Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 291415

Area Forecast Discussion
National Weather Service Boston/Norton MA
1015 AM EDT Wed May 29 2024

Dry weather through this morning and then scattered showers and
thunderstorms develop this afternoon, with rain chances
continuing into Thursday morning, with a drying trend in the
afternoon, west to east. Drier weather with plentiful sunshine
for Friday. After a few days of temperatures at or slightly
cooler than normal, the weekend into Monday offers a warming
trend toward above normal temperatures and still tolerable
humidity levels. Drier weather should generally prevail this
weekend into at least Tuesday.



10 AM update...

Not much change from previous forecast. Beautiful morning in
progress, with sunshine, pleasant temperatures and comfortable
dew pts in the upper 40s inland to the mid to upper 50s along
the south coast.

Cold air and cyclonic flow aloft beginning to result in diurnal
scu/cu forming. This cyclonic flow will combine with anomalous
cold air at 500 mb (-18C) to yield scattered afternoon
showers/thunderstorms. Departing upper jet level offshore will
provide 25-35 kt of deep layer shear over RI and southeast MA.
This shear combined with cold temps aloft may yield a few storms
producing small hail this afternoon. Seabreeze convergence will
help initiate convection on the leading edge of seabreeze
front, but SSTs in the mid to upper 50s will provide low level
stability along and near the coastline. Thus, highest chance for
stronger storms will be across interior CT/RI and southeast MA,
possibly up to the MA Pike.

Some moisture pooling taking place over CT/RI and south of the
MA Pike, with PWATs climbing to about 1.2 inch. Therefore,
brief heavy downpours possible. However, PWATs just over 1 inch
in late May is not very anomalous. Also, dew pts capped in the
50s today will also limit rainfall potential. HREF qpf max only
1.5 inches. While this could happen in a small window of 1-2
hours, not anticipating any flash flooding today, more
nuisance/minor flood potential. Otherwise,previous forecast
remains on track with highs 75-80, but somewhat comfortable
given dew pts in the 50s. Light WNW winds this morning become
light/variable this afternoon, locally onshore. Earlier
discussion below.



* Sunshine to start, scattered showers and storms this afternoon.

* Seasonable highs in the 70s with a few spots pushing 80F.

Sunshine this morning will give way to developing cumulus clouds and
scattered afternoon thunderstorms. BUFKIT forecast soundings show
steep low-level lapse rates, mid-late morning are around 9-10 C/km,
this should get diurnal clouds popping, away from the coast. HREF
shows modest MUCAPE over much of Connecticut, Rhode Island, and
southeast Massachusetts - in the ballpark of 500-750 J/kg. 00z CAMs
indicate that initiation of the convection around 16z and 18z. The
greatest Bulk Shear is over southeast Connecticut, Rhode Island and
southeast Massachusetts as well, this is where storms could become
locally stronger and given the cold air aloft, small hail with these
storms are possible. Will be an area to keep an eye on as mid-level
lapse rates are around 6.5C/km and may lead to an over performing
storm or two. Areas northwest likely to have sub-severe storms. As
we get late into the afternoon and lose the daytime heating, storms
begin to weaken and we dry out temporarily. Not everyone will see a
shower or storm this afternoon, given the expected scattered nature.
With PWATs between 1.0 and 1.2, which is near normal for the end of
May, these storms could produce locally heavy rainfall. HREF six
hour (18z-00z) ensemble max does show very isolated pockets of 1.0
to 1.5 inch, more or less as a worst case scenario. While the mean is
around 0.5 inch. It is worth mentioning, WPC does have Connecticut,
Rhode Island and southeast Massachusetts in a marginal risk for
excessive rainfall for a 24 hour period, 12z today to 12 Thursday.
The risk here would be the potential for any training storms.

Afternoon highs are generally 75-80 degrees, with the immediate
coast in the low 70s.



* Weak surface low pressure brings second round of rain overnight
  into Thursday, along with below normal temperatures.

Our next round of rain comes through late tonight and into part of
Thursday, this second round will be more synoptically forced as a
weak surface low pressure interacts with a cold front. There seems
to be good consensus among the GFS, ECMWF, and GEM - showing more or
less widespread rainfall sometime after 06z. That said, the NAM12km
and NAM3km are less in line with the other global models, and trends
to a drier forecast, with most the rain occurring along the southern
coast of CT/RI/MA.

PWATs are still in the neighborhood of 1.0 to 1.2 inches with good
forcing, we should be able to ring out showers overnight into
part of Thursday. Limited instability, mainly over the southern
waters could allow for thunder overnight into early Thursday
morning too. Surface low and mid-level shortwave exit east of
the waters Thursday afternoon as dry air is advected in from
the west - thus ending the threat of showers. In all, an
additional 0.25 to 0.50 of precip is possible, with the greatest
totals likely south and east of BDL-ORH-BVY. Locally higher
amounts are possible. HREF six hour ensemble max, does indicate
the potential for isolated amounts of 1.5 to 2.0 inches, this
would be the reasonable worst case scenario.

Will need to be vigilant if these downpours develop in areas
prone to urban and poor drainage flooding. As I mentioned above
WPC kept our region in a marginal risk for excessive rainfall
ending Thursday morning for Connecticut, Rhode Island and
southeast Massachusetts.

Temperatures tonight hold int he mid 50s, while Thursday`s highs
will largely depend on how much cloud cover and the duration of the
rain. In addition, winds become northerly across the western half of
the CWA while eastern half has more of a north/northeast wind. With
the wind off the water, this would promote cooler temperatures.
Thinking highs in eastern MA and RI are in the low to middle 60s.
And western MA and CT are in the upper 60s and low 70s.


Key Points...

* Temperatures will be increasing steadily each day through next

* An extended stretch of dry weather is expected Friday through at
  least Tuesday.


A pattern change is on its way to SNE this weekend as a deep mid
level trough is replaced by a ridge of high pressure. This will
bring both warmer and drier weather to the region. Friday is a
transition day, as surface high pressure nudges into the region
beneath the lingering trough. This leads to plenty of sun and warmer
temperatures. Inland temps will be very comfortable, in the mid 70s,
while sea breezes will keep locations along the coast cooler. As we
get further into the month, though, ocean temperatures also continue
to warm, with temps as warm as 60F near Boston harbor which will
decrease the amount of cooling compared to April or early May.

For the weekend expect more of the same as heights rise and mid
level ridging moves overhead while surface high pressure looks to
remain in place through at least Tuesday. 850 mb temps around +7C on
Friday will increase to around 15C by Tuesday, so expect steadily
warming temperatures and marginally more humidity as we go into next
week. High temps should rise above average, into the mid 80s for
inland spots by Monday and Tuesday. Ensemble guidance indicates only
a 30-40% chance of temps AOA 80F for Sunday but odds increase to 70-
80% for Monday and Tuesday.


Forecast Confidence Levels:

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

15z TAF Update: no significant changes to previous TAFs.
Scattered afternoon showers and thunderstorms, most numerous
across CT/RI and along/south of the Mass Pike. A few stronger
storms may contain small hail and brief heavy downpours between
noon and 8 pm. Given dew pts are in the 50s, not expecting any
worse than MVFR conditions in showers and storms. Light WNW
winds become light/variable this afternoon, except locally
onshore. Earlier discussion below.


VFR through at least early tonight. Isolated -TSRA expected this
afternoon, mainly 18z-23z across CT, RI, and MA near and south
of I-90, where we are including PROB30 but any activity will be
widely scattered in nature. One or two storms could produce
small hail but strong winds not expected. Sea breezes get
underway along E Coastal MA including BOS around 16z (E) and
along South Coast 17z-19z (SW).

Winds shift to N tonight 10kt or less which may give way to
MVFR ceilings toward daybreak Thu along with light rain. Higher
confidence is near BDL/BAF but should impact more of area during
day Thu. Ceilings could lower to IFR near Islands but more
confident in lower-end MVFR for now. Should see improvement to
VFR Thu afternoon.

KBOS TAF...Moderate confidence (60%). Scattered TS more likely
to remain south of terminal but due to uncertainty kept Prob30
going from 19z-23z. Higher confidence on timing of sea breeze.

KBDL TAF...High confidence (70%). Scattered TS may be more
focused in eastern CT but due to uncertainty kept Prob30 going
from 18z-22z.

Outlook /Thursday Night through Sunday/...

Thursday Night through Sunday: VFR.


Forecaster Confidence Levels:

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

Wednesday through Thursday...

Today: Outside of some patchy marine fog around the Island of
Nantucket do expect a fairly sunny day with scattered showers and/or
thunderstorm during the afternoon. Most numerous near shore and may
contain small hail. Seas and winds diminish. Over the southern
waters wind direction is southwest less than 20 knots, while eastern
waters direction is out of the south less than 15 knots. In/around
Boston Harbor winds are southeast to east southeast and are 10 to 15
knots. Near shore, waves are generally less than 2 feet, while outer
waters are 3-4 feet.

Tonight: A renewed threat for rain overnight, perhaps embedded
rumbles of thunder, with the greatest concentration over the
southern waters a weak low pressure moves in. Southwest winds
becoming northerly by the end of the night. Strongest winds are
concentrated over the northeast waters - adjacent to Cape Ann, but
are still well below advisory criteria wind gusts less than 20

Thursday: Rain showers linger across the waters as weak low pressure
system moves east. Northeast winds 10 to 15 knots, seas 2-4 feet.

Outlook /Thursday Night through Sunday/...

Thursday: Winds less than 25 kt. Chance of rain showers.

Thursday Night through Saturday Night: Winds less than 25 kt.

Sunday: Winds less than 25 kt. Seas locally approaching 5 ft.