126
FXUS61 KBOX 210256
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1056 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/...
Main forecast challenge overnight is the northward extent of
stratus and fog. As of this writing, this stratus had just
begun to reach the South Shore, as well as the South Coast of
MA. Much of the Cape and islands were already blanketed as well.
Still have high confidence in stratus overspreading much of RI
and southeast MA. However, have lower confidence in the timing.
Will be monitoring that overnight, as it will have implications
for temperatures during the day tomorrow.

Minor tweaks to bring the forecast back in line with observed
trends.

315 PM Update...

* Low Clouds and 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 /03Z TUESDAY THROUGH SATURDAY/...
Forecast Confidence Levels:

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

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 as
S/SW winds increase to 10-15 knots just after sunrise. 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 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...Moderate Confidence

High confidence in IFR ceilings developing around/after 06Z.
There is a low risk for LIFR ceilings around 400 feet after 09Z
along with potential for MVFR vsbys in patchy fog near the
terminal. Can`t rule out a possible sea-breeze for a brief
period Tuesday afternoon, but confidence is low given pretty
strong southwest winds so have left out of TAF for now.

KBDL TAF...High confidence

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...Belk/Frank
SHORT TERM...Frank
LONG TERM...RM
AVIATION...Frank/RM
MARINE...Frank/RM