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-- Discussion containing changed information from previous version are highlighted. --
079
FXUS61 KBOX 200813
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
413 AM EDT Thu Jul 20 2017

.SYNOPSIS...
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Dry and very warm afternoons expected Friday and Saturday, but with
lower humidity.  Unsettled weather with showers at times are
expected Sunday through Tuesday with below normal temperatures.
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&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

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400 AM Update...

Boundary still set up over southern New England as dewpoints are
still in the 70s across RI and SE MA. North of the boundary,
dewpoints are in the mid to low 60s as winds aloft are in a more
westerly direction.

These higher dewpoints has allowed for fog and stratus to remain
over the CAPE and Islands. Aside from some minor updates the
forecast, overall trend remains on track for this morning.

Today...

A warm start today as temperatures will warm well into the upper 80s
to low 90s. In fact, a few sites may be warmer than yesterday as
850mb temps warm to 18C. Some dry air will push into the region,
which will help increase mixing potential. A few sites may see
westerly winds increase to 15-20 MPH. These winds will allow for
mixing especially across the western half of the region. Thus have
lowered dewpoints in that region. However across RI and SE MA, 70+
dews will pool during the afternoon. This combined with the warm
temperatures will push Heat Indicies values to 95F. Went ahead an
issued a heat advisory for regions that have the best shot for 95F
heat indicies and that observed them yesterday.

Aside from the heat and humidity, falling heights through the day
may trigger a few isolated showers and/or thunderstorms. Best region
for this to occur will be across the MA east coastline where sea
breeze boundary will the area of focus. A strong storm cannot be
rule out as CAPE values will reach above 1000 j/kg and increasing
shear values. Gusty winds and heavy rain will be the main threat.
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&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM FRIDAY/...

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Tonight...

Hi risk, low confidence forecast for tonight as there is still some
model spread on the convection potential tonight. Models have
trended farther south with a potential cluster of showers and
thunderstorms propagating from the Great Lakes region. With an upper
level ridge firmly entrenched across the central CONUS, thinking is
that convection over the western Great Lakes region will track more
southeast than eastward, following the better instability and flow.

Biggest question is where this flow will set-up. The latest EC and
UKMET continue to bring the cluster through the region, while the Hi-
Res guidance and NAM have trended south of the Pike. Will trend the
forecast towards the later as the bulk of the instability axis is
south as well as the nose of southerly LLJ and the flow of the
Corfidi vectors. 850 mb theta-e gradient also has trended a bit
further south than 24 hours ago.

Best timing for the cluster of thunderstorms to move through the
region will be after 7PM and exit around 2 or 3AM. Steep lapse rates
above 6 C/km combined with 40-50 kts of 0-6 km Bulk shear values and
falling heights ahead of the complex will lead to a few strong to
severe storms. Greatest risk will be gusty winds and heavy rainfall
as PWAT are well above 1.5 inches. Agree with SPC on the marginal
risk as the storms could be strong to perhaps severe. Again greatest
potential will be south of the Pike.

Will need to continue to monitor trends for the complex, especially
this afternoon once the storms eventually develop. This will help
determine SNE actual risk for this evening and early tonight.
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&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...

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Highlights...

* Fri & Sat: Dry and very warm afternoons but with lower humidity
* Sun into Tue: Cooler than normal + unsettled with showers at times
* Drying trend likely by the middle of next week

Details...

Friday and Saturday...

Anomalous closed upper level low across eastern Canada will result
in northwest flow aloft into southern New England Fri & Sat.  The
result will be dry weather into at least Saturday afternoon.  Plenty
of sunshine should push high temps well into the 80s on both days,
and a few locations may reach 90 on Friday.  However, drier air will
mix down resulting in lower humidity than what we have experienced
the last few days.

Sunday through Tuesday...

The anomalous closed low across the Canadian Maritimes will help to
carve out an upper level trough across the northeast.  The result
will be temps averaging below normal.  Specific daily high temps are
uncertain and will depend on timing, cloud cover, and location of
any surface boundaries. Highs will mainly be in the 70s over this
time frame, to perhaps a few lower 80s.  It is also possible that a
some locations have a day where it struggles to break 70 as a result
of a surface boundary to the south coupled with northeast winds/low
clouds.

Plenty of shortwave energy rotating around the upper trough will
result in periods of showers and perhaps isolated thunderstorms at
times. It does look rather unsettled, but the entire period will not
be a washout it is just impossible to try to pin down timing this
far out.

Wednesday...

Forecast highly uncertain in this time range, but guidance shows
upper level trough finally lifting out which would result in dry and
pleasant weather.
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&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
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Forecaster Confidence Levels...

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

Short Term /through Friday/...High confidence.

Before 12z...High confidence. Mainly VFR with patchy fog in
typical low lying terminals.

Today...High confidence. VFR for most terminals. Shallow sea
breezes may the focus for a isolated showers/-TSRA during the
afternoon.

Tonight...Moderate confidence. Chance SHRA/TSRA for the entire
region. TSRA may contain strong to damaging winds along with
+RA. TEMPO MVFR-IFR impacts. Majority of impacts evening and
towards midnight, clearing out during the early morning hours
with SW winds becoming W. Great risk is terminals south of the
Pike.

KBOS Terminal...VFR. Could see a sea-breeze around 15z that
holds into the afternoon hours before mixing out and returning
W. A spot SHRA/-TSRA is possible this afternoon.

KBDL Terminal...SHRA/TSRA impacts possible Thursday evening.

Outlook /Friday through Monday/...

Friday and Saturday...High confidence in VFR conditions.

Sunday and Monday...Moderate confidence. MVFR to IFR conditions
likely at times in periods of showers, low clouds and fog patches.
VFR conditions will also likely occur, but timing of all this is
uncertain.
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&&

.MARINE...
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FORECASTER CONFIDENCE LEVELS...

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

Short Term /through Friday/...

Today and Tonight...High confidence.

SW flow becoming W towards the morning before returning SW.
During daytime periods and along the S/SE near shore, gusts up
to 20 kts are expected.

Possibility of showers and thunderstorms sweeping the waters
tonight. Southern waters have the greatest chance for this to
occur. Seas remaining below 4 feet.

Outlook /Friday through Monday/...

Friday and Saturday...High confidence.  Pleasant boating weather as
weak pressure gradient will keep winds/seas below small craft
thresholds.

Sunday and Monday...Moderate to high confidence.  East to northeast
winds may gust between 15 and 20 knots at times, but probably remain
below small craft thresholds for the most part.  Areas of fog and
showers may reduce visibilities for mariners at times.
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&&

.BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None.
MA...Heat Advisory from 1 PM this afternoon to 6 PM EDT this
     evening for MAZ017>021.
RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for RIZ001>008.
     Heat Advisory from 1 PM this afternoon to 6 PM EDT this
     evening for RIZ001>004-006.
MARINE...None.
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&&

$$
SYNOPSIS...Frank/Dunten
NEAR TERM...Dunten
SHORT TERM...Dunten
LONG TERM...Frank
AVIATION...Frank/Dunten
MARINE...Frank/Dunten