NWS Boston/Norton, MA wrote its last area forecast discussion before the first event at Jun 21, 12:56 PM EST, 1 hour 48 minutes earlier.
Area forecast discussion, Jun 21, 12:56 PM EST, 1 hour 48 minutes before
Area Forecast Discussion
National Weather Service Boston/Norton MA
156 PM EDT Sun Jun 21 2020
.SYNOPSIS...
Father's Day features a rather persistent forecast. We expect
above normal temperatures and the isolated risk for a shower or
thunderstorm. Very warm and humid weather continues through the
middle of next week. A cold front brings greater chances for
showers and thunderstorms Wednesday night or Thursday. Warm
temperatures and lower humidity levels Friday and Saturday.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
145 PM Update...
Stratus had lifted across the south coast and Cape, but lingered
at times over Nantucket. There may be periods of some low
stratus on Nantucket during this afternoon. Elsewhere, a mix of
diurnal cumulus and also cirrus moving into our area from the
offshore low bring mostly to partly sunny skies. Forecast highs
look on track, upper 80s to low 90s across much of the area,
cooler along the immediate shoreline where local seas breezes
will continue.
With dew points in the 60s, the humidity will be noticeable.
Widely scattered slow-moving showers and thunderstorms possible
inland this afternoon/evening.
Early morning discussion follows...
* Another warm and muggy day, cooler on the S Coast. Sea breeze
could also develop on the E MA coast to keep highs in the mid 80s
instead of low 90s.
* Scattered rain chances with an isolated heavy downpour or
thunderstorm in the afternoon, especially over interior MA and CT.
Fog has developed along the Cape and Islands as well as the fog-
prone areas in the interior river valleys, although the latter is
more isolated in nature. Expect fog to burn off rather quickly after
sunrise. However, expect partly to mostly cloudy conditions on the
Islands and perhaps parts of the immediate South Coast and the Cape
as well thanks to an elongated 582 Dm upper low moving off the Mid
Atlantic coast, tracking east and becoming closed off near the
70W/40N benchmark. The upper low appears too far offshore for any
meaningful rainfall on land but it will help keep temperatures lower
on the Cape and the Islands. In fact, the Islands and parts of the
immediate South Coast would probably stay in the 70s the whole day.
Further north and inland, rising heights from a building upper ridge
means yet another warm and muggy day is in store. NAEFS Ensemble
Situational Awareness table shows the 850mb and 1000mb temperatures
between the 90th and 99th percentile of model climatology. Bufkit
soundings show mixing up to 800mb, meaning temperatures could yet
again overperform in areas removed from marine influence. With the
wind field aloft being very weak (5 kts wind up to 300 mb), expect
sea breezes to develop along the Eastern MA coast despite the deep
mixing. So this would likely put a cap to afternoon highs for places
like Boston, which should see highs top out in the mid 80s instead
of today's low 90s. With another day of south to southwest flow,
expect northern MA to be the warm spots again and South Coast to
stay relatively cool thanks to SSTs being in the mid to upper 60s.
With 850mb temperatures of +15 to +17C, expect highs in the mid 80s
to low 90s for much of the inland area.
Also like on Saturday, expect scattered showers with localized
downpours and even a thunderstorm to develop during the afternoon
hours, with best chances over Western MA and CT, as well as northern
MA. Without any significant forcing, the main trigger mechanisms for
any storms that develop looks to be differential heating between the
higher terrain and valleys or lower elevations as well as the
outflow boundaries that form from decaying storms. While SBCAPE
could rise to 1000 J/kg, there is almost non-existent bulk shear and
helicity. Any storms that form would be pop-up and isolated in
nature and would be relatively short-lived as they rain on
themselves and shut off the updraft. And thanks to the near-
stationary nature of the upper level flow and PWATs between 1.25 and
1.5 inches, any storms that form could produce locally heavy
downpours. In fact, our Skywarn observer relayed to us that Orange,
MA saw 0.6 inches of rain in just 20 minutes on Saturday. Overall,
there is no need to cancel any outdoor plans for Father's Day but do
stay weather aware, especially in the afternoon hours for any
scattered heavy downpour or thunderstorm that may form.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
345 AM Update...
Tonight...
More widespread coverage of clouds and fog expected thanks to a 580
Dm closed upper low drifting close to the 70W/40N benchmark. Weak
boundary layer flow would also help to favor fog development thanks
to a weak pressure gradient. Besides the South Coast and the Cape
and Islands, expect areas of patchy fog along the South Shore and
Cape Ann. Otherwise, expect another mild and muggy night with lows
in the 60s.
Monday...
* Pretty much a repeat of Sunday but warmest locations look to be in
the CT River valley with highs in the low 90s. Eastern MA should
top out in the mid 80s thanks to clouds and sea breeze. Again,
coolest spots on the South Coast and the Cape & Islands.
Monday looks to be almost a repeat of Sunday given that we still
have the same air mass in place. The closed upper low moves further
offshore but does not make too much progress and the cyclonic flow
continues to send lobes of vorticity towards Eastern MA. Whereas in
the interior over Western MA and CT, building heights should mean
another toasty day, perhaps warmer in the interior than on the
coast. The wind fields look weak again, which means that sea breezes
should again develop over the Eastern MA coast. With mixing again up
to 800 to 850mb, we again look to 850mb temperatures to forecast
temperatures at the surface. Again, very similar temperatures as on
Sunday with +15C to +18C, higher over the interior thanks to the
rising heights. Overall, this means that the CT River valley should
see highs top out in the low 90s. Likewise for northern MA away from
the coast thanks to downsloping southwest winds leading to
compressional heating. Over the Eastern MA coast, highs should top
out in the mid to upper 80s thanks to likely sea breeze development.
And like Sunday, expect scattered showers and thunderstorm chances
in the afternoon with localized brief heavy downpours possible.
&&
.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Big Picture...
Upper low from Manitoba moves slowly east much of the week,
eventually moving through Quebec late in the week. This feature
influences our weather for most of the week, with smaller-scale
shortwaves moving through the flow.
Contour heights remain above normal Tuesday and Wednesday, then
lower to normal the rest of the week. This suggests warmer than
normal temperatures through Wednesday, then temperatures
closer to normal Thursday through Saturday.
Model mass fields are similar through Wednesday, then develop
differences Thursday through Saturday. The GFS is slowest, the GGEM
fastest. Confidence is high through Wednesday, then moderate the
remainder of the week.
Daily Concerns...
Monday night...Any showers taper off around sunset. Skies will be
mainly clear with light southwest wind. With dew points in the 60s,
expect min temps to be in the 60s to near 70.
Tuesday... Upper ridge overhead during the morning, but this moves
offshore during the afternoon. PW values around 1.5 inches and
lifted index at -3 to -4. So signs of moisture and instability, but
favorable dynamics will be out over the Great Lakes. Will favor a
mostly rain-free day, but with scattered to widely scattered
thunderstorms possible over western and central MA and northern CT.
The mixed layer reaches 800 mb, with temps 13-15C...equiv to 18-20C
at 850-mb and supporting 90-95. This is much lower than guidance, so
the forecast will nudge values up several degrees with a range of
upper 80s to mid 90s.
Wednesday-Thursday... Upper trough moves east, with the right
entrance region of the supporting upper jet reaching New England
either late Wednesday or Thursday. These will push a surface cold
front east from the Great Lakes and into New England, reaching
Southern New England Wednesday night or Thursday. PW of 1.5 to 2.0
inches and LI of -3 to -5 each day all favor scattered
showers/thunderstorms over a broader portion of Southern New
England both days. Temperatures at 850-mb will be 17-18C
Wednesday and 16C Thursday. This suggests highs around 90
Wednesday and mid to upper 80s Thursday.
Friday-Saturday... If the cold front moves offshore fast enough, as
per the GFS, then Friday will be dry weather. If it lingers, there
may be again be a chance of afternoon showers/tstms. On Saturday,
another shortwave approaches from the west, bringing a chance of
showers/tstms. As it is so far out in the future, timing of the
shortwave and its effects will probably vary. But the forecast
offers a first estimate.
&&
.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...
Low - less than 30 percent. Moderate - 30 to 60 percent. High -
greater than 60 percent.
18Z Update...
Through 00Z...High confidence, except moderate confidence Nantucket.
VFR with local seabreezes. Periods of IFR stratus possible ACK.
Isolated showers or a thunderstorm cannot be ruled out especially
over interior MA/CT terminals during the afternoon hours.
Tonight...moderate confidence.
Stratus CIGs/VSBYs developing/lowering to MVFR/IFR by 00Z ACK
and south coastal RI/MA and Cape Cod by 06z. VFR elsewhere in
the interior MA and CT terminals, with patchy MVFR/IFR vsbys in
fog. There is a low probability of stratus along and offshore of
coastal Maine dipping southward into the North Shore and
vicinity of BOS. ESE wind around 10 kts expected 14Z Mon and on.
Monday...high confidence.
Any leftover MFR/IFR CIGs/VSBYs will quickly lift and dissipate,
improving to VFR. Exception will be over immediate south coasts
of MA/RI and Cape/Islands, where it may take longer in the
morning to lift and dissipate. Low probability of stratus along
far northeast MA early in the morning. Isolated to widely
scattered SHRA/-TSRA during the afternoon.
KBOS Terminal...High confidence in TAF, except moderate
confidence late tonight and early Monday. There is a low
probability of marine stratus to the N drifting S into BOS
vicinity late tonight/early Mon.
KBDL Terminal...High confidence in TAF.
Outlook /Monday Night through Thursday/...
Monday Night: Mainly VFR, with local IFR possible.
Tuesday: Mainly VFR, with local MVFR possible. Chance SHRA,
chance TSRA.
Tuesday Night: Mainly VFR, with local MVFR possible. Slight
chance SHRA, slight chance TSRA.
Wednesday through Wednesday Night: Mainly VFR, with areas MVFR
possible. Breezy. Chance SHRA, chance TSRA.
Thursday: Mainly VFR, with areas MVFR possible. Chance SHRA,
chance TSRA.
&&
.MARINE...
Today/Tonight/Monday/...
High pressure southeast of Nantucket will gradually move
offshore and give way to an area of weak low pressure. Winds
and seas will remain below small craft advisory levels through
Monday with local sea breezes today and Monday. Areas of fog
redevelop tonight, gradually lifting Monday morning.
Outlook /Monday Night through Thursday/...
Monday Night: Winds less than 25 kt. Seas locally approaching
5 ft.
Tuesday: Winds less than 25 kt.
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, slight chance of
thunderstorms.
Wednesday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, chance of
thunderstorms.
Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Chance of rain showers, chance of thunderstorms.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.
&&
SYNOPSIS...WTB/Chai
NEAR TERM...WTB/Chai/NMB
SHORT TERM...Chai
LONG TERM...WTB
AVIATION...WTB/Chai/NMB
MARINE...WTB/Chai/NMB