Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 051404
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1004 AM EDT Wed Aug 5 2020

.SYNOPSIS...
A weak cold front moves through Southern New England late in the
day, and may generate isolated showers. High pressure then
builds over the region tonight and Thursday with very warm and
drier air. While generally dry and seasonable Thursday night
into Saturday, cloudy and unsettled conditions for the South
Coast of MA and RI and Cape Cod. Warm and dry weather for the
rest of weekend. Our weather then turns very warm and
increasingly muggy into next week, with daily chances for
showers and thunderstorms.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
10 AM Update:

Only a few minor tweeks to the near term forecast this morning.
We`ve updated sky conditions to reflect the few clouds that
remain along the coast of RI and the south coast of MA;
including the Cape and Island. Otherwise it`s a mainly dry and
sunny day. As previously discussed a few pop-up showers are
possible across eastern MA/RI - coverage will be isolated.
Otherwise it`s a warm and muggy day with temperatures in the
upper 80s and low 90s.

7 AM Update:

No significant changes needed to the going forecast with this
update. Upper shortwave disturbance aloft passing to our
northwest with its main effect being a modest increase in
coverage of cumulus. Possibility that a few could pop a shower
or two but it`s a fairly remote possibility and many will stay
dry today. Turning quite warm today and given that folks may
head to the beach, a high surf advisory is in effect for south-
facing beaches due to sea/swell from Isaias leading to a higher
risk of rip current and hazardous surf conditions. Highs look on
track in the 80s to lower 90s with generally decreasing humidity
levels.

Previous discussion...

Upper low over the Great Lakes will swing a shortwave across Srn
New England today. The associated upper jet will place its
right entrance region overhead, providing some lift that could
generate a few showers. PW values are around 2 inches along the
South Coast, but diminishing farther north and west. Favorable
CAPE is limited to the South Coast. With limited moisture and
instability, confidence in precip is limited. Will continue to
forecast widely scattered showers with no mention of thunder.

Mixing is forecast to reach 800-mb, where temperatures will be
11-13C. These temperatures will support surface max temps of 85
to 90. Dew points in the 60s, so the humidity will be noticeable
but much more comfortable than yesterday.

&&

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

The weak cold front moves offshore, and surface winds turn out
of the northwest. Dew points will continue to fall to more
comfortable levels, reaching the mid 50s to mid 60s. Expect min
sfc temps in the upper 50s and 60s, possibly mid 50s in extreme
northwest MA.

Thursday...

High pressure over the region will bring dry weather and
continued comfortable humidity. Mixing to 850-mb will tap
temperatures of 14-15C, supporting max sfc temps in the mid 80s
except a little cooler on the South Coast.

&&

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

* Unsettled showers South Coast, Cape and Islands Thurs nite thru
  Sat, generally dry for remainder of SNE. Seasonable temps.

* Warm and dry weather for the rest of the weekend.

* Increasing heat and humidity early next week with rising heat
  indices. Daily thunder chances, better t-storm coverage Tues-Wed.

Details...

Thursday Night into Saturday:

SW flow aloft regime prevails in mid-levels through this period,
established by an upstream mid-level trough which moves from the
Great Lakes into the Northeast states late in the week into Sat.
While a broad surface ridge associated with dry weather and low
humidity levels is expected to extend across a part of interior New
England, the primary surface feature is a SW-NE oriented stationary
boundary from the mid-Atlantic states into our far southern offshore
waters. Weak large-scale ascent acting on the stalled boundary
appears to be enough across most of the 00z NWP suite to permit a
weak surface wave to develop on the boundary near the DE/NJ coast
early Fri. As this feature ripples NE along the boundary, it shifts
its effective position slightly northward as a warm front.

Main source of uncertainty in this period is timing when and how far
north will the stationary feature return as a warm front, spreading
clouds and related showers into a part of Southern New England. In
general, the ECMWF/Canadian GEM keep the feature suppressed to the
south enough to allow for little if any measurable precip. The GFS
and the SREF on the other hand keep much of this period
wet/unsettled, bringing showers to the South Coast/Cape by Thurs
nite and spreading the precip shield northward across much of
Southern New England thru Fri/Sat. Ensemble means also offer no
preferred outcome and generally lend support to their respective
deterministic model solution.

Can`t dismiss a wetter solution yet, but am thinking given the
weaker surface low shown across most models that it probably won`t
be enough to really produce as much northward advance to the
boundary/precip as depicted in the wetter guidance. Thus following
along the lines of the ECMWF/GEM. Will thus keep most of MA,
northern CT into northern RI generally dry, and confine more clouds
with Chance-ish PoPs to the MA/RI South Coast, Cape and adjacent
waters. That said, doesn`t look to be a washout by any stretch but
enough to potentially ruin any beach days.

With more clouds/showers south than north, temperatures should have
more of a diurnal range to them (e.g. warmer highs/cooler lows) as
one moves north from the South Coast/Cape. More clouds and rains to
the south will allow for narrower diurnal ranges in SE MA and
coastal RI. Overall temps should be fairly seasonable, though highs
may run cooler than average towards the South Coast/Cape.

Saturday Night into Sunday Night:

Much nicer stretch of weather for the rest of the weekend with high
pressure building in from the Great Lakes Saturday night, then
moving offshore Southern New England late Sunday into Sunday night.
Look for dry conditions with plenty of sun, and should be great for
any plans outdoors. 850 mb temps initially around the mid teens (+13
to +15C) with warm advection on Sunday rise to the upper teens
Sunday (+16 to +18C). Should see highs in the mid to upper 80s,
though a few sites in the CT Valley and eastern MA could reach 90
given WSW downsloping. Will also start to see a rise in humidity
levels come Sunday but nothing too humid or oppressive.

Monday through Wednesday:

Pattern aloft turns zonal across CONUS, allowing for significant
heat (upper teens to low-20s Celsius 850 mb air) to advect eastward
into Southern New England. Humidity levels will rise to very humid
to oppressive levels especially by Tuesday, so the main story in
this period will be increasing heat and humidity. With high temps in
the upper 80s to lower 90s and dewpoints looking to rise into the
upper 60s to mid 70s, heat indices approaching Advisory levels
appear possible in parts of this period.

Given the heat and humidity, daily thunder chances should then
resume. Difficult at this time range to pinpoint how strong they may
be and/or on what days. Do note the GFS advecting in some steeper
mid-level lapse rates that could really boost instability but
appears flow aloft is very weak for most of the period. GFS/ECMWF
differ on timing a cold front through which would offer greater
convective coverage, but that may be sometime either Wednesday
(ECMWF) or Thursday (GFS). Will follow blended PoP guidance this
period.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...

10 AM Update: No major changes to the TAFs.

Forecaster Confidence Levels...

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

12z TAF Update:

Today: High confidence.

VFR all terminals with bases around 050. Outside shot at
showers but too remote a possibility to mention in the TAFs.
Daytime mixing will also draw southwest wind gusts of 20 kt
down from aloft.

Tonight...High confidence.

VFR. Possible IFR/LIFR cigs/vsbys in fog and low clouds moving
off the ocean across ACK and possibly Cape Cod.

Thursday...High confidence.

VFR. Light northwest wind.

KBOS Terminal...High confidence in the TAF.

KBDL Terminal...High confidence in the TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday: VFR. Slight chance SHRA.

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

Saturday through Sunday: VFR.

&&

.MARINE...

10 AM Update: We`ve allowed the Small Craft Advisory to expire
for waters east of Boston. Otherwise, moderate to high risk for
rip currents on southern facing waters.

Today...

Southwest winds with gusts to 20 kt. Lingering rough seas on the
outer waters and RI/Block Island Sounds, with 10-12 feet. Expect
these seas to diminish this afternoon.

Tonight...

Southwest winds shift from the northwest by morning, as a weak
cold front moves across the waters. Seas will continue to
diminish, dropping below 5 feet overnight.

Thursday...

High pressure builds over the waters. Light northwest winds
become variable. Seas remain less than 5 feet.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday Night: Winds less than 25 kt.
Chance of rain showers.

Saturday: Winds less than 25 kt. Slight chance of rain showers.

Saturday Night through Sunday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Surf Advisory until 6 PM EDT this evening for MAZ020-023-
     024.
RI...High Surf Advisory until 6 PM EDT this evening for RIZ006>008.
MARINE...Small Craft Advisory until 6 PM EDT this evening for ANZ235-
     237.
     Small Craft Advisory until 10 AM EDT this morning for ANZ250.
     Small Craft Advisory until 6 AM EDT Thursday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Loconto
NEAR TERM...WTB/Loconto/Gaucher
SHORT TERM...WTB
LONG TERM...Loconto
AVIATION...WTB/Loconto/Gaucher
MARINE...WTB/Loconto/Gaucher


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