Old Res Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 181422
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1022 AM EDT Fri Jun 18 2021

.SYNOPSIS...
Warmer today with low humidity. A warm front may bring a few 
showers tonight into Saturday morning, then another round of 
showers and thunderstorms is possible Saturday afternoon/evening
ahead of a cold front. Dry, very warm but less humid conditions
follow on Sunday. Heat and increasing humidity returns early 
next week along with the risk of tropical downpours Monday 
night/Tuesday. Trending cooler, drier and less humid mid week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

1010 AM Update...

High pressure centered off the Mid Atlantic coast will maintain
dry weather for most of Southern New England today. Visible satellite
shows mid and high clouds streaming into Upstate NY and Northern
VT, out ahead of a MCS moving across southern WI and northwest
OH. For us here in Southern New England, increasing southwest
flow out ahead of the aforementioned shortwave will lead to
somewhat breezy conditions and temperatures rising into the low
to mid 80s, cooler on the coast as well as the Cape and Islands.
A well-mixed boundary layer should minimize the development of
sea breezes. Plentiful sunshine although clouds will be on the
increase from the west by the afternoon hours. Overall, a very 
delightful and low humidity early summer day. Enjoy! 

Previous discussion...

After a cool start temperatures will quickly warm up today.High
pressure slides south and east of southern New England 
resulting in prevailing winds shifting to the southwest. This 
will result in higher dewpoints in the upper 40s to low 50s 
being advected into the region. Despite the higher dewpoints 
conditions will remain fairly dry with lowest RH this afternoon
in the 30-40 percent range. High temperatures are expected to 
be slightly above normal for this time of year as 20C temps at 
925 hPa will support temperatures in the low-mid 80sF at the 
surface. As deep- layer moisture is advected over the region 
from the south and west we'll also see increasing cloud cover by
mid-late afternoon. However skies should be mostly clear for 
the majority of the day with the exception of some diurnal 
cu/stratocu.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
Highlights

* Showers possible Tonight into Saturday

* Isolated to scattered showers/thunderstorms Saturday.

Tonight

By Friday night moisture advection over southern New England will
jack PWATs up close to 1.5". This falls just below the 90th 
percentile (1.55") compared to sounding climatology. With ample 
moisture in place and a weak-short wave at 500 hPa moving over 
head, we will have a chance for some scattered showers and 
possibly a rumble of thunder tonight into Saturday. Best chance
for thunder will be out west where there is expected to be weak
instability (100-250 J/Kg of MUCAPE). With overcast skies 
settling over the region Friday evening, temperatures will stay 
mild with lows falling into the low-mid 60s.

Tomorrow and Tomorrow night


SPC has placed a large portion of the Northeast (including 
southern New England) under a marginal risk for severe 
thunderstorms on Saturday afternoon. As scattered-showers from 
Friday night exit the region Saturday morning, a second more 
robust short-wave trough will approach New England from The 
Great Lakes region. Moisture (dewpoints in the mid 60s, PWATs 
~1.5"), deep-layer shear (40+ knots), and instability (500+ J/kg
MLCAPE) will all be available to support the development of 
thunderstorms that could become severe. The only ingredient 
missing is a source of lift, which in this instance will be the 
approaching short-wave and associated surface front. 

Latest guidance does not have these features arriving in 
southern New England until Saturday evening when the time of 
maximum instability will have passed. Thus, any widespread 
convective activity is not favorable at this point. This 
forecast is further supported by latest hi-res model guidance 
from the HREF which is hinting at shower/thunderstorm activity 
that would be more isolated to scattered in nature. Should the 
approaching short-wave arrive sooner we could be looking at a 
different outcome, so we'll be monitoring this feature closely 
over the next 24 hours. Should severe thunderstorms develop
Saturday afternoon, damaging wind gusts would be the primary
hazard. As the short- wave exits east overnight Saturday into 
Sunday morning, we'll see the atmosphere dry out a bit with 
clearing skies. Mild temperatures Sunday morning in the low
mid-60s.

&&

.LONG TERM /SUNDAY THROUGH THURSDAY/...
Big Picture...

Weak upper ridge builds over the Northeast USA Sunday, cresting 
Monday. Tropical system along the Gulf Coast moves through the 
Southeast USA to the Southern New England coast by Tuesday. This 
draws large amounts of moisture up the coast in the form of rain. At 
the same time, a cold front sweeps east from the Great Lakes, 
crossing New England on Tuesday Great Plains high pressure then 
brings dry weather for Wednesday and Thursday.

Mass fields are similar Sunday and Monday.  Differences arise with 
the tropical system, with the GFS/Canadian/ECMWF showing different 
timing on the 500-mb shortwave and surface system...the GFS quite 
different with an inland surface system. There are also increasing 
differences between the models regarding a broad upper trough over 
Central Canada and the Great Lakes. Confidence is moderate-high 
Sunday and Monday, trending to moderate midweek.

Details...

Sunday and Monday...

High pressure builds Sunday and moves overhead Monday morning. Dew 
points drop back a little on Sunday...mid 50s NW to mid 60s South 
Coast...then climb into the mid-upper 60s Monday afternoon as the 
high moves offshore and the flow turns from the south and southwest. 
Temps at the top of the mixed layer Sunday 16-17C which would 
support mid to upper 80s. Temps at the top of the layer Monday 17-
20C which supports 90-95 except cooler 80s along the south coast.

Monday night and Tuesday...

Warm and humid airmass in place, associated with the flow of air 
from the tropical weather system. Dew points remain in the mid to 
upper 60s during the period. PW values will be at least 1.5 inches 
and possibly up to 2 inches. 

Shortwave trough embedded in the upper trough to our west...this 
sweeps across New England Tuesday afternoon and evening. Surface 
cold front moves through with the shortwave. Convective parameters 
show buoyancy/instability with CAPE 1500-2500 J/Kg and Totals in the 
upper 40s. Lifted Index values minus 3 to minus 6 ahead of the 
shortwave and front. 

Based on this, expect showers and scattered thunder mainly during 
the afternoon and evening. The high moisture values suggest 
potential for downpours. Once the cold front moves through, 
conditions should improve with showers ending and dew points 
lowering into the 50s. 

Wednesday and Thursday...

High pressure builds in with subsidence and dew points mid 40s to 
mid 50s. Temps aloft 8-11C support max temps in the 70s and low 80s. 
Comfortable each day with seasonable temperatures.

&&

.AVIATION /14Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels.

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

Today...High confidence. VFR. Winds becoming more SW.

Tonight...High confidence. VFR. VCSH moving in after 04Z. Gusty
southwest winds 20+ knots kick in at The Cape and Island 
terminals after 00Z.

Saturday...High confidence in large scale, but moderate
confidence in timing. VFR with patches of MVFR in isolated
heavier showers and thunderstorms in the afternoon. A few 
showers and thunderstorms, some with reduced visibility in 
heavier downpours. Southwest winds 10 to 15 kt. 

Saturday night...High confidence. Cold front moves through.
Winds shift out of the west 10 kt. VFR.

KBOS TAF...High Confidence in TAF. 

KBDL TAF...High Confidence in TAF. 

Outlook /Sunday through Tuesday/...

Sunday through Sunday Night: VFR. 

Monday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA, isolated TSRA.

Monday Night: Mainly MVFR, with local IFR possible. Breezy.
Chance SHRA, slight chance TSRA.

Tuesday: Mainly MVFR, with local IFR possible. Breezy. Chance
SHRA, chance TSRA.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Saturday Night/...High Confidence.

Today

High pressure slides south and east of the coastal waters today
resulting in a wind shift to the southwest. Wind speeds pick up
this afternoon to the 15-2 knot range. Seas from 0-3 feet.

Tonight

Southwest winds at 15 knots continue tonight with gusts greater
than 20 knots. Seas increase to 3-5 feet for the outer marine
zones which has prompted a small craft advisory beginning at
00Z and ending at 22Z Saturday.

Saturday and Saturday night

5 foot seas continue for the outer waters south of The Islands
Saturday afternoon. SCA set to expire at 22Z but may need to be
extended should 5 foot seas linger. Weak cold frontal passage
may produce isolated showers/thunderstorms over the coastal
waters Saturday night into Sunday.

Outlook /Sunday through Tuesday/...

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

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

Monday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers. 

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Chance of
rain showers, slight chance of thunderstorms. 

Tuesday: Winds less than 25 kt. Rough seas up to 9 ft. Chance
of rain showers, chance of thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 10 AM to 6 PM EDT Saturday for 
     ANZ254.
     Small Craft Advisory from 5 AM to 6 PM EDT Saturday for ANZ255.
     Small Craft Advisory from 11 PM this evening to 6 PM EDT 
     Saturday for ANZ256.

&&

$$
SYNOPSIS...WTB/RM
NEAR TERM...WTB/Chai/RM 
SHORT TERM...RM 
LONG TERM...WTB 
AVIATION...WTB/RM 
MARINE...WTB/RM