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Area forecast discussion
National Weather Service Taunton Massachusetts
155 am EDT Fri Jul 28 2017

Synopsis...
a cold front moves into the region this evening and will be
accompanied by scattered showers followed by a drying trend behind
the front later tonight. The front weakens or washes out Friday
giving way to mainly dry weather along with seasonably warm
temperatures and humidity. Low pressure then tracks south of New
England Saturday, likely bringing a windswept rain to the cape and
islands but possibly dry weather for the remainder of the region.
A coastal storm will push off the mid Atlantic coast and track
south of New England Saturday, which will bring rain and gusty
onshore winds with cool temperatures especially close to the
South Coast. Another low passing off the coast may renew the
chance for rain across portions of the region later Monday into
Tuesday.

&&

Near term /until 6 am this morning/...
940 PM update...

Last of the showers have moved offshore with mid level drier
air nosing down from the north. However, deeper moisture and
higher k indices in the mid 30s remain near the South Coast
so can't rule out a shower here through the early morning hours
before moisture pushes south of the new eng. As previous
forecaster indicated, low level moisture and higher dewpoint air
will remain suggesting potential for stratus and patchy fog
developing.

Previous discussion...

Later this evening and especially overnight the short wave trough
moves east of the region with its attending cold front sliding south
and reaching the South Coast by sunrise. Good drying through the
column from top down as k indices fall from the mid 30s this evening
to mid teens by 12z Fri. This will result in a drying trend from
north to south tonight. However this mid level dry air will not
reach the surface with dewpts remaining in the 60s regionwide
overnight. So while showers will taper off fog will likely
develop as the night progresses.

Given the relatively high dewpt air it will be much milder
tonight than previous nights with lows in the 60s.

&&

Short term /6 am this morning through 6 am Saturday/...
4 PM update...

Friday...

Behind departing short wave tonight mid level confluent flow
develops across the area. This provides mid level subsidence over
the region weakens or washes out the frontal boundary near the South
Coast. This mid level drying and associated subsidence provides a
cap tomorrow and should yield a mainly dry day. Given dew pts will
remain in the 60s there will be some sb instability present tomorrow
afternoon along with some weak low level convergence from
seabreezes. Thus can't rule out an isolated low top shower during
the afternoon. Given the mid level cap and limit instability (sb and
aloft) decided not to include any thunder.

So much of the day looks dry tomorrow along with seasonably warm
temps in the upper 70s to low 80s and typical Summer humidity with
dew pts in the 60s. A combination of some mid level cloudiness along
with diurnal clouds should allow for at least a few breaks of
sunshine.

Friday night...

Other than an isolated low top shower the evening hours should be
mainly dry as mid level cap and dry air aloft hold til about 03z or
so. However thereafter closed mid level low near Pittsburgh moves
east and backs the mid level flow along the eastern Seaboard,
advecting deeper moisture northward into southern New England,
especially the South Coast. This will result in the risk of
overrunning rain /yeah, it really is late July/ to invade the South
Coast including Cape Cod and the islands. The remainder of the
region likely remains dry especially north of the Massachusetts Pike.
Seasonably mild with lows in the 60s.

&&

Long term /Saturday through Friday/...
highlights...

* risk continues for showers and a few thunderstorms possible Fri
night and Sat, best chance S coast
* long east-NE fetch off the Gulf of Maine leads to continued strong
winds especially along the coast from S of Boston to the S coast
this weekend into early next week
* another low passes offshore late Monday into Tuesday with
more light rain for eastern areas

Overview and model preferences...

While examining the 12z operational models, noted changes to a
slower solution along the mid Atlantic and northeast U.S. With
respect to overall motion of surface systems especially from
Sunday through Tuesday. Continued wide solution spread, however,
though in the broad sense they keep an overall east-NE wind flow
in place with a front stalled to the S and high pressure, albeit
weak, to the north. Timing of surface waves along the front
remains tough, as some guidance has a couple of waves early in
this period, while others have only one from late Sat into early
Mon.

The biggest question appears to be how each individual model member
handles the 500 mb pattern, with some developing a broad cutoff 500 mb low
(12z GFS/00z ecmwf) generally across the mid Atlc states to about
the S coast of CT/Long Island, while others keep broad positive
tilted troughing. What really makes this forecast particularly
difficult is, even if cutoff 500 mb low forms, there is still quite a
bit of dry air to the north which could lead to variable sensible
weather into early next week. Where this all sets up, and where the
dry vs. Somewhat wetter conditions develop, is another big dilemma.

At this point, with so much uncertainty in place, will be leaning
toward available model and ensemble guidance blend for this forecast
package. Depending upon the final blend, may enhance with some more
agreeable model blending, such as around the Sunday-Monday timeframe
which does overall appear drier for most areas. Then, may see
another low pres wave move along the stalled front, but could lift a
bit further north which could affect the eastern part of the forecast
area Monday night and Tuesday.

Beyond Tuesday, looks like broad trough continues across the eastern
U.S., Though weak surface ridge should keep mainly dry conditions in
place for Wednesday and next Thursday.

Details...

Saturday through Sunday night...
as low pres slowly passes S of the region Sat, will see area of
rain move in across north CT/RI/se mass, with some patchy light
rain further north from there. Pretty sharp dry are across north mass,
though, so may remain dry there. Precip pushes offshore Sat
night with drier conditions.

Will see gusty NE wind especially along the coast with gusts up
to 30-40 mph, highest along the S coast, Cape Cod and the
islands. May see the northern fringe of heavier rainfall reach
portions of Cape Cod and the islands, with some local rainfall
of around an inch. Have left mention of thunder out of the
forecast with the cooler, stable onshore winds.

Expect temps both days to run up to around 10 degrees below
seasonal normals.

Monday and Tuesday...
500 mb long wave trough lingers along the eastern Seaboard during
this time. Another low forms offshore and passes close enough to
the 40n/70w benchmark to bring another round of rain across
central/east mass into Rhode Island and NE CT Monday afternoon through part
of Tuesday. Continued cool NE winds in place, but not quite as
strong as over the weekend. As the low passes Tuesday, rain
should taper off from west to east.

Temps will recover closer to seasonal norms with highs reaching
the lower 80s well inland but will remain cooler along the
shore.

Wednesday and Thursday...
lower confidence due to wide model solution spread. For now,
looks dry and warm on Wednesday, then another front may approach
during Thu though timing is uncertain.

&&

Aviation /06z Friday through Tuesday/...
forecaster confidence levels...

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

Through 12z...highly variable conditions across southern New England from
IFR/LIFR to VFR. MVFR/IFR will be most prevalent in the CT
valley and southeast new eng coast.

Today...moderate confidence.
Any morning IFR-MVFR conditions will lift to VFR during the
morning. An isold afternoon shower is possible across Rhode Island and southeast
Massachusetts.

Tonight...moderate confidence.
Mainly VFR cigs with isolated shower threat in the evening may
trend toward MVFR toward daybreak along the South Coast as rain
lifts north into the region. Patchy fog may develop across southeast
new eng.

Saturday...moderate confidence.
Highest confidence of rain and MVFR conditions will be near the
South Coast with mainly VFR and dry conditions further to
north, especially Mass Pike northward. Increasing NE winds along
the coast with gusts to 25-30 kt developing.

Kbos terminal...high confidence. Sea breeze should develop by
midday.

Kbdl terminal...low confidence on IFR cig potential through 12z.
High confidence after 12z.

Outlook /Saturday night through Wednesday/...

Sunday...moderate confidence. May see patchy -ra lingering
along the S coast through midday sun with local MVFR-IFR
cigs/vsbys. Otherwise, should see mainly VFR conditions. NE
winds gusting to 20-30 kt along the coast, highest across Cape
Cod and the islands, through sun then should diminish from north-S
Sun night.

Monday-Tuesday...moderate confidence.
Area of -ra may push back across east mass/RI/NE CT with local
MVFR ceilings/visibilities mainly along the coast Mon afternoon into Tue,
then may improve Tue night from W-E. Isold thunderstorms and rain possible along
the coast Tue. Otherwise mainly VFR.

&&

Marine...
forecaster confidence levels...

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

Short term /through tonight/...high confidence.

720 PM update...

Tonight...
light S-SW winds this evening becoming west toward daybreak.
Showers and patchy fog will reduce visibility late this evening
and overnight. Tranquil seas continue.

Friday...
light southeast winds and mainly dry weather. Vsby may be reduced in
morning fog but improving by midday. Tranquil seas continue.

Friday night...
winds become NE and increase to 20-25 kt by daybreak Sat. Seas
increase rapidly given the long fetch. Chance of rain increases
especially southern waters.

Outlook /Saturday through Wednesday/...

Saturday and Sunday...moderate confidence.
As low pressure passes S of the waters, will likely see NE wind
gusts to 30-35 kt during this timeframe mainly from east of
Cape Cod southward. Gale watch has been issued for those areas
through Sat night, but could continue into sun. With long
onshore fetch, seas will build up to 6-10 ft, possibly higher
across the outer waters. Area of rain and patchy fog will likely
bring reduced visibilities, which may briefly improve Sun
afternoon/night.

Monday...moderate confidence.
NE winds will continue to gust up to 25-30 kt, then start to
back to north Mon night as the low pushes offshore. Seas remain at
or above 5 ft on the outer and southern near shore waters.
Another area of rain and patchy fog moves in during Mon with
reduced visibilities.

Tuesday...moderate confidence.
North winds gusting to around 25 kt Tue morning, then diminishing.
Seas at or above 5 ft should subside during Tue night. Areas of
rain/fog with reduced visibilities should improve from west-east Tue
night.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...gale watch from Saturday morning through late Saturday night
for anz231>235-237-254>256.

&&

$$
Synopsis...nocera/evt
near term...kjc/nocera

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