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Area forecast discussion 
National Weather Service Taunton Massachusetts
1039 PM EDT Friday Jul 3 2015

Synopsis...
high pressure over southern New England will shift offshore
overnight. Low pressure races south of New England on July 4th
and may bringing some light rain to south coastal areas. High
pressure returns Sunday and lingers through Monday. A warm front
on Tuesday and cold front on Wednesday will bring a risk of
scattered showers and thunderstorms.

&&

Near term /until 6 am Saturday morning/...
1039 PM update...

High pressure shifts east and satellite imagery shows high clouds
spilling over shortwave ridge. Will remain dry overnight...with
increasing clouds above 10 thousand feet in advance of an
approaching trough.

With the overall forecast still on track...only made minor tweaks
to bring forecast back in line with observed trends.

&&

Short term /6 am Saturday morning through Saturday night/...
Independence day...

Low pressure along the middle Atlantic coast races south of our area
on Saturday. The low will be supported by an upper shortwave

Have had a chance to take a look at the 18z GFS run and it
suppresses the rain a little further south and more in line with
other models. The current forecast of probability of precipitation still looks reasonable.
However...at this time and have not modified from prior forecast.

Previous discussion...
model forecasts have been bouncing around the past 24 hours
regarding the handling of coastal low that passes south of New
England Saturday. GFS is most aggressive in bringing precipitation across
all of southern New England while the NAM and ggem clip Cape Cod and
islands. The European model (ecmwf) compromises with precipitation across Rhode Island and southeast mass. We
favored the European model (ecmwf) placement in setting up our probability of precipitation...likely probability of precipitation
on the cape/islands/South Coast...chance probability of precipitation in southeast mass/RI/NE
CT...slight chance NE mass and ncentral mass. Convection from New
York may cross into western mass during the afternoon so we have
chance probability of precipitation in western mass.

Saturday night...
the coastal low crosses 40n/70w in the evening and moves south of
Nova Scotia overnight. This should bring any precipitation to an end
during the early part of the night. The supporting upper trough
moves east of US by morning. This should bring a clearing trend
after midnight with winds turning northwest.

&&

Long term /Sunday through Friday/...
highlights...

* high pressure should keep area dry with seasonable temperatures
through Monday.
* Unsettled conditions return Tuesday and Wednesday...possibly
lingering into the latter part of next week.

Overview...as one would expect in July...any robust short waves pass
north of region with heights relatively high across southern New
England. The flow looks to be relatively zonal but with a tendency
for weak troughing over the eastern Continental U.S.. main issues include
timing of shortwaves...often more problematic in more zonal
flow...and whether or not weak surface waves may form along
passing cold fronts.

Details...

Sunday through Monday...models are in reasonable agreement for a short
wave ridge aloft and weak surface high keeping the area dry
through this period with seasonable temperatures.

Tuesday...warm front with possibly modest instability burst aloft
per k index signature moves through region. Air mass moistens
with precipitable waters increasing to near 2 inches on the GFS. Thinking chance
probability of precipitation northwest two thirds of area where a hint of a little better
instability and slight chance most of southeast third. Believe less
chance of any showers/thunderstorms by late afternoon except may see a few
air mass thunderstorms late afternoon/early evening in northwest Massachusetts.
Wednesday...uncertainty in timing of short wave trough and
associated weak cold front in turn leads to some associated
uncertainty in shower/thunderstorm chances for Wednesday. Even the faster
GFS...however...depicts some lingering surface convergence and
relatively weak instability through the afternoon. Thus...have
chance probability of precipitation most areas on Wednesday. May see clusters of thunderstorms that move
west to east during the afternoon but relatively low shear
suggests for now that convection will not be all that organized.
Thursday...a fair amount of uncertainty exists for Thursday given model
differences. European model (ecmwf) develops wave along the front and holds rafl
back across much of the area whereas the GFS is much more
progressive. Have opted to go with probability of precipitation in the upper end of the
slight chance category for Thursday...reflecting the relatively high
uncertainty.

Friday...have gone with continuity of dry weather for Friday but
confidence is low. The latest run of the European model (ecmwf) amplifies a passing
short wave trough and develops still another surface low pressure
center along the front. This was not reflected in the 00z version.
If the latest European model (ecmwf) run were taken literally...it would have a
soaking rain spread across southern Rhode Island and southeast Massachusetts for Thursday night
into Friday. The operational GFS run in contrast keeps any frontal
wave activity well south of the region with only a few ensemble
runs suggesting anything close to the European model (ecmwf) solution. May need to
adjust the Friday forecast as we draw closer and get a better handle
on model trends.

&&

Aviation /02z Saturday through Wednesday/...
forecaster confidence levels...

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

Short-term /through Friday night/...

Tonight...high confidence. VFR. Winds generally light from S
overnight. Increasing high clouds above 10 thousand feet.

Saturday...moderate confidence.
VFR to start. MVFR possible in scattered areas of light rain late
in the day...best chance is at terminals along the South Coast. We
are monitoring the possibility of low conditions at Boston toward
evening depending on the strength of developing east/northeast
flow at that time.

Saturday night...moderate to high confidence.
Rain over southeast mass/Rhode Island should taper off during the first part
of the night. MVFR conditions possible in any areas of
rain/fog...otherwise VFR with cloud bases lifting/clearing
overnight.

Kbos terminal...high confidence in taf. Expect VFR through the
period.

Kbdl terminal...high confidence in taf with VFR through the
period...even if a few sprinkles late Sat afternoon.

Outlook /Sunday through Wednesday/...

Sunday through Monday...high confidence of VFR conditions.

Tuesday through Wednesday...moderate confidence. Anticipate mostly VFR
conditions but with scattered MVFR ceilings/visibilities in scattered showers
and thunderstorms. Pockets of late night/early morning IFR ceilings/fog along
the South Coast.

&&

Marine...
forecaster confidence levels...

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

High confidence tonight...moderate confidence Saturday and
Saturday night.

Tonight...
high pressure over the region moves east of the waters by Saturday
morning. Expect light winds and diminishing seas.

Saturday...
low pressure developing along the middle Atlantic coast will pass
over or south of the southern outer waters Saturday afternoon and
evening. This will bring lower visibilities in rain to the southern
waters with east winds increasing to 15-20 knots. Farther north
the winds will become east but at lighter speeds.

Saturday night...persistent east to northeast winds will build
seas to 5 feet on the southern outer waters. This may eventually
require a Small Craft Advisory on those waters. As the low
pressure area moves off to the east the winds will diminish and
turn north overnight.

Outlook /Sunday through Wednesday/...

Moderate to high confidence.

Subsiding swell Sun morning...probably just under 5 feet...over the
outer S and southeast coastal waters. Otherwise...anticipate winds and
seas below Small Craft Advisory thresholds through the period.
Areas of visibilities below 3 miles in fog possible Tuesday and
Wednesday nights. Scattered thunderstorms are possible over the waters
Wednesday afternoon and evening.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...wtb/Thompson
near term...wtb/Belk/doody/Thompson
short term...wtb
long term...Thompson
aviation...wtb/Belk/Thompson
marine...wtb/Belk/Thompson

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