temperatures.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
Mid level trough and shortwave will depart off to the east
tonight into Wednesday. Winds turn westerly behind an occluding
front during this time, too. This will eventually mean drier 
air overspreading southern New England after midnight and 
through Wednesday.

Expecting any lingering showers and thunderstorms to diminish
quickly after sunset this evening. Dry weather dominates
Wednesday. Despite clearing skies tonight and more sunshine 
Wednesday, still expecting temperatures to be near to slightly
below normal for mid June.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...

Big Picture... 

Upper ridge sits over the Rockies and Southwest USA with above-
normal heights and expected hot temperatures. Meanwhile, upper jet 
stretches across the Northern Tier of the USA with a few shortwaves 
embedded in the flow and moving east toward New England. One 
shortwave axis is overhead of New England Thursday, moving off 
to the east Thursday night. A second shortwave moves across 
Western/Central Canada during the mid-late week, then sweeps 
across New England over the weekend. High pressure ridge 
rebuilds over this area for Monday.

Heights at 500-mb are forecast in the 560s Thursday, and the 570s 
Friday and Saturday. The field may trend to the low 580s Monday. 
Seasonal normals are in the 570s Dm.  This suggests the deep layer 
will be cooler than normal Thursday, but near normal and
trending higher the rest of the period. 

Good agreement among the model mass fields late this week, then 
diverging solutions among the models during the weekend and early 
next week. Confidence is high Thursday and Friday, trending to low-
moderate over the weekend and early Monday.

Details...

Wednesday night through Friday...

Upper trough overhead Thursday with some lingering moisture between 
800 and 700-mb. Mixing reaches near 750-mb. This suggests diurnal 
clouds from late morning through afternoon but with enough dry air 
above and below to maintain a rain-free day. Temperatures around 
freezing at 750-mb, which supports max temps mid to upper 70s. For 
both Wednesday night and Thursday night, dew points will be in the 
40s, except 50-55 Cape and Islands. Light winds. This should allow 
min temps to reach the 40s and low 50s, except 55 to 60 along the 
coast.

Ridge builds over the region Friday morning and offshore afternoon, 
bringing drier air at all levels and thus fewer clouds. Also expect 
surface winds to pick up out of the southwest by afternoon. Mixed 
layer is a little shallower, 850-mb, with temps around 12-14C. This 
suggests max temperatures in the upper 70s to mid 80s, cooler on the 
south coast.

Saturday through Monday...

Upper shortwave and surface cold front move across Southern New 
England during the late afternoon/early night. Dew points increase 
into the 60s during the day, so will be noticeable but not 
excessive. PW values climb to 1.75 inches in the afternoon, with 
water vapor transport at 1000 units or higher. Afternoon CAPE is 
forecast around 1000 J/Kg, mainly Merrimack Valley to CT Valley. 
Total-totals forecast upper 40s to low 50s. Overall, expect at least 
scattered showers Saturday afternoon/early night. Potential but not 
certainty of thunder.

Drier weather returns for Sunday and much of Monday. PW values drop, 
and high pressure builds over the region. As the high moves off to 
the east on Monday, low-level moisture increases and brings 
potential for low clouds/fog late in the day especially along the 
south coast. Possible showers overnight.

&&

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

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

Today...Moderate confidence. Scattered SHRA and isolated TSRA 
becoming more likely across the area. Mix of MVFR and VFR for 
most of our region after the morning lower clouds burn off. 
However, IFR conditions will remain possible towards the south 
coast, Cape and Islands.

Tonight...Moderate confidence. Conditions improve to VFR for
most of the region. Areas of IFR and LIFR towards the Cape and
islands late tonight in stratus and fog.

Wednesday...High confidence. VFR most of the day. Areas MVFR in
the morning as low clouds and areas of fog dissipate.

KBOS TAF...Moderate Confidence in TAF. 

KBDL TAF...Moderate confidence in TAF. 

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Friday: VFR. 

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

Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SHRA, slight chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Wednesday/...High Confidence.

Southerly flow across the waters ahead of an occluding front.
This front should cross the southern coastal waters today, and
the eastern coastal waters tonight. A secondary cool front is
anticipated to cross the waters tonight, too. This will shift
winds to the west to northwest tonight. Pressure gradient is not
overly strong, so local sea breezes possible Wednesday
afternoon. Seas may be a bit choppy at times, but generally 
remain below Small Craft Advisory thresholds. Showers and 
perhaps a few thunderstorms are possible today with areas of 
stratus and fog still expected, especially tonight into
Wednesday morning. Reduced visibility is likely in fog this
morning, and again tonight into Wednesday morning. Reduced in
showers and thunderstorms also possible, but that should be be
more short-lived.

Outlook /Wednesday Night through Saturday/...

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

Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.

Thursday Night through Friday: Winds less than 25 kt. 

Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft. 

Saturday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Chance of rain showers, slight
chance of thunderstorms. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&