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Area forecast discussion
National Weather Service Taunton Massachusetts
720 PM EDT Wed Sep 20 2017

Synopsis...
Jose continues northeast overnight, then turns east and circles
well southeast of Nantucket through Saturday. This may be close
enough to maintain rain and strong wind for a couple of days
over southeast mass, favoring Cape Cod and islands as well as
the adjacent ocean waters. High pressure builds in from the
Great Lakes with dry weather early next week.

&&

Near term /until 6 am Thursday morning/...
** tropical storm warnings continue for some waters, Cape Cod
and the islands. **

7pm update...
drier air continues to filter in as winds shift around to north.
This will allow the bands of leftover rain showers and dz to gradually
dissipate through the remainder of the evening and all but
eliminate the widespread fog we have experienced over the last
several nights. Pops reflect this by gradually dropping off
across interior portions of S New England. Otherwise, the rain
bands directly related to Jose continue to spill over the
cape/islands but heaviest echos remain offshore. However,
outside of the mesoscale guidance, large-scale models seem to be
far too low for pops and are not capturing this banding well.
This will have to be watched during the overnight hours, as this
could increase rain totals in these areas.

Forecast on track other than these pop issues. Temps/dwpts were
brought up to current trends, but were not far off. Winds will
gradually increase overnight, but not peak until the early am
hours and especially after diurnal mixing is able to begin after
sunrise.

Previous discussion follows...

Jose rests, at the time of this writing, about 170 miles south-southeast of
Nantucket and 90 miles southeast of the 40/70 benchmark. Satellite
presentation has actually improved diurnally, similar to
yesterday with convection wrapping around the center, assuming
a slightly more symmetrical shape. In fact, as of the latest
advisory, NHC has been able to establish a Max of 60 kt
sustained. Jose will continue to slide east-northeast through the overnight
hours before once again sliding slightly west given the lack of
any synoptic pushing mechanism, riding right along the Gulf
Stream SST gradient, helping to maintain it. Given this close
proximity, as has been forecast now for some time, Jose will
continue to influence our sensible wx for some time, possibly
until the weekend before completely letting go. NHC suggests
extra-tropical conditions in 36-48 hours.

Winds...
tropical storm warnings continue for mainly the waters open to
the ocean and Cape Cod/islands. These remain the only locations
where sustained winds to the 39mph (34 kt) threshold are
possible while other locations are likely to see gusts to this
magnitude. Although some locations, particularly Block Island
and Nantucket, have already gusted to 40-45 mph with a few spots
(block island again) reaching near 40 mph sustained. However,
the strongest winds are yet to come. This is in part due to a
slight amplification of the mid lvl ridge, which will allow the
sfc anticyclone across north New England to respond and build
slightly, enhancing the pres gradient between Jose and the high
late tonight into early tomorrow. At the same time, slight
northerly shift in isallobaric flow will allow drier air to
filter into S New England from the N, which will help enhance
the surface-h92 lapse rates especially during daylight hours
tomorrow, allowing a mix down of at least some momentum from the
low level jet at h92 which peaks tomorrow morning between 45-55kt. All of
this is an admittedly long winded means of stating that the peak
winds actually won't arrive until late tonight and linger
through tomorrow night and slowly climb through tonight.

At peak, sustained winds between 35-45 mph are possible mainly
for those locations currently within tropical storm warnings,
with 25-35 along and southeast of the I-95 corridor, and 15-25 for
interior S New England. Gusts 45-60 mph, 35-50 mph, and 20-35
are possible respectively. Will continue, and actually extend
tropical storm warnings (tsw), wind advisories for I-95 and
points southeast not in thunderstorm with snow showers through tomorrow evening at least. Fully
leaved trees will exacerbate the issue as we have already seen
some spotty tree damage with lesser values.

Rainfall...
the biggest change this afternoon will be to drop the Flash
Flood Watch (ffw) for Cape Cod. With the slight shift E, and
tightening of the gradient/rain bands even as it shifts back west
should keep the heaviest and convectively drive bands too far east
for significant heavy rainfall over a short period of time. Not
to say that additional 1-2 inches aren't possible, but it would
be over the next 24-48 hours. Meanwhile, Nantucket is still
close enough to where guidance still suggests direct rain bands
that it warrants keeping the watch up and extending it into
tomorrow night as quantitative precipitation forecast bullseyes on meso-scale guidance do impact
the island through tomorrow. Still holding on rain totals
(inclusive of what has fallen) of 3-5 inches, which could be
exacerbated by the inability to drain tidal surges over the next
few high tide cycles. Interior showers and drizzle will taper
off overnight as the winds shift north and allow drier air
infiltrate some of the lower levels.

High surf...
with Jose now nearing its closest point to S New England, and
expecting to linger nearby through late week, a building
southerly swell, combined with strong north-NE flow will lead to
building seas through tomorrow night at least, and then maintain
high seas into the weekend. This will create high surf at local
beaches outside of Boston Harbor which remains sheltered in this
setup. 20-25 ft waves are possible well offshore. This will also
lead to dangerous rip currents. High surf advisories continue
into Friday for these conditions outside of tropical storm
warnings which cover this risk.

For more on the coastal flooding and beach erosion risk, see
that section below.

Otherwise across S New England, temperatures will remain near
normal across portions of west Massachusetts/CT where some breaks to sunshine
are possible tomorrow and Friday. Meanwhile cloud shield will
limit sunshine further E, leading to highs generally below
normal.

&&

Short term /6 am Thursday morning through Thursday night/...
Jose continues to slide back west allowing the maintenance of
strong pres gradient. Will extend wind headlines into the
overnight hours to match, and may need to push them into Friday
given only a little change. Although do note some weakening of
the northwest quadrant low level jet by daylight on Fri. Otherwise, Jose
continues to bring some rain to the extremities of Cape/Cod and
Nantucket and the risk for tropical storm conditions, so these
too will be maintained into early Friday. More to come in future
updates.

&&

Long term /Friday through Wednesday/...
highlights...
* gusty winds persist on Cape Cod and islands Friday
* dry and seasonable over weekend and early next week
* monitor NHC forecasts as Maria heads through Caribbean

With a tilted axis of high pressure building over the Great Lakes
Friday, Jose will be pushed southwest but ultimately remain
southeast of Nantucket.

Dry conditions will return to the region through early next week.
However, tropical storm force winds will continue across the cape
and islands through Friday. Winds will be slow to diminish as Jose
remains to our south.

A persistent ridge points to a warm period, especially Friday
through Monday.

Details...

Friday through Wednesday...

Both the European model (ecmwf) and the GFS are in agreement on the position of Jose
through Monday morning. Jose will essentially sit at 40 north through 12z
Monday, but depending on the position of the high pressure and shear
aloft, Jose may considerably weaken and therefore have very minimal
impact on the region.

High pressure builds in from the west Friday, bringing clearing
skies and lower humidity.

Unfortunately, with Jose remaining in close proximity, an extended
wind field will continue through Friday. The cape and islands will
see gusts surpassing 40 knots well into Saturday.

Models start to differ when it comes to Jose on day 5. The European model (ecmwf)
model now takes Jose further southeast and out to sea as a high
continues to build to its west. The 12z run of the GFS however
suggests that the high pressure currently building over the Ohio
Valley will eventually push Jose to the southwest, making landfall
along the New Jersey coast. Jose would be considerably weaker by
this time period, likely a Post-tropical depression.

The eventual track of Jose will have impacts the track of
Maria, currently located west Puerto Rico. If Jose does make
landfall near the New Jersey coast, Maria will likely be forced
out to sea and therefore having very little impact on southern
New England.

Temperatures through the weekend will be well above normal with
highs forecast to be in the mid 80s. The normal high for this time
of year is 70 degrees.

&&

Aviation /00z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through Thursday night/...

Overnight...high confidence in trends, lower confidence in
exact timing.
Gradual improvement, but mainly MVFR ceilings east of Worcester Hills
while IFR ceilings remain across Cape/Island terminals. Low vsbys
mainly confined to the cape/islands as well as rain showers/fog
dissipates inland this evening. VFR west of the Worcester Hills
should generally stay from now on. Winds gradually increase
through the night, but should peak mainly after sunrise.
Sustained winds remain around 25-35 cape/islands tonight with
gusts 35-45, 15-25 inland with gusts mainly 30-35 kt until
sunrise. Llws, especially east Massachusetts and Rhode Island with 40-50 kt off the
deck.

Tomorrow...
gradual improvement in ceilings from west to east with areas
off- cape becoming VFR by afternoon. Lingering MVFR cigs over
Cape Cod and islands. North-northeast winds continue to gust
near 20 knots in the west, 20-30 knots in the central hills, and
30-40 knots in Rhode Island and eastern mass with some 45-55 kt possible
cape/islands.

Kbos taf...high confidence in taf trends, lower confidence in
timing. Sustained north-northeast winds around 15-20 knots, and gusts
increasing to 25-35 knots.

Kbdl taf...high confidence in taf trends, lower confidence in
timing.

Outlook /Friday through Monday/...

High confidence.

Thu night and Fri...lingering MVFR ceilings in eastern MA,
otherwise VFR. North winds gusting to 25-35kt on Cape Cod/islands.

Sat and sun...VFR.

&&

Marine...
forecaster confidence levels...

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

Short term /through Thursday night/...

Tropical Storm Jose passes southeast of the waters today, then
lingers well offshore through at least Thursday. This will allow
strong winds to linger over the waters through at least Thursday,
with gusts of 35-45 knots over most of the waters, with gusts 25-30
knots on Narragansett Bay and mass Bay/Boston Harbor.

Building swells have already reached as high as 16 ft at buoy 44008
southeast of Nantucket. Expect high swell will continue through
Thursday, especially over the waters with a southern exposure.

Showers and fog will also linger through Thursday, creating poor
vsbys.

Outlook /Friday through Monday/...

Moderate confidence.

North winds remain strong through Saturday morning, with gusts of
35-40kt on south coastal waters as Jose drifts S and away from
southern New England. Winds diminish further as high pressure
builds over region this weekend, but seas will take most of
weekend to subside on open south coastal waters.

&&

Tides/coastal flooding...
our thinking in regards to coastal impacts remain generally
unchanged. We anticipate the greatest impact to be along the
Nantucket, Cape Cod, and Martha's Vineyard shorelines where tropical
storm warnings remain in place. Our primary message remains the very
long duration of beach erosion for this event. Jose is expected to
remain quasi-stationary southeast of New England through the end of
the week with slow weakening by the weekend.

Relentless wave action will continue for the rest of this week. We
anticipate some beach erosion along all ocean exposed Sandy
shorelines of Massachusetts and RI, but the most severe beach erosion is
expected along the east side of Nantucket and the Outer Cape ocean
side from Eastham to Orleans to Chatham. Another area of significant
beach erosion is expected to be the south sides of Nantucket and
Martha's Vineyard due to the impact of a persistent southeast swell.

As far as coastal flooding, the area of greatest concern remains the
Nantucket Harbor area where we may experience a high end minor event
late tonight and possibly moderate coastal flooding for the Thursday
early afternoon and late night high tide cycles. Minor coastal
flooding may linger along Nantucket Harbor for the Friday early
afternoon high tide. Elsewhere across Cape Cod, Martha's Vineyard,
and the Plymouth County coastline, we anticipate areas of minor
coastal flooding for the late night and early afternoon high tide
cycles through at least Thursday night.

We have extended the high surf advisory outside of the Tropical
Storm Warning area through Friday. The advisory covers all ocean
exposed coastlines in Massachusetts and Rhode Island outside of the Tropical Storm
Warning and 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.

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.

&&

Equipment...
kbox WSR-88D z-r relationship remains in tropical to get better
rainfall estimates than with convective.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...Tropical Storm Warning for maz022>024.
High surf advisory until 6 PM EDT Friday for maz007-019-020.
Coastal flood advisory until 4 am EDT Friday for maz019.
Wind Advisory until 6 am EDT Friday for maz006-007-013>021.
Flash Flood Watch through Thursday evening for maz024.
Rhode Island...Tropical Storm Warning for riz008.
High surf advisory until 6 PM EDT Friday for riz006-007.
Wind Advisory until 6 am EDT Friday for riz002>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/correia/Thompson
near term...doody
short term...doody
long term...correia

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