Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 070201
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1001 PM EDT Mon Apr 6 2020

.SYNOPSIS...
High pressure remains in control through Tuesday, though a passing
upper-level disturbance will lead to an increase in clouds tonight.
Later Tuesday night, a warm front will spread a period of light
rains across most of southern New England. A fast moving low
will develop along that warm front and bring a period of rain
through Wednesday. Another system slides in on Thursday bringing
more rain for Thursday. Scattered shower activity possible
Thursday night through Friday. Drier weather possible for the
weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

10 PM Update...

No major changes from previous forecast. Quite the difference in
dew pts across the region, where seabreeze/maritime layer
penetrated dew pts are in the 30s across RI and southeast MA.
Elsewhere very dry airmass exist with dew pts in the teens.
Enough of a WNW breeze to preclude boundary layer from
decoupling and as a result temps haven`t fallen that quickly.

Short wave energy approaching in the NW flow aloft will spread
mid/high clouds across the area later tonight. This should also
slow the rate of cooling overnight. The net result will be for
temps to run about 5-8 degs warmer than normal with lows
overnight mainly in the upper 30s to lower 40s. Previous
forecast captures this well so no changes with this update.

Previous discussion...

A weak 1020 mb bubble high centered over Lake Erie extends a ridge
eastward across much of Southern New England this afternoon. This is
leading to clear skies with a very dry air mass across much of New
England. Cyclonic flow aloft regime is in place aloft, leading to a
seasonably strong northwest mid to upper flow/jet. So despite the
surface ridge, a mid-level disturbance currently leading to some
scattered to overcast mid level clouds across the northern Great
Lakes will ripple SE through the flow aloft towards our area later
tonight. This will increase mid-level cloud cover across much of the
region tonight, with low-levels being quite dry (from 850 mb to sfc)
so not expecting any precip from this passing disturbance. Given the
initial clear skies and drier air, expect a sharp fall in
temperatures after sunset thru late evening/midnight. As clouds come
in, strong nocturnal cooling should start to slow after midnight
into the pre-dawn hours. Biggest forecast challenge tonight will be
on how low temperatures may get, given the net increase in cloud
cover especially the second half of the night. Opted for lows in the
mid 30s to low 40s, but if the clouds should prove more delayed in
arriving into Southern New England, temperatures could end up being
several degrees cooler.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...

Tuesday:

12z NAM/GFS guidance shifts the passing mid-/upper disturbance aloft
into our coastal waters early in the day. Mid-level RH tends to
subside through mid-/late morning per model cross sections, which
should lead to more sun than clouds by late morning onward. The
combination of westerly downsloping winds (other than near the
coast, with another day of sea-breezes indicated in some guidance)
and greater sunshine in the afternoon should produce highs into the
low to mid 60s (a few upper 60s towards the CT Valley). Given the
sea-breeze influence, cooler highs into the upper 50s near the
coast.

From a fire weather perspective, lowest RHs are projected to be a
bit higher across the interior (around 25-35 percent, due to higher
afternoon mixed-dewpoints than today) and west winds should be a bit
weaker with only occasional gusts to 20 mph. See the updated Fire
Weather section below for more info.

Tuesday Night:

Look for an increase in clouds again Tuesday evening, as a weak area
of low pressure moves across Southern New England. With this low is
an attendant warm front which tries to build NE through the night.
Regarding the warm front, there is some level of uncertainty on how
far northeast the feature actually progresses, given some leftover
confluent flow aloft. Better chances for rains (solid Likely PoPs)
mainly along and SW of a Greenfield to Worcester to Plymouth line,
with lower end Likely PoPs towards NE MA. Rain amounts range from
between a few hundredths for NE MA with a tapered increase to
just under a quarter-inch towards northern CT eastward into
central/southern RI.

Winds should become fairly light into the first half of the night as
the warm front moves northward, though towards morning will start to
see an increase in NE wind speeds. Given that tides are
astronomically high with the upcoming Supermoon, this could result
in some splashover or minor coastal flooding at least for eastern MA
and adjacent coastal areas along Cape Cod Bay. More on this in the
Coastal Flooding section.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...

Highlights

* Active pattern with several opportunities for precipitation.

* Potential for minor coastal flooding Wednesday to Friday around
  high tide across the east coast of MA and Nantucket.  See the
  Tides and Coastal Flooding section below for more details.

* Temperatures running near to below normal through the extended.

Wednesday...

A shortwave trough will lift from the eastern Great Lakes through
southern New England and offshore by Wednesday evening. A secondary
low will develop south of southern New England and deepen as it
lifts east of Cape Cod later in the day.

Good agreement amongst deterministic and ensemble guidance with the
track and intensity of the surface low sliding through. Best shot
for rain is during the morning and expect precipitation to end from
west to east as the system moves through. Good clustering via
ensemble plume guidance with total QPF values ranging from 0.2
inches to roughly 0.4 inches.

Some uncertainty in regards to temperatures on Wednesday. Based on
flow at 925 hPa and the surface expecting winds to be NE/E which
will advect in air from the ocean. Knocked down high temperatures on
Wednesday toward the 25th percentile of guidance. Held off from
going with the coldest guidance as there are some questions on if
the sun can break out during the afternoon, which may allow
temperatures to quickly bounce up. High temperatures range from the
mid 40s along the east coast of MA to the low 50s in the CT River
Valley.

Wednesday Night and Thursday...

Weak mid level ridge axis builds in from the eastern Great Lakes and
slides through by early Thursday morning. Behind the ridge a cutoff
low will rotate from the western Great Lakes into the eastern Great
Lakes by late on Thursday. A weak high will initially build in
bringing a period of dry weather Wednesday night into early Thursday
morning. Low pressure will slide from the central Great Lakes into
the eastern Great Lakes by early Thursday. Southern New England will
remain near the triple point associated with the system and could
potentially see a secondary low develop during the afternoon/evening
over portions of the area.

No concerns for Wednesday night with the weak high building in. This
will bring partial clearing, but do not anticipate a good
radiational cooling night as cloud cover will be on the increase
especially after midnight. Lows will generally be in the upper 30s
and low 40s.

Some discrepancy on how quickly precipitation spreads into the
region between deterministic guidance. The GFS/NAM is slower than
the ECMWF/GEM guidance at this point in time. Have leaned toward the
ECWMF and GEM guidance at this point in time, which matches well
with the latest NBM. Will need to watch as a secondary low develops
over the region during the afternoon. The pieces of guidance
previously mentioned all suggest that there will be a few hundred
J/kg of MUCAPE, which could allow for some thunderstorm activity to
develop. There is also sufficient shear aloft, but any storms that
develop would be elevated in nature. Bears worth keeping an eye on,
but would like to see how things look with some CAMs. Have not
included any thunderstorm mention at this point in time.

As the low pressure deepens also anticipating that a strong 50+ kt
low level jet develops. This will bring gusty winds across the
region. Some uncertainty on much of these stronger winds mix down.
May need to increase gusts in future updates. As of right now looks
like gusts of 30 mph to potentially 40 mph will be possible that
afternoon. Highs top out in the 40s and low 50s across southern New
England.

Thursday Night through Friday night...

Confidence in the forecast lowers after this point in time. There
are significant differences amongst guidance with how quickly the
cutoff system lifts out of the region. The GFS being the quickest
and the ECMWF/GEM much slower. Think that the ECMWF/GEM seem more
reasonable and tend to handle these situations much better than the
too fast GFS.

This setup will keep southern New England under cyclonic flow, which
with the moisture in place over the region will allow for scattered
shower activity. Some of the higher elevations will even see some
snow shower activity, but do not anticipate much if any snowfall
accumulation. This continues Thursday night through Friday. The best
opportunity for scattered showers will be during the day Friday
where the combination of cyclonic flow and diurnal heating will
bring more widespread activity. Any diurnally driven showers will
diminish in coverage Friday night, however some of the higher
elevations may see showers linger.

Low temperatures on Thursday night into Friday morning and Friday
night into Saturday morning will generally be in the 30s. Highs on
Friday top out into the 40s and low 50s.

Saturday and Sunday...

Potentially drier conditions across southern New England as high
pressure builds in. The GFS hints at some showers being possible on
Saturday with a trough sliding through, whereas the ECMWF/GEM keep a
ridge in place over the region. Have kept a blend of guidance due to
the uncertainty at this point in time. Better shot for drier weather
appears to be on Sunday.

&&

.AVIATION /02Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

02Z Update ... no major changes to previous TAFs. Previous
discussion below.

===========================================================

Tonight...High confidence

VFR conditions overnight with 3-8 kt winds, generally out of
the W/WNW. Will see SCT to BKN cloud cover with cloud heights
around 10-15 kft overnight as a shortwave swings through
northern New England.

Tuesday...High confidence

VFR conditions through this period with gusty winds developing.
Any of the mid clouds scatter out as high pressure re-asserts
itself into the region. Have hinted at fair weather CU
developing late in the morning, but looks more likely during the
afternoon per NAMNest/RAP Bufkit soundings. Ceilings should be
around 5-6 kft. Expect W to NW winds around 10 kts with gusts
between 15-20 kts. If the gusty winds do not develop or when the
gusty winds decouple am expecting sea-breezes to develop along
the eastern MA coast. Since the pattern is very similar have
timed gusty winds ending around the same time they ended today.

Tuesday Night...High confidence

Warm front brings in lower ceilings toward MVFR and potentially
IFR levels. Best opportunity for IFR is across the eastern
slopes of the Berkshires, much of CT and the south coast of RI.
Light rain spreading in. Best chance for any visibility
reductions would be across the south coast. W winds become
light and shift to the NE 4-8 kt.

KBOS Terminal...High confidence in the TAF. Confidence in the
wind direction lowers to moderate during the afternoon/evening.
Uncertain on when the sea breeze will develop and shift the wind
direction.

KBDL Terminal...High confidence in the TAF.

Outlook /Wednesday through Saturday/...

Wednesday: Mainly MVFR, with areas IFR possible. Chance RA.

Wednesday Night: MVFR/IFR conditions possible. Slight chance
RA.

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

Thursday Night: Mainly VFR, with local IFR possible. Windy with
gusts up to 35 kt.

Friday: Mainly VFR, with areas MVFR possible. Breezy. Slight
chance SHRA.

Friday Night: VFR. Breezy.

Saturday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA.

&&

.MARINE...

With high pressure in control, relatively quiet conditions overall
on the waters into Tuesday, with winds and seas below SCA
thresholds. Mainly looking at westerly wind gusts under 20 kt with
seas 2-4` on the outer waters and around 1-3 ft near shore.

A warm front will spread a period of light showers across the waters
later Tuesday night. Vsbys in rain should be 3-6 miles. While
seas should remain in the 2-4 ft range Tuesday night, some
enhancement to NE winds could bring a period of marginal-SCA
gusts to the eastern MA waters toward pre-dawn/early Wed.

Outlook /Wednesday through Saturday/...

Wednesday: Winds less than 25 kt. Rain likely.

Wednesday Night: Winds less than 25 kt.

Thursday: Low risk for gale force winds with gusts up to 35 kt.
Areas of rough seas. Rain likely.

Thursday Night: Low risk for gale force winds with gusts up to
35 kt. Rough seas up to 9 ft. Slight chance of rain showers.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Rough seas up to 9 ft. Slight chance of rain
showers.

Friday Night: Winds less than 25 kt. Areas of rough seas.

Saturday: Winds less than 25 kt. Local rough seas.

&&

.FIRE WEATHER...

Relative humidities this afternoon across the interior have fallen
into the 15 to 25 percent range with occasional gusts up to 25 mph.
An SPS for elevated fire weather potential continues until 6 PM,
with good recovery in RH expected after sunset as temperatures cool.

For Tuesday, with slightly less sunshine than today, lowest
afternoon relative humidities across interior MA, CT and RI project
around 25 to 35 percent, a little bit higher than today. Will also
have some west winds though gust speeds only around 20 mph or less.
These conditions are only slightly different than today. Another SPS
for elevated fire weather potential could again be needed for
tomorrow pending input from forestry partners.

&&

.TIDES/COASTAL FLOODING...

Astronomical tides will start to rise into mid to late week (across
multiple tidal cycles) with the upcoming Supermoon. The first period
to watch at least for some possible splashover would be Tuesday
night, at least for the eastern coasts. Astro tide for Boston Harbor
is 11.6 ft at 1143 PM Tuesday night, and is 3.7 ft for Nantucket
Harbor at 1241 AM Wednesday morning. While these tides are high,
poor wave action (seas under 4`) and weak onshore winds should
preclude a greater coastal flood threat. However, guidance from
Stevens Institute indicates a worst-case minor flooding scenario
Tuesday night for eastern MA. There may be a need for a coastal
flood statement for minor splashover Tuesday night for eastern MA,
especially if northeast winds late Tuesday night could be stronger
than currently projected and coincide near or around the
aforementioned Tuesday evening high tides.

Wednesday through Friday...

Very high astronomical tides continue into mid to late this
week. Best shot for onshore flow is on Wednesday, but wave
heights remain below 4 ft. Expect higher seas on Thursday with a
system moving in from the west and low pressure deepening
near/just north of the region. Think Thursday has the best shot
for any flooding across eastern MA and Nantucket in combination
with the Stevens Institute guidance. Seas remain elevated
across the south coast and eastern outer waters on Friday. Not
out of the question there are still some minor flooding issues
at Nantucket.

&&

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

&&

$$
SYNOPSIS...BL/Loconto
NEAR TERM...Nocera/Loconto
SHORT TERM...Loconto
LONG TERM...BL
AVIATION...Nocera/BL/Loconto
MARINE...BL/Loconto
FIRE WEATHER...staff
TIDES/COASTAL FLOODING...



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