691
FXUS61 KBOX 031742
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
142 PM EDT Fri May 3 2024

.SYNOPSIS...

High pressure supplies dry weather with cooler onshore breezes
today and Saturday. Increasing clouds Saturday night with
showers likely and cooler temperatures Sunday into Sunday night.
High pressure then brings dry and warm weather Monday afternoon
into Tuesday. Unsettled weather is possible later Wednesday
into Thursday, with the risk of showers but likely remaining
mild.

&&

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

A blanket of low clouds across the region, with the exception
being over western MA/CT, where breaks in the overcast are
occurring per latest visible satellite imagery and surface
observations. This low level moisture is trapped beneath
subsidence inversion across the area, courtesy of 1020+
maritime high. However, strong May sunshine will slowly erode
the clouds and give way to breaks of afternoon sunshine,
especially mid to late afternoon.

Seasonable temps but feeling cooler across eastern MA with
maritime NE flow, capping highs in the 50s this afternoon,
response to water temps only in the 40s. Also, low level NE jet
currently yielding gusts up to 25 mph over Cape Cod and the
Islands, will diminish this afternoon. Hence, less wind this
afternoon than this morning. Previous forecast captures these
details nicely, therefore no major changes with this update.
Earlier discussion below.

Previous Discussion...

A broad anticyclone centered to the north/northeast of southern New
England takes control today. A steady northeast flow will advect a
cooler air mass over southern New England this afternoon with 925
hPa temps dropping to 5C or lower. With diurnal mixing this will
translate to surface temps in the low to mid 60s across the interior
and mid to upper 50s along the coast. Downsloping from easterly flow
over The Worcester Hills will support warmer temperatures in the
upper 60s in the CT River Valley. After a cloudy start, a mid-level
ridge axis building in from the west will be accompanied by a much
drier air mass that will allow some clearing my late morning/early
afternoon. Still expect FEW to SCT diurnal clouds this afternoon,
but there should be a good amount of sunshine as well. Overall a
seasonable day in southern New England.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...

Tonight and Tomorrow

High pressure supports dry/quiet weather across southern New England
tonight. Winds become light and variable which should allow for
fairly efficient radiational cooling across the region, though there
is a question as to how much cloud cover there will be and thus
cooling potential. Decided to blend the NBM25th percentile with
CONSMOS to yield low temps in the low to mid 40s which is near to
slightly below normal for early May in southern New England.

High pressure remains in control on Saturday, so we continue to
expect dry/quiet weather. However, continued onshore flow will
support cloudiness across the region. Very little change in the air
mass for Saturday, so expect high temperatures similar to what
should be observed Friday afternoon with cooler temps along the
coast in the mid 50s and warmer temps across the interior and
CT River Valley ranging from the low to upper 60s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...

Highlights:

* Cool but dry Saturday night into Sunday, then numerous showers
  Sunday afternoon into Monday morning.

* Dry & warm Monday afternoon into Tuesday

* Showers possible late Wed/Wed night into Thursday.

Synoptic Overview...

Tranquil stretch of weather in response to east coast ridging/above
normal heights much of this forecast period. A brief interruption to
the pattern, as northern stream trough briefly erodes this ridge
with a risk of showers later Sunday into Monday morning. Then dry
post frontal NW flow returns later Monday and into Tue. Progressive
pattern follows with ridge building back into the northeast, but may
be accompanied by warm frontal showers later Wed/Thu.

Temperatures...

Cool maritime ridge Sat night into Sunday, with lows in the 40s and
highs 55-60. 850 mb ensemble temp anomalies peak 18z Monday over
southeast MA, in the transition to post frontal. Thus, looking at a
warm day (provided clouds & showers exit before the PM hours) with
WNW flow combined with warm temps aloft coupled with downsloping
winds. Hence, coastline warms up too, including Cape Cod and
Islands. Also, dew pts in the 50s will provide mild/warm feel to the
airmass. Not much of a drop off in temps in the post frontal
airmass, thus, another warm day Tuesday with NW flow, warm temps
aloft combined with downsloping NW winds. Likely not as warm Wed/Thu
but probably above normal(60s).

Precipitation...

High amplitude, deep layer ridge with 1030 mb high from the
maritimes into eastern MA, provides dry weather Sat night into
Sunday morning. Then approaching trough provides cyclonic flow and
increasing/above normal PWATs yield numerous showers Sunday
afternoon into Monday morning. Ensembles offering likely probs (60-
70% for 0.25 inches of rainfall, with chance probs (30-40%) for up
to 0.50 inches of rainfall. Timing for widespread showers, highest
PWATs from 06z-12z Monday per ensembles. Post frontal with dry slot
during Monday afternoon. Drier than normal PWATs linger into Tue,
then becoming unsettled Wed/Thu with moisture plume advecting across
the region (PWATs up to 200% of normal). Too far out in time for
details, but looking at height anomalies from the ensembles,
anomalous lows over Newfoundland and the Dakotas, may result in a
slower ridge advecting across SNE mid of next week. Thus, high
amplitude pattern may support dry weather lingering thru much of
Wed, especially eastern MA/RI.

&&

.AVIATION /18Z FRIDAY THROUGH TUESDAY/...

Forecast Confidence Levels:

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

Through 00Z...High confidence in trends, moderate in exact
timing.

VFR across the CT River Valley, but still working on eroding the
MVFR/borderline VFR conditions across central MA/RI into eastern
MA. Should see these ceilings continue to lift/scatter out by
roughly 19-21Z. This should take the longest across the Cape
terminals, though think ACK remains MVFR through the forecast
with onshore flow.

Tonight...Moderate confidence.

VFR conditions to start with light and variable winds. There is
a possibility for MVFR stratus to spread back in with enough
onshore flow aloft. Timing wise think it would be after 06Z.
Could see patchy fog across the typical prone areas in the
Merrimack Valley, but have only hinted at for now.

Tomorrow...Moderate confidence.

Any lingering MVFR may improve to VFR for much of the day. Some
uncertainty here given we are still stuck in persistent onshore
easterly flow at 5-10 kts. Confidence higher for VFR away from
eastern MA coastline.

KBOS TAF...High confidence this afternoon. Lowers to moderate
tonight.

Improving to VFR by 19Z with NE to E winds. Winds light and
variable tonight with enough onshore flow aloft that MVFR
stratus could spread back in roughly 04-06Z. Could improve to
VFR as boundary layer mixes roughly 13-15Z, but uncertain on
this given the onshore wind component.

KBDL TAF...High confidence.

VFR through the forecast. Could see some borderline MVFR
ceilings spread in 02-04Z tonight.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Mainly VFR, with local IFR possible. Chance
SHRA.

Sunday: Chance SHRA.

Sunday Night: Breezy. Chance SHRA.

Monday: Chance SHRA.

Monday Night through Tuesday: Slight chance SHRA.

&&

.MARINE...

Forecaster Confidence Levels:

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

Marine

Today through Tomorrow

High pressure supports fairly calm conditions across the coastal
waters through Saturday. Winds will prevail out of the northeast
from 10 to 15 knots with some gusts to 20 knots possible. Seas
generally 1 to 3 feet.



Outlook /Saturday Night through Tuesday/...

Saturday: Winds less than 25 kt.

Saturday Night through Sunday: Winds less than 25 kt. Chance of
rain showers.

Sunday Night through Monday: Winds less than 25 kt. Seas
locally approaching 5 ft. Chance of rain showers.

Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers.

Tuesday: Winds less than 25 kt. Slight chance of rain showers.

&&

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

&&

$$

SYNOPSIS...Nocera/RM
NEAR TERM...Nocera/BW/RM
SHORT TERM...RM
LONG TERM...Nocera
AVIATION...Loconto/BW/BL
MARINE...Nocera/RM