Area Forecast Discussion
Issued by NWS Boston, MA

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332
FXUS61 KBOX 211109
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
709 AM EDT Sat Jul 21 2018

.SYNOPSIS...
High pressure over the Gulf of Maine provides another dry day across
New England along with comfortable humidity. Low pressure with tropical
moisture moves northward along the New Jersey coast tonight with
its attending warm front approaching New England. This setup will
bringing showers and thunderstorms with torrential downpours and
gusty winds tonight into Sunday. Bermuda high pressure then builds
west toward southern New England next week bringing tropical humidity
and warm temperatures. Scattered showers and thunderstorms are possible
Monday and again Thursday with drier conditions likely Tue and Wed.

&&

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

7 AM update...

Another gorgeous morning in progress with pleasant temperatures
and dew pts in the 50s. Mostly sunny for much of the region
however surface observations and GOES16 satellite imagery
indicates some strato-cu and stratus clouds inland. However
this low level moisture should thin as very dry air aloft per
PWATs only 0.5 inches along with negative K values, mixes down
to the surface once strong July surface heating commences. Also
satellite and surface obs indicate low clouds offshore pushing
into the outer Cape and Nantucket/Marthas Vineyard. This low
level moisture is trapped beneath subsidence inversion. However
all guidance indicates this moisture will come to a halt and not
progress much farther inland as drier air aloft mixing to
surface once strong heating commences. Thus have updated the
forecast to better reflect these current conditions.

Otherwise one more tranquil/pleasant weather day with highs
75-80 and dew pts in the 50s providing comfortable humidity.
Earlier forecast captures these nicely so no major changes to
the afternoon portion of the forecast. Previous discussion
below.

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

Today...

One more nice, refreshing Summer day. Quiet, dry weather. With
sunrise, any locally dense, shallow ground fog immediately lifts,
burning off while oncoming marine stratus / fog holds along the
shore, perhaps enveloping portions of the Cape and Islands. Greater
low-level moisture content with continued onshore flow, scattered,
potentially broken cumulus across the interior. Otherwise drier air
holding in place aloft, noting negative K indices. Hedge dewpoints
lower with boundary layer mixing. By late, anticipating increasing
mid-high cloud through the mean S/SE wind profile in advance of the
approaching coastal low. Towards sunset, marine stratus / fog builds
W with an increasing low-level advective onshore wind profile.

&&

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

Tonight into Sunday...

*/ Highlights...

 - Highly anomalous synoptic setup for mid July
 - Tropical downpours, excessive rainfall, potential flooding
 - Embedded thunderstorms, possible severe weather elements
 - High shear, low instability, potential for brief spin-ups
 - However a measure of uncertainty, set-up bears watching

*/ Overview...

Sub-tropical low rotating round a negatively tilted cut-off closed
low across the Mid-Atlantic coast NW into Upstate NY. An attendant
anomalous S to N low-level wind / precipitable water axis, low-level
convergent beneath broad diffluence, along a lifting warm front to
the sub-tropical low. Deep moist profile with high shear, especially
0-1 km, low instability. Forecasting a slug of tropical downpour /
thunderstorm activity during the early morning hours, potentially
evolving further later Sunday within the warm sector as sunshine
breaks out over S New England. Potential severe weather elements,
flooding possible, a highly anomalous situation for mid July.

However, sub-tropical low focus rotating into, as mentioned prior,
the NYC tri-state region into Upstate NY. A low-level focus is still
possible into S New England, however the system is transitional as a
mid to upper level ridge builds into the region, heights rising. A
measure of uncertainty.

Split between mesoscale and global runs for the 36-48 hour period.
Difference in axis of heavy rainfall with global guidance leaning E
over the W leaning mesoscale guidance. It is perhaps an outcome of
mesoscale guidance beyond 36 hours which tends to do poorly. Not
much clarity from ensemble means / probabilistics, as well as CIPS
Analogs. Threats are there, but nailing them down remains difficult.
Break down the potential threats below.

*/ Discussion...

Flooding...
Potential for tropical downpours, pockets of +1"/hr rainfall rates,
yielding 1-2" total rainfall amounts, locally higher. Leaning with
an initial burst of rainfall along a convergent low-level jet, then
S to N training of tropical elements. Deep, warm column moisture,
precipitable waters 2.0-2.5", high freezing level heights, efficient
rainfall processes. Sultry environment with mid 70s dewpoints, H85
dewpoints +15C. Instability present, conditionally unstable profile,
invoking updrafts, heavy rain potential. Localized flooding is
possible.

However low confidence. Spread continues within mesoscale and global
forecast guidance. No clear signals within ensemble probabilistic
guidance and means. Greater heavy rain focus may occur towards the
Delaware River Valley. Meanwhile H5 ridge / rising heights building
into the region during the forecast period. Can`t rule out a S-N
training of tropical elements, potential flooding over New England,
but not enough clarity / consistency to warrant any flood headlines.
Still a challenge to nail things down.

Winds...
Combined with potential heavy rain and/or severe weather elements,
forecast S gusts around 35 mph. Roughly around 2 kft agl sustained
winds 40-50 mph per forecast model consensus ahead of the sub-trop
low. GEFS / SREF +5 standard deviation low-level S flow, highly
anomalous for mid July. Waters not as cold, low-level inversions not
as stout. Can`t rule out locally higher gusts up as high as 45 mph
especially with potential heavy rain / thunderstorms. Impacts
especially along S-coastal New England around Sunday morning
possibly continuing through the day with any breakout sunshine
within the warm sector.

Tornado / Waterspout...
Potential for a brief spin-up. A high shear, low instability setup.
Echoes of recent Concord and Revere early morning tornadoes which
had a similar environment. Allegory to a landfalling tropical rain-
band associated with a cyclone, the interaction with terrain and
subsequent frictional component could potentially aid in a brief
spin-up for the early morning period with any onshore thunderstorm.
Then focus on later in the day within the warm sector. Continued low-
level jet / shear within weak, subtle synoptic scale forcing as
heights rise with the enhancing Atlantic ridge. Can`t rule out
additional break-out midday into the afternoon activity. Low LCLs
throughout with dewpoints getting into the mid 70s. SREF probs
highlighting a high probability of exceeding 0-1 km helicity over
150 m2/s2, and some indication of sigtor`s over 1.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...

Highlights...

* Tropical humidity through at least Thursday
* Greatest risk for showers/T-storms Sun ngt-Mon & again Thu
* Drier weather possible Tue and Wed

Sunday night...

Tropical moisture plume aligned along the eastern seaboard into New
England via deep southerly flow courtesy of anomalous 597 dam ridge
south of the Maritimes and a closed low over the eastern Great
Lakes/OH Valley. Within this tropical moist conveyor belt scattered
showers and embedded thunder will continue across southern New
England. Global guidance and the NAM indicating torrential downpours
possible. This combined with ongoing convection during the day
Sunday will yield a low risk of localized flash flood threat. WPC
recognizes this with a slight risk in their excessive rainfall
graphic. However given the time range here still enough uncertainty
to hold off on a Flash Flood Watch but later shifts may have to
consider pending model trends. Otherwise oppressive tropical
humidity within this airmass along with warm overnight temps given
dew pts of 70-75. Breezy as well in response to lingering southerly
low level jet.

Monday...

Anomalous subtropical ridge retrogrades westward into New England
with 594 dam heights into the region! These rising heights shifts
tropical moisture plume westward into NY state as the day
progresses. Thus drier weather advecting into the area from east to
west as seen in model PWAT and K indices fields decreasing. Yes, not
your typical advection pattern. So risk for showers in the morning
and then a drying trend from off the ocean into eastern MA trending
westward in the afternoon. Given rising heights/warming temps aloft
will not include thunder in the forecast. Very tropical feel to the
day with highs 80-85 and dew pts in the mid 70s!

Tuesday and Wednesday...

Good agreement among ensembles and deterministic data sets for
subtropical ridge to continue building westward into New England
with 500 mb heights of 591-594 dam across southern New England. This
will shunt tropical moisture plume north and west of our region.
Thus Tue and Wed could turn out to be mainly dry given subsidence
and deep layer dry air aloft. However can/t rule out a few tropical
isolated low top showers. Tropical airmass will remain over the
region with highs 80-85 and dew pts 70-75. Other than some patchy
morning fog possible, could be looking at two good beach days
especially across Cape Cod and Nantucket, closest to the subtropical
ridge...a taste of Bermuda weather.
.
Thursday...

Ensembles and deterministic guidance both suggest northern stream
trough enters the Great lakes. This provides cyclonic flow into New
England along with cold front approaching New England. Thus
probability of showers and T-storms on the increase. Warm and humid
conditions continue with highs 80-85 and dew pts 70-75.

Friday...

Ensembles suggest FROPA sometime Fri-Sat time period so expecting
the tropical humidity to finally break. However still warm with
ensembles offering +16C or so at 850 mb over New England.

&&

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

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

Short Term /through Sunday/...High confidence.

11z update...

VFR most terminals today along with dry weather. Spotty IFR/MVFR
inland at sunrise will lift to a VFR deck of clouds by midday.
IFR/LIFR conditions over Cape Cod/Nantucket and Marthas Vineyard
not as straight forward. Indications suggest these low clouds
and fog not moving any farther inland and should lift to
MVFR/VFR or drift just offshore once strong surface heating
commences later this morning into the afternoon. Previous
discussion below.

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

Today...
Interior VFR, IFR-LIFR fog eroding, lifting, becoming SCT 4-5
kft cumulus, increasing mid-high clouds late. Meanwhile along
E/SE coastal terminals, onshore marine stratus / fog. Right
along the immediate coast, could be contending with MVFR-IFR
conditions, definitely towards evening and lowering.

Tonight...
Lowering IFR-LIFR, widespread. RA with embedded +RA/TSRA towards
early morning Sunday, roughly after 6z. Gusty winds possible
25-35 kts with TSRA. Both CIG and VSBY impacts.

Sunday...
IFR-LIFR slowly lifting through the day towards MFVR to low-end
VFR. Some locations may remain IFR throughout. Morning widespread
RA with embedded +RA/TSRA becoming more localized along a S-N
band. Continued strong to severe weather potential with gusty
winds.

KBOS Terminal...
Hold VFR, keep IFR marine stratus / fog immediately offshore
during the morning, pushing in during the later-half of the day.
Increasing E onshore flow. Onset wet-weather towards early
morning Sunday with gusty winds, potential +RA/TSRA.

KBDL Terminal...
IFR FG possible overnight roughly 8-11z, lifting and improving
quickly during the morning hours. Lowering conditions overnight
Saturday into Sunday morning with onset of widespread RA, embedded
+RA/TSRA elements with TSRA potentially being strong to severe.

Outlook /Sunday Night through Wednesday/...

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

Monday: Mainly MVFR, with areas VFR possible. Breezy. Chance
SHRA, patchy BR.

Monday Night: Mainly MVFR, with local IFR possible. Chance
SHRA.

Tuesday through Tuesday Night: Mainly VFR, with areas MVFR
possible. Chance SHRA.

Wednesday: Mainly MVFR, with areas VFR possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

*/ Highlights...

 - A Gale Watch continues for Narragansett Bay and the waters
   south of Rhode Island for tonight and Sunday.

 - A Small Craft Advisory has been issued for all other waters
   for tonight and Sunday.

*/ Discussion...

Today...
Increasing onshore E flow ahead of a lifting warm front. Will
see marine stratus / fog envelop the waters, advect towards the
shore during the day. Low visibility possible.

Tonight through Sunday...
A storm system will be moving N along the mid-Atlantic coast
and passing well W of the waters by Sunday. A strong warm front
will be moving across the waters tonight into Sunday morning.
Showers and thunderstorms with torrential rain and gusty winds
are expected late Saturday night into Sunday.

Gale force wind gusts are possible, hence the Gale Watch in the
waters along the RI coast. Winds are forecast to be upwards of
30 knots over all other waters late tonight and Sunday, but it
is possible that portions of these areas may need to be upgraded
to Gales in subsequent forecasts. In addition, there is the
threat of a brief spin of a waterspout.

Continued marine stratus / fog, the threat of low visibility.
Strong wind and wave action, there is the threat of high surf
especially along S-coastal New England, wave action building up
to 10 feet. This will also pose the threat of dangerous rip
currents.

Outlook /Sunday Night through Wednesday/...

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Rough seas up to 9 ft. Chance of rain
showers, slight chance of thunderstorms, patchy fog. Visibility
1 to 3 nm.

Monday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain showers, patchy fog. Local visibility 1 to 3 nm.

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers.

Tuesday through Tuesday Night: Winds less than 25 kt. Seas up
to 5 ft.

Wednesday: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 2 AM to 6 PM EDT Sunday for
     ANZ230>234-250-251-254-255.
     Gale Watch from late tonight through Sunday afternoon for
     ANZ235>237-256.

&&

$$
SYNOPSIS...Nocera/Sipprell
NEAR TERM...Nocera/Sipprell
SHORT TERM...Sipprell
LONG TERM...Nocera/Sipprell
AVIATION...Nocera/Sipprell
MARINE...Nocera/Sipprell



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