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Area forecast discussion
National Weather Service Taunton Massachusetts
431 PM EDT Thu Jul 27 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 Friday morning/...
4 PM update...

Low amplitude mid level trough entering western New York per latest Storm Prediction Center
mesoanalysis. Good moisture advection ahead of this feature with
pwats up to 1.6 inches across western CT/Massachusetts and eastern New York along
with k indices in the mid 30s. This combined with some weak synoptic
scale lift has been sufficient to generate scattered showers across
the region this afternoon.

This will continue into early this evening. Most of the shower
activity will be light given the weak forcing and lack of
instability (both sb & aloft). But given the abundant moisture can't
rule out isolated and brief downpour.

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 dew pts remaining in the 60s regionwide
overnight. So while showers will taper off fog will likely develop
as the night progresses.

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

&&

Short term /6 am Friday morning through Friday night/...
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 Thursday/...
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
*

Overview and model preferences...

While examining the 12z operational models as they came in, noting
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 /21z Thursday through Tuesday/...
forecaster confidence levels...

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

18z update...

Through 00z...high forecast confidence.

VFR conditions will trend to MVFR as the day progresses.
Scattered showers will continue to impact the region.

After 00z...high confidence on trends but lower on exact timing.

Showers and isolated T-storm will shift southward with a drying
trend developing from north to south as the night progresses.
IFR- MVFR this evening in showers and fog will trend toward VFR
late especially away from the South Coast.

Friday...moderate confidence.

Any morning IFR-MVFR along the South Coast should lift to VFR by
midday. Then marginal MVFR-VFR develops in the afternoon
regionwide with isolated shower possible especially near the
South Coast.

Friday night...moderate confidence.

Marginal VFR-MVFR in isolated shower threat during the evening
will trend toward IFR late especially South Coast as risk of
rain increases.

Kbos terminal...high confidence on trends but some uncertainty
on exact timing of lowering cigs this evening then timing
improvement overnight.

Kbdl terminal...high confidence on trends but lower confidence
on exact timing of changing categories.

Outlook /Saturday through Tuesday/...

Saturday and Saturday night...moderate confidence.
Area of rain across north CT/RI/se mass with MVFR-IFR
ceilings/occasional MVFR vsbys, with patchy heavier rain along the
immediate S coast possible. Patchy fog with local IFR visibilities late
Sat/Sat night. May see patchy -ra/-shra reach across most of
the remainder of the area but should be mainly VFR. East-NE winds
gusting to 25-35 kt along coast, highest along S coast, Cape Cod
and the islands. Precip may start to taper off from north-S late
Sat night with improving visibilities.

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 Friday night/...high confidence.

4 PM update...

Tonight...

Light south-southwest 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 Tuesday/...

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
Provincetown 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...nocera

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