Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 280543
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
143 AM EDT Thu May 28 2020

.SYNOPSIS...
High pressure offshore will result in continued summerlike warmth
and mugginess away from the south coast for the rest of the work
week. More humid weather conditions ahead of a cold front will
lead to an increasing threat of showers and thunderstorms later
Friday into Friday night, possibly lingering into Saturday.
Cooler and much drier weather then returns for late in the
weekend into early next week. After Saturday, temperatures trend
cooler than normal into the first couple days in June.

&&

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

130 AM Update...

Forecast is on track so no changes made.

PREVIOUS DISCUSSION...

10 PM Update...

Nighttime microphysics satellite imagery shows fog bank on the
South Coast and the Cape and Islands spreading NNE thanks to
the predominant SSW near-surface flow. Near-term Bufkit
soundings show a well-mixed enough boundary layer up to 975 mb
away from the immediate South Coast to reduce the likelihood of
the widespread fog we saw last night. As the fog spreads
northeastwards, it will tend to become more of a low stratus
deck as opposed to ground fog. As such, the current Dense Fog
Advisory coverage looks good. Nonetheless, will have to monitor
the trends through the overnight hours to see if the Advisory
needs to be expanded. A short-fused SPS to cover the possibility
of patchy dense fog is also an option later tonight.

Otherwise, current forecast is largely on track. The main change
was to increase the dew points in the near-term to better match
the obs.

745 PM Update:

Fairly nice early-evening across a large part of Southern New
England with south-southwest winds around 10 mph. However
webcam images and surface obs across the South Coast, Cape Cod
and the Islands show persistent fog and low clouds/stratus where
the Dense Fog Advisory is in effect. While stratus to eventually
expand north and west, following the latest 18z guidance
appears stronger (just enough) boundary layer winds should
preclude dense fog. No expansion to the dense fog advisory was
needed. A more possible area for fog in the interior is in the
CT Valley, which did see areas of fog last night but here too,
dense fog doesn`t look likely.

Otherwise, a fairly mild night with lows upper 50s to mid 60s,
generally coolest towards the South Coast and the Cape.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM FRIDAY/...
Thursday...

Areas of fog focused near the south coast will burn off Thursday
morning. However, model cross sections indicate a low more low level
moisture persisting through the afternoon. So even though
visibilities should improve, considerable cloudiness should persist
but a few peeks of sunshine will also occur especially in northeast
MA. Other than areas of drizzle in the morning, there is still not
much synoptic scale forcing/deep moisture for precipitation.
However, a brief passing shower or two is possible in western MA and
northern CT.

The abundance of clouds should keep afternoon high temperatures
mainly in the upper 70s to the lower 80s with perhaps a few middle
80s in the Merrimack Valley. Dewpoints in the 60s though will make
it feel somewhat muggy. Cooler conditions are expected on the south
coast, Cape and Islands where southwest winds blowing off the
relatively cold ocean will keep afternoon temps mainly in the middle
60s to the lower 70s.

Thursday night...

A rather mild and muggy night is on tap for the region. Low level
moisture will continue to be on the increase. As a result, overnight
low temps should only bottom in the middle to upper 60s in most
locales. The marine influence given southwest flow will result in
somewhat cooler overnight low temps in the upper 50s to the lower
60s on the south coast, Cape and Islands.

Low clouds will be widespread with ample low level moisture. Some
more fog is expected too, focused again near the south coast. A
modest southwest low level jet with some elevated instability will
also result in the risk of a few showers, particularly after
midnight near the south coast.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Highlights...

* Turning a bit more humid Friday. Increasing chances for showers
  and thunderstorms ahead of a slow-moving cold front late Friday
  into Friday night. Some potentially with heavy downpours.
  Possibility front may linger into Saturday with uncertainty in
  when front clears.

* Cooler and much drier conditions return for Sunday with high
  pressure building in.

* First couple days in June look dry with plenty of sun, but cooler
  than average temperatures and low humidity levels.

Details...

Friday into Saturday:

Bermuda high both sfc and aloft shifts to the east, allowing for
feed of southerly moisture. Biggest uncertainty in this period
continues to be the timing of the cold front, and that will end up
being critical in determining outcomes especially when it comes to
PoPs/Wx. Recent guidance has trended slower and given an amplified
pattern, slower trend may be favored.

Will focus higher PoPs tied to the front into the late Friday night
into Saturday timeframe. Given the very humid air mass (PWATs over
1.75" given persistent SW flow), showers and thunderstorms may be
capable of localized downpours. If frontal timing comes in during
the peak-heating hours on Saturday, the threat of stronger
thunderstorms would stand to increase (slower NAM has 40 kts of
effective shear and about 1000 J/kg of most-unstable CAPE). This
still appears too uncertain at this point but will need to be
monitored in the coming days.

Less cloudiness will lead to cooler high temperatures in the 70s to
mid 80s each day. However dewpoints look to be in the mid to upper
60s in the warm sector ahead of the cold front, and that will be
raising the humidity levels to summerlike levels. This will also
lead to fairly muggy conditions Friday night in the low-mid 60s.

Sunday:

Better conditions for Sunday as high pressure starts to nose into
the region. Plenty of sun and northwest breezes will allow for
falling dewpoints leading resulting comfortable humidity levels
(dewpoints in the 40s to lower 50s). Much cooler night Sunday night
with at least very good radiational cooling progged under clear
skies and a dry air mass. Lows Sunday night in the 40s in the
interior with lighter winds, and into the low 50s near the coast
where northerly winds may be a touch stronger.

Monday into Tuesday:

First couple days of June 2020 project cooler and much drier per
guidance consensus, as a large upper trough establishes itself over
New England. High pressure and northerly flow will afford plenty of
sun and very comfortable humidity levels (dewpoints in the 40s to
lower 50s) and potentially sizable diurnal temperature ranges.
Guidance does vary on how cool 850 mb temps get, with the ECMWF
offering values as low as 0 to -3C, but could be as mild as +3C.
Despite this variance, pleasant but certainly not very summerlike
with below-average high temperatures in the upper 60s and lows in
the upper 40s to mid 50s.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

0530z Update...

No major changes made to the 00z TAFs.

PREVIOUS DISCUSSION...

00z TAF Update:

Rest of Tonight: High confidence on trends, lower on exact
timing.

Initially IFR to VLIFR in fog and stratus towards the South
Coast/Cape and Islands, with VFR through at least 03z in the
interior and towards BOS/BED. Deterioration to MVFR-IFR levels
mainly from stratus thereafter (with fog at BED and the CT
Valley where wind speeds project lightest). Continued IFR to
VLIFR fog and stratus for the balance of the night across the
South Coast/Cape and Islands.

Thursday...Moderate confidence. Low clouds and fog early
Thursday morning will undergo gradual improvement in many areas
into mid afternoon. Unlike today though, many areas may see
MVFR cigs persist through much of the afternoon.
Best chance of improvement to a period of VFR conditions will be
across northeast MA. SW winds of 10 to 15 knots with some lower
20 knots possible on the coastal plain during the afternoon.

Thursday night...Moderate to high confidence. Mainly MVFR-IFR
conditions in lower clouds and fog patches anticipated. A few
showers are possible too especially after midnight. S winds 5
to 15 knots.

KBOS Terminal...Moderate to high confidence in TAF. SW wind
should mitigate fog but stratus (possibly at LIFR levels at
times) between 04-12z.

KBDL Terminal...Moderate to high confidence in TAF.

Outlook /Friday Night through Monday/...

Friday: Mainly MVFR, with local IFR possible. Breezy. Chance
SHRA, isolated TSRA.

Friday Night: Mainly MVFR, with areas IFR possible. Breezy.
SHRA likely, isolated TSRA.

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

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

Sunday through Sunday Night: VFR.

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

&&

.MARINE...

130 AM Update...

Tonight through Thursday night...High confidence. The main concern
for mariners will be for areas of dense fog especially during the
overnight to mid morning hours. Some nearshore lower 20 knot wind
gusts are expected near the south coast Thursday afternoon.
Otherwise, winds/seas should pretty much remain below SCA thresholds
but persistent long SW fetch may yield some 5 foot seas toward
daybreak Friday across our southern most waters.

Outlook /Friday Night through Monday/...

Friday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Seas up to 5 ft. Chance of rain showers, isolated
thunderstorms.

Friday Night: Winds less than 25 kt. Areas of rough seas. Rain
showers likely, isolated thunderstorms.

Saturday: Winds less than 25 kt. Local rough seas. Chance of
rain showers, isolated thunderstorms.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Slight chance of rain showers.

Sunday: Winds less than 25 kt. Seas up to 5 ft.

Sunday Night through Monday: Winds less than 25 kt. Areas of
seas approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Dense Fog Advisory until 8 AM EDT this morning for MAZ020>024.
RI...Dense Fog Advisory until 8 AM EDT this morning for RIZ005>008.
MARINE...None.

&&

$$
SYNOPSIS...Frank/Loconto
NEAR TERM...Frank/Loconto/Chai
SHORT TERM...Frank
LONG TERM...Loconto
AVIATION...Frank/Loconto
MARINE...Frank/Loconto



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