Lexington Weather

Lexington, Massachusetts, USA

70°F
8/17/2022 6:34pm 
  • Lexington Conditions: Overcast
  • Temperature: 69.8°F / 21.0°CColder 0.2°F than last hour.
  • Dew Point: 58.8°FIncreased 0.3°Fsince last hour.
  • Relative Humidity: 68%Increased 1.0% since last hour.
  • Wind: Calm, 10-min avg: Calm, gust: 3 mph
  • Barometer: 29.85 in Steady
  • Visibility: 10 miles
  • Rain Today: 0.00 in
  •   

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 150757
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
357 AM EDT Mon Aug 15 2022

.SYNOPSIS...
High pressure remains in control today, providing mainly dry 
and warm weather, along with comfortable humidity. By Tuesday,
coastal low approaches our area resulting in clouds and possibly
a few showers across eastern Massachusetts. The low pressure
system could still bring periods of rain Wednesday into Thursday
for eastern and northern Massachusetts, along with onshore 
breezes and building seas. While cooler than normal temperatures
are favored for most of the week, temperatures and humidity 
levels rise Friday into Monday to summerlike levels with limited
chances for pop- up showers or storms. 

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

350 AM Update...

* Mostly dry and breezy today with seasonable temperatures. 

A highly anomalous H5 trough (10th percentile according to NAEFS 
ensemble situational awareness table) will dig even deeper into the 
Mid Atlantic states during the day today. With anomalous ridging 
developing east of Newfoundland and Labrador, this will help induce 
a surface pressure gradient from northeast to southwest. Long story 
short, easterly to northeasterly onshore flow will be the main story 
for Monday. Surface high pressure moves offshore during the day, 
allowing for a low pressure system currently located off the 
Carolina coast to begin its voyage up the Atlantic seaboard. Its 
track will be closely monitored over the next few days as it is a 
fine line for us between getting widespread beneficial rainfall or a 
shutout. As for today, more clouds than sun but remaining mostly 
dry, although a few showers could not be ruled out, especially 
across the Worcester Hills and East slopes due to upslope effect. 
Some of the 06z CAM guidance also try to show a few sprinkles making 
it up into southeast MA by this afternoon out ahead of the main 
precipitation shield associated with the aforementioned low pressure 
system but this is a low probability scenario. In other words, most 
locations should stay dry. East winds will keep eastern MA coast 
cooler than further inland. Still quite pleasant with temps just 
below to near normal. Highs upper 70s to mid 80s.  

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...

* Coastal low brings clouds and possibly a few showers to
  eastern MA along with cooler than normal temperatures. 

Shortwave energy rounding the longwave trough into coastal Mid 
Atlantic looks to induce weak cyclogenesis over or near the 70W/40N 
benchmark. An anomalously strong H5 trough will transition from 
being positively tilted to negatively tilted by this evening into 
the overnight hours. Theoretically, this should allow the surface 
low currently located off the Carolina coast to be pulled towards 
southern New England like during the cold season and bring us 
beneficial rainfall with much of the region currently in D2-D3 
drought conditions. However, there remains quite a bit of model 
spread in how strong the trough is and how quickly/where it closes 
off. If the trough digs too deep, areas to our south will get the 
bulk of the rainfall on Tuesday and then areas to our north and east 
get soaked on Wednesday/Thursday while we get shut out. Conversely, 
if the trough is a little weaker, then we are in play for widespread 
beneficial rainfall. 

Long story short, there remains quite a bit of uncertainty how 
Tuesday will turn out even though we are within 36 hours time frame. 
The complex mid-level pattern means that it remains a fine line 
between a coastal low passing close enough to our region bringing 
breezy onshore winds as well as significant beneficial rainfall to 
our area and passing well offshore to bring us mainly dry weather. 
Interestingly enough, GEFS/EPS ensemble members actually show better 
clustering with regards to the track of low pressure center later in 
the week converging in the Gulf of Maine (refer to the long term 
section for more details) than on Tuesday. I think part of the 
challenge is that the models are not too sure how to handle such an 
anomalous system during the time of the year when we 
climatologically have the least baroclinicity. 

Of course, given that we are now in fact within 36 hours of 
potential wind/rain, we do need to produce a forecast. Taking the 
reasonable middle ground approach would mean eastern MA has the best 
chance for beneficial rainfall, especially later Tuesday into 
overnight. Surface low pressure would organize itself near the 
70W/40N benchmark and then retrograde northwestward towards coastal 
New England. I used a blend of Superblend and SREF for PoPs, with 
the best chances for a few showers on Cape Cod, particularly later 
in the day. Even though the GFS and ECMWF deterministic guidance 
shows a shut out for our area on Tuesday, the GEFS and EPS do have 
precipitation shield reaching into eastern MA and especially Cape 
Cod by Tuesday afternoon. This is supported by both NAM 3km and 12km 
guidance as well as the typically reliable CAM member, ARW guidance. 
Granted the NAM is the extreme outlier out of all the guidance, but 
it is nonetheless a non-zero probability. We will see which model 
prevails inside 24 hours, which is the time period where NAM shines 
during cold season east coast cyclogenesis. 

Temperature forecast is also tricky since the daytime highs for 
eastern MA and RI will obviously depend on the rainfall and cloud 
cover, which hinge on the track and evolution of the coastal low. 
Did end up going with the 75th percentile guidance (but cooler for 
eastern MA due to likely cloud cover/possible rain) for Tuesday 
highs since interior MA and CT may actually end up having a rather 
nice and sunny day. With 925mb temps +17 to +19C, CT valley should 
see mid 80s with a good amount of sunshine but eastern MA could 
struggle to get out of the mid 70s, especially across Cape and the 
Islands. As for overnight Monday lows, ended up going with the 50th 
percentile guidance. Air mass is still dry enough across much of the 
interior on a light northerly flow for lows to bottom out in the mid 
to upper 50s but low to mid 60s are expected for the coast due to 
cloud cover and a more moist air mass. 

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...

Highlights: 

* While a closer track to coastal New England is possible, an 
  offshore low track is favored bringing rain Wed-Thurs to eastern 
  and northern MA and less in the interior. Onshore NE breezes and 
  the potential for rip currents Wed/Thu along eastern MA beaches.

* Turns warmer and more humid Fri into Mon, with limited chances for 
  pop-up showers/t-storms.

Details:

Tuesday Night through Thursday Night:

Continuing to monitor details and developments pertaining to a 
coastal low pressure and parent 500 mb closed circulation that may 
impact SNE in this period. 

Over the last 24 hours, trends in EPS/GEFS ensemble mean low 
pressure and membership low pressure clusters has made a pretty 
evident eastward/offshore track adjustment. This tendency lends 
support to the offshore track offered by the 00z ECMWF/00z GFS 
solutions, and casts the stronger and closer to the coast 00z NAM as 
an outlier. While some rains are possible mainly Wed into early 
Thurs for eastern, northern and northeast MA potentially on an 
inverted trough, based on the ensemble consensus and clustering it 
now appears as though a significant welcomed rain to all of SNE on 
Wed/Thurs is looking less likely. Since it weights earlier model 
runs, NBM's PoP came in higher than what the 00z guidance and 
ensemble consensus would support and reduced these to the 
15-30% range, higher on Wed and particularly into eastern and 
northeastern MA. We are also looking at a couple days of 
enhanced onshore ENE/NE breezes. Cyclone looks to pull away on 
Thurs.

Thus it appears a greater impact is to boaters, mariners and those 
along the eastern coast. Short of a tropical cyclone, it's otherwise 
not too common in mid-August to see a pronounced NE/ENE fetch 
building waves to the 7-9 ft range. While it may not be a great 
beach day, our RipCurrentRisk forecast grids for Wednesday convey 
widespread moderate rip current risk along the entire eastern coast 
with a high rip current risk for Wed for the Outer Cape beaches. 
Thus pending low track details, its possible that we'll need to 
consider rip current statements (more likely) or high surf 
advisories (less likely, if seas remain above 7 ft) for Wed and/or 
Thurs. In addition, astro tides are around 10 to 10.5 ft MLLW and 
thus needing a 2-2.5 ft storm surge for minor coastal flooding. 
Highly unlikely we reach these surge levels, but still think 
splashover or very minor coastal flooding is possible as NE/ENE 
winds over several tide cycles oppose periods of outgoing high 
tide(s). 

Temperatures remain near to cooler than seasonable through the 
period. Highs in the 70s to low 80s, lows in the mid 50s to lower 
60s.

Friday through Monday Night:

Overall no significant change in this period, with a warming trend 
toward levels more typical of summer as Bermuda ridge builds back 
westward. Highs should return to mid 80s-low 90s levels given mid 
teens 850 mb temps and SWly flow. Dewpoints don't appear to really 
start to reach the above-60F sticky/muggy level until Sat. 

Shower/storm chances are overall fairly limited and likely more pop-
up/hit-or-miss/diurnal in character. Enhanced vort max energy 
rounding a OH Valley trough around Sunday could lead to a greater 
storm coverage then, but expect drier weather to prevail for most 
more often than not. 

&&

.AVIATION /08Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels:

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

07Z TAF Update...

Rest of tonight...High confidence. 

Mostly VFR. N winds across interior terminals but SE winds for Cape 
terminals. Winds 5 to 10 kt. 

Mostly VFR. Winds light and variable. 

Today...High confidence.

Mostly VFR. E winds at 5 to 10 kt. 

Tonight...High confidence. 

Mostly VFR. E winds turning to N and NE at 5 to 10 kt, except up to 
15 kt across Cape terminals. 

Tuesday...Moderate confidence. 

Mostly VFR. Best chance for low VFR to MVFR ceilings is across Cape 
terminals. There could be a few showers across eastern MA on Tuesday 
afternoon but that is contingent on the track of a coastal low 
pressure system. NW winds 5-10 kt across western MA, northern CT and 
RI but 10-20 kt across eastern MA, especially southeast MA. 

KBOS...High confidence. 

KBDL...High confidence.

Outlook /Tuesday Night through Friday/...

Tuesday Night: Mainly VFR, with local MVFR possible. Windy with
local gusts up to 30 kt. Slight chance RA.

Wednesday: VFR. Windy with local gusts up to 30 kt. Slight
chance RA.

Wednesday Night through Friday: VFR. 

&&

.MARINE...
Forecaster Confidence Levels:

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

350 AM Update...

Through Monday night...High confidence.

Seas and winds will remain below SCA thresholds for the forecast 
period. S winds will turn E today. Seas 1-3 ft. 

Tonight into Tuesday...Moderate confidence. 

As a coastal low approaches our area, seas and winds will be on the 
increase. During the overnight hours, winds will switch from the 
east to the northeast. Wind speeds increase to 10-15 kt across 
northern waters and 15-20 kt across the outer waters. 

Due to uncertainty in the track of the coastal low, the marine 
forecast is subject to change. Highest confidence in SCA conditions 
by Tuesday afternoon is currently across the southern and 
southeastern outer waters. Seas 2-4 ft for inner waters and
northeast outer waters but 5-6 ft for outer southern and
southeastern waters. 

Outlook /Tuesday Night through Friday/... 

Tuesday Night: Moderate risk for Small Craft Advisory winds
with local gusts up to 30 kt. Seas up to 5 ft. Slight chance of
rain. 

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 8 ft. Chance of rain. 

Wednesday Night: Winds less than 25 kt. Local rough seas. 

Thursday: Winds less than 25 kt. Areas of seas approaching
5 ft. 

Thursday Night through Friday: Winds less than 25 kt. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 PM Tuesday to 8 AM EDT Thursday 
     for ANZ254.
     Small Craft Advisory from 2 PM Tuesday to 8 AM EDT Thursday 
     for ANZ255-256.

&&

$$
SYNOPSIS...Loconto/Chai
NEAR TERM...Chai
SHORT TERM...Chai
LONG TERM...Loconto
AVIATION...Loconto/Chai
MARINE...Loconto/Chai
      

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