Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 212359
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
759 PM EDT Thu Sep 21 2017

.SYNOPSIS...
Jose will slowly weaken and drift very slowly toward the
southwest the next few days. It will remain well southeast and
south of Nantucket. This will cause strong gusty winds,
occasional rain and rough seas through Friday for Cape Cod, the
Islands,and adjacent ocean waters. High pressure builds in from
the Great Lakes with dry weather over the weekend, before a
front brings a few showers early next week.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

730 PM Update...

Radar imagery was depicting a fairly impressive band of moderate
to heavy rain rotating about the Cape and Islands and at least
brushing across a portion of SE MA northwest of the Canal.
Fortunately, the band looks to be transitory enough to preclude
serious flood concerns beyond local street flooding on the Cape
and Islands. High resolution models and to some extent other
models indicate that areas of showers may work their way further
NW into SE MA overnight and possibly reaching the Boston-
Providence corridor toward daybreak and have expanded likely
POPs further NW to cover most of SE MA and much southern RI.

Did make some adjustments to winds but not too dramatically from
the prior forecast. Anticipate strong winds to persist over the
Cape and Islands to about 03Z and then gradually diminish some
overnight but will remain gusty into Friday. It is possible that
a Wind Advisory will have to be issued once the Tropical Storm
Warning is taken down.

Just minor adjustments were made to temperatures.

Previous Discussion...
Strongest winds are expected through early evening, and then
they should very gradually diminish a little over night.
Tropical Storm Warnings remain in effect for the Cape,
Nantucket, Martha`s Vineyard, and Block Island and will probably
need to do so through tonight. As anticipated, deeper mixing has
helped bring some of those stronger gusts to the surface across
southeast Massachusetts, especially where there was thining of
low clouds.

Satellite imagery suggests a slow WSW drift of Jose`s center.
Radar was indicating at 415 PM moderate to heave rain bands
moving onto the outer Cape and Islands. These bands will
probably rotate as far west as the Cape Cod Canal and possibly
brush southern Plymouth and southern Bristol County this
evening.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
Jose will continue to weaken but not really move out of the
area. Thus, we anticipate a continued fairly strong N to NNE
flow but not as strong as today given weakening in Jose`s
overall circulation and significantly poorer vertical mixing
through the boundary layer. Nonetheless, we anticipate 35 to 45
mph gusts to persist well into Friday over Nantucket and 30 to
40 mph gusts over Cape Cod, Martha`s Vineyard, and Block Island.

Model vertical moisture profiles depict an increase in low level
moisture below 10 thousand feet late tonight through much of
Friday. Along with the moisture and onshore flow component, we
anticipate periods of rain or drizzle across much of eastern MA
into at least southern RI including the Providence area on
Friday. The clouds and precipitation will also keep temperatures
in the 60s on Friday. It should remain drier further west and
temperatures should make it into the mid 70s across the CT River
Valley on Friday.

We anticipate a slow drier trend Friday night and a slow
diminishing of surface winds as Jose continues to decay. There
will be enough cloudiness to inhibit radiational cooling and
temperatures will likely only fall to the mid 50s to around 60
across most of the region.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights...

 * Dry over weekend, some showers possible early next week
 * Monitor forecasts as Maria heads through Caribbean

Jose, nearly stationary will begin its slow dissipation
process, especially after it transitions to extra-tropical given
a lack of synoptic support in the form of any shortwave energy.
Wedged between subtropical Bermuda high and a very broad ridge
across most of the E CONUS, there is little to steer it away as
it weakens. This will help to maintain at least some gusty winds
into the weekend, along with high surf and a risk for rip
currents. The broad ridging, following the exit/end of Jose
defines the weather well into next week, yielding a blocking sfc
high pres and the approach of a shearing wave to impact Maria`s
track late week.

Sat through Mon...
Jose loosens its grip on New England as it both dissipates and
shifts slightly further offshore. Weakening pres gradient
suggests a few gusty winds on Sat, but not nearly as strong as
today or Fri. Improving conditions expected throughout S New
England, with a gradual warming trend as well within the
amplifying ridge. H85 temps warm through to about +19C by Mon.
Therefore, highs especially will likely be well above seasonal
normals, in the low 80s Sat except SE MA where cloud cover
could limit might. Mid-upper 80s on Sun and Mon except where
onshore flow develops on Mon. Dwpts suggests mins drop to the
low-mid 60s each night. The warm temps could lead to increased
attraction to area beaches for late September, but high surf and
rip currents will remain a risk through the weekend as the
swells from Jose will be slow to subside.

Tue and Wed...
Early indications suggest a backdoor cold front could shift
across the region allowing cooler E-NE flow across the region.
Where SSTs remain in the low-mid 60s in the Gulf of Maine, this
would bring ambient temperatures closer to seasonal normals
after the weekend warm spell. Higher risk for clouds as
shortwave energy approaches from the W.

Late week...
Most eyes on Maria. Current indications that after a slow
northward progression about 200-300 miles off the E seaboard,
that as the shearing wave to the W, combined with blocking high
pres will begin to push it out to sea. However, it is still too
early to completely rule out any possibility with its track, so
stay tuned as the players come in line. In any case, the N track
and Great Circle trajectory indicate another round of increasing
swells with some marine/surf impacts possible.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Tonight and Friday...

Not too much change has been made to prior reasoning except to
spread areas of rain and/or drizzle a little further NW to
possibly reaching the BOS-PVD corridor during the early morning
hours. Still think that MVFR ceilings will probably make their
way to BOS and PVD after 06Z.

Prior Discussion...
Anticipate MVFR ceilings to spread NW after midnight across
eastern MA and southern RI and remain low MVFR or IFR across
Cape Cod and Nantucket. Relatively strong and gusty NNE winds
will linger but gradually diminish during Friday. Gusts of 40
to 50 knots still possible Nantucket this evening and then 30 to
40 knots overnight. Gusts 35 to 40 knots over Cape Cod and
Martha`s Vineyard this evening and then gradually diminishing to
30 to 35 knots overnight.

KBOS TAF...NNE winds should begin to slowlydiminish after 00Z.
Anticipate with moderate confidence MVFR cigs moving back to
BOS after 06Z.

KBDL TAF...High confidence.

Outlook /Saturday through Tuesday/...

High confidence.

Sat...
Lingering MVFR CIGS across SE MA should dissipate through the
day giving way to VFR everywhere. N winds may gust 20-30 kt at
times.

Sun and Mon...
VFR. Light winds.

Tue...
Some MVFR possible in lower clouds, otherwise VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Seas will remain rough through Friday night but very gradually
begin to subside late tonight through Friday night but still
probably an issue for small craft well into if not through the
weekend.

Bands of showers, drizzle, and fog will also linger through
Friday creating poor vsbys.

Outlook /Saturday through Tuesday/...

Moderate confidence.

Mon...
Gusty N winds continue through early Sat with gusts to Gales
lowering to sub Small Craft levels through the day. Seas too
will be subsiding with time, but remain elevated due to
lingering swells. Small craft advisories are likely to continue.

Sun through Tue...
E coastal waters may drop below the 5 ft threshold, however
lingering swells from Jose, combined with increasing swells from
Maria may lead to high seas above the 5 ft threshold especially
on the S waters and SE waters well into early next week.

&&

.TIDES/COASTAL FLOODING...
High surf will continue into at least Saturday along the east
side of Cape Cod and Nantucket with lingering beach erosion.
Some beach erosion may also persist along south facing ocean
exposures.

Surge is still rising some at both Nantucket and Boston, and
anticipate another round of minor coastal flooding across
southeast MA vulnerable locations for the early morning Friday
high tide. There is still a possibility that Nantucket may
experience a high end minor or minor to moderate coastal flood
event early Friday morning due to the wind fetch toward the
harbor and a surge that may peak out near or just shy of 2 feet.
Total water level may top out a few tenths either side of 5.5
feet MLLW.

It may be necessary to issue/extend Coastal Flood Advisory for
the Friday afternoon high tide, but confidence is not all that
high presently. One bit of caution is that there will continue
to be a persistent chain of long period swells coming to the
shoreline through Friday.

High Surf Advisory remains posted outside of the Tropical Storm
Warning area through Friday night. The advisory covers threats
from both high surf and dangerous rip currents. We may end up
having to extend the High Surf Advisory into at least part of
the weekend for some of the ocean exposed coastline.

Swells from Maria may re-increase the surf and associated rip
current risk across our ocean exposed south coast sometime
during the first half of next week even though Maria may
eventually recurve out to sea before getting too close to New
England.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Tropical Storm Warning for MAZ022>024.
     High Surf Advisory until 6 AM EDT Saturday for MAZ007-019-020.
     Coastal Flood Advisory until 4 AM EDT Friday for MAZ019.
     Wind Advisory until 6 AM EDT Friday for MAZ007-015-016-018>021.
RI...Tropical Storm Warning for RIZ008.
     High Surf Advisory until 6 AM EDT Saturday for RIZ006-007.
     Wind Advisory until 6 AM EDT Friday for RIZ005>007.
MARINE...Tropical Storm Warning for ANZ231>235-237-250-254>256.
     Small Craft Advisory until 4 PM EDT Friday for ANZ230-236-251.

&&

$$
SYNOPSIS...Doody/Thompson
NEAR TERM...Thompson
SHORT TERM...Thompson
LONG TERM...Doody
AVIATION...Doody/Thompson
MARINE...Doody/Thompson
TIDES/COASTAL FLOODING...Thompson


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