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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
255 am EDT Mon may 28 2018

Synopsis...
drier, warmer trend ahead of a sweeping cold frontal boundary
Monday into Tuesday. Warmer weather continues through Thursday.
An approaching weather system brings showers late Thursday into
Friday.

&&

Near term /until 6 PM this evening/...
230 am update...

Cool, dreary, drizzly conditions exiting east. Surface low pressure
against a ridge of high pressure extending over southeast Canada to the
adjacent S ocean waters is lending to shallow persistent NE flow
beneath a building dry-subsidence inversion aloft. Trapped moisture
within a cool boundary layer, a measure of lift given upslope,
onshore flow, subsequent drizzle.

Going towards morning, NE flow weakening, turning S, ahead of
additional "kicking" energy through the north-stream cyclonic flow
regime across the NE conus, a surface-reflection trailing cold
front out of the Hudson Bay region of Canada. Upslope, moist flow
concludes as the subsidence inversion deepens top-down through the
column. Re-iterating from earlier discussion, evident is the
subsidence inversion interrogating west to east 0z soundings from Buffalo
New York to Chatham Massachusetts. Along 290-310k surfaces do we see conclusion
of E, onshore upslope followed by west dry advection. Low clouds
persist however visibility should improve, aside from the high
terrain enveloped in clouds.

Temperatures remaining stable, hovering around the upper 40s to
low 50s into morning.

Today...

Clearing trend west to E, however east/se-coastal communities may remain
socked in through early evening. Tricky forecast as to timing. The
subsidence inversion in place, trapping moisture, with winds turning
S ahead of a pre-frontal trof axis over northwest New England extending
SW through PA, warm air advection proceeding, a combination of
boundary layer mixing and perhaps mechanical mixing at the top
of the layer given top-down subsidence yields a west to east trend of
clearing conditions, the late-may sun contributing to increased
warming and eroding of the west-edge of clouds east. Locations in the
CT River Valley look to warm into the 70s, whereas further east
remains in the 60s, low 60s along the coast, cape and islands.

Trapped cool, moist conditions, visibility impacts with mist, some
lingering drizzle early on, expected over east/se-coastal Massachusetts and Rhode Island
a good portion of the day, but difficult to ascertain when this
will all erode. May not come until drier air surges S into Tuesday
as discussed below.

&&

Short term /6 PM this evening through 6 PM Tuesday/...
tonight...

Cooling overnight, low clouds look to make their return for
portions of S New England. Forecast model consensus exhibits
boundary layer cooling beneath the continued Stout dry-inversion
aloft. Trapped low-level moisture, cooling towards condensation
lends to low cloud decks. Greater confidence along S-coastal
New England with greater moisture, and along the high terrain
with more favorable cooling.

Challenges inherent as a pre-frontal trof axis drifts S across
the region with a stronger cold front right on its heels. Faster
cooling within the mid-levels would lessen the inversion, perhaps
limit low cloud formation. Better idea as we get closer. Will
play a role in overnight lows given light winds, presently forecast
down around the mid to upper 50s.

Tuesday...

Clearing out and warming up abruptly. Strong advective push of
cooler, drier air behind a Stout vortmax rotating through southeast
Canada. Surge of high pressure over the lower Hudson Bay, north flow
throughout the column results in the cold front capturing the
pre-frontal trof from today, however washing out in a region of
subsidence as Alberto pushes N, it's energy neighboring into the
mid-Atlantic, Heights rising across our region.

Late may sunshine combined with the frontal push as 850 mb temperatures
hover around +14-16c, can't rule out temperatures warming well
into the 80s near 90 given boundary layer mixing h8-9. But as
the frontal boundary pushes S, winds behind it less Stout, and
opportunity for sea-breezes, and a potential clash with onshore
moisture, can't rule out a spot shower along the S-coast. Will
lean with slight chance pops across S-coastal New England,
especially towards CT coincident with k-indices as noted by
prior forecasters.

An abundance of dry air within the column and weak wind fields,
whatever may generate would be expected to be brief and simply
nuisance. At a minimum, scattered to broken cloud decks around 5
kft at boundary collision.

&&

Long term /Tuesday night through Sunday/...

Big picture...

Lots of blockiness over southern North America, interfering with the
southern jet. Meanwhile the northern jet flows from the Pacific
northwest up across central Canada and then southeast across the
Maritimes. That puts New England on the warm side of the upper flow.

Twin high pressure areas channel the remnants of Alberto north to
the Great Lakes during the week, where it merges with low pressure
ejecting from the western USA. The merged system then lingers over
the northeast USA into the weekend.

Model solutions remain similar through Thursday, with a ridge over
New England and the merging systems to our west. All solutions show
some form of closed low/upper trough for Friday and the weekend, but
diverging into different flavors with time. High confidence in warm
and dry weather for Wednesday and Thursday. Good confidence in
unsettled weather Friday and the weekend, but low-moderate
confidence in the form that it will take place. Will evaluate as
moderate confidence for now.

Concerns...

Tuesday night through Thursday...

High pressure from Canada builds over New England and controls our
midweek weather. East flow Tuesday night while the high is centered
to our north...then a light flow Wednesday as the the center moves
over the Gulf of Maine. This positioning favors an east/southeast
flow especially along the coast. Light airflow turns from the south
Thursday, although the pressure gradiant may remain light enough for
a sea breeze to develop on the eastern mass coast.

Onshore flow may buffer Max temps each day, especially in the
coastal plain. Even so, interior temps from the model data looks
a little subdued for a light flow regime. Will bump 00z
guidance up a little with warmest temps in northern/western
mass. Meanwhile clear skies and light flow should allow good
nighttime cooling. Dew points in the mid 40s to mid 50s Tuesday
night, climbing into the mid and upper 50s Thursday night.
Expect min temps close to this, perhaps upper 40s to mid 50s
Tuesday night and 50s Wednesday night.

Thursday night through Sunday...

Guidance actually brings showers into southern New England Thursday
afternoon, which is rather fast considering the placement of the
upper ridge. We prefer a slower onset with low-end chance pops
after midnight followed by chance pops Friday. Precipitable
water values still flag a sharp rise Thursday afternoon and
maintain 1.75 to 2.0 inch values Friday and Saturday, which is
2+ std deviations above normal for early June. This points to a
wet period to end the week with high moisture content and
potential for heavy downpours.

Model surface patterns Show Low pressure, presumably the combined
remnants of Alberto and the western trough, passing to our north
Friday and Saturday. But the supporting upper trough drifts slowly
south across the northeast USA during the weekend so it is
reasonable to consider an eventual redeveloped low along the mid
Atlantic coast by Sunday. Low confidence on such specificity, but
enough confidence to continue chance pops through Sunday.

&&

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

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

Short term /through Tuesday/...moderate confidence.

Through morning...
MVFR-LIFR cigs, lowest conditions especially with br/fog over
southeast-coastal terminals, also across high terrain. North/NE winds,
diminishing overnight, down to 5-10 kts with gusts upwards of 20
kts over east/se-coastal terminals.

Today...
MVFR-LIFR cigs early, lower vsbys along southeast-coastal terminals,
eroding east through the day as winds shift S, conditions improving
to VFR. However, southeast-coastal terminals may remain in a IFR soup
throughout the period, not breaking until early-evening if at
all.

Tonight...
overall VFR but will have to watch S/se-coastal terminals as
well as high terrain as to whether low clouds re-emerge,
potential low visibility issues. Overall light S/SW winds over
the interior, bit more brisk along the S-coast.

Tuesday...
winds shifting north early on, conditions rapidly improving VFR north to
S. However light, an opportunity for onshore sea-breezes, could
see a spot rain showers along S-coastal New England late in the day with
minor impacts.

Kbos terminal...
IFR through the morning push, then gradual improvement midday
into afternoon towards VFR as winds turn S. Low confidence with
respect to exact timing.

Kbdl terminal...
MVFR with periods of IFR through mid-morning, improving through
midday into afternoon towards VFR as winds turn out of the S.
The possibility lower conditions could return overnight.

Outlook /Tuesday night through Friday/...moderate confidence.

Tuesday night through thursday: VFR.

Thursday night: mainly VFR, with areas MVFR possible overnight.
Slight chance rain showers.

Friday: mainly MVFR. Chance shra, slight chance thunderstorms and rain.

&&

Marine...
forecaster confidence levels...

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

Short term /through Tuesday/...high confidence.

East winds diminishing, becoming light S ahead of a sweeping cold
front late tonight into Tuesday, seas allowed to diminish
allowing small craft advisories to conclude.

Bigger issue over the water is lower visibility due to fog / mist
in addition to drizzle. While drizzle will conclude though the
morning, reduced visibility due to mist is likely to linger into
the evening hours, likely to improve towards Tuesday morning
with the aforementioned sweeping cold front, push of drier air.

Outlook /Tuesday night through Friday/...moderate confidence.

Tuesday night through thursday: winds less than 25 kt. Seas less
than 5 feet.

Thursday night: winds less than 25 kt. Seas less than 5 feet.
Slight chance of rain showers.

Friday: winds less than 25 kt. Seas less than 5 feet. Chance of
rain showers.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 2 PM EDT this afternoon for anz250-
251-254-255.
Small Craft Advisory until 8 am EDT this morning for anz256.

&&

$$
Synopsis...wtb/sipprell
near term...sipprell

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