Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 231051
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
651 AM EDT Wed May 23 2018

.SYNOPSIS...
Isolated showers move offshore this morning, with a second set
of showers possible this evening. Expect dry and warming
weather Thursday and Friday. Saturday will also be warm, just
before a cold front sweeps south across Southern New England
generating a few showers and scattered thunderstorms. Although
it will not be a washout, the potential for showers will linger
through the remainder of the Memorial Day weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

650 AM Update...

Visible imagery shows breaks over Western Mass and North-central
CT with signs of additional clearing in central NH and NE New
York. Radar shows showers over Cape Cod and Islands moving
offshore and additional showers over Northern New Jersey and NE
PA. All are moving east.

Could be leftover showers in parts of CT this morning, but there
is a diminishing trend to the showers this morning. Temperatures
and dew points are mostly within a degree or two of the
forecast. No significant changes in this update.

Previous discussion...

Pleasant, dry day on tap, highs upper 70s to low 80s. Mid level trof
axis having swept through, deamplifying in advance with stout ridging
W, weak cold frontal boundary becomes diffuse. Light winds, midday
sea-breezes allowed prior to late day into evening N push associated
with clipping stout N-stream vort-lobe across SE Canada / N New England.
Forcing mechanism upon available moisture, daytime heating yielding
boundary layer destabilization, perhaps a spot shower or two beneath
cyclonic flow. However, got to get the right conditions. Fairly dry
while a bit of a cap around H6-7. If anything pops, it`ll be brief.

&&

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

Isolated shower activity over N/E MA this evening, otherwise dry,
turning cool. H3 jet streak amplified, diving S in response to
enhanced upstream ridging, a N-stream vort-lobe and attendant colder
H85 temperatures skirt by. N surface wind response, onshore along
the coast. Drier airmass pushing across the region, lowering both
dewpoints and temperatures. Push lows down into the upper 40s for
many locations, potential impacts to sensitive vegetation.

Thursday...

High pressure in control, another pleasant day on tap. However
beneath continued influence of skirting N-stream vort-lobe / cooler
airmass. Light winds overall, allowance of sea-breezes along the
coast. Mixing up to around H85, can`t rule out a few W wind gusts
along the interior, especially over high terrain. Mixing drier air
as well, dewpoints falling into the 30s for interior locations.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Big Picture...

Upper ridge over the Northeast USA de-amplifies with time by the
weekend, leaving a more zonal flow over the weekend and early next
week. A shortwave trough ejects from the Western USA into this zonal
flow, eventually moving over the Northeast USA early next week.
Southern stream upper trough hovers over the Gulf of Mexico and
brings tropical moisture to the Gulf coast. The ECMWF keeps the
tropical moisture south of Mason-Dixon through the period, while the
GFS makes a stronger effort to move it up the East Coast but then
stalls out south of Long Island.

Contour fields from Thursday onward remain above normal, although
not excessively so. Thermal fields are near normal Thursday, then
above normal most of the time through next Tuesday. The thermal
fields do dip close to normal Sunday and Monday especially over NE
Mass. An easterly flow Sunday and Monday due to high pressure over
the Maritimes suggests a cooler surface layer underneath the warm
deep layer, thus supporting the thermal indicators.

Model mass and thermal fields are in general agreement through
Saturday, then show growing differences early next week. Forecast
confidence is good through Saturday, then diminishes to low early
next week.

Concerns...

Thursday night through Saturday...

High pressure offshore provides a west-southwest surface wind, so
most places should warm up. The only air conditioning will be where
this flow is an ocean flow, such as the Outer Cape and parts of Cape
Ann. Mixing is forecast to reach about 825 mb each day, temps at
that level support max sfc temps in the mid to upper 80s. Can`t
rule out a 90, especially if the mixing goes a little deeper
than expected.

A cold front moves south Saturday. Precipitable water values build
ahead of the front with GFS values reaching 1.5 inches Saturday
afternoon/evening. Model consensus shows Total-totals in the upper
40s at that same time; mid level lapse rates are forecast at 6 to
6.5 C/Km, and lifted indices away from the South Coast go sub-zero.
So the potential is there for scattered thunder along with the
showers. The front moves through early Saturday night, with winds
shifting from the northeast behind the front.

Sunday-Tuesday...

The cold front stalls just south of the region Sunday, with northern
stream high pressure building over the Maritimes. This should bring
cooler temperatures Sunday and Monday, most notable in Eastern Mass.
With the front stalled nearby and weak disturbances moving
through the flow, a chance of showers will linger each day with
best chance along the South Coast. As noted above, there remains
uncertainty with how much tropical moisture works up the coast
during the early week as well as if it reaches our area. This
event, or non-event, remains in the low confidence part of the
forecast. So we will maintain chance pops for this part of the
forecast.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

This morning... Low conditions in the Hartford-Springfield
area, and parts of RI and Southeast Mass. Wind shift moving into
Western Mass at 6 AM marks the approximate location of the cold
front, with NW winds and drier weather behind. This will mean
drying and improving ceilings from west to east during the
morning.

This afternoon...VFR most of Southern New England. Some IFR/LIFR
cigs in fog/stratus is possible across the Islands, but should
show improvement. Light NW flow allowing sea- breezes along the
coast. Another cold front moves through toward evening, with a
wind shift out of the north. Could be an isolated SHRA along
this front, mainly northeast MA and RI.

Tonight... VFR.

Thursday...
VFR. Sea-breezes along the coast.

KBOS Terminal...
Potential sea-breeze around midday. NW winds becoming E for a
brief period. Isolated SHRA possible around late afternoon into
evening. N wind shift behind SHRA activity into Wednesday night.

KBDL Terminal...
Low CIGs during the morning push, quickly improving 8-9 am.


Outlook /Thursday Night through Sunday/...Moderate confidence.

Thursday Night through Friday Night: VFR. Breezy.

Saturday: Mainly VFR, with areas MVFR possible. Breezy. Chance
SHRA, slight chance TSRA.

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

Sunday: Mainly MVFR, with local IFR possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

A series of cold fronts later this morning and again this
evening, high pressure thereafter which will shift gradually E
into the end of the week. Low clouds, fog, visibility issues
ahead of the front with scattered showers, some thunder possible
over the S waters this morning. Clearing out, high pressure
building in, a weak pressure gradient, winds for the most part
will be light allowing near-shore sea-breezes during the day.

Outlook /Thursday Night through Sunday/...Moderate confidence.

Thursday Night: Winds less than 25 kt. Seas locally approaching
5 ft.

Friday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas.

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. Areas of seas
approaching 5 ft. Chance of rain showers, slight chance of
thunderstorms.

Sunday: Winds less than 25 kt. Local rough seas. Chance of rain
showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

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



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