Old Res Weather

Lexington, Massachusetts, USA

5/15/2021 6:26am 
  • Lexington Conditions: Clear
  • Temperature: 52.9°F / 11.6°CWarmer 0.2°F than last hour.
  • Dew Point: 44.1°FDecreased 0.1°Fsince last hour.
  • Relative Humidity: 72%Decreased 1.0% since last hour.
  • Wind: Calm, 10-min avg: Calm, gust: None
  • Barometer: 30.21 inRising 0.02  inHg/hr Rising Slowly
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  • UV Index: $VPuv None

Area Forecast Discussion
for Boston, MA

FXUS61 KBOX 150728

Area Forecast Discussion
National Weather Service Boston/Norton MA
328 AM EDT Sat May 15 2021

High pressure will remain centered near southern New England
over the next several days, resulting in mainly dry weather,
warm afternoons, and cool nights. An isolated shower is 
possible most afternoons, with best chances for pop up showers 
or thunderstorms on Sunday and Monday. Even warmer temperatures
and low humidity levels for the rest of the workweek. A few 80 
degree days are looking more likely in interior New England by 
the middle of next week.


High pressure centered over the Appalachians will extend
subsidence over Southern New England. Cross sections continue to
show a layer of moisture between 850-mb and 650-mb, available to
be converted to clouds when introduced to some lift. Solar
heating during the day will generate that lift. So expect mostly
sunny skies turning into partly sunny, but overall a dry weather

Mixing yesterday was quite deep, reaching about 700-mb per the
00Z Albany sounding. If one allowed for near-adiabatic lapse
rates, the mixing in that sounding would reach near 600-mb. As
mentioned... deep. Temperatures at 700-mb today are forecast
around -6C, equiv to +9C at 850-mb...which supports max temps in
the mid 70s. Light winds may allow a sea breeze along the coast,
which may generate cooler max temps. Will forecast highs in the
70s, cooler along the coast.

The air itself will remain dry. Dew points in the 30s would
create a temperature-dewpoint difference of nearly 40 degrees.
Showers trying to fall through that should mainly evaporate on
the way down. A very light shower or sprinkle might reach the
ground, but little to speak of.


Overview...High pressure remains in place at the surface. But
a weak shortwave aloft will race over New England later tonight
and Sunday. 


Enough moisture aloft for patchy clouds, but rather dry near the
surface. So no forecast of showers...just a fair, dry night. Dew
points will be in the 40s, so expect min temps in the 40s and
low 50s.


Sunshine again kicks off heating of the air and development of
diurnal clouds. Mixing will support max temps reaching the low
to mid 70s. Much of the day looks dry, and the temperature-
dewpoint difference will again reach 30 degrees. 

But other factors suggest a period of scattered or widely 
scattered showers during the afternoon. The heating, the lift 
from the passing upper shortwave, the lingering cold air 
aloft...minus 24C to minus 26C at 500-mb...and favorable 
stability parameters will assist in generating buoyancy. CAPE is
forecast around 250 to 500 J/Kg, Lifted Index is forecast at 
minus 1 to minus 2. Total-totals are forecast in the mid 50s. 
With this instability in place, the forecast will continue to 
mention a chance of thunder.



* Afternoon slow-moving sct showers/t-storms Mon in the interior. 
  Brief gusts, small hail possible in stronger storms. Severe 
  weather not expected.

* Turning drier and even warmer most of the rest of the workweek. 
  Low-mid 80s high temperatures in the interior Wed and Thurs, 
  with comfortable humidity levels. 

Sunday Night into Monday Night: 

The same general diurnal trends we've seen over the last 
several days will continue into Monday. Weak shortwave ridging 
Sunday night into early Monday weakens as another weak shortwave
trough moves SE in NW flow. This should again lead to increased
afternoon cloudiness with scattered showers and thunderstorms 
blossoming, following the diurnal trend. We still have fairly 
steep 700-500 mb lapse rates around 7 C/km, and while low-levels
are still well mixed with an inverted-V look on GFS/NAM 
soundings, surface dewpoints are higher than prior days (mid 40s
to low 50s). This generates higher surface-based CAPE values 
ranging from 800-1200 J/kg. With drier air in mid-levels, do 
have to watch CAPE values falling on the lower end of that range
as dewpoints mix out/drop, which is the uncertainty in the 
convective forecast. Still should be enough for scattered storms
that will be very slow moving, with less than 15 kt of flow 
progged through a deep depth of atmosphere. Strongest of storms 
capable of brief gusty outflow given the inverted-V profile, 
perhaps some small hail as well given low freezing levels/wet-
bulb 0C heights. Thinking whack-a-mole SPSable type storms at 
worst, best chance in the interior and especially vicinity of 
the terrain. Showers/storms should fizzle after sundown toward 
mostly clear skies. With the slightly higher dewpoints, patchy 
fog possible for the South Coast and Cape Cod. 

Tuesday through Friday: 

Pattern amplifies across CONUS for much of the upcoming workweek. 
Most of the global NWP now feature a highly amplfied 500 mb ridge 
centered over the central Appalachians by Wed, with a ridge axis 
northward into Hudson Bay! West of the ridge axis in more 
conditionally unstable air, much of the Plains states should be 
rather convectively-active. While there are no synoptically-salient 
or apparent precip events advertised in the models for SNE, we do 
have to keep tabs on any convectively-augmented vorts/shortwaves 
rounding the top of the ridge, but too unpredictable at this range. 
Thus kept a mainly dry forecast, but will have to be cognizant of 
anything that can round/topple the ridge axis around Wed/Thurs.

The main story in this period will be increasing warmth, especially 
for Wed and Thurs and possibly into Fri as well. There's strong 
multi-model consensus in 850 mb temps around the mid teens Celsius 
by Wed, which should bring highs in the interior into the low to mid 
80s Wed and Thurs following the NBM, cooler 70s near the coasts.
In most cases (Hartford did get to 80 degrees back on May 2nd),
it should be the first appearance of 80 degree high 
temperatures that SNE has seen since the last such occurrence 
around late September/early October 2020. Don't really see any 
potential for backdoor cool fronts, so I see no compelling 
reason to deviate from these temps. While we will see building 
warmth, humidity levels are comfortable still with dewpoints in 
the 40s- lower 50s. All in all, a taste of early- summer warmth 
with lots of sun and generally dry weather.


Forecaster Confidence Levels.

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


VFR with light wind. Diurnal clouds will develop midday. 
Showers may form this afternoon at 10 thousand feet AGL, but 
the dryness of the air closer to the surface should evaporate 
most of the drops that fall. 

The light flow may allow sea breezes to form near the coast
toward midday and in the afternoon.


VFR with patches of clouds at 10 thousand feet. Light wind.


VFR with light wind. Clouds will again form with scattered
showers and widely scattered thunderstorms possible. 

KBOS TAF...High confidence in TAF. VFR. Moderate confidence on 
the timing of the coastal sea breeze.

KBDL TAF...High confidence in TAF. VFR.

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR. 

Monday: VFR. Isolated SHRA, isolated TSRA.

Monday Night: VFR. 

Tuesday through Wednesday: VFR. Breezy. 


Short Term /through Sunday/...

High pressure centered west of Srn New England will maintain 
light winds and seas through Sunday. Local sea breezes close to
shore will redevelop today from midday through evening.

A few light showers are possible today, but expect a mainly dry
day. Scattered showers, widely scattered thunderstorms possible
on Sunday.

Outlook /Sunday Night through Wednesday/... 

Sunday Night through Monday Night: Winds less than 25 kt. 

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

Tuesday Night through Wednesday: Winds less than 25 kt. Areas
of seas approaching 5 ft. 




LONG TERM...Loconto

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