070
FXUS61 KBOX 201916
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
316 PM EDT Mon May 20 2024

.SYNOPSIS...

A ridge of high pressure sinks to our south tonight...setting the
stage for summerlike warmth Tuesday through Thursday at least away
from the immediate south coast. Generally dry weather is expected
Tuesday and Wednesday...but an approaching cold front may bring a
round of scattered showers and thunderstorms Thursday afternoon and
evening. This will usher in more seasonable but generally pleasant
weather for the Holiday Weekend.

&&

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

315 PM Update...

* Low Clouds & fog develop tonight with lows upper 40s/50s
* Fog may become dense across parts of RI/SE MA tonight

A ridge of high pressure across southern New England gradually
shifts to our southwest tonight...but remains in control of our
weather. Light southerly flow and a cooling boundary layer will
allow areas of low clouds and fog to redevelop from south to north
after sunset. Give that the surface winds will be light/calm...some
of the fog may become locally dense across RI/SE MA given 50+
dewpoints. It is possible later shifts may need to consider a Dense
Fog Advisory for a portion of this region.

Overnight low temps should bottom out mainly in the lower to middle
50s...but may see readings drop to between 45 and 50 towards the
Cape and Islands given lower dewpoints in that region.

&&

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

Key Points...

* Summerlike Warmth Tue away from the south coast
* Plenty of sun Tue with highs in the 80s NW of I-95
* Rather mild Tue night with lows in the 50s/lower 60s

Details...

Tuesday...

A ridge of high pressure off the mid-Atlantic coast will set the
stage for summerlike warmth on Tue...at least away from the cooling
marine influence near the south coast. S-SW surface winds should
allow the low clouds and fog patches to burn off earlier than today.
850T warming to +15C should allow highs to reach the middle 80s in
many locations northwest of I-95 and perhaps even a few spots flirt
with the upper 80s. The S-SW winds will keep things a bit cooler
southeast of I-95 which will probably hold highs in the 70s to near
80. In fact...on the very immediate south coast/Cape and Islands
highs will only be in the 60s and perhaps a struggle to reach 60 in
Nantucket. Bottom line though is summer-like warmth is on tap for
most of the region especially northwest of I-95.

A shortwave does track across northern New England. This coupled
with diurnal heating/instability should trigger some scattered
convection to our north. However...the dynamics/deeper moisture will
remain across northern New England. This should result in generally
a dry and warm Tuesday...but there is the risk of a brief spot
shower or two during the afternoon/early evening hours across
northwest and north central MA.

Tuesday night...

The ridge of high pressure off the mid-Atlantic coast will continue
to generate a mild southwest flow of air Tue night. While we
probably will see some low clouds and fog patches
redevelop...thinking is the main threat for these will be confined
towards RI/SE MA. The southwest wind component will probably limit
the northward extent compared to what we are expecting tonight. Low
temps Tue night will be mild...mainly in the 50s to the lower 60s.
The mildest of those low temps will be found northwest of I-95,
furthest away from the cooling marine influence given the southwest
wind trajectory.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...

Highlights

* Above normal temperatures this week with highs ranging from the
  low to upper 80s Wednesday through Friday

* A cold front may trigger strong to severe thunderstorms west of I-
  495 Thursday afternoon/evening.

* Seasonable/dry weather next weekend

Wednesday and Thursday

A broad upper-level ridge and associated surface high pressure over
the eastern US will support a persistent deep southwest flow into
The Northeast this week. As a result, the region is expecting well
above normal temperatures with 925 hPa temps above 20 degrees
Celsius through the end of the week. With diurnal mixing, this will
translate to surface high temperatures ranging from the low to upper
80s Wednesday through Friday. Wednesday will be the warmest day this
week, with many locations across the interior approaching the upper
80s to low 90s. Increasing cloudiness ahead of a cold front on
Thursday will result in slightly cooler temperatures with highs in
the low to mid 80s. The south coast, Cape, and Islands will be
substantially cooler with onshore flow from the cooler ocean keeping
highs in the 60s to low 70s Wednesday and Thursday.

In addition to the heat there will be a risk for thunderstorms on
both Wednesday and Thursday afternoon. Latest guidance suggest any
thunderstorm activity on Wednesday would be isolated an confined to
western MA. Forcing looks to be weak on Wednesday, but there will be
a modest amount of shear and instability in the atmosphere. Of
greater concern is Thursday afternoon when strong to severe storms
may be possible as a weak surface cold front pushes through The
Northeast. Plenty of moisture and instability will be in place with
dewpoints in the mid 60s and surface temps in the upper 80s to low
90s supporting SBCAPE values as high as 1000 J/kg. Furthermore, an
upper-level jetstream to the north will support deep layer shear
values between 30 and 40 knots. Should the values of these
parameters verify, the ingredients would be in place for some areas
of organized convection. Model guidance suggests the greatest height
falls and forcing for ascent would be focused across the interior
west of I-495. CIPS severe weather probability guidance is
indicating a 30 percent chance of severe thunderstorm winds and a 15
percent chance of severe hail. CSU machine learning models are also
hinting at severe thunderstorm potential with a 15 percent chance
across western MA and CT. So while the heat will catch a lot of
attention this week, we will also need to pay close attention to how
the forecast unfolds for Thursday afternoon/evening.

Friday

Temperatures remain above normal on Friday with highs in the upper
70s to low 80s. Though the air should be more comfortable than on
Wednesday/Thursday as dewpoints fall into the 50s behind a weak
surface cold front. The column dries out behind this front as well
with PWATs dropping to around 0.5 inches. This should result to a
sunny/pleasant end to the work week.

Next Weekend

Forecast looks to be on the quiet side for Saturday with a
dry/seasonable air mass supporting normal conditions across southern
New England for late May. Sunday may become more unsettled as model
guidance is hinting at high pressure offshore supporting an easterly
wind over the region. This could come with cooler temperatures,
clouds, and possibly light rain/drizzle. Details are vague at the 7
day time range, so stay tuned for more details later this week.

&&

.AVIATION /19Z MONDAY THROUGH SATURDAY/...

Forecast Confidence Levels:

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

18z TAF Update...

This afternoon...High Confidence.

Low clouds have scoured out to the northwest of I-95 yielding
VFR conditions early this afternoon. Still MVFR ceiling
southeast of I-95...but probably see temporary improvement at
least away from the Cape and Islands over the next few hours.
Light E winds generally becoming more SE-S in many spots toward
evening.

Tonight...High Confidence in trends but lower confidence in
specific timing.

Low end MVFR to IFR/LIFR conditions will ovespreads area near
the Springfield...to Worcester to Beverly corridor...but may
struggle to get too far northwest of that region. The greatest
risk for LIFR conditions will be across RI/SE MA where low
clouds arrive earlier and there also will be fog
developing...which may become locally dense. Calm/light S winds.

Tuesday...High Confidence.

Low clouds and fog patches should burn off Tuesday morning. This
process will probably be faster than today...given more of a
S-SW winds 10-15 knots. May also see a few hours of localized
sea breezes along the very immediate coast...but this should be
overcome during the afternoon as gradient strengthens.

Tuesday night...Moderate Confidence.

Low clouds and fog patches will probably redevelop Tue
night...but probably only impact areas south of I-90 with the
best chance across RI/SE MA. Surface winds out of the SW at 5-15
knots will probably keep the low clouds/fog from becoming as
extensive and spreading as far north as tonight.

KBOS TAF...High confidence in trends but lower confidence in
timing. Thinking low clouds with low end MVFR to IFR conditions
return after 02z or so tonight and burn off by mid-morning Tue.

KBDL TAF...High confidence in trends but lower confidence in
timing.

Outlook /Wednesday through Saturday/...

Wednesday through Wednesday Night: VFR. Breezy.

Thursday: VFR. Breezy. Chance SHRA, isolated TSRA.

Thursday Night: VFR. Breezy. Chance SHRA.

Friday: VFR. Slight chance SHRA.

Friday Night through Saturday: VFR.

&&

.MARINE...

Forecaster Confidence Levels:

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

Tonight through Tuesday night...High Confidence.

High pressure across the region this afternoon sinks south of the
region tonight. This high then remains off the mid-Atlantic coast
Tue and Tue night. This keeps the pressure gradient weak enough and
winds/seas should generally remain below small craft advisory
thresholds. That being said...we may see some 20+ knot S-SW wind
gusts Tue afternoon and Tue night so some choppy nearshore seas are
possible. The biggest concern for mariners though will be the
formation of fog tonight...which may become locally dense tonight
especially across the southern waters. This fog should burn off by
Tue afternoon.

Outlook /Wednesday through Saturday/...

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

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

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

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

Friday Night through Saturday: Winds less than 25 kt.

&&

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

&&

$$

SYNOPSIS...Frank/RM
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...RM
AVIATION...Frank/RM
MARINE...Frank/RM