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FXUS61 KBOX 231351
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
951 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/...

950 am update...

Low clouds/fog patches earlier this morning have quickly given
way to partial sunshine in most locations. 850T between +11C
and +12C along with good mixing should yield highs in the
lower 80s in many locations...except a bit cooler where
localized sea breezes occur.

Dry weather generally prevails...but a shortwave may trigger a
brief spot shower or two late this afternoon/evening across
eastern MA. Nothing more than than expected given lack of
low level moisture.

&&

.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 /14Z 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.

Today...Most locations have already become VFR and any lingering
lower cigs/vsbys near the south coast should improve by lunch
time. Generally looking at NW winds of 5 to 15 knots but
localized sea breezes likely for a time this afternoon.

Tonight...VFR.

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

KBOS Terminal...High confidence in VFR conditions. Potential
sea- breeze around midday but probably is short-lived and done
by early evening.

KBDL Terminal...High confidence in VFR conditions.

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...Frank
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...Frank/Sipprell/WTB
MARINE...WTB/Sipprell
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