Area Forecast Discussion
National Weather Service Boston/Norton MA
704 AM EDT Wed Jul 15 2020

.SYNOPSIS...
Dry, cool, comfortable and cloudy due to onshore flow despite
the high pressure building in today. The cooler and dry weather
continues on Thursday. A warm front will bring a few showers 
and thunderstorms Friday, then hot and humid weather returns 
this weekend into early next week. A slow approaching cold front
may trigger more showers and storms early next week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
7 AM update...

A few light showers rotating south across eastern MA/RI will
erode this morning but clouds will linger with moist low level 
NE flow in place. HREF suggesting low clouds will persist
through the morning with partly sunny skies developing in the 
afternoon as low level moisture diminishes. Adjusted cloud cover
and PoPs to reflect current trends, otherwise forecast on 
track. 

Previous discussion...

Synoptically southern New England between a trough offshore and
a ridge axis over the central/eastern Great Lakes. The trough
will lift offshore and the ridge will build into the eastern 
Great Lakes. This will keep southern England cloudy and cool due
to onshore flow despite a surface high building in. May have 
some isolated showers across the eastern slopes of the 
Berkshires due to upslope and heating.

Leaned toward the GFS and high resolution guidance versus the
ECWMF/GEM. This keeps the 925 hPa winds in more of an onshore
component and advects the cooler air in. As a result should see
more cloud cover especially during the first half of the day. 
There is a possibility that the July sunshine is able to work 
away at these clouds and break the sun out during the afternoon 
as indicated by high res guidance. Am skeptical given winds will
still be onshore, but drier air is advecting in. Should see dew
points falling into the upper 50s by late afternoon across
eastern MA and RI. 

Temperatures will be much cooler in comparison to the past
couple of days with highs topping out in the 70s across the
majority of southern New England. Still could see some low 80
degree readings across the CT River Valley.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
Highlights 

* Isolated showers still possible across the eastern slopes of
  the Berkshires tonight. Rest of southern New England dry, cool
  and quiet. 

* Dry and quiet weather persists on Thursday across southern New
  England.

Ridge axis builds into New England tonight. The ridge axis will
build east of New England into the Gulf of Maine on Thursday.
The surface high will nudge into southern New England tonight
from northern Maine. The surface high will move into the Gulf of
Maine on Thursday.

The position of the surface high will keep flow onshore tonight.
This will continue advecting cool air into the region. This flow
in combination with the low level moisture in place and upslope
may bring isolated showers to the eastern slopes of the
Berkshires. The rest of the region will remain dry with some
cloud cover. Will be comfortable with dew points falling into
the mid to upper 50s. Low temperatures will generally be in the
low 60s.

As the surface high moves into the Gulf of Maine expect low
level winds to shift from the east to the southeast on Thursday.
Still expecting some cloudiness to the region, but not as much
as today. This will result in similar, but slightly warmer
temperatures across southern New England. Highs generally in the
70s across the region. Still could see some low 80 degree
readings across the CT River Valley.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Highlights...

* Chance of showers/t-storms Friday 
* Hot and humid this weekend into early next week
* Increasing risk of showers/t-storms Mon/Tue

Thursday night into Friday night...

Northern stream shortwave tracking east from upper Lakes region will 
pass north of New Eng during Fri. Warm front approaches from the 
west with increasing moisture leading to chance of showers/t-storms 
during Fri. Surface based instability axis will remain to the west 
Fri but enough elevated instability to support a few t-storms. Focus 
for scattered showers/t-storms may shift to the south coast Fri 
night along a weak boundary. Main risk with any convection will be 
locally heavy rainfall as PWATs increase to 1.75-2 inches. 
Increasing humidity Fri as dewpoints rise into mid/upper 60s. 

Saturday through Tuesday...

SNE will be on the northern periphery of the central/southern CONUS 
subtropical ridge this weekend which will mean a return to heat and 
humidity. Heat advisories may be needed for parts of SNE by Sun as 
high temps in the low/mid 90s combine with dewpoints 70+. Convection 
not expected this weekend as a cap will be in place. 

Next northern stream shortwave from the upper Lakes expected to 
flatten the ridge across New Eng early next week. Increasing risk of 
showers/t-storms as a pre-frontal trough moves through Mon ahead of 
a cold front which approaches Tue. Heat and oppressive humidity 
likely continues Mon with heat advisory potential. Dewpoints may 
peak Mon in low/mid 70s. Some cooling possible by Tue.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

12z update...

Today...Moderate confidence

Gradual improvement from MVFR/IFR to VFR. Timing uncertain,
especially in eastern New Eng given onshore flow but heating
will result in cigs lifting with sct-bkn clouds developing in
the afternoon. In addition, may have some isolated shower 
activity across the eastern slopes of the Berkshires due to 
orographic lifting. Not confident enough in the coverage at this
point in time to include in BAF or BDL TAF. Winds shifting to 
the E with speeds around 5 to 10 kts.

Wednesday Night...High confidence

VFR across the majority of southern New England. Could see MVFR
ceilings across the eastern slopes of the Berkshires. Winds
generally less than 5 kts out of the E or SE. 

Thursday...High confidence

VFR at all locations except for the higher elevations where E to
SE winds will bring MVFR ceilings.  

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. Breezy. Slight chance SHRA.

Friday: Mainly VFR, with local MVFR possible. Breezy. Chance
SHRA, chance TSRA.

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

Saturday through Sunday: VFR.

&&

.MARINE...
Small Craft Advisories expanded to include Mass and Ipswich Bay
due to seas building to 5 ft. Small Craft Advisories continue
across the eastern outer waters with seas of 5 to 6 ft. Could
have a few gusts around 25 kts early this morning, but gusts
will quickly diminish below 25kt as the close low moves further
offshore. Did extend the Small Craft Advisory time into this
evening as seas take a bit longer to subside. 

Dry weather expected today into Thursday.

Outlook /Thursday Night through Sunday/...

Thursday Night: Winds less than 25 kt. 

Friday: Winds less than 25 kt. Chance of thunderstorms, slight
chance of rain showers. 

Friday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, chance of thunderstorms. 

Saturday through Sunday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 10 PM EDT this evening for ANZ250-
     254.
     Small Craft Advisory until 6 PM EDT this evening for ANZ251.

&&