Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 280415

Area Forecast Discussion
National Weather Service Taunton MA
1215 AM EDT SAT MAY 28 2016

Humid weather continues through the overnight with any lingering
showers diminishing. More scattered showers and thunderstorms may
develop toward daybreak along the south coast. A backdoor cold
front will bring cooler temperatures to eastern MA Sunday but it
will remain very warm across the interior, where scattered showers
and thunderstorms likely develop during the afternoon. The
potential exists for a period of heavy rain very late Sunday night
into Memorial Day. Dry weather likely follows Tuesday through
Thursday, with above normal temperatures trending back to
seasonable levels by late in the week. A cold front approaches on
Friday with a chance of showers.



1150 PM Update...

A few spotty showers, one with a quick rumble of thunder, have
crossed into far NW Mass at 03Z. Expect these to weaken as they
shift E over the next few hours.

00Z high resolution models as well as the GFS/NAM are all
indicating the development of scattered showers/thunderstorms
around or after 06Z as a strong W-SW low level jet moves near and
just S of the S coast. Starting to note a few showers developing
on the twin forks of Long Island on latest KBOX 88D radar.

Will also see fog redevelop across the southern waters, possibly
reaching the S coast, Cape Cod and the islands overnight. GOES-E
experimental aviation MVFR/IFR/LIFR percentages, and start to see
at least MVFR conditions could move into the Cape and islands
overnight as the low clouds and fog develop.

Remainder of the forecast looks in pretty good shape, but have
updated to bring conditions current.

Previous Discussion...

With light winds and increasing moisture in the air, we are
likely to have another round of fog. This will favor coastal areas
as well as typical inland fog spots. The fog may be locally dense,
and may require a dense fog advisory once confidence is higher in
its location.

Otherwise, mins only fall into the low-mid 60s again tonight
thanks to high crossover temps and lingering clouds.


Mid lvl ridging continues with rising heights across the region
and plenty of sunshine. No trouble mixing to at least H85 where
temps will approach +16C by afternoon peak heating. Weak return
flow will limit sea breezes to mainly the south coast. Therefore,
still expecting probably the hottest day we have seen thus far
with highs topping out in the low-mid 90s across much of the
region away from S coastal areas. Dwpts will remain elevated as
well, mainly in the low-mid 60s even during peak mixing.

The other issue will be convective potential. Plenty of SFC/mixed
lvl CAPE would be available, except the rising heights and modest
subsidence it/s associated with is likely to yield a slight
capping inversion and limit updraft development. Lapse rates are
modest, mainly approaching 6.0C/KM in the mid lvls, but this is
most likely a reflection of the mid lvl dry air. Even the latest
12Z ECMWF now is almost NIL for QPF during the afternoon and
evening hours. Definitely not going NIL given the rising PWATs and
potential energy available if the cap is broken, but will limit it
mainly to slight and low end chance across the terrain and along
developing sea breezes where at least some lift will be available.
T-storms will be mainly airmass-type as shear is also quite

Saturday night...
Continued increase in low-mid lvl moisture will actually help to
erode the cap and allow for some maintenance of any shra/t-storms
that develop, so will be slow to drop overnight POPs. Otherwise,
the increase in moisture beneath the inversion increases the risk
for overnight fog development which once again has the risk of
being locally dense at times and may require headlines. Otherwise,
another mild night with mins only falling back into the 60s.


Big Picture...

Longwave scale shows East Coast upper ridge over the weekend. The
ridge axis shifts west and deamplifies Sunday and Monday. Pattern
shifts to ridge West and trough East during mid to late week.

Shortwave scale shows East Coast ridge Sunday shifting out to sea
and opening a moist flow up the coast from a disturbance off
Florida. All models hold some of this disturbance off the SE USA
coast early week while sending pieces of it up the coast and past
New England early week. Timing of these pieces remains in question
but model consensus would point to Monday-Monday night passage.
Northern jet stream shows one shortwave racing past New England
Monday night, and a second approaching next Friday.



Maritime high pressure builds and pushes a backdoor front into
Southern New England. With the upper ridge shifting overhead the
surface front will at some point be parallel to the upper flow
and may stall over the region. Stability parameters such as the
Total- Totals and the Lifted Index become marginally unstable by
the afternoon across Western and Central MA. Any sunshine heating
this airmass together with the low level convergence along the
front may lead to thunderstorm development. Best chance would be
in Western/Central MA, but this could expand across Northern CT as
well depending on placement of the backdoor front. Precipitable
water values near 1.5 inches, well above the May 29 average of
0.88, suggest some potential for local heavy downpours.

The backdoor front shifts north Sunday night plunging our area
more fully into mild and humid air.

Temperatures at the top of the mixed layer are expected to be
equiv to 15-16C at 850 mb. Full mixing of this would support highs
in the 80s. East/marine flow on the cool side of the backdoor
front may keep temps in Eastern MA in the 70s. Increasing clouds
and dew points of 55 to 65 should keep Sunday night temps no lower
than 55 to 65.


Surface low pressure, part of the system off the SE USA coast,
moves up the coast and spreads a good supply of moisture into New
England. The coastal low will bring some dynamic support for lift.
At the same time, a shortwave moves east from the Great Lakes and
drives a cold front toward New England. This shortwave will move
New England under the lift-friendly right entrance region of its
upper jet. It will also draw plenty of moisture north with
precipitable water values at 1.75 inches and potentially near 2
inches. As with the Sunday values, this is well above normal for
the Memorial Day weekend. This will mean more showers, scattered
thunder, and some locally heavy downpours. Much of the concern for
Monday will be the developing potential for local flooding from
any heavy downpours.

Tuesday through Friday...

High pressure brings drier air in on Tuesday and lingers through
the week. Westerly winds Tuesday will allow temps to reach the low
to mid 80s. Later in the week, as the high shifts offshore and the
surface flow becomes south to southeast, max temperatures will be
held to the 70s with some 60s at the coast.

By Friday a cold front in the St Lawrence Valley will bring
showers in New York and Northern New England. The question will be
if any of those showers can reach Southern New England. For now we
will go with consensus values with slight chance pops during the
day and chance pops in some locations Friday night. This timing
will likely change.


Forecaster Confidence Levels.

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

Tonight...Moderate confidence. Mainly VFR through about 06Z
inland, then patchy fog possible especially at typically prone
airports with localized MVFR/IFR conditions. Another risk for
LIFR along Cape Cod and Island terminals overnight into Saturday

Saturday and Saturday night...Moderate confidence. Mainly VFR.
Local MVFR-IFR conditions in patchy fog through mid morning. Low
probability of brief MVFR conditions in SCT afternoon SHRA/TSRA
across the interior. Another risk for overnight fog tomorrow

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...

Sunday...Moderate Confidence.

VFR. Areas of MVFR in afternoon showers and scattered

Sunday night-Monday...Moderate confidence.

Conditions lowering to MVFR and IFR in fog overnight. MVFR-IFR
conditions in showers late Sunday night through Monday. Locally
heavy downpours. Winds become east Sunday night and South on

Tuesday-Wednesday...Moderate to high confidence.

VFR conditions except local IFR in fog each late night and


Forecaster Confidence Levels.

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

Short Term /through Sunday/...High confidence.

Winds and seas remain below small craft criteria. There will be
some fog to contend with. The fog could be quite dense at times
and may take some time to burn off even after sunrise.

Saturday and Saturday night...
Seas on the southern outer waters may briefly reach 5 feet at
times but should diminish again at night. South to southwest
winds will remain less than 20 knots. More fog may develop
Saturday night.

Outlook /Sunday Night through Wednesday/...

Sunday...High confidence.

Winds and seas remain less than Small Craft Advisory thresholds.
Locally poor vsbys in fog Sunday morning.

Sunday night-Monday...Moderate confidence.

Winds and seas increase but still remain below SCA thresholds.
Areas of poor vsbys in rain and fog.

Monday night through Wednesday...

Seas build to 5-6 feet on the exposed Southern Waters Monday
night and Tuesday. South winds near 20 knots early Monday night
shift out of the West and diminish overnight. Winds remain below
SCA thresholds through Wednesday. A Small Craft Advisory may be
needed for hazardous seas Monday night and Tuesday.


Some record highs may be challenged as temperatures will be the
warmest on Saturday.

Record highs for Saturday May 28...

Boston...92 set in 1931
Providence...91 set in 1931
Hartford...93 set in 1977
Worcester...88 set in 1911 and 1929
Milton/Blue Hill...90 set in 1929


CT...Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for CTZ003-004.
MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this
     evening for MAZ004>007-009>018-026.


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