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Area forecast discussion 
National Weather Service Taunton Massachusetts
654 PM EDT Wednesday Jul 27 2016

Synopsis...

One more day of heat and humidity Thursday before a cold front
slowly drops south across the region, possibly bringing scattered
showers and thunderstorms. An area of low pressure along with rain
and possible thunderstorms is forecast around Friday which could
be heavy at times. With that there is the potential for localized
flooding. Seasonable and dry for Saturday. Shower chances return
for Sunday into Monday. Looking dry with a moderating warming,
muggy trend through the remainder of the week.

&&

Near term /through Thursday/...

7 PM update...

Few to scattered clouds diminish giving way to mostly clear
conditions, and thus ends another 90 degree day for most, the 7th
consecutive day for some, whereas Boston, under the influence of a
sea-breeze broke its heat wave with only a high of 87 degrees
thanks to persistent onshore flow parent with a sea-breeze.

Sea-breezes wash out as light S/SW winds prevail. With light winds
and clear conditions forecast, expect another decent setup of
radiational cooling which will allow lows to drop back into the
60s, especially for low-lying sheltered valleys such as those
throughout the CT river. It is in these low lying spots that
patchy dense fog could develop. Warmer along the coast and within
the urban centers with lows close to around 70.

&&

Short term /Thursday night through 6 PM Thursday/...

SW flow gets underway Thu as high pressure moves offshore and cold
front approaches from northern New England. Some of models such as
12z NAM/GFS want to bring showers and storms into southern New
England during afternoon and front draws closer. However, it
appears most of the instability and better lift stays north of our
area, which is where we expect most of activity to be favored.
This is in agreement with most ensemble solutions, including
high-res ncar ensemble, which show better chances for storms
across Vermont and New Hampshire. It's certainly possible we see few storms manage
their way into western or north central Massachusetts late in day, so will
keep chance pops in these areas. Not looking at severe threat
given lack of strong 0-6km shear and weak mid level lapse rates.

We are more confident in one more day of heat and humidity as
highs reach 90s in many locations (aside from higher terrain and
immediate south coast). Even east Massachusetts beaches should reach 90 given SW
flow. Dewpoints will be a bit higher than today, getting into
upper 60s to near 70, which results in heat index into low to mid
90s.

&&

Long term /Thursday night through Wednesday/...

*/ highlights...

- low confidence forecast for Thursday night through Friday night
- still some possibility of heavy rain, flooding
- seasonable and dry for Saturday
- chance of wet weather for Sunday into Monday
- warming and becoming more muggy through the remainder of next week

*/ discussion...

Thursday night into Friday night...

Overall, an interesting synoptic setup atypical with respect to the
pattern as of late depending on which Camp of solutions one prefers.
GFS/UKMET vs. NAM/ec/Canadian/sref?

S-stream sub-tropical energy brought north captured within the north-stream
trough pattern and cyclonic flow undergoes surface cyclogenesis as
the trough undergoes a negative-tilt. Maturing, closing off beneath
h7, conveyor-belt motions strengthen drawing North Rich sub-tropical
moisture with pwats up to 2.5 inches while undergoing deep-layer
synoptic forcing ahead of mid-level vortex energy and beneath the
right-rear-quadrant of the upper-level-jet. Much of the sub-tropical
moisture focused within the forward-leading anticyclonic conveyor
belt flow, looking at a front-end thump with particular attention to
the NE quadrant of the 850 mb low where frontogenesis and deep-layer
forcing combine along a SW to NE axis. It is here heavy rain is
anticipated with rainfall amounts at least 1-inch, locally higher
with around 2 to 3 inches possible.

But challenges remain: are there complications of and between N-
stream and S-stream systems? The depth, timing and orientation of
the 500 mb trough axis? Where and when will the forecast heavy rain axis
setup? Will it impact S new england? Will thunderstorms be an issue?
Over how long will this event take place, if it does impact S new
england? And the biggest question, are there any flood threats?

Evaluating ensemble probabilistics, cips analogs, and sref guidance,
a considerable spread of outcomes lending to low confidence. Notable
differences in the 27.09z sref run with arw core solutions wetter
than nmb, while exhibiting a considerable spread per plumes in both
camps. But when looking looking at the past 4 sref solutions, S of
the Mass-Pike has trended with a considerably wetter forecast, while
there are indications of a north-S gradient of rainfall. Gefs plumes too
has a considerably large spread with the operational GFS run towards
the bottom (driest) of solutions for S New England.

Collaborating with wpc, they're going with a GFS/UKMET blend holding
with a quicker solution with the attributing vortex energy becoming
sheared within the overall flow, that all other solutions which are
slower subsequent of the deeper developing surface low are outliers.
But they too have low confidence. While the 27.12z ec has trended S
from its prior run, it still has significant outcomes for S New
England. Will go ahead with a 50/50 blend of the two camps. Upon
doing so, likely pops prevail for all of the region. Will keep with
chance thunderstorms. Conservative with precipitation amounts. An
overall low confidence forecast.

In closing, there are some points to consider depending on which
Camp of solutions are preferred: 1) thunderstorms will be possible
given elevated instability with moist adiabatic, conditionally
unstable profiles. 2) atmospheric column is forecast to be above
freezing below h6 (give or take 12-13 kft) in areas of heaviest
rain. 3) and storm motion is roughly east/NE at 20 to 30 mph. All
combined, it is likely within areas of forecast heaviest rain that
warm-rain processes should be anticipated and this could yield
significant rainfall rates on the order of 1-inch, possibly as high
as 2-inches, per hour. Thus the concern with this synoptic setup
with regards to timing, strength, and specifically where the area is
of greatest risk for possible flooding. Will highlight this in the
hazardous weather outlook while preparing for all possible outcomes.

Saturday...

High pressure in control with a slightly cooler airmass aloft with
850 mb temperatures around +12-14c. Looking at seasonable temperatures
with light and variable winds likely yielding sea-breezes along the
shores. Thinking just some scattered mid to upper level cloud decks
from upstream convection and troughing over the Ohio River valley,
otherwise quiet weather.

Sunday...

Not ready to call it completely dry. While high pressure exiting to
the east with a measure of dry and air low to mid level anticyclonic
flow lingering, there is a fairly potent mid-level 500 mb trough axis
and attendant vortex energy sweeping the NE Continental U.S.. tapping into S
moisture and likely more favorable destabilization short wave of our region
feel some rainfall is possible for S New England late into the
overnight period.

Monday...

Timing of the mid-level 500 mb trough critical. There may be a window of
opportunity in where the E-half of New England destabilizes ahead of
synoptics. Thus pop chances continue but low confidence as forecast
solutions differ on outcomes and timing. Some measure of rainfall
does seem possible so long as we can tap into S moisture rather than
being robbed by a possible emerging mid-Atlantic low.

Rest of the week...

Keep it dry though diurnally forced convection is not out of the
question. Greater confidence to the north/west as northwest flow aloft persists
pushing cooler temperatures aloft across the region. Looking at a
period of warming temperatures with a return of muggier conditions.
Next system Worth of note may not be till Thursday.

&&

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/...high confidence.

0z update...no major changes.

VFR. Sea breezes decay this evening around sunset with S/SW flow
tonight. Local MVFR/IFR in patchy valley fog overnight. Continued
S/SW flow Thursday. Scattered thunderstorms and rain possible after 18z mainly
across western and north central Massachusetts.

Kbos taf...high confidence.

Kbdl taf...high confidence.

Outlook /Thursday night through Monday/...

Thursday night through Friday night...low confidence.

+Ra possible with thunderstorms and rain at times. Accompanying reductions in
visibility, in some instances due to br/fg. MVFR to LIFR impacts.
S winds backing out of the N/NW.

Saturday...moderate confidence.

VFR. Light west/northwest winds with sea-breezes developing along the shores.

Sunday into Monday...low confidence.

Chance rain showers with S winds. Will hold VFR.

&&

Marine...

Forecaster confidence levels.

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

Short term /through Thursday/...high confidence.

7 PM update...no major changes to the forecast.

Winds remain below 25 knots and seas below 5 feet through the
period. Sea breezes will diminish around sunset and give way to
S/SW flow tonight and Thu.

Outlook /Thursday night through Monday/...

Thursday night through Friday night...low confidence.

Rain likely, heavy at times, with possible with thunderstorms. S
winds with gusts up to 15 to 20 kts backing out of the north/northwest as
the low passes near or across the S waters. Wave heights forecast
below 5-feet. Soupy conditions and fog expected with rain reducing
visibilities at times.

Saturday...moderate confidence.

Light west/northwest winds with sea-breezes developing along the shores. Waves
below 5 feet. Good boating weather. Perhaps some showery weather
lingering over the southeast waters during the morning.

Sunday into Monday...low confidence.

Chance showers with winds out of the S. Possible thunderstorms. Will
hold waves below 5 feet.

&&

Fire weather...

Dry conditions will persist Thursday, except possibly in western
and north central Massachusetts where scattered showers/storms may bring
localized rainfall Thu afternoon.

Minimum relative humidity values of 25 to 35 percent are expected Thursday across
much of southern New England, except 40 to 60 percent on the South
Coast, Cape Cod, and the islands. South to southwest winds will
stay under 20 mph.

&&

Box watches/warnings/advisories...

CT...air quality alert from 11 am to 11 PM EDT Thursday for ctz002.
Massachusetts...none.
Rhode Island...none.
Marine...none.

&&

$$

Synopsis...sipprell/jwd
near term...sipprell/jwd
short term...jwd
long term...sipprell
aviation...sipprell/jwd
marine...sipprell/jwd
fire weather...weather forecast office box staff

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