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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
1003 PM EDT Sat may 26 2018

Synopsis...

Scattered showers continue overnight as a back door cold front
pushes SW. Much cooler weather with cloudy skies and areas of
rain and fog will be the rule Sunday as low pressure along the
front passes south of the region. Improving conditions by Monday.
High pressure brings mainly dry and warmer weather Tuesday through
Thursday. An approaching weather system brings showers Friday.

&&

Near term /until 6 am Sunday morning/...

10 PM update...

Scattered shower activity persisting overnight. A lot of energy
and a lot of moisture floating across the NE Continental U.S. As mid-level
height rises continue subtly as building surface high pressure
over southeast Canada pushes SW considerably cooler air marked along
the leading edge as a back door cold front. Drier air mixed in
as well given subsidence between the pieces of energy as noted
below.

Chance pops given the challenges of nailing down where and when
scattered shower activity will emerge, whether ascent can over-
come sinking mid-level air squeezed between a ribbon of mid-
level energy draped northwest-east over New England as an additional area
of energy emerges out of the Ohio River valley. Lower confidence
concerning thunder as nocturnal cooling proceeds and rap-model
trends exhibits diminishing instability / convective indices.
Some aid via orographic support (as presently observed as of 10
PM over the berkshires).

But going towards morning, increasing leading isentropic ascent
out ahead of Ohio River valley energy up against the back-door
cold front pressed S of New England. Airmass confluence, with
cooler, drier undercutting air from NE up against Continental
tropical air from the west/SW, beneath a diffluent 300 mb jet streak,
seeing increasing areal coverage of rain emerging, especially S
of the MA-Pike. Leaning likely pops accordingly, greatest
confidence over the S-coast.

One last thing worthy of note, prior mentioned undercutting cool,
dry air, beneath over-running warmer air up around h8, expecting
lowering clouds overnight, some areas of patchy fog, but with
steepening lapse rates within the boundary layer, can't rule out
some drizzle conditions with higher confidence along the coast.

&&

Short term /6 am Sunday morning through Sunday night/...

Sunday and Sunday night...

Low pressure will be traveling along the front on Sunday, which
will be well to the south of New England. High pressure will be
ridging across our region from ME to western CT. This keeps US
in a moist and breezy east-NE flow. Winds of 15-20 mph are forecast
with some higher gusts at the coast. Low cloudiness will
persist throughout the day on Sunday and Sunday night. Rain is
likely during the day across northern CT, RI, and southeastern
Massachusetts with a chance of some rain farther north into interior MA,
mainly south of the Mass Pike. Rain chances will diminish Sunday
night as the low pulls farther out to sea to our southeast.

Right now, have not yet included thunderstorms in the forecast,
but cannot rule out a few thunderstorms working their way into
CT from NY, as per some mesoscale models.

Temperatures will be more than 30 degrees colder than today,
with highs in the mid 50s east to lower 60s west in the CT
valley. Lows Sunday night will dip to the upper 40s to lower
50s.

&&

Long term /Monday through Saturday/...

Highlights...

* becoming warmer and dry Tue through Thu
* return of unsettled and wet weather late week

Overview...
although S New England remains on the periphery of the north
stream, linked to a persistent vortex near baffin island, as the
previous forecaster noted, low-mid lvl heights remain generally
above normal through most of the long term. In fact height
anomalies actually increase after Tue, as north periphery ridging
continues from subtropical storm Alberto across the S central
Continental U.S.. this ridging builds into New England mid-late week,
yielding positive temp anomalies as well. It is not until
Alberto becomes encapsulated by a longwave trof currently along
the Pacific coast, and begins to more rapidly shift east that
conditions change. Remnant moisture, and subtropical energy will
lead to a transition to unsettled and wet wx for the late
week/weekend. Guidance is actually in fairly good agreement with
this, in spite of its track record handling tropical
interactions. Will use a consensus blend for this update.

Daily details...

Mon...
inverted ridging is somewhat relieved as return flow develops
ahead of sharpening trof upstream. This should allow for warmer
highs, with temps reaching back into the 70s especially across
the west interior. Lingering clouds, but little in the way of
precipitation expected as column is not particularly saturated.

Tue...
early frontal passage is mainly dry as BUFKIT suggests a dry
column. Mid lvl temps actually warm it's wake however thanks to
influence of the upstream ridge and warm advection. In fact, buy
00z Wed (tue eve) 850 mb temps should be approaching +14c.
Therefore, expecting a return to seasonably mild temps. Mainly
in the low-mid 80s for highs.

Wed and Thu...
gradual airmass modification leads to 850 mb temps warming to near
+16c through the mid week period. This should allow daytime
highs to once again reach the mid 80s, potentially upper 80s
where some downsloping can be realized. High pres cresting will
likely allow for sea breeze development each day, leading to
cooler coastlines. This is especially true for the S coast on
Thu, in which the S component to the gradient winds will enhance
the cooling. Dry, as high pres yields decent subsidence.

Fri and Sat...
remnant moisture and upper lvl wave from Alberto transitions east
late week. Noting that even ensembles support high pwats
potentially exceeding 1.5in (nearly 3 std deviations above
normal). The overall trof does weaken initially but then may
actually deepen again as it approaches the Atlantic coastline.
Given these factors, wet and unsettled conditions are likely to
return, with a risk for pockets of heavier rainfall given the
high moisture content. Still some factors to be resolved, but
this will be a period to watch.

&&

Aviation /02z Sunday through Thursday/...

Forecaster confidence levels...

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

Short term /through Sunday night/...high confidence.

Tonight...
VFR with gradual reduction to predominantly IFR ceilings between 02z
and 06z from NE to SW. Some IFR/LIFR vsbys along the southeast coast
especially Cape/Island terminals after 04z especially. A few
rain showers possible through the night, but these will be widely scattered.
Winds also continue to shift to the east-NE from east to W, with a few
gusts to around 20 kt with the wind shift.

Sunday...
MVFR-IFR ceilings linger through most of the day. Rain developing
across mainly the southern half of southern New England. Cannot
rule out an isolated thunderstorm in CT. Patchy fog along S
coast through the day.

Kbos terminal...high confidence in taf. Backdoor cold front
shifts winds to the northeast early this evening with a wind
shift from west to NE. Wind gusts to around 20+ kt at times
tonight.

Kbdl terminal...high confidence in taf. Backdoor cold front
should hold off until around 02-03z when wind shifts to NE.

Outlook /Monday through Thursday/...moderate to high

Tuesday through thursday: VFR.

Thursday night: VFR. Slight chance rain showers.

Friday: 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 Sunday night/...high confidence.

Tonight...
small craft advisories are in effect for east coastal waters
for winds shifting to the northeast and gusting to 20-25 knots.
Seas will also build to 5 to 6 ft well to the east of Cape Ann
and Boston toward Sunday morning. Isolated showers. Cannot rule
out an isolated thunderstorm north of Cape Ann this evening.
Areas of fog will reduce visibility to 1 to 3 miles. Areas of
dense fog with near 0 visibility possible, especially southeast
of Nantucket.

Sunday...
east-NE winds increase to 20 kt across all waters but some gusts to
25-30 kt over the southeastern waters. Seas will become rough
with 5 to 8 feet over the eastern waters and 4-5 feet over the
southern waters. As a result, small craft advisories are in
effect on Sunday for all waters except Boston Harbor and
Narragansett Bay. Areas of fog will persist, especially over the
southeastern waters...with reduced visibilities.

Sunday night...
a the low pulls away farther to our southeast, east-NE winds slowly
diminish to 10-15 kt late and seas slowly subside to 4-6 feet
on the eastern waters and 3-5 feet on the southern waters.

Outlook /Monday through Thursday/...moderate

Tuesday: winds less than 25 kt. Seas locally approaching 5 ft.

Tuesday night through thursday: winds less than 25 kt.

Thursday night through friday: winds less than 25 kt. Slight
chance of rain showers.

&&

Box watches/warnings/advisories...

CT...air quality alert until 11 PM EDT this evening for ctz002>004.
Massachusetts...air quality alert until 11 PM EDT this evening for maz011>013-
016>021.
Rhode Island...air quality alert until 11 PM EDT this evening for riz001>008.
Marine...Small Craft Advisory from 7 am Sunday to 6 am EDT Monday for
anz232>235-237-255-256.
Small Craft Advisory until 6 am EDT Monday for anz231-250-251-
254.

&&

$$

Synopsis...doody/gaf
near term...doody/sipprell/gaf

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