Lexington Weather

Lexington, Massachusetts, USA

64°F

5/20/2024 5:48pm

Lexington Conditions: Clear
Temperature: 63.7°F / 17.6°C
Dew Point: 56.3°F
Relative Humidity: 77%
Wind: Calm, 10-min avg: Calm, gust: 1 mph
Barometer: 29.96 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 151554
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1154 AM EDT Wed May 15 2024

.SYNOPSIS...
Isolated to scattered showers today before more widespread
showers develop as low pressure moves south of SNE tonight.
Drying trend on Thursday. Weather pattern for late in the week 
into the weekend is a cloudy and unsettled one with onshore 
breezes forcing cooler temperatures near the coastlines. While 
we can't rule out a few showers from time to time, drier weather
generally prevails late this week into the weekend. Trending 
drier with a potential warm-up into early to mid next week.

&&

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

1130 AM Update...

No major changes in the immediate near term in the latest
update. However, did increase our precip changes and QPF amounts
late tonight into early Thu. A bit concerning with a roughly
stationary front nearby, PWATs around 1-1.25 inches and some
instability in place. The 925 hPa jet is anomalous and guidance
has trended an area of convection further north and east
compared to previous runs. This could result in heavy rainfall
falling over roughly at 12 hour period late tonight into early
Thu across CT, RI and perhaps into SE MA. The latest GEFS/GEPS
show low/mod probs (10-40 percent) of 24 hr QPF of 1+ inches,
whereas the EPS indicating mod/high probs (50-80+ percent of 24
hr QPF of 1+ inches and even some low 2+ inch probs. Something
we will need to closely monitor as some convective guidance,
NAM, HREF and NCAR ensembles showing the possibility of heavy
rain across the area aforementioned. Will hopefully have a
clearer picture for this afternoons package.

705 AM Update: 

Updated PoPs/Wx to match current radar/observations and expected
near-term trends with this update. While these rains are not
expected to be significant or notable, forecast confidence in 
the timing of an anticipated second round of rain is pretty low.

At least variable amounts of mid-level cloud cover early this
morning across all of Southern New England, but cloud bases
lower and thicken with westward extent associated with a band of
light rains across the Berkshires and into western/southwestern
CT. 

Latest guidance indicates two general periods of rain today. The
first is the ongoing area of light rains over interior western 
New England, which could expand a bit further east into central 
MA and eastern CT but expecting much of if not all of eastern MA
and RI to be dry through much of today. This lead area of rains
seems to be being driven by moisture advection in light SE 
flow, is not amounting to much in terms of QPF and is not
expected to yield much. This lifts northwestward during the 
morning to early afternoon period, with a potential brief break 
for the afternoon. The bigger uncertainty then comes with an 
anticipated second round of rains associated with low pressure 
near the Chesapeake Bay area. Most solutions show this rain 
reaching our southern coastline but varies as far as when, with 
some guidance including past few HRRR cycles indicating an 
arrival by late afternoon (4-6 pm timeframe) while better 
consensus in rain arriving later tonight. That will need to be 
better pinned down but I show another round of PoPs increasing 
during the later afternoon period along the south coast. Though 
I couldn't rule out a rumble of thunder given meager 
instability, rain showers to predominate. All in all, a better 
day further east one goes, but still will be a generally cloudy 
day.

Previous discussion:

Light returns on radar have been very slow to translate into 
observed precipitation overnight as shower activity advances 
northward from Long Island/NYC into the Hudson River Valley/Central 
CT given dry air from the sfc to ~800mb. Scattered showers are 
expected to fill in across Connecticut and southwestern MA this 
morning, but lingering dry air associated with weak persistent 
ridging (that brought highs into the 80s for some yesterday!) should 
be enough to stave off significant shower activity for the eastern 
half of the CWA. CAMs, and model soundings, do introduce a bit of 
uncertainty, particularly prior to 18Z this afternoon, regarding how 
far east shower activity can advance but all in all not expecting a 
washout of a day for any particular location. Will note that there 
is just a tiny bit of instability, about 150J/kg CAPE, that could 
translate into an isolated rumble or thunder or two. Should see a 
dry period develop later this afternoon ahead of increasing precip 
chances tonight. 

Temperatures aloft remain mild but do cool through the day, with 
850mb temps falling from ~10C to ~7C this afternoon. Given 
significant cloud cover, its unlikely we mix much above 925mb, where 
temps hover around 12C, so highs will top out in the mid 60s to low 
70s, warmest across eastern MA where showers are delayed the 
longest.

&&

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

Low pressure moves northward from the mid-Atlantic coast to just 
south of southern New England overnight which will pivot a more 
robust precipitation shield towards the region after 03/04Z tonight. 
As mentioned by the previous forecaster, the lack of baroclinicity 
this time of year introduces significant uncertainty regarding how 
far northward more stratiform precipitation can extend with low 
pressure centered south of the benchmark, but overnight guidance has 
trended, just slightly, towards a more northern and "wetter" 
solution, wetter in quotations as less than a half inch of rain is 
expected for far southern New England through the period. 

Widespread shower activity will linger through at least the first 
half of Thursday before low pressure pulls seaward, which will allow 
PoPs to diminish through the afternoon. Concurrent with lessening 
shower chances, may see a few breaks of sunshine develop across the 
far interior by sunset.

With surface flow shifting to the E/ENE, onshore flow will impact 
highs, with mid to perhaps upper 60s expected across the interior 
and low 60s along the immediate eastern coastline. It will be a 
breezy day, with a 925mb jet of 35kt draped south of I-90, 
translating to gusts between 20-25kt along the south shore.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Highlights: 

* Unsettled late-week into the weekend, with onshore flow favoring 
  generally dry weather but with considerable cloudiness on most 
  days. Better, although limited, chance for rain is around late 
  Sunday but no significant rains or washouts are expected. 

* Possible pattern change toward even milder weather around midweek?

Details:

00z suite of global ensembles still depict a generally unsettled 
weather pattern in this period, with two closed-off upper level lows 
moving through the mid-Atlc states, thus progged to be suppressed to 
our south. One of these features sags ESE Thursday night into Friday 
south of our waters, with another anticipated to pass to our south 
around Sunday night or into Monday. Related moisture tied to these 
disturbances will keep a generous amt of cloud cover around on most 
days; while we can't rule out a spot shower or two, the 
southward/suppressed tendency of these disturbances depicted in most 
of the ensembles should favor drier weather more often than not. 
Onshore flow being produced by a surface ridge of high pressure 
nosing in from Nova Scotia would tend to favor cooler temps near the 
coast and several degrees warmer inland for late in the week through 
the weekend. There is more uncertainty in the pattern as we move 
into early to mid next week, with some solutions offering a warmup 
and less cloud cover, while the Canadian GEM indicating a 
continuation of unsettled conditions. 

Thursday Night through Friday: 

Coastal low pressure which is expected to bring showers to areas 
largely south of the Mass Pike on Thurs will continue to shift 
southward Thurs night into early Fri, with surface ridge of high 
pressure over Nova Scotia gradually taking hold. We should see 
decreasing rain chances into the evening and overnight hrs, with Fri 
trending drier but still with partly to mostly cloudy conditions and 
onshore breezes, with greater cloud cover south and east. Rain amts 
on Thurs night are light though. For temperatures, highs could 
struggle to reach the lower 60s near the coasts on Fri with the 
cooler onshore flow, but seem likely to reach into the lower to mid 
70s further inland. Above-normal lows in the low to mid 50s for 
Thurs and Fri nights.

The Weekend into Monday: 

Saturday is still looking cloudy but generally dry under continued 
surface ridge of high pressure, with cooler onshore breezes and 
similar high temps to those from Fri. 

Next upper level low pressure tries to move in around later Sunday 
or into Mon. The Canadian GEM is the most bullish in bringing steady 
rains in, while the GFS keeps any rains to far southwestern areas, 
and the ECMWF is the driest and would favor dry weather for all of 
SNE. So still uncertainty in the specifics to work through; I opted 
to keep a slight to low chance mention of showers in the forecast 
for now. PoPs will need adjusting once there is better cross-model 
agreement. Kept highs in the 60s in all areas both days, again 
coolest near the coast.

As of now, Saturday looks to be the pick of the weekend but it still 
will feature a good amt of cloud cover. 

Tuesday and Wednesday: 

While the Canadian solution in this period maintains continued 
cloudy conditions with periodic showers, indications in the ECMWF 
and GFS solutions depict a potential warmup toward early-summerlike 
temps and less cloud cover. For now maintained NBM for this portion 
of the forecast given the disparity in outcomes, though with a 
little lean toward the GFS/ECMWF drier/warmer idea.

&&

.AVIATION /16Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

12z TAF Update: 

Today: High confidence in TAF/trends but moderate on rain
timing.

Mainly VFR for most, with VFR/MVFR mix over the CT Valley
airports with embedded light -SHRA thru 17z. VFR should
generally continue elsewhere, although confidence in timing the
next round of rain coming up from the southern waters is low.
Ceilings should then trend sub-VFR as this second round of rain
develops, but could happen as soon as mid-afternoon for the Cape
airports and PVD but more likely after 00z Thu. South winds
under 10 kt today; sea breeze ESE winds along the eastern MA
coast by mid/late morning should trend back to south later in
the day.

Wednesday night...Moderate Confidence.

Light S-SE winds become east overnight. VFR/MVFR conditions become 
MVFR overnight, as more organized rain shield comes onshore to the 
south coast of CT/RI/MA generally after 04Z. Localized IFR/LIFR 
possible. Forecast uncertainty in how far north this rain shield 
tracks overnight.

Thursday... Moderate Confidence

Uncertainty surrounds precipitation shield on Thursday but expecting 
widespread shower activity through at least 12Z.
Showers possible all day but chances diminish through the afternoon. 
Generally MVFR with pockets of IFR, improving to VFR late Thursday. 
Breezy winds from the ENE/E gusting to 25kt along the south coast. 

KBOS TAF...Moderate confidence in TAF. OVC VFR with mainly dry
weather through early afternoon. S winds trend SE/ESE early this
afternoon, before flipping back to S again later today. More 
widespread showers after 03Z Thursday as ENE flow develops.

KBDL TAF...Moderate confidence in TAF. VFR to MVFR today with
light SHRA this morning, lifting N while briefly trending dry
for mid to late afternoon. More widespread rain moves into the 
terminal after 03z tonight.

Outlook /Thursday Night through Sunday/... 

Thursday: Mainly VFR, with local IFR possible. Breezy. Chance
SHRA.

Thursday Night: VFR. Breezy. Slight chance SHRA.

Friday: VFR. Breezy. 

Friday Night: VFR. 

Saturday: VFR. Slight chance SHRA.

Saturday Night: Mainly VFR, with areas IFR possible. Slight
chance SHRA.

Sunday: Mainly VFR, with local IFR possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

3 AM Update...

Wednesday...

Weak high pressure offshore results in light S-SE winds. Mainly dry 
weather and any morning fog burns off by midday with good vsby 
thereafter. Scattered showers across the southern waters. 

Wednesday night...

Low pressure south of New England drifts north, with increasing east 
winds 15-20 kt toward Thu morning. Vsby lowers to 1-3 miles in areas 
of fog and rain, especially southern MA and RI waters. 

Thursday... 

E/ENE winds increase as low pressure tracks closer to SNE, gusts to 
30kt possible. Areas of rough seas with increased rain chances. 

Outlook /Thursday Night 

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Chance of rain showers, isolated
thunderstorms. Local visibility 1 to 3 nm.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Chance of rain showers.

Friday through Saturday: Winds less than 25 kt. Seas up to
5 ft. 

Saturday Night through Sunday: Winds less than 25 kt. Seas up
to 5 ft. Slight chance of rain showers. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 AM to 8 PM EDT Thursday for ANZ232.
     Small Craft Advisory from 2 AM to 11 PM EDT Thursday for 
     ANZ235-237-254.
     Small Craft Advisory from 11 PM this evening to 11 PM EDT 
     Thursday for ANZ255-256.

&&

$$

SYNOPSIS...Loconto/KS
NEAR TERM...Loconto/BL/KS
SHORT TERM...KS
LONG TERM...Loconto
AVIATION...Loconto/KS
MARINE...Loconto/KS