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Area forecast discussion 
National Weather Service Taunton Massachusetts
347 am EDT Friday Jul 29 2016

Synopsis...
a weak front will slip south of New England by midday. Low
pressure along the mid Atlantic coast will pass offshore late in
the day. This will bring showers and isolated thunderstorms today.
Weak high pressure brings drier weather Saturday. Another risk for
showers follows on Sunday and Monday.

&&

Near term /until 6 PM this evening/...

Low pressure over eastern Maryland this morning with a primarily
zonal flow aloft. There is a faint signal of a shortwave in this
flow that could draw the surface low farther north, but most of
the push looks out to sea. Also note the 92-knot upper jet
crossing New England with our area in the right entrance region
through early afternoon. This will support lift and pcpn
generation over southern New England... if there is enough
moisture to tap.

Precipitable water values continue to show 2+ inches along the New
England South Coast with 1.5 inches north & west. Surface dew
points in Rhode Island and southeast Massachusetts are 70-74. On the other hand, convection
over the mid Atlantic is moving more toward the ocean than toward
New England. This would intercept the inflow of additional
moisture from the south. Broad area of showers over eastern PA is
showing a northeast trend and should bring US wet weather this
morning, but its trailing edge is progressing east and should
limit the time for precipitation.

Model trends early this morning have also shifted the forecast a
little farther south. This shifts the heavier pcpn over the South
Coast or offshore, so this should be our best chance of higher
rainfall amounts. We have shifted rainfall amounts south, with 1
inch or higher south of a Willimantic-Plymouth line.

Flash Flood Watch continues in Rhode Island and southeast MA, but has been
discontinued in northern CT where expected total rainfall has
diminished. Even so, remember that extremely dry soils can also
lead to rapid runoff, which could result in rapid flooding of some
small streams and rivers, again primarily due to urban runoff.

The Storm Prediction Center marginal risk for severe weather remains along the
immediate South Coast today. Sb cape is more limited with this
run, with values of 200-500 j/kg depending on the model of choice.
Storm Prediction Center rap helicity values are forecast to stay well offshore. The
farther-south track of the low will not help, nor will the low
lapse rates. But mesoscale features could still generate an
isolated damaging wind gust.

Rain should fall this morning and early afternoon but taper off
from west to east during afternoon, with most of it done toward evening
on east Massachusetts coast. Highs will top out in 70s to around 80.

&&

Short term /6 PM this evening through 6 PM Saturday/...

As the offshore low pressure moves off to the east, weak high
pressure over Canada will build with somewhat drier air. Dew
points will range from the mid 50s north to mid 60s south, down
from the 70s in Rhode Island/southeast Massachusetts today but still noticeable humidity in
southern areas. The moisture and light wind will allow fog patches
especially in the more humid south.

Temps aloft of 14-16c will support Max temps in the 80s Saturday.
The light wind flow will allow sea breezes to form, keeping
coastal areas cooler.

Additional concern is potential for convection upstream in NE PA
that could move east into Connecticut late in the day Saturday. Li
values will be sub-zero across PA and the lower Hudson Valley with
SBCAPE values near 1000 j/kg. We have included chance pops for
showers/tstms in CT and greater Springfield MA, mostly 5-8 PM
Saturday.

&&

Long term /Saturday night through Thursday/...
highlights...

* showers and storms possible Sunday and Monday
* drier and seasonable weather mid-late next week

Overview and model preferences... interesting weakly split flow
regime defines the mid-term period. Weak ridge-over-trof split flow
develops and gradually moves across NE conus, finally giving way to
joined ridging toward the latter half of the forecast period. The
trof is the primary wx-maker for late weekend and early next week,
providing a Focal Point for continued shra/tstm activity. Timing
disagreements are minimal with the 12z updates and even show
continuity with European model (ecmwf) eps and gefs means. Therefore, will use a
consensus blend for this afternoon's long-term update.

Details...

Sun and Mon... tricky forecast period as diffuse open wave and
modest sfc trof rest across much of the mid-Atlantic region and
into S New England. This should provide a Focal Point for
continued shra/tstm development especially to the west and S, where
highest moisture lies. Pwats linger around 2.0 inches to the southeast
but then drop into New England. Gradient winds mainly east-NE which,
off the cooler Gulf of Maine waters which is likely to limit
convective (particularly sfc based). Therefore not confident in
how much thunder activity S New England will experience, however
leftover shra is possible. Cips analogs support this thinking,
focusing the highest risk for convective activity, SW of New
England.

Will need to watch a secondary low pres wave development which could
provide better low lvl convergence and f-gen. European model (ecmwf) is closest to
the S coast with this feature Sun night, but if it remains further
offshore, less rain will impact S New England. Will need to monitor
as we approach as it has similar look to the low pres which could
bring heavy rain to the region tomorrow.

Tue through Thu... in spite of what occurs earlier in the week, the
split flow finally joins in the form of a ridge upstream which
gradually shifts to the east. This should bring about a period of
mainly quiet wx with sfc high pres in place. However, the
orientation of high pres brings in some cooler/drier air with 850 mb
temps really only hovering around +12c, so temps should remain much
closer to seasonable levels than they have been for the last week or
so.

&&

Aviation /07z Friday through Tuesday/...
forecaster confidence levels.

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

Moderate confidence today and tonight. High confidence Saturday.

Areas of IFR/LIFR along the South Coast and islands due to a broad
area of fog and low clouds along the shore.

Showers and isolated thunderstorms moving toward east-northeast from the mid
Atlantic will bring a period of MVFR vsbys and possibly cigs
during the morning and early afternoon. Locally heavy downpours
could bring vsbys briefly to IFR. The showers taper off from west
to east this afternoon and evening. Conditions will return to VFR
tonight, except for local MVFR/IFR in patchy fog.

High pressure will maintain VFR on Saturday. The light flow will
allow sea breezes late morning through afternoon. There is also
be showers developing west of CT that could move into the state
during the late afternoon. Possible MVFR vsbys in any
showers/tstms.

Kbos taf...moderate confidence. Heavier rain and lower conditions
may stay farther south today and miss the terminal. Higher
confidence tonight.

Kbdl taf...moderate confidence. Heavier rain and lower conditions
may stay farther south today and miss the terminal. Higher
confidence tonight.

Outlook /Saturday night through Tuesday/...

Sunday and Monday...moderate confidence.
Some showers and a few thunderstorms possible through the period.
Occasional MVFR conditions, but VFR likely dominates.

Tuesday...high confidence.
Mainly VFR.

&&

Marine...
forecaster confidence levels.

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

Short term /through Saturday/...high confidence.

Areas of low vsbys in fog along the southern waters, especially
the nearshore bays and sounds. Locally dense fog will have vsbys
less than 1/4 mile.

Main concern is potential for strong or even severe storms over
south coastal waters today, mainly south of islands and east of
Nantucket where brief waterspouts are possible this afternoon.
A marine weather statement continues to highlight this potential,
especially for recreational boaters.

Otherwise winds will remain below 25 knots and seas below 5 feet.
Rain and fog will reduce visibility today and this evening.

Outlook /Saturday night through Tuesday/...high confidence.

Mainly quiet boating weather as high pressure slowly builds over the
waters. Winds and seas below Small Craft Advisory thresholds. Only
caveat could be an isolated shower or thunderstorm mainly Sunday or
Monday.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...Flash Flood Watch through this evening for maz017>024.
Rhode Island...Flash Flood Watch through this evening for riz001>008.
Marine...none.

&&

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

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