Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 131406

Area Forecast Discussion
National Weather Service Boston/Norton MA
1006 AM EDT Thu May 13 2021

Some clouds, but mainly dry conditions today. Mostly dry Friday,
but a few pop up afternoon showers possible. The weekend
features a mix of sun and clouds with daytime pop-up showers or
thundershowers, with clearing at night. However mainly dry
weather prevails from the weekend into the midweek period, with
warm afternoons and cool nights.



10 AM Update:

Spectacular spring weather today with 850 mb temps warming to
about +4C and boundary layer mixing to about 800 mb, will
support highs from the upper 60s to lower 70s. Low to mid 60s
for highs along the coast before cooling seabreezes develop by
late morning/early afternoon. Calm wind at MQE a good signal for
seabreeze development shortly. Nonetheless, gorgeous spring
weather with dew pts in the upper 20s and lower 30s combined
with light winds.

Satellite already showing diurnal cumulus developing over the
high terrain at 10 am, and this will spill into the coastal
plain this afternoon. Therefore morning sunshine giving way to
partly sunny conditions this afternoon. Low risk of an isolated
shower given cold air aloft at 500 mb with temps around
-25C, yielding steep mid level lapse rates. Although not
 expecting any small hail like yesterday given temps not quite
 as cold aloft as Wed, along with less cyclonic curvature to the
 upper flow and surface dew pts lower today. Thus, any shower
 activity will be very isolated with most areas remaining dry
 today, especially with large dew pt depressions likely to see




Dry air and subsidence will maintain a mainly clear night with
light wind, supporting radiational cooling. Expect min temps in
the upper 30s and 40s.


Ridge maintains fair weather for most of Friday. The forecast
does note a weak shortwave diving southeast into Nrn New
England. This along with cold air aloft...not as cold as it was
recently...may generate widely scattered showers Friday
afternoon. The surface layer remains dry, so expect any
precipitation to be very light.

Temps in the mixed layer will support max sfc temps in the upper
60s to mid 70s. The light surface flow will again suggest sea
breezes along the coast.



* Mix of sun and clouds, sct pop-up diurnally-driven showers Sat
  thru Mon, clearing at night. Thunder possible each aftn but
  more likely showers. Somewhat better chances for showers Sunday,
  though no day is a washout. Milder than avg temps.

* More uncertainty in the details for midweek, but mainly dry
  weather prevails. Likely continuation of above-normal temps
  though do have to watch for a backdoor cool front Wed.


Friday Night thru Monday Night:

Rather stable, persistent pattern in the offing for the weekend into
Monday with sensible wx varying little on a day to day basis. Most
of the days will feature increasing diurnally-driven cumulus clouds,
with chances for showers or thundershowers following typical diurnal
trends (isolated to widely scattered coverage aftn, less after
sundown). Despite some relatively steep mid-level lapse rates
around 6.5-7 C/km each day, given lack of robust moisture (both
in magnitude and at depth), will leave grids as showers. And
those pop-up showers in and of themselves won`t generally be
enough to put a damper on outdoor plans, with QPF being light
to, frankly, scant. Clearing skies each night with high and low
temperatures trending slightly above average. Rinse and repeat,
essentially. Highs upper 60s to mid 70s each day, lows in the
upper 40s to low 50s. Do have to watch for highs possibly
overachieving on some days, with a general lack of significant
precip allowing for drier soils and the warmer feedback that
tends to have on high temperatures. It`s envisioned that bias
corrected temps should perform quite well in this period.

If there is some variation to that general theme, it`s on Sunday.
The main difference here to the other days is the 00z NWP swing
a somewhat stronger shortwave disturbance in broad WNW flow
aloft from the Gt Lakes. This in conjunction with similar
limited diurnal convective instability should encourage more in
the way of shower/possible t-shower coverage. While Saturday
and Monday feature PoPs in the 20-30 percent range, Sunday is
more in the solid-Chance neighborhood around 40-50 percent.

Tuesday into Midweek:

Mid-level pattern across CONUS trends to a split-flow pattern, with
stronger polar jet retreating into Canada and a large ridge over the
northern Plains/Corn Belt. Building warmth across the center of the
country may find it difficult to make much northeast advance with
northern-stream dominated WNW flow regime across the Northeast,
though. The international suite of guidance show a rather strong
shortwave disturbance in the NW flow on Wed, which would support a
potential risk for showers or thunderstorms. This isn`t generally
supported by the GFS which builds ridging/warmer temps east into the
Northeast. In addition, this is a fairly big shift in run to run
continuity from the ECMWF.

Thus for most of the period, no significant precip events look to be
in the cards. Tougher call on temperatures, though they should trend
above-normal on the whole. The GFS offers much milder temps, while
the ECMWF/GEM solutions point to a possible backdoor front
around midweek.


Forecaster Confidence Levels.

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

14z TAF Update: only change to previous thinking is for isolated
afternoon shower/virga. Although most or all terminals remain
dry. Earlier discussion below.


Today: High confidence.

VFR. Sea breezes develop toward midday thru early evening along
the coast. Light northwest flow inland.


VFR with light wind.


VFR. Light surface wind with sea breezes again possible midday
to afternoon along the coast. Widely scattered showers/sprinkles
possible in the afternoon/evening.

KBOS TAF...High confidence in TAF. Sea-breeze develops around
15-16z with winds becoming SE.

KBDL TAF...High confidence in TAF.

Outlook /Friday Night through Monday/...

Friday Night: VFR.

Saturday: VFR. Slight chance SHRA.

Saturday Night: VFR.

Sunday: VFR. Chance SHRA.

Sunday Night: VFR.

Monday: VFR. Slight chance SHRA.



10 am update...

Very tranquil spring boating conditions, light and variable
winds, sharp horizon with just a low risk of an isolated shower
near shore this afternoon. Earlier discussion below.


Fair skies and light winds through Friday. Sea breezes
developing near the coast this afternoon and Friday afternoon.
Isolated showers/sprinkles possible near the coast Friday

Outlook /Friday Night through Monday/...

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

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

Sunday Night through Monday: Winds less than 25 kt.




NEAR TERM...WTB/Nocera/Loconto
LONG TERM...Loconto
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