Lexington Weather

Lexington, Massachusetts, USA

5/12/2021 4:38am 
  • Lexington Conditions: Clear
  • Temperature: 44.4°F / 6.9°CColder 0.4°F than last hour.
  • Dew Point: 33.4°FIncreased 0.9°Fsince last hour.
  • Relative Humidity: 65%Increased 3.0% since last hour.
  • Wind: Wind from NW NW 1 mph, 10-min avg: 1 mph, gust: 7 mph
  • Barometer: 29.92 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in

Area Forecast Discussion
for Boston, MA

FXUS61 KBOX 120714

Area Forecast Discussion
National Weather Service Boston/Norton MA
314 AM EDT Wed May 12 2021

Weak low pressure slowly moves off through the Maritimes. This
will draw colder air across Southern New England, which may
cause a few showers and possibly a thunder this afternoon. 
High pressure appoaching from the west will bring dry and 
milder conditions Thursday. Friday through the weekend feature 
slightly above average temperatures and scattered daytime pop 
up showers or thundershowers but not a washout. Trending drier 
for early next week with continued slightly above average 
temperatures, but forecast confidence is low especially on 
timing rain chances.


Early morning satellite imagery showed one patch of clouds moving 
east of the Islands. A second area of clouds was moving south into 
Western Mass. Clear skies between.

Upper shortwave moves off through the Maritimes today. But its cold 
pool, complete with -26C to -28C temperatures, will linger overhead 
through the day. Moisture cross sections also show support for 
clouds between 925-mb and 700-mb. Some of this should burn off after 
sunrise, but expect some lingering clouds during the morning. The 
buoyancy generated by the upper cold pool and low level solar 
heating will work on the moist layer to generate more clouds this 
afternoon and a few showers. Stability parameters support this, such 
as Total-totals in the lower 50s. Will mention mainly slight chance 
POPs for showers and isolated thunder. All of this will dissipate 
toward evening. 

The projected depth of the mixed layer reaches 800-mb today, with 
winds of up to 20 kt in the layer available for gusts. Temperatures 
in the mixed layer suppport max sfc temperatures in the low to mid 


The cold pool aloft will be tough to fully dislodge tonight. But 
high pressure builds in from the west. The resulting subsidence will 
provide clear skies and lighter winds tonight. Dew points in the 30s 
will give room for interior low temps to reach 36 to 40, while 
coastal temps reach 40 to 47. The coldest temps will be in
Western Mass, and will be close to frost levels... but not
enough to include a Frost Advisory at this time.

High pressure builds further into Srn New England Thursday. Expect 
mostly sunny skies. Mixing is projected to reach 750-mb, possibly 
700-mb. Temps at those levels are supporting of sfc max temps in the 
mid 60s to low 70s.



* Mix of sun and clouds, sct pop-up diurnally-driven showers Fri 
  thru Sun. Thunder possible each aftn but more likely showers. 
  Seasonal to slightly above avg temps. 

* Low confidence fcst for early next week. Guidance trending drier 
  with seasonable temps, but sizable uncertainty in the pattern for 

Thursday Night thru Sunday Night: 

Essentially a modified persistence forecast for Friday thru the 
weekend, with the salient features and overall pattern being little 
changed on a day-by-day basis this period with good consistency 
across the available 00z NWP suite. Broadly cyclonic NW flow pattern 
aloft, with advection of somewhat steep mid-level lapse rates of 
about 7 C/km from the northern Plains boosting total-totals indices 
in the low-50s. That said, moisture depth is shallow, flow is 
weak NWly, low-levels are well mixed and a likeihood of more 
mesoscale triggering sources (terrain, sea-breezes) for 
scattered diurnal showers, perhaps with some rogue thunder. 
Would probably anticipate more showers than thunder given the 
paltry dewpoints/moisture but can't be ruled out given a few 
hundred J/kg MUCAPE and the steeper mid-level lapse rates. Kept 
PoPs following a heavy diurnal-convective trend (higher/20-40% 
in the day, trending lower/sub-mentionable below 15% for the 
evenings). Even if any thunder materializes, QPF each day looks 
scant and not a washout on any one day. Kept daytime highs close
to NBM in the mid 60s to mid 70s (cooler coasts), lows in the 
mid to upper 40s each evening. On temperatures, with the pattern
not changing much if at all, could be an target of opportunity 
to utilize bias-corrected temps as we get closer. 

Early Next Week (Monday thru Tuesday):  

Predictability dwindles markedly with a low-confidence forecast for 
early next week. 00z GFS/ECMWF/Canadian GEM really show vast 
differences in handling of shortwave disturbances emanating from the 
central Rockies/Plains states. There's really no coherent across-
model or run-to-run trends as well; earlier guidance had advertised 
a warm front that may have been our next best chance at more 
widespread precip, but latest indications suggest the warm front 
remains SW of SNE, to either a mostly dry forecast or a continuation 
of the daytime pop-up shower threat seen in the late-
week/weekend. Stuck mostly with NBM here for temps and PoPs/Wx.
However in deference to prior continuity, PoPs were capped at 
slight chance levels to maintain a mention of showers, while 
recognizing that most 00z models are currently dry. These PoPs 
can then be adjusted as models start to become more congruent on
a pattern.


Forecaster Confidence Levels.

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

Today through Thursday...

VFR. WNW winds with gusts to 20 kts today. Winds diminish in the 
evening as the sun sets. Lighter Northwest winds Thursday.

KBOS TAF...High confidence in TAF. 

KBDL TAF...High confidence in TAF.

Outlook /Thursday Night through Sunday/...

Thursday Night: VFR. 

Friday: VFR. Slight chance SHRA.

Friday Night: VFR. 

Saturday: VFR. Slight chance SHRA, isolated TSRA.

Saturday Night: VFR. 

Sunday: VFR. Slight chance SHRA, isolated TSRA.


Lingering winds and seas today, so Small Craft Advisory continues. 
After 8 AM it will be limited to the outer waters. Buoy observations 
are actually lower than forecast values, but there are not enough of 
these observations to be sure this is consistent. Winds during the 
day should be gusting into the lower 20s, close to criteria. With 
that uncertainty, will maintain the headline on the outer waters 
through the day.

High pressure approaches from the west tonight and Thursday. This 
will bring diminishing northwest winds and seas.

Outlook /Thursday Night through Sunday/...

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

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

Saturday Night: Winds less than 25 kt. 

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


MARINE...Small Craft Advisory until 8 AM EDT this morning for 
     Small Craft Advisory until noon EDT today for ANZ250.
     Small Craft Advisory until 4 AM EDT Thursday for ANZ254>256.


LONG TERM...Loconto

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