Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 241707
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
107 PM EDT Thu May 24 2018

.SYNOPSIS...
Dry and cooler weather will prevail today as high pressure
builds into the region. Warmer and more humid conditions then
return through Saturday. A cold front approaches from northern
New England Saturday afternoon, bringing scattered showers and
thunderstorms. The front stalls south of the region Sunday, with
cool east winds and patchy showers lingering through the
remainder of the holiday weekend. High pressure moves in with
drier weather for the midweek.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
* 1 PM Update *

Low clouds were pushing south of New England and should remain
over ocean through early evening before retreating back toward
land. Otherwise plenty of sunshine this afternoon with just
some patchy mid or high clouds around NE MA. Onshore flow will
keep coastal areas in upper 50s to lower 60s this afternoon,
while it warms well into 70s inland.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
Tonight...

Clear conditions, return light S flow, opportunity for radiational
cooling with boundary layer decoupling early on before higher
dewpoint air pushes N. Perhaps some low cloud and/or fog issues
along the immediate S-shoreline of New England, otherwise leaning
with coldest forecast guidance with lows upper 40s to low 50s.

Friday...

Mild day on tap with increasing humidity. High pressure SE, W/SW
flow prevails. Warm air advection within low-mid levels, ridge
builds. Up against N-stream cyclonic flow / Baffin Bay low, in
the confluence of which low-level SW winds increase. Daytime
heating, boundary layer mixing, mix-down of drier air, faster
winds. Looking at highs around the mid to upper 80s, the warm
spot over the Merrimack River Valley and Boston-metro where
we`ll likely see some locations top out over 90. The deep
mixing, can`t rule out SW gusts upwards of 30 mph.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Big Picture...

Zonal flow with embedded shortwaves in place across Canada and the
Northern tier of the USA. Upper flow also features closed lows over
the Gulf of Mexico and the Western USA. The Western low opens and
feeds into the zonal stream early next week. The Gulf low opens into
the zonal flow the middle of next week. One shortwave embedded in
the zonal flow will move across New England early next week.

Upper contour fields are in general agreement through Monday, while
the MSLP fields show differences in details starting Sunday. Thermal
fields also start showing differences starting Sunday. Overall
forecast confidence is moderate-high through Sunday, moderate for
Monday-Tuesday. Agreement on a building ridge Wednesday suggests
higher confidence, but it is a day 7 forecast which limits such
confidence.

Contour fields from Saturday onward remain above normal through the
forecast period. Thermal fields are also above normal, although fall
to near normal Wednesday. Low level flow turns from the east and
northeast Saturday night, then remains that way for Sunday and much
of Monday. This will affect the surface layer with cooler
temperatures while the deeper layer remains above normal.

Concerns...

Friday night and Saturday...

High pressure over the West Atlantic maintains dry weather over
Southern New England Friday night and Saturday morning. Low pressure
passing across Northern Maine combines with the high to create a
pressure gradient favoring west-southwest winds in our area. This
flow brings in higher moisture air, with precipitable water values
around 1.3 inches...more than 1 std dev above normal but less than
2. Dew points should climb through the 50s into the lower 60s.

Temps aloft in the mixed layer favor max sfc temps in the mid to
upper 80s inland and cooler along the South Coast.

As the Northern Maine low pressure moves off through the
Maritimes, it pushes a cold front south through our area during
the late afternoon and early night. PW values continue to rise,
reaching 1.5 to 1.7 inches which would be near 2 std dev.
Stability parameters become more favorable for convection, with
CAPE 500-1000 J/Kg and Totals around 50. Mid-level lapse rates
are forecast at 6-6.5C/Km. Theta-e ridge is indicated from
Southern NY through CT and Nrn RI. All of this supports showers
and scattered t-storms with potential for local downpours. Most
favored area would be CT and Nrn RI, but all areas would have a
chance. Winds aloft ahead of the front are rather weak, so not
favoring strong wind gusts at this time.

The cold front moves south through our area Saturday night.
Increasing east winds behind the front may lead to gusty winds
especially over eastern Mass.

Sunday-Wednesday...

Cold front stalls south of New England, aligned with the zonal flow.
Shortwave trough moves east through the upper flow, passing New
England around Monday. This may generate a weak wave along the front
that moves past us on Monday. Thus we expect continued unsettled
conditions Sunday and Monday with a chance of showers each day.

Another cold front pushes SE out of central Canada during Tuesday.
This May generate a few showers before pushing the moisture
offshore.

High pressure builds over the region Wednesday, bringing dry weather.

&&

.AVIATION /17Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

* 1 PM Update *

Short Term /through Friday/...

High confidence. VFR through early tonight as low cloud deck
remains over ocean south of New England. Sea breezes diminish
around sunset and give way to S/SW winds tonight. Should see
IFR/LIFR conditions return to south coastal areas, including
Cape and Islands tonight, before conditions improve Friday
morning.

Increasing SW winds Friday with 25-30kt gusts, especially across
RI and eastern MA. VFR with diminishing SW winds Friday night.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Friday Night through Monday/...Moderate confidence.

Friday Night: VFR. Breezy.

Saturday: Mainly VFR, with local MVFR possible in afternoon
showers/scattered thunderstorms. Breezy.

Saturday Night: Mainly MVFR, with local IFR possible. Chance
SHRA, slight chance TSRA.

Sunday: Mainly MVFR, with areas IFR possible. Breezy. Chance
SHRA.

Sunday Night: Mainly IFR, with areas MVFR possible. Chance
SHRA, patchy BR.

Memorial Day: Mainly MVFR, with areas IFR possible. Chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

* 1 PM Update *

Short Term /through Friday/...High confidence.

Small Craft Advisories posted for all waters for increasing SW
winds tonight (NE MA waters) and Friday (remainder of MA/RI
waters).

S/SE flow this afternoon with local sea breezes. Should see
some 15-20kt gusts along south coastal near shore waters with
locally choppy seas due to incoming high tide. Fog and low
clouds linger on waters south of islands.

High pressure moving off the coast will bring increasing SW
winds tonight and Friday. Enough of a low level jet tonight to
bring marginal SCA conditions to NE MA waters, otherwise winds
increase during day Friday across all waters with strongest
gusts (30kt) near shore. Winds and seas gradually subside Friday
night.

Outlook /Saturday through Monday/...Moderate confidence.

Saturday: Winds less than 25 kt. Areas of seas approaching 5
ft. Slight chance of rain showers, slight chance of
thunderstorms.

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain showers, slight chance of thunderstorms.

Sunday: Winds less than 25 kt. Local rough seas. Chance of rain
showers, slight chance of thunderstorms.

Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, slight chance of thunderstorms.

Memorial Day: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 10 AM Friday to 2 AM EDT Saturday
     for ANZ231>234.
     Small Craft Advisory from 11 AM to 8 PM EDT Friday for ANZ230-
     236-251.
     Small Craft Advisory from 11 AM Friday to 2 AM EDT Saturday
     for ANZ235-237.
     Small Craft Advisory from 8 PM this evening to 8 PM EDT Friday
     for ANZ250.
     Small Craft Advisory from 8 AM to 8 PM EDT Friday for
     ANZ254>256.

&&

$$
SYNOPSIS...WTB/Sipprell
NEAR TERM...JWD
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...JWD
MARINE...JWD



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