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000
FXUS61 KBOX 200602
AFDBOX

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

.SYNOPSIS...
Mainly dry but hot and humid weather persists through Thursday
afternoon. There is growing concern for a round of showers and
thunderstorms Thursday night, a few of which might be strong to
severe and produce heavy rainfall. Then dry and warm Fri and Sat but
turning less humid. Unsettled weather in the form of showers
are possible at times Sunday into Tue. A drying trend likely
develops middle of next week with mild days and cool nights.

&&

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

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Overall trend in the forecast remains on track early this
morning. High level clouds continue to move across the region
as weak wave passes through the upper levels. At the surface,
very moist airmass remains across RI and SE MA with dewpoints
over 70. This has resulted in some lowering vsbys across the
Cape and Islands. In fact ACk continues to be below a mile.
However latest surface winds suggest backing to a more westerly
direction. This may allow enough dry air to hold off a dense fog
adv. Will have to watch for patchy fog across the CT valley as
winds have become light at BAF and CEF. Temp/dewpoint spread is
also within a few degrees so something to watch for the morning
commute.
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&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/...
Thursday...

A near-repeat of Wednesday, heat returns. In wake of the overnight
into morning mid level trough, with daytime heating will see the
return of W flow across the interior and SW flow closer to the
coast. Beneath a slightly warmer airmass with H85 temperatures
around +17C, the level around which the boundary layer mixes out,
will see the mix down of drier air and faster winds, with gusts
up to 20 mph, which will again define the SW-NE dewpoint boundary.
Convergent lift of sufficient low-level moisture, can`t rule out
an isolated shower around the Hartford-Providence-Boston corridor.

Otherwise scattered cumulus and temperatures warming up into the
upper 80s to low 90s. For those areas around and S of the boundary
in higher dewpoint air, heat indices can get back into the mid 90s.
Higher confidence across CT, RI, into Southeast MA. Evaluating heat
indices that have occurred today, aforementioned areas may need a
HEAT ADVISORY.

Thursday Night...

The chance of showers and thunderstorms associated with a mesoscale
convective system (MCS), that if it originates, has the potential of
producing strong to damaging winds along with frequent lightning and
heavy rain (as well as the possibility of flooding) during the
evening into overnight period.

Looking at the ingredients that would maintain a MCS, particularly
winds and shear through the layer coupled with low level inflow of
robust moisture in an environment of favorably steep lapse rates and
subsequent availability of instability. Good model agreement on a
region of strong synoptic forcing and ascent sweeping the region
with strong, parallel mean wind and shear vectors which would aid in
damaging wind potential with any strong to severe storms. However
unclear is the measure of low-level moist inflow, notably around
H85, as well as moisture availability within the H5-7 layer. While
a majority of guidance suggests 3-8 km lapse rates around 6C/km, if
the layer ends up drier, slightly warmer, then there`s less mid-
level instability available to maintain a MCS as the 19.12z GFS
would suggest.

Having interrogated closely all available guidance, feel a stronger
MCS will originate out of the U of Michigan prevailing S/E with
longevity with the Corfidi vector flow towards better moist-inflow
and instability, only dissipating further S as flow becomes more W
away from a quasi-stationary low-level thermal boundary. That boundary
is more in question across the NE CONUS, how effectively if it can
focus SW low-level inflow into the NE CONUS or rather it remains
more W with less focus.

However, given the atypical magnitude of synoptics over the area
with a slightly more amplified trough pattern not usually seen for
mid-July that supports decent venting aloft above a model consensus
mid-level unstable environment, though marginal, can`t rule out some
shower and thunderstorm activity. Dependence falls on moisture in
the column as well as the magnitude of low-level inflow. If the non-
GFS camp is correct yielding potential instability above 1500 J/kg
within a strong-sheared environment with parallel mean wind vectors,
then there is a the possibility of evening into overnight strong to
severe thunderstorms with the main threat being damaging winds with
secondary threats of heavy rain and potential flooding along with
frequent lightning.

With this forecast, will go with chance PoPs for now and won`t go
into any special wording. There`s a lot of mesoscale elements,
including the upstream anticipated MCS, and a lot of variables that
are involved such that confidence is not high enough to go with
stronger wording at this time.

Will continue to highlight the possibility in the HAZARDOUS WEATHER
OUTLOOK.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Updated 430 PM...

Highlights...

* Fri & Sat: ideal summer weather, dry, warm but less humid
* Sun into Tue: unsettled with showers at times & cooler
* Midweek: trend toward drier weather along w/mild days-cool nights

Pattern Overview...

Vortex traverse across northern Quebec Fri and Sat providing dry WNW
flow aloft along with slight height falls across New England, but
very anomalous height and temps anomalies across southeast Quebec
with -3 standard deviations from climo! This will yield dry, warm
but less humid weather across the area Fri and Sat. Then the overall
upper air pattern continues to amplify with the northern stream
becoming suppressed and the westerlies straddling the U.S.- Canadian
border. Very atypical pattern for mid to late July.  This will yield
a period of active weather here Sunday into early next week with
several rounds of showers.

Daily Details...

Friday and Saturday...

Dry WNW flow across the area this period. Convective complex may
come close to the region Sat per past several runs of the EC.
However WNW flow aloft appears to suppress the 850 mb Theta-E ridge
south of New England. Thus any convective complexes should traverse
southwest of the area. Regarding temps, airmass remains warm across
the area Fri and Sat (esp Fri) with 850 temps still +15C/+16C, so
highs well into the 80s (85-90 Fri, 80-85 Sat) both days, but less
humid especially Sat with dew pts falling into the low and mid 60s
Fri, then into the 50s Sat away from the south coast. Overall, two
nice summer days ahead.

Sunday thru Tuesday...

Very anomalous pattern unfolding with suppressed jet/storm track
this period along with a robust short wave trough ejecting out of
the Great Lakes Sunday. This will set the stage for a series of weak
lows to track over or south of the region. Yes, the calendar reads
July but looks more like a Fall or spring pattern. So risk for
periods of rain/showers Sun thru Tue along with cooler temps given
cloud cover, precip and onshore flow. There will likely be some dry
periods as well in between waves however too difficult to pinpoint
at this time range.

Midweek...

Mid level mean trough axis likely moves offshore with a period of
dry and cooler weather overspreading the region. However this may be
short lived as both ensembles and deterministic guid agree that
northeast trough reloads shortly thereafter.

&&

.AVIATION /06Z 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...
There is the possibility tonight of some patchy shallow ground fog,
however confidence is low at this time. SHRA/TSRA impacts possible
Thursday evening.

Outlook /Friday Night through Monday/...

Friday and Saturday...VFR and dry weather likely. High forecast
confidence.

Sunday and Monday...a mix of MVFR and IFR likely along with
scattered showers. Moderate forecast confidence.
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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/...

Tonight through Thursday...High confidence.

SW flow becoming W towards Thursday morning before returning SW.
During daytime periods and along the S/SE near shore, gusts up
to 20 kts are expected. Its also in this region that there is
the potential for dense fog redeveloping overnight, burning off
Thursday morning. Seas remaining below 4 feet.

Thursday Night...Moderate confidence.

Possibility of showers and thunderstorms sweeping the waters
late ahead of which SW flow maintains with gusts up to 20 kts,
slowly veering out of the W into Friday morning.

Outlook /Friday Night through Monday/...

Friday and Saturday...high forecast confidence.

Nice summer boating weather with tranquil seas and winds along with
mainly dry conditions.

Sunday and Monday...moderate confidence.

Unsettled weather with several weak lows tracking over or south of
New England. Yes, atypical for mid to late July. One of these waves
may yield modest to strong NE winds so we have to watch this
closely. Vsby likely limited in showers and areas of fog.
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&&

.BOX WATCHES/WARNINGS/ADVISORIES...
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CT...None.
MA...None.
RI...Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for RIZ001>008.
MARINE...None.
-- End Changed Discussion --

&&

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