820
FXUS61 KBOX 162321
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
721 PM EDT Fri May 16 2025

.SYNOPSIS...
Weakening low pressure will bring scattered showers or thunderstorms
at times through the end of the week as it reaches New England, but
a majority of the time will be dry. A cold front moves through
Saturday night behind the storms, drying and cooling the region.
Even with mostly dry conditions, scattered showers remain
possible Sunday with a cold pool aloft, then the pattern turns
unsettled again toward midweek.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Key Messages:

Key Messages:

* Isolated showers and storms possible late afternoon-early evening,
  mainly for western MA and CT.

* Low clouds and patchy fog develop tonight.

Clouds have scattered out in most spots this afternoon which has
helped support some marginal instability. SPC mesoanalysis shows
around 500 J/kg of MLCAPE across west and central MA. Although
lacking in much forcing, this should be enough to support the risk
for a isolated shower or storm in those mentioned areas late this
afternoon through early evening.

Main story for tonight will be low clouds and patchy fog. Similar
conditions to last night with a moist airmass and light winds to
support the re-development of low stratus and areas of patchy fog.
Can`t rule out an isolated shower or sprinkle overnight.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY/...
Key Messages:

* Fog and stratus develop again tonight, and it may linger for
  quite a while into Saturday.

* Possible round of scattered t-storms Saturday morning in most
  areas.

* Better chance at t-storms between 2-8 PM west of Worcester.
  Some could become strong.

Details:

An upper level low and trough slowly approaches southern New England
from the Great Lakes region. This will generate better synoptic
support for ascent across the region. The already moist airmass will
provide a primed environment for convective showers and
thunderstorms and potentially even a couple strong to severe ones
later on in the day.

The forecast is (lack of a better word) "messy" for tomorrow. This
owes to the very marginally unstable and sufficiently moist airmass
that prevails Friday night and Saturday. High resolution guidance
has been having difficulty in resolving showers/storms accurately in
timing/location over the past few days. Given this marginal airmass,
it hasn`t taken much to trigger a shower or storm with a little bit
of instability and weak lift. The suite of high resolution guidance
continues to lack consensus among each other with the details of the
convection on Saturday (timing, location). Some models show the
potential for some elevated showers and storms in the morning on
Saturday over portions of southern New England with lightning and
brief heavy downpours as the main threat.

The next element to consider will be how well we clear out. Most
model guidance shows that the morning low stratus will be
slow/gradual to improve with breaks in the clouds developing in the
afternoon across west and central southern New England. This means
it will take time for instability to build. If we can break the
cloud cover enough, models show a favorable environment for
thunderstorms and a couple strong to severe storms as well. If
clouds stick around too long, we may not destabilize in time for
peak heating while ahead of the approaching cold front.

This has been signaled pretty consistently among the CSU, NSSL ML
guidance. Moisture is plentiful with decent boundary layer
moisture as well given dewpoints in the low to mid 60s. As a
cold front approaches from the west, this will steepen the mid-
level lapse rates and support MLCAPE values in the 500-1100 J/kg
range across west and central southern New England. Sufficient
shear as well, especially later in the afternoon as a low level
just pushes in from the west. This should be enough to support
multi cell convection with a few of those storms strong to
severe with damaging wind and large hail (~1"). Can`t rule out
the low risk for a tornado as well with low LCLs, steep low
level lapse rates, and sufficient 0-1 helicity.

Another element to the "messier" forecast will be the timing of the
convection. There isn`t a lot of agreement among high resolution
guidance on the timing of the convection. Overall, there will be a
round or two of thunderstorms that develop across western southern
New England mid afternoon through early evening. The second round of
storms will likely hold the greater risk for severe with the HREF
showing a line developing across western MA early evening moving
eastward. So bottom line, the risk for thunderstorms in west and
central New England increases in the afternoon with the higher
chance for seeing strong to severe storms late afternoon-early
evening. It is that time of year where there are plenty of outdoor
events, so stay updated on the forecast and stay aware of incoming
weather.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Key Messages

* Mild day Sunday with a risk for some scattered showers across
  interior southern New England

* Warm/dry day on Monday with seasonable temperature

* Trending cooler Tuesday through the end of next week with daily
  highs/lows in the upper 50s/lows and mid to upper 40s
  respectively

* Mid-week coastal storm may bring substantial precipitation and
  gusty winds to southern New England.


Sunday and Monday

An upper-level low moves over southern New England on Sunday. Cold
pool aloft and some elevated instability may produce a few scattered
showers in the afternoon mainly across the interior of southern New
England. Otherwise expecting mild temperatures in the upper 60s-low
70s. The associated upper trough axis shifts east of southern New
England Sunday night into Monday supporting height rises and a
dry/mild day on Monday with seasonable high temperatures in the
upper 60s to low 70s.

Tuesday

As the aforementioned upper-level low shifts east of southern New
England Monday into Tuesday, a deep northwest flow pattern develops
over southern New England. This will support cooler/drier conditions
on Tuesday with high temperatures likely confined to the low 60s and
perhaps mid 60s in the CT River Valley.


Wednesday - Friday

By Wednesday, short-wave energy digging south over The Great Lakes
will likely support the development of a coastal low-pressure system
over The Mid-Atlantic states. This system is forecast to travel up
the east coast and produce a late spring Nor`Easter for southern New
England. This would mean a period of gusty northeast winds and
substantial rainfall. Model guidance differs on the timing with some
ensemble members supporting a faster solution that would result in a
wet Wednesday afternoon. Other members support a late arrival with a
mostly dry day Wednesday followed by a wet Wednesday night/Thursday.
As the system departs the region toward the end of next week we`ll
be left with broad cyclonic flow aloft that will support
cool/unsettled weather into next weekend. Stay tuned for more
details.

&&

.AVIATION /23Z FRIDAY THROUGH WEDNESDAY/...
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

00Z TAF Update

Tonight: Moderate confidence.

IFR-LIFR ceilings and visibilities return from the coastal
waters mainly after 00z. Low chance of an isolated SHRA after
06z.

Saturday: Moderate confidence overall, but lower on timing both
sub-VFR improvement and on timing SHRA/TS.

IFR-LIFR stratus likely to begin Sat. Slow improvement late
morning through early afternoon with MVFR, improving over the
west terminals first then eastward. West terminals improve
toward VFR mid afternoon. Lower confidence in timing of further
improvements across the east terminals in the afternoon, likely
not until late afternoon if it does scatter out. Convective
showers/storms possible in the morning 13-16z, but will be
isolated- widely scattered. Another round for western airports
after 18z and closer to 00z. Timing lower confidence. Some
storms could become strong, with a better chance for strong
activity in the 2nd round of storms for BAF- BDL- ORH. Winds
mainly SE to S around 5-10 kt.

KBOS TAF...Moderate confidence. IFR cigs move in after 03z with
lowering visibilities from BR. Slow improvements in ceilings
Saturday morning with MVFR cigs. Low chance for convective
showers 14-16z around the terminal. SE winds 5-12 kts.

KBDL TAF...Moderate confidence. IFR/LIFR overnight. Gradual
improvements after 15z toward MVFR and VFR later in afternoon.
Few rounds of thunderstorms possible in the afternoon and early
evening. Timing lower confidence. Few stronger storms possible,
more likely with the early evening round of storms.

Outlook /Sunday Night through Wednesday/...

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

Monday: VFR. Breezy.

Monday Night: VFR. Breezy. Slight chance SHRA.

Tuesday: VFR. Breezy.

Tuesday Night: Mainly VFR, with areas MVFR possible.

Wednesday: MVFR/IFR conditions possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

High confidence overall.

Fog and reduced visibility will be the biggest potential hazard
for mariners tonight, with S winds around 10 kt and seas 4 ft
or less. Risk for thunderstorms could develop as soon as pre-
dawn Saturday over the southern waters, with another round of
storms possible into Saturday afternoon. Winds 10-15 kts out of
SE Saturday switching to W Saturday night. Waves across southern
outer waters increase to 4-5 ft Sunday which will need SCAs as
we get closer.

Outlook /Sunday Night through Wednesday/...

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

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

Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers.

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

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

&&

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

&&

$$

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