Area Forecast Discussion
National Weather Service Boston/Norton MA
702 AM EDT Thu Aug 5 2021

.SYNOPSIS...
Low pressure moves across RI and eastern MA today, resulting in 
showers and thunderstorms with locally very heavy rainfall and 
possible flooding. The low departs tonight with dry weather to 
follow Fri into Sat, along with warming temperatures. Then low 
pressure tracks south of New England Sat night thru Sun night, 
yielding a risk of scattered showers and cooler conditions. High 
pressure over the Maritimes provides mainly dry weather Monday along 
with cool temperatures continuing. Then summer heat and 
humidity return to southern New England Tue and Wed.

&&

.NEAR TERM /UNTIL 7 PM THIS EVENING/...

7 AM update...

Rain shield over RI, southeast MA and south of Long Island, NY
continues to expand with intensifying rainfall rates. This lines
up well with cooling cloud tops on satellite imagery along with
an uptick on lightning activity. The culprit, a vigorous mid
level short wave over the Mid Atlantic acting on a tropical
plume of 2+ inch PWAT streaming into RI and southeast MA.
Meanwhile, confluent flow across southeast Quebec yielding a 100
kt upper level jet with its RRQ over SNE, providing strong Q/G
forcing. Very strong jet dynamics for early Aug. Thus an
additional 1-3" of rainfall this morning and early afternoon.
Heaviest rainfall overnight was southeast of I-95 in RI and
southeast MA, with 1-3" rainfall. This next pulse of rain for
the remainder of this morning and early afternoon, will fall
just to the NW, more along and in the I-95 corridor of RI and
eastern MA. Thus, highest probs for flooding will be along and
southeast of I-95 remainder of today. 

Also, need to watch embedded convective features. With
increasing low level jet and approaching surface wave, low level
helicity increases later this morning across RI and southeast
MA. This combined with high dew pts in the low 70s and low LCLs,
a few rotating storms are possible. Previous discussion below.

==================================================================

WV and IR imagery show well defined tropical connection in place 
with cooling cloud tops developing off the mid Atlc coast. Axis of 
2"+ PWATs lifting north across SE New Eng with right entrance of 
region of upper jet in place for much of today which will set the 
stage for heavy rainfall across eastern New Eng. 

Locally heavy showers ongoing across portions of RI and SE MA with 
additional heavy showers developing over the ocean at the nose of a 
strengthening low level jet. Expect these showers to become more 
organized with locally heavy rainfall moving up across eastern
half New Eng through the morning hours as the low level jet 
approaches. Consensus of the global and hi-res guidance favor 
axis of heaviest rainfall along and east of the I-95 corridor 
across RI and SE MA which looks reasonable given location of low
level jet and PWAT axis which favors SE New Eng. Also, surface 
boundary will be moving into SE MA as wave of low pres tracks 
along it which will enhance low level convergence. Heaviest 
rainfall will occur through the morning but showers will likely 
persist well into the afternoon in eastern New Eng with deep 
moisture and forcing for ascent persisting within the right 
entrance region of the upper jet. MRMS rainfall estimates show 
1-2 inches of rain have fallen over S RI and SE MA. Expect an 
additional 1-2 inches of rain over a broad area in RI and 
eastern MA. Localized total rainfall up to 4+ inches possible in
portions of RI and SE MA. Flash flood watches will continue for
RI and eastern MA where highest confidence of heavy rainfall 
and areas of street and highway flooding. Dropped the watches 
for eastern CT and southern Worcester county which will likely 
remain west of the axis of heaviest rainfall. 

A few t-storms are possible across SE New Eng today where marginal 
instability will be present in the warm sector. While a very low 
probability, can't rule out a rotating storm later this morning into 
early afternoon as low level jet approaches, especially Cape/Islands 
and possibly SE MA near and east of the boundary which will be a 
source for enhanced surface vorticity. Low LCLs and favorable 0-3km 
CAPE present in the warm sector and HREF indicating low probs of 
updraft helicity tracks so low risk for an isolated brief tornado. 

Temps will reach the mid 70s with dewpoints near 70 in the warm 
sector across Cape/Islands into SE MA, with upper 60s along and west 
of I-95 with cooler northerly flow on the west side of the boundary. 
Some lower 70s also possible in the CT valley. A period of gusty 
winds expected to impact Cape/Islands later morning into the 
afternoon as low level jet approaches given more favorable mixing in 
warm sector. Can't rule out a few gusts to 30-40 mph. 

High surf: We issued a high surf advisory for south coast 
beaches today as seas build which will result in increasing 
surf and risk for dangerous rip currents.

&&

.SHORT TERM /7 PM THIS EVENING THROUGH 6 PM FRIDAY/...

Showers near the coast will be exiting by evening, then expect 
partial clearing developing from west to east during the night as 
good mid level drying moves in from the west. Low clouds may linger 
a bit, especially in the east and patchy late night fog possible in 
the interior valleys. Lows mainly upper 50s to lower 60s with mid 
60s Cape/Islands.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...

Highlights...

* Fri/Sat - dry and turning warmer
* Sun/Mon - not as warm with scattered showers Sunday
* Tue/Wed - return to summer heat and humidity

Fri/Sat...

Rising heights/anticylonic flow will support dry weather and warming 
temps. Good column drying Fri with PWATs dropping to about 80% of 
normal per ensembles. This dry airmass lingers into much of Sat, 
although can't rule out a low risk of an isolated late day 
shower/thunderstorm across western MA/CT. Warming trend this period 
with 925 mb temps up to +21C/+22C Fri. This combined with WSW flow 
will yield highs in the mid to upper 80s. Additional warming Sat 
with 925 mb temps climbing to +22C/+23C, which should support highs 
in the upper 80s to lower 90s away from the south coast. 

Sat night/Sun/Sun night...

Southern and northern stream moisture converge on southern New 
England this period. Models differ on stream interaction and 
amplitude, so definitely some uncertainty this period. Shouldn't be 
a washout, but scattered showers are possible. Not as warm as Fri or 
Sat, given increased cloud cover, risk of showers and and light 
onshore flow as surface wave likely tracks south of New England.  

Monday...

1025 mb surface high over the Maritimes, with ridge axis extending 
southwest into New England should support mainly dry weather. 
Although maritime airmass combined with easterly onshore flow, will 
result in cooler temps especially in eastern MA. Large temp 
difference possible across the area, with highs 75-80 in eastern MA 
but mid to upper 80s possibly in the CT river valley, including 
Hartford and Springfield. 

Tue/Wed...

Subtropical ridge builds northward, placing southern New England on 
its northern periphery, with ensembles showing 850 mb temps warming 
to +17C Tue and then +18C/+19C Wed. This should support highs 85-90 
Tue and upper 80s to lower 90s Wed. Ensembles suggest there could be 
enough ridging over southern New England to support mainly dry 
weather.

&&

.AVIATION /12Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

11z update...no major changes from previous TAFs. Earlier
discussion below.

==============================================================

Through Today...Moderate confidence.

Widespread MVFR/IFR with areas of LIFR across eastern New Eng
with multiple pulses of heavy rainfall moving up from the 
south. Isolated t-storms possible across RI and SE MA. Further 
west toward the CT valley, mainly MVFR cigs with a few showers 
possible, but steadier and heavier rainfall to the east. A 
period of southerly wind gusts 25-35 kt possible over the Cape 
Islands 15-20z as the low level jet moves up across the region. 
Along and west of I-95 corridor, NE winds becoming N.  

Tonight...Moderate confidence.

Showers exiting to the east by evening. Otherwise MVFR/IFR cigs
in the east with VFR CT valley, improving to VFR in the east
overnight. Patchy late night fog possible in the interior 
valleys.

KBOS Terminal...Moderate confidence. Pulses of heavy showers
through the morning with IFR conditions. Showers becoming more 
scattered during the afternoon but improving cigs may be 
delayed until evening. 

KBDL Terminal...Moderate confidence. Areas of MVFR cigs today
with a few showers possible, improving to VFR this evening. 

Outlook /Friday Night through Monday/...

Friday Night through Saturday: VFR. 

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

Sunday: Mainly VFR, with areas MVFR possible. Chance SHRA,
chance TSRA.

Sunday Night: VFR. 

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

&&

.MARINE...
Forecaster Confidence Levels.

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

High confidence through tonight.

Increasing southerly winds across S/SE waters as low level jet 
approaches. A period of 25-30 kt expected into the afternoon. Winds 
shift to W/SW tonight with diminishing speeds. Seas build to 
6-8 ft over southern waters. Vsbys reduced in occasional heavy 
showers and isolated t-storms. Improving conditions tonight. 

/Outlook /Friday Night through Monday/...

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

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

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers. 

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

Sunday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers, slight chance of
thunderstorms. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Flash Flood Watch until 6 PM EDT this evening for MAZ005>007-
     013>021.
     High Surf Advisory until 8 PM EDT this evening for MAZ020-023-
     024.
RI...Flash Flood Watch until 6 PM EDT this evening for RIZ001>008.
     High Surf Advisory until 8 PM EDT this evening for RIZ006>008.
MARINE...Small Craft Advisory until 8 PM EDT this evening for 
     ANZ232>234.
     Small Craft Advisory until 8 PM EDT this evening for ANZ231.
     Small Craft Advisory until 8 AM EDT Friday for ANZ235-237.
     Small Craft Advisory until 2 AM EDT Friday for ANZ250.
     Small Craft Advisory until 8 PM EDT Friday for ANZ254>256.

&&