Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 240330

Area Forecast Discussion
National Weather Service Taunton MA
1130 PM EDT Sat Sep 23 2017

High pressure, both at the surface and aloft, will extend from the
eastern Great Lakes to New England overnight through early next
week. It will bring unseasonably warm weather to the region
into the middle of next week. Maria is expected to pass
southeast of New England late in the week with strongest winds
well offshore.



1130 pm update...

Will be adding Dense Fog Advisory to Cape Cod and Islands
including Block Island. Considered Plymouth County, but
satellite trends show thinning of the stratus, so suspect the
fog will thin shortly.

Previous discussion...

Fog over much of Eastern Massachusetts, with the GOES 16 fog
product defining this area quite well. This same product shows
some weakening of the stratus over the Gulf of Maine, but
continued expansion over land and especially south of Boston.
This fog should move into eastern RI in the next 1-2 hours. Also
signs of potential development in the CT Valley and Quabbin
area, as well as a few other spots, which would be expected
under these conditions.

Otherwise surface map shows high pressure over the Eastern USA
with a light NNE flow over Southern New England. This will
continue overnight and provide an environment favorable for fog
expansion. Most observed visibilities are around 1/2 mile or
higher, a little higher than we`d like to see for a Dense Fog
Advisory. The exceptions toward the lower are Logan Airport and
Marshfield which report 1/4 mile and less than 1/4 mile,
respectively. We will hold off on a headline at this time but
re-evaluate before midnight.

Evening temperatures in the 60s and dew points in the mid 50s to
mid 60s. With fair skies and light wind, expect a min
temperature range in the mid 50s to lower 60s.



Winds diminish tonight, which with clear skies will allow
temperatures to cool to dew point. We trended toward the mos values
for min temp, which was a couple of degrees lower than the standard
guidance. Radiation fog possible in the interior, and marine
fog/stratus is possible along the Cape/Islands/South Coast.



High pressure over the region, both at the surface and aloft,
will mean sunshine and hot temperatures away from the coast.
Heights at 500 mb are forecast at or above 590 Dm, and 850
temps are forecast at 18-20C. 925 mb temps are forecast to
reach +24C. Expecting highs reaching the lower 90s in the CT
River Valley, which would be a record...see Climate section at
the end of this forecast discussion. Expecting upper 80s
elsewhere in interior southern New England. Light winds under
the high pressure will allow daytime sea breezes, which will
keep temps much cooler along the shoreline....only 70s to near

The one big exception to the above will be Cape Cod and the
islands, which are likely to begin the day with cloud cover and
areas of fog, which may linger through a big chunk of the
morning, before becoming partly sunny. Expecting highs on the
Cape and islands to range from the upper 60s on Nantucket to
the mid 70s on the upper Cape.

With dewpoint temperatures only in the lower to mid 60s, the
heat indices are forecast to reach 90 to 94, just shy of heat
advisory criteria - although that criteria requires it to last
for two days. Will need to keep an eye on this.

Sunday night...

Expecting mostly clear skies. Could see marine stratus again
late at night over the Cape and islands. Patchy fog possible
late in some interior areas. Lows in the 60s.

Swells from Hurricane Maria will be reaching south coastal
areas, so a High Surf Advisory remains posted for those areas.



* Summer warmth expected to the middle of next week
* Maria expected to sharply recurve out to sea southeast of New
  England late next week

No major surprises in the 23/12Z guidance. Still expecting an
anomalous mid level ridge to hover over our region through the
middle of next week, although it should weaken with time after
Tuesday. The strength of this ridge will be crucial in delaying the
northward progress of Maria towards our region.

High pressure with dry weather expected Monday and Tuesday.
Temperatures should be 10-15 degrees above normal Monday, but not
quite as much Tuesday or Wednesday. A few interior spots may reach
90 degrees Monday. Seabreezes should develop along both coasts,
lowering daytime max temperatures there.

A potential weak low level front just south of our region, may be
enough to trigger a few showers Tuesday night into Wednesday. A
greater risk for showers should be ahead of a cold front sweeping
through our region Wednesday night into Thursday morning. This
cold front and a potent mid level trough are expected to be the
final kickers which steers Maria out to sea well to our south.

Drier weather should prevail next Friday into Saturday, although
cannot dismiss the possibility of some stray showers given the
proximity of a mid level trough. The bigger change will be the
temperatures, which should snap back to near normal levels during
this time.

The primary impact on our region from Maria will likely be another
prolonged period of high surf and dangerous rip currents, mainly
along ocean-exposed south coast beaches. For several days, Maria
tracks near a great circle route in our direction and hence we will
likely see significant long period swells from the S or SE propagate
into our coastal waters most of next week. The slow northward speed
of Maria for the next several days may enhance the swell activity
during this period.


Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...

Tonight...Moderate confidence, mainly due to timing.

IFR/LIFR conditions in fog in Eastern Mass, which should persist
overnight. Satellite trends show expansion into RI and Metrowest
Boston. Models show that this will persist through the night,
with the fog becoming locally dense. Satellite trends also show
potential for fog formation in the CT River Valley especially
north of Springfield. IFR/LIFR possible in these areas.

Elsewhere, VFR conditions. Patchy fog with isolated IFR/LIFR
conditions possible over portions of the interior late.

Sunday...High confidence.

Any interior fog burns off early in the morning. IFR/LIFR in low
clouds and fog will be slower to lift over Cape Cod and the
islands and may persist through the morning. Otherwise VFR most
locations with light wind. Sea breezes expected along the
coastlines, including KBOS.

Sunday night...Moderate to high confidence.

VFR most areas, however, could see marine IFR developing again
on Cape Cod and the islands. Patchy areas of fog may develop
late over portions of the interior.

KBOS TAF...Moderate confidence. Marine stratus and fog will
likely persist into the first half of the morning push.
Conditions improve to VFR Sunday morning, with light seabreezes
developing from late morning and lasting through the afternoon.

KBDL TAF...High confidence in TAF. Patchy fog may develop 07Z-
11Z tonight, although the chance of this is stronger over
valley locations north of Springfield.

Outlook /Monday through Thursday/...Confidence...High.

Monday through Tuesday...VFR expected.

Tuesday night...Mostly VFR except MVFR in patchy stratus/fog
some interior valleys and near the south coast.

Wednesday...Generally VFR except MVFR ceilings/visibilities
possible in scattered late day showers and patchy late night/early
morning stratus/fog.

Thursday...Generally VFR, with MVFR in scattered showers during
the morning.


Forecaster Confidence Levels...

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

Tonight...Moderate confidence.

Leftover swell was diminishing along the east coast of MA.
Low clouds and fog will continue along the southern and
southeast coasts tonight with poor visibilities. Winds will
diminish to less than 15 knots, but seas will linger at 5-6 feet
on the southern outer waters.

Sunday...Moderate confidence.

High pressure building over the region will bring light wind,
with sea breezes developing at the shoreline. Poor vsbys in fog
early, slowly improving by late morning or early afternoon over
the southeastern waters. Seas of 5-6 feet will linger on the
southern outer waters. A south swell sent north from Maria will
start showing itself in our southern waters late Sunday, with
3-4 feet swell heights in the afternoon.

Sunday night...High confidence.

Seas build to 5 to 7 feet over the southern outer waters, mainly
due to swells from distant Hurricane Maria. High surf conditions
will exist on area beaches along the south coast. Patchy areas
of fog.

Outlook /Monday through Thursday/...Confidence...High

Relatively light winds are expected through this period. However,
swells from Maria will propagate into the southern coastal waters,
beginning late Sunday night or Monday. Small Craft Advisories will
likely need to be extended due to rough seas for the waters south of
the MA and RI coast for all of this period.

The swells from Maria will likely produce another round of high surf
and dangerous rip currents.


High Surf Advisories will expired along the east coast of MA
this evening, but they will remain in effect through Monday
along the south coast. Swells from Maria will likely increase
the surf and associated rip current risk across our ocean-
exposed south coast starting late Sunday and continuing through
the week. This will be true even though Maria may eventually
recurves out to sea well southeast of New England.


Best chance for record high temps will be Sunday and Monday,
when records are 85-90. Records jump up Tuesday to 89-95, while
less mixing will lead to lower max temps. So the chance for new
records diminishes Tuesday. The current record max temps for
the three days are

BOS  90/1959  89/1926 95/1881
ORH  85/2010  85/1970 91/1930
BDL  89/1959  90/2007 93/2007
PVD  87/1959  89/1920 89/2007


MA...Dense Fog Advisory until 9 AM EDT Sunday for MAZ022>024.
     High Surf Advisory until 6 PM EDT Monday for MAZ020-022>024.
RI...Dense Fog Advisory until 9 AM EDT Sunday for RIZ008.
     High Surf Advisory until 6 PM EDT Monday for RIZ006>008.
MARINE...Small Craft Advisory from 2 AM to 6 PM EDT Monday for ANZ235-
     Small Craft Advisory until 6 PM EDT Monday for ANZ254>256.


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