Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 121644
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1244 PM EDT Mon Apr 12 2021

.SYNOPSIS...
A ridge of high pressure over interior northern New England will
again allow for another period of onshore flow. This will bring
overcast conditions, northeast winds and cooler temperatures
through tonight. Seasonable temperatures expected for the rest
of the week except for the coolest temperatures on Friday. An
unsettled weather pattern will provide a threat for showers late
into the week, then drier on the weekend.

&&

.NEAR TERM /UNTIL 7 AM Tuesday/

... 1230 PM Update...

Persistent onshore flow has resulted in lower cloud bases and
scattered areas of light rain/drizzle across most of eastern
MA. Radar observations suggest that this weak area of precip
will continue to move WSW into interior southern New England
this afternoon. Adjusted PoPs/QPF to reflect the latest
observational trends and brought temperatures/wind in line with
latest observations as well. Precipitation should lose steam by
early evening leaving us with overcast skies tonight and lows
in the upper 30s/low 40s. &&

.SHORT TERM /7 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Tonight:

For tonight, the closed low to our west across PA then begins to
peel ESE into the waters south of Long Island. A band of rain along
a weak mid-level deformation zone northeast of the closed low across
NY into far western CT should shift SE away from Southern New
England. Guidance has trended further south and west with the
placement of this rain shield with successive runs, and as such,
I`ve left the forecast dry for the region. And while overcast skies
should exist early tonight, a east to west lifting to cloud bases
and even some possible scattering of cloudiness stands to occur as
winds turn from NE to N as surface ridging shifts westward and
allows for drier air to filter back in. However late in the
overnight into early Tuesday, another increase in clouds is expected
across eastern MA though cloud cover may prove more scattered to
broken then overcast. Continued gradual PBL cooling and somewhat
less cloud cover should allow for lows to cool off into the upper
30s to low 40s.

Tuesday:

Surface ridging continues to remain in place across Southern New
England on Tuesday. Looking somewhat more optimistic a from cloud
cover perspective, as mostly cloudy skies to begin should start to
scatter to more partly cloudy skies and more sunshine as the day
progresses. Northerly pressure gradient looks to become a little
stronger, with breezier conditions and coolest temperatures expected
across coastal eastern MA. Away from the coast, will see lighter
winds and also more continental than onshore; this will also allow
for milder temperatures. Have highs in the upper 40s to mid 50s near
the eastern MA coast. For much of central/western MA, CT into RI
expecting mid 50s to lower 60s highs. Overall somewhat more improved
conditions than compared to Sunday or today, if not like what we
saw for much of last week.

&&

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

Upper low over the Great Lakes will determine our weather for much
of the late week period. Upper shortwave ridge and surface high
pressure move over New England Tuesday night/Wednesday. Great Lakes
upper low drifts southeast, generating a coastal low that passes
near the Srn New England coast late Thursday and Friday. Another
shortwave ridge moves across on Saturday, with signs of another
shortwave and surface cold front approaching for Sunday.

Upper contours are below normal on Thursday through Saturday, near
normal otherwise. Expect seasonable temperatures Wednesday and
Sunday, and temps below normal for the end of this week.

Mass fields surface and aloft are similar through the upcoming week.
They may not be exact, but they indicate similar weather. Confidence
is moderate.

Details...

Tuesday night-Wednesday...

Lingering moisture below 700 mb, but also a ridge building in with
subsidence during the period. The ridge moves off to the east
Wednesday afternoon, at which time the Great Lakes closed low
approaches from the west. Expect mixed clouds/sun with the most
clouds during the afternoon/evening and favoring the Central Hills
and CT Valley/East Slope regions. Dew points and low-level temps
favor min temps in the upper 30s to mid 40s both Tuesday night and
Wednesday night. During Wednesday, light winds inland should allow
full mixing, while a northeast surface wind will buffer coastal
temps. Temps of 2-4C at 850-mb supports inland temps in the 60s.
Temps of  +4C at 950-mb along the east coast suggests eastern temps
in the upper 40s to mid 50s.

Wednesday night through Friday...

Great Lakes upper low sweeps southeast, with increasing low level
jet over Eastern NY clipping parts of Western MA/CT. The upper low
and its surface component swing just south of our region Thursday
night and Friday, then move off to the east Friday night. Increasing
cyclonic flow aloft will generate lift, and the southerly flow
leading the upper low will draw more moisture up the coast...PW
values climb to 0.75 inches by Friday which is above normal for mid-
April. Based on this, the forecast will show chance pops for rain
showers. Temps at 950 mb are near 0C Friday, and sub-zero above 950-
mb. These cold temps are mostly found over the western and central
hills of MA, and may support snow showers in those areas from early
Friday through the evening. The cool temps aloft will support max
sfc temps in the low to mid 50s Thursday and the mid 40s to low 50s
Friday.

As coastal low pressure passes offshore Friday Thursday night
and Friday, expect northeast winds to become stronger especially
along the coast. Winds at 900-950 mb are expected to reach 40-50
kt, which would support surface gusts of 25 to 30 kt along the
coast and the waters, slightly less farther inland.

Saturday-Sunday...

Low level moisture lingers, but a weak high pressure ridge builds
in. Expect any lingering showers Saturday morning to move offshore.
Drier weather follows, but with a mix of sky cover.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels.

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

Today: Moderate confidence.

VFR ceilings deteriorate down to MVFR-IFR levels from NE to SW.
Though exact timing is somewhat unclear thinking this occurs
~13-15z for eastern airports, around ~15-18z for western
airports. Best chance IFR ceilings ORH-PVD east. Intervals of
patchy -DZ in stratus but given strengthening NE/ENE winds fog
should not develop. NE winds increase to 10-16 kt, gusts 20-24
kt. Strongest winds east/southeast.

Tonight: Moderate confidence.

IFR/MVFR ceilings then gradually lift to VFR thru 06z from NE
to SW. Ceilings then trend lower again after 06z toward lower-
end VFR to perhaps MVFR eastern airports. NE winds lighten and
back to N/NNE around 5-10 kt, strongest near the eastern coast.

Tuesday: High confidence.

Early VFR/MVFR ceilings become all VFR thru the AM hrs for most
except later at Cape Cod/ACK. N to NNE winds 6-10 kt interior
TAFs, 10-15 kt gusts to 17-24 kt coastal TAFs.

KBOS TAF...Moderate confidence in TAF. VFR bases this morning,
then lowering to MVFR-IFR ~13-15z/9-11AM.

KBDL TAF...Moderate confidence in TAF. VFR bases this morning,
lowering to MVFR ~15-17z/11AM-1PM.

Outlook /Tuesday Night through Friday/...

Tuesday Night: VFR.

Wednesday through Wednesday Night: Mainly VFR, with local MVFR
possible. Slight chance SHRA.

Thursday: Mainly MVFR, with local VFR possible. Breezy. Chance
SHRA.

Thursday Night: Mainly MVFR, with areas IFR possible. Windy
with gusts up to 30 kt. Chance SHRA.

Friday: Mainly MVFR, with local IFR possible. Windy with gusts
up to 35 kt. Chance SHRA.

&&

.MARINE...
Small craft advisories continue on the offshore waters through
tonight and early Tuesday on the southern waters. Have also
opted to hoist small craft advisories effective around noontime
and ending later tonight for most of the near- shore zones
except for Boston Harbor and Narragansett Bay. NE flow will
allow for building seas and strengthening winds.

For today: NE winds strengthen to 15-20 kts with gusts 25-30 kt on
most waters except Boston Harbor and Narragansett Bay. Higher seas
in the 4-7 ft range on the offshore waters and around 2-5 ft
nearshore.

Tonight, winds become northerly and tend to decrease to 10-15 kt
with gusts in the low 20s kt range. Similar wave heights expected
tonight on most waters, though there may be enough of a decrease on
the northeast waters which could allow existing SCAs on these waters
to be dropped.

Into Tuesday, N winds will tend stronger furthest south and east to
around 20-25 kt in gusts. Seas 2-4 ft nearshore and 4-6 ft offshore,
trending higher east of Cape Cod.


Outlook /Tuesday Night through Friday/...

Tuesday Night: Winds less than 25 kt. Local rough seas.

Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft.

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

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

Thursday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Rough seas up to 10 ft. Chance of rain
showers.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Low risk of 35 kt gale-force gusts. Rough seas up
to 14 ft. Chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 11 PM EDT this evening for
     ANZ232>234-251.
     Small Craft Advisory until 8 PM EDT this evening for ANZ231.
     Small Craft Advisory until 2 AM EDT Tuesday for ANZ235-237-250.
     Small Craft Advisory until 8 AM EDT Tuesday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Loconto
NEAR TERM...RM
SHORT TERM...Loconto
LONG TERM...WTB
AVIATION...WTB/Loconto
MARINE...WTB/Loconto


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