Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 270210
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1010 PM EDT Fri May 26 2017

.SYNOPSIS...

Low pressure moves offshore tonight followed by weak high pressure
this weekend which will bring mild days and cool nights. Low
pressure in the Great Lakes generates a coastal low late Monday
that passes off Nantucket on Tuesday. This will bring wet weather
on Memorial Day. Daytime clouds and scattered showers will pop
up each day Tuesday through Friday.

&&

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

10 pm update...

Comma-head occlusion continues to peel off to the E round which
there is some emphasis of boundary-layer focus of airmasses
brought on by mid-level lobing energy round the backside of the
low. Broken to overcast low cloud decks lingering along with
scattered shower activity. This as cooler air surges rearward
and onshore as discerned via latest radar imagery. Temperatures
dropping towards their respective wet-bulb, it is behind the
cool-frontal surge that patchy fog and drizzle development are
possible, especially over E MA.

But some improvement on the horizon. Heights rising as weak
mid-level ridging builds in from the W. Building high pressure
along with drier air, should see showers dissipate E as we go
through the overnight hours. Winds should relax becoming more
northerly with time. Lows bottoming around the upper 40s to low
50s.

&&

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

Saturday...
Weak high pres builds across the region but low level moisture
remains abundant below the inversion which indicates clouds will
dominate with bkn-ovc cloud cover. Mid level shortwave approaches
from the west by late in the day which will also bring increasing
mid level clouds and can`t rule out a brief shower toward evening
across CT. Soundings show mixing to about 925 mb with highs mostly
in the 60s with a few lower 70s possible lower CT valley. Seabreezes
expected in the afternoon which may hold temps in the upper 50s
along east coastal MA.

Saturday night...
A brief shower possible from CT to southern RI in the evening as
shortwave passes to the south, otherwise dry weather as weak high
pres remains in control. Mostly cloudy skies persisting. Mins mid
40s to lower 50s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...

Big Picture...

Longwave scale maintains a ridge west/trough east pattern for much
of the long term period.  Shortwave scale shows a closed low slowly
moving from Canada across the Great Lakes, eventually reaching New
England late in the week.  Two jets in place for much of the period.
The northern jet sweeps around the closed low from Western Canada
across the Northeast USA. The southern jet swings from Baja into the
Southern Plains and Southeast USA.  The two streams interact over
the Ohio Valley and Mid Atlantic early next week.

Model mass fields are similar through Tuesday...the GFS and ECMWF
are similar through Thursday. GFS and ECMWF moisture and thermal
fields are similar during the same period while showing differences
in detail. This brings increased confidence in the overall pattern
through much of next week, although lingering uncertainty regarding
timing of individual shortwaves in the flow.

Details...

Sunday...

High pressure is in control Sunday with subsidence and light wind
flow. Moisture fields show an area of higher RH below 800 mb and
suggests a cloud/sun mix with the fair weather. Temperatures in the
mixed layer are equiv to 5-8C, suggesting max temps upper 60s to mid
70s. Sea breezes along the coast will hold temps there in the 60s.

Fair skies linger early in the night, but the next approaching
shortwave supports increasing clouds...expect this overnight. Dew
points will be roughly 45-50 which with light flow and initial fair
skies should allow temps to reach the lower 50s and possibly the mid
40s.

Monday-Tuesday...

Interaction between the northern and southern streams leads to a 120-
knot jet over the Ohio Valley which races east ahead of northern
stream shortwave during Monday. This crosses the surface cold front
and generates a triple-point low as it crosses the Mid Atlantic
coast. This in turn spins up a weak southerly low level jet that
aims for Srn New England during the afternoon/evening. Precipitable
water values reach 1.0-1.2 inches which is above normal but not
excessive.

Model consensus slows some slowing of the onset of measurable
pcpn in Southern New England. The GFS holds off on measurable
until after 12Z Monday while the GGEM and ECMWF bring it to the
CT Valley by 12Z and possibly a little farther. We opted to slow
pcpn onset to a compromise time with chance pops to the CT
Valley around 09z and to Worcester and the RI Hills 12Z...then
to most of Eastern MA by 14Z.

We will bring likely pops to the western sections for late
morning/early afternoon and for eastern sections for the afternoon.
Stability parameters are somewhat favorable for convection, with
totals around 50 while surface LI values will be near zero. We will
include scattered to isolated tstms Monday afternoon/evening.

The coastal low passes offshore Tuesday morning. Expect leftover
showers diminishing Tuesday morning, then expanding again as another
shortwave moves through.

Wednesday through Friday...

Upper low slowly sweeps across Eastern Canada during the period,
with shortwaves moving through the flow across New England. Cold
pool advection aloft will destabilize the airmass and support
scattered daytime showers, especially in areas north of the Mass
Pike. Temperatures in the mixed layer will support max temps upper
60s to mid 70s each day. Fair skies and dew points in the 50s will
support mins in the 50s each night...possibly cooling to mid 40s to
low 50s late in the week.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...

Forecaster Confidence Levels...

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

Short Term /through Saturday Night/...Moderate Confidence.

2z update...

Tonight...
MVFR CIGs mainly with low risk IFR, while an improving trend
from the W with low-end VFR CIGs. Lowest conditions across E MA
with the threat of DZ/FG as winds have turned NE onshore. Should
see improving conditions W to E overnight, diminishing winds
becoming northerly. SHRA activity concluding.

Saturday and Saturday night...
Mix of VFR/MVFR cigs. Seabreezes developing late morning into
the afternoon. Low risk for a brief shower late Sat/Sat evening
south of the Mass Pike.

KBOS TAF...NE flow persisting as a cool front has pushed onshore.
Should see NE winds diminish towards midnight becoming more N
overnight. The brief period of IFR should be just that, the rest
of the overnight period MVFR.

KBDL TAF...BKN low-end VFR CIGs persisting. N flow.

Outlook /Sunday through Wednesday/...Moderate confidence.

Sunday...
VFR. Light S wind becoming onshore on the Mass East Coast by
midday.

Monday...
VFR at first, lowering to MVFR in the afternoon and IFR/LIFR at
night. SE winds less than 20 kts. RA/SHRA developing during the
morning and midday, with potential SCT TSRA in the afternoon into
evening. Areas of fog at night.

Tuesday-Wednesday...

IFR Tuesday morning, improving to VFR Tuesday afternoon through
Wednesday. SCT SHRA, mostly during the daytime. Light E wind
Tuesday becoming S in the afternoon, then W/SW Wednesday. Speeds
less than 15 kts.

&&

.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.

10 pm update....

Tonight...
Have modified headlines to account for hazardous seas which
persist on the outer waters and S sounds of RI and Block Island.
With high pressure building in from the W, should see mainly N
winds which will be light allowing seas to diminish further into
the Saturday morning timeframe.

Saturday...
Winds becoming E/SE with speeds below 15 kt. Seas will subside
below 5 ft over southern waters.

Saturday night...
Quiet weather with light winds and seas below SCA.

Outlook /Sunday through Wednesday/...Moderate confidence.

Winds less than 20 knots and seas less than 5 feet through the
period. Showers and isolated tstms Monday, showers and fog Monday
night and early Tuesday.

&&

.TIDES/COASTAL FLOODING...

Still observing a 0.5 to 1 ft surge along both coasts which
ESTOFS and ETSS guidance is underestimating. Surge expected to
diminish to around 0.2 to 0.5 ft at the time of high tide later
tonight as winds and swell diminish. High tides this evening are
quite high (Boston 12.41 feet around 1230 AM, Providence 6.45
feet around 930 pm), Even with minimal wave action, total water
level above 12.5 ft in Boston typically produces very minor
inundation for the notorious low-lying locations (i.e, Morrissey
Boulevard in Boston). In addition, minor inundation possible
along the south coast if surge still around 0.5 ft. We will
issue a coastal flood statement to address these concerns.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 10 AM EDT
     Saturday for ANZ235-237.
     Small Craft Advisory for hazardous seas until 6 AM EDT
     Saturday for ANZ250.
     Small Craft Advisory for hazardous seas until noon EDT
     Saturday for ANZ254>256.

&&

$$

SYNOPSIS...WTB/KJC
NEAR TERM...WTB/KJC/Sipprell
SHORT TERM...KJC
LONG TERM...WTB
AVIATION...WTB/KJC/Sipprell
MARINE...WTB/KJC/Sipprell
TIDES/COASTAL FLOODING...WFO BOX Staff



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