Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 272253
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
653 PM EDT FRI MAY 27 2016

.SYNOPSIS...
Humid weather continues tonight with any lingering showers
diminishing but fog possible overnight. 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.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...

The remaining showers should diminish this evening. Concern now
turns to fog formation overnight. Current dew points 55 to 65
while water temps east and south are in the 50s. Satellite imagery
continues to show fog along the Maine coast and farther offshore.

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.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Tomorrow...
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
limited.

Tomorrow 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.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
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.

Details...

Sunday...

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.

Monday...

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.

&&

.AVIATION /00Z SATURDAY THROUGH WEDNESDAY/...
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 morning.

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 night.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Sunday through Wednesday/...

Sunday...Moderate Confidence.

VFR. Areas of MVFR in afternoon showers and scattered thunderstorms.

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 Monday.

Tuesday-Wednesday...Moderate to high confidence.

VFR conditions except local IFR in fog each late night and
morning.

&&

.MARINE...
Forecaster Confidence Levels.

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

Short Term /through Saturday Night/...High confidence.

Tonight...
Winds and seas remain uneventful tonight. 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 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.

&&

.CLIMATE...
Still expect highest temperatures of the week on Saturday. Some
record highs may be challenged. However, might even see temps
getting close to records today as well.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Air Quality Alert from 11 AM to 11 PM EDT Saturday for CTZ003-
     004.
     Air Quality Alert until 11 PM EDT this evening for CTZ002>004.
MA...Air Quality Alert from 11 AM to 11 PM EDT Saturday for
     MAZ004>007-009>018-026.
     Air Quality Alert until 11 PM EDT this evening for MAZ002>006-
     008>013-026.
RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>005.
MARINE...None.

&&

$$
SYNOPSIS...WTB/Doody
NEAR TERM...WTB/Doody
SHORT TERM...Doody
LONG TERM...WTB
AVIATION...WTB/Doody
MARINE...WTB/Doody
CLIMATE...



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