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Area Forecast Discussion
Issued by NWS Boston, MA
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FXUS61 KBOX 241431
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1031 AM EDT Wed Apr 24 2024

.SYNOPSIS...
Showers and perhaps an isolated thunderstorm today with the
passage of a cold front. Dry but cold overnight, which prompt a
Freeze Watch for parts of Connecticut, Rhode Island, and
southeast Massachusetts. More seasonable conditions develop for
the weekend. Another chance for showers comes mid to late
weekend then again late Mon or Tue.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
1030 AM update...

Scattered light showers ahead of a robust mid level trough are
moving through SNE. These showers will exit by this afternoon
then will have to watch for development of scattered convective
showers along the cold front this afternoon. Partial sunshine
expected to develop this afternoon ahead of the front and
surface heating below steepening mid level lapse rates will
help to contribute to marginal instability with CAPES up to a
few hundred J/kg. This will help fuel scattered convective
showers along and just ahead of the cold front and can`t rule
out an isolated t-storm or 2. Small hail is possible given
steepening mid level lapse rates assocd with the cold pool
aloft. Timing of the convection will be 18-20z in the interior
and 20-22z across RI and SE MA, possibly a bit later over Cape
Cod.

Previous discussion...

Highlights:

* Showers and perhaps an isolated weak thunderstorm through this
  afternoon with near seasonable temperatures.

Robust mid-level shortwave and surface cold front will bring sct`d
light rain during the first-half of the day, followed by a secondary
round of sct`d to iso`d convective showers this afternoon. Showers
early this morning need to overcome surface dry air, we`ve watched
the dewpoint/temperature depression decrease with some activity of
light rain in far western Massachusetts. High-res models hint at the
first round of precip between 13z-19z, then the secondary convective
showers 19z-23z - all from west to east. SPC placed all of southern
New England in general thunder this afternoon, no risk for severe.
But, given the flow aloft is between 25-35 knots, at best a
convective shower could produce an iso`d gust between 30 and 40
mph. In general, QPF is a few hundredths of an inch to a tenth
of an inch, though any convective showers could produce locally
higher amounts up to a 0.25 inch.

Southwest winds this morning become westerly this afternoon behind
the cold front, with gusts to 20 to 30 mph. Afternoon highs in the
low 60s, slightly cooler at the immediate coast and higher terrain
of northern/western Massachusetts.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
Highlights:

* Freeze Watch expanded for Wednesday night, and then potential
  elevated fire weather concerns on Thursday.

As advertised, the overnight hours will be rather cold. Clouds clear
quickly as the cold front races off shore and PWATs fall. The wind
direction shifts, west to north, advecting an unseasonably cold air
mass over southern New England.

925mb temperatures range between -3C and -5C. As mentioned, the
previous forecaster brought up a good point, there are two ways
we can achieve unseasonably low overnight temperatures - one
being traditional radiational cooling - second is to mix down
the colder air aloft. Right now, most of the night looks to have
a well mixed boundary layer with steady north wind. This will
transport those colder temperatures aloft to the surface. By the
end of the night, we could decouple for the last two or three
hours - with clear sky cover this will help maintain and or
lower the temperatures further. Overnight lows are in the low
and middle 20s across northern Massachusetts, southern coast
could have overnight lows in the upper 20s to low 30s. For
temperatures, did us a blend of the forecast and the BC of the
MAV. Cape Cod and Islands is tough, do think the SST will help
to keep above freezing, but could be close. Just last night, MVY
saw the temperature freefall into the upper 20s with calm wind
and clear sky. During this update we expanded the Freeze Watch
to southern Rhode Island and south coast of Massachusetts. We
did omit the islands and Cape Cod. Day crew will need to
evaluate guidance and decide whether to go with a freeze warning
or frost advisory in the area where the frost/freeze program is
active.

Heading into Thursday we are dry but noticeably cooler with
highs only reaching into the middle 50s. A dry air mass settles
over the region thanks to 1030mb high moving south out of
Canada. Winds are northwest and less than 10 mph, possibly
becoming more light and variable by late afternoon. Elevated
fire weather concerns because of very low values of RH due to
low dewpoint temperatures in the teens. Minimum values of RH are
in the upper teens to upper 20s away from the coast.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Key Points...

* Dry weather through Saturday.

* Chance of showers Saturday night then again around Tuesday.

* Temperatures increase each day through Monday when some could see
  highs near 80.

Details...

Surface high pressure centered over New York Thursday night remains
in control through Saturday, crossing New England on Friday.
Subsidence beneath the high and very little moisture in the column
will lead to dry conditions and sunny skies each day through
Saturday (though clouds will begin moving in to start the weekend).
Given the high pressure overhead winds will be light and variable.
Thus, we`ll see a few nights of good radiational cooling with the
typical cool spots dropping into the upper 20s to low 30s. By Friday
the mid level ridge axis shifts over or just east of SNE with
low/mid level winds backing to the SW bringing warm advection. 850
mb temps rise from -3C on Thursday to 0C on Friday and +3C by
Saturday afternoon. This will translate to highs steadily increasing
each day, into the mid to upper 60s in the interior by Saturday.
Areas along the south coast will be cooler (50s) thanks to onshore
flow.

The dry pattern breaks down briefly for the second half of the
weekend as a passing warm front will bring not only increasing
cloudcover Saturday afternoon but the chance for widespread light
showers Saturday night. It will be trailing behind a surface low
which lifts into Canada; with little forcing and mid level ridging
overhead, not expecting much out of these showers. A more
substantial system looks to arrive with a shortwave on or around
Tuesday. Something of note for the start of the week will be the
very warm day on Monday (well into the 70s in the interior). The
chance for a backdoor front to spoil things is looking less likely,
though a seabreeze will likely keep those along the coast in the
60s. Meanwhile, the CT Valley may see temperature approaching 80F!
The odds temps AOA 80F are between 40-50% per the EPS/GEFS ensemble
guidance.

&&

.AVIATION /14Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

Today...Moderate confidence.

VFR through 12z, MVFR conditions develop west to east as a cold
front approaches the region with scattered rain showers between
14z and 18z. Localized IFR is possible. A second round of hit or
miss showers 20z to 23z could contain isolated risk of thunder.
Southwest winds shift to the west behind the frontal passage
mid day, winds gusting to as high as 20kt.

Tonight...High confidence.

Rapid clearing, becoming VFR from west to east behind the
departing front. Wind shift to the north/northwest 5 to 10
knots, though could remain gusty with winds to 20 knots along
the Cape and Islands.

Thursday...High confidence.

VFR. North/northwest winds 5 to 10 knots. Winds ease and
direction becomes variable late afternoon.

KBOS TAF...Moderate confidence.

Cold frontal passage brings showers to the terminal between
15z-19z. Low risk for TSRA between 20-23Z this afternoon before
rapid clearing. Winds are southwest this morning and west this
afternoon behind the frontal passage. Winds eventually become
north late tonight, winds will be greatest for coastal
terminals, up to 25kt.

KBDL TAF...Moderate confidence.

Cold frontal passage brings showers to the terminal between
14z-18z. Winds are south/southwest this morning become west
behind the front this afternoon, then north overnight. Low risk
for TSRA between 20-23Z this afternoon before rapid clearing.

Outlook /Thursday Night through Sunday/...

Thursday Night through Friday Night: VFR.

Saturday: VFR. Breezy.

Saturday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance SHRA.

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

&&

.MARINE...
Forecaster Confidence Levels:

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

Today through Thursday...High Confidence.

Rain showers and an isolated thunderstorm with a cold front passage
later today. Cold front will bring a wind shift from southwest to
west/northwest this afternoon, then becoming northerly overnight.
Wind gusts increasing 25 to 30 knots and increasing seas to over 5
feet, this lead to a Small Craft Advisory through late Thursday
morning. Surface high pressure comes down from Canada with easing
winds and diminishing seas for Thursday afternoon.

Outlook /Thursday Night through Sunday/...

Thursday Night through Saturday: Winds less than 25 kt.

Saturday Night through Sunday: Winds less than 25 kt. Slight
chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Freeze Watch from late tonight through Thursday morning for
     CTZ002>004.
MA...Freeze Watch from late tonight through Thursday morning for
     MAZ017-018-020-021.
RI...Freeze Watch from late tonight through Thursday morning for
     RIZ001>007.
MARINE...Small Craft Advisory from 2 PM this afternoon to 9 AM EDT
     Thursday for ANZ231>235-251.
     Small Craft Advisory until 3 PM EDT Thursday for ANZ250-
     254>256.

&&

$$

SYNOPSIS...BW/Dooley
NEAR TERM...KJC/Dooley
SHORT TERM...Dooley
LONG TERM...BW
AVIATION...BW/Dooley
MARINE...BW/Dooley
National Weather Service Regional Headquarters
