Lexington Weather

Lexington, Massachusetts, USA

44°F

4/18/2024 11:02pm

Lexington Conditions: Overcast
Temperature: 43.5°F / 6.4°C
Dew Point: 41.4°F
Relative Humidity: 92%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 30.16 in Steady
Visibility: 10 miles
Rain Today: 0.18 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 171052
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
652 AM EDT Wed Apr 17 2024

.SYNOPSIS...

Sunshine will become filtered by some mid and high level cloudiness 
this afternoon with developing sea breezes resulting in much cooler 
temperatures along the coast. Some showers will impact mainly 
western Massachusetts and northern Connecticut tonight into 
Thursday. Dry for much of Friday with high pressure in control. A cold front 
brings another round of light rain showers late Friday into early 
Saturday. Dry for the rest of Saturday with gusty winds. The dry 
stretch of weather continues as we head into early next week.

&&

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

7 AM Update...

Forecast remains on track. No changes in the latest update.

330 AM Update...

* Sunshine filtered through some mid-high level clouds this afternoon
* Cooler today...highs in the 50s along the coast & 60s inland

An upper level ridge axis will be approaching southern New England 
from the west today. This will result in plenty of sunshine to start 
the day...but some mid-high will spill in from the west this 
afternoon especially across interior southern New England. This a 
result of an approaching of a mid level warm front increasing the 
forcing for ascent. 

It will be cooler today...especially along the coast as the upper 
level ridge axis results in a weak pressure gradient. This will 
allow for sea breeze development...holding highs in the lower to 
middle 50s along portions of the immediate coast. Further inland 
away from the impact of the sea breezes...enough sunshine will allow 
for high temps to reach the 60s with the mildest of those readings 
in the lower CT River Valley.

&&

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

Key Points...

* Some showers tonight & Thu...bulk of them west of ORE-ORH-PVD line
* Cool Thu...a few mainly interior showers highs upper 40s-middle 50s

Details...

Tonight and Thursday...

An approaching mid level warm front will bring some showers tonight 
and into Thu...mainly southwest of an ORE-ORH-PVD line. The 
approaching shortwave from the west runs into the upper level ridge 
axis and will be deamplifying. In addition...high pressure across 
Quebec will result in some low level dry air across eastern New 
England.

Therefore...the main focus for some showers will be mainly near and 
southwest of an ORE-ORH-PVD line tonight & Thu. We probably will see 
a few brief light showers impact eastern MA/RI...but again the main 
threat for showers will be across the interior. As for 
temperatures...the clouds will keep overnight low temps mainly in 
the lower to middle 40s. Onshore flow on Thu with lots of clouds 
will keep high temps in the upper 40s to the middle 50s.

&&

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

Highlights

* Showers/drizzle coming to an end Thu Night.

* Dry for much of Fri with temps returning to seasonable levels.

* Scattered rain showers late Fri into early Sat. Dry, mild and 
  breezy for the rest of Sat and Sun.

* Still appears dry Mon and Tue. 

Thursday Night...

Initially will have a shortwave over eastern NY/northern New England 
Thu Night and a ridge axis over the eastern Great Lakes. The 
shortwave lifts toward northern Maine by early Fri, while the ridge 
builds into western New England. A frontal boundary will weaken 
during this timeframe as high pressure re-establishes itself over 
the Gulf of Maine.

Will have decreasing chances of precipitation as the night 
progresses with the high building in. Still will have onshore E to 
SE flow through this period. Did add some drizzle into the forecast 
as it appears the NAM/FV3 guidance holds onto showers after 
midnight. This could perhaps be the case, but with the high nudging 
in we do lose some low level (1000-850 hPa) moisture. This is really 
the only deviation from the NBM for this timeframe. Lows range from 
the upper 30s to the low 40s.

Friday through Saturday...

The ridge axis will be overhead/nearby on early Fri. The ridge 
builds offshore by late on Fri. In its wake a cutoff digs from 
Ontario early on Fri into the central Great Lakes by late in the 
day. Will have to watch a convectively driven shortwave lift from 
the OH Valley into our region by late Fri. Though there is some 
uncertainty with how things evolve with this shortwave. The deeper 
trough digs into the eastern Great Lakes/northern New England by 
late Sat. High pressure remains in control through much of Fri. Late 
on Fri into early Sat a cold front sweeps through. Another high 
nudges in from the OH Valley/Great Lakes for late Sat. 

High pressure remains in control through much of Fri. The high 
shifts further offshore late in the day. This allows a cold front to 
begin sliding in from the west late in the day. Should see 
temperatures rebound on Fri as flow becomes southerly. This will 
advect 4-9 degree Celsius air at 925 hPa. Should see highs on Fri 
top out in the 50s with perhaps some low 60s across the CT/Merrimack 
Valley.

Did dial back the default NBM precip chances to later as it takes a 
bit for the low levels (1000-850 hPa) to moisten. The NAM/GFS are 
much more progressive and spread the precip in faster than the 
international guidance (ECMWF/UKMET/GEM and ICON). For now have 
leaned toward the slower solutions. Better shots for precip come 
late Fri into early Sat as the front is sliding through. This 
coincides when a 0.75-1 inch PWAT plume pushes through. Should see 
this plume offshore and cold front by late Sat AM/early Sat 
afternoon. Really not expecting a whole lot of precip given the 
progressive nature of front. Thinking a few hundredths to perhaps 0.1 
inches of QPF possible, which makes sense given ensembles only 
showing mod probs (30-60 percent) of 24 hr QPF AOA 0.1 inches.

Depending on how much precip is realized from the front there could 
be fire weather concerns on Sat. In the wake of the front will have 
W to WNW flow at 850 hPa. Expecting a roughly 20-40 kt low level jet 
in place. At this point it does not appear that it will be too 
difficult to mix down these winds per the latest GFS Bukfit 
soundings. Due to this have increased wind speeds/gusts a bit, but 
may actually not be enough if we fully tap into the LLJ. Given the 
excellent boundary layer mixing have also increased our temps and 
lowered our dew points/RH values given we are still in pre-greenup. 
Went toward the 10th percentile of guidance for RH values and temps 
at the 75th. Not out of the question that there could be elevated 
fire weather concerns depending on how much precip we realize. High 
temps in the 60s on Sat with a few spots perhaps hitting the low 70s.

Sunday through Tuesday...

Cyclonic flow in place through this timeframe. Though will have a 
mid level ridge build from the Upper Mississippi River Valley into 
the Great Lakes and New England for early in the week. High pressure 
will generally be in place through much of this period. 

Dry and quiet weather anticipated through much of this timeframe. 
There is a lot of uncertainty with how things evolve in the Tue 
timeframe, so have just stuck with the NBM for now. This brings in 
some rain chances late in the day.

Should note that it still could be a bit breezy with a well mixed 
boundary layer on Sun. Likely another day where we could overachieve 
on temps and drier conditions, so adjusted our temps up to the 75th 
percentile of guidance and RH values/dew points to the lower end. 
Highs top out in the upper 50s to low 60s on Sun. Temps will be near 
to slightly warmer than seasonable levels for Mon and Tue.

&&

.AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...

Forecast Confidence Levels:

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

06z TAF Update...

Today...High Confidence.

VFR with increasing mid to high clouds across the interior this
afternoon. Light NNE winds becoming light S this afternoon, except
for localized seas breezes along the coastline.

Tonight and Thursday...High Confidence. 

Some MVFR ceilings and light showers develop tonight across 
western MA/CT with the best chance near and west of the CT 
River. These conditions will persist at times into Thursday. 
Meanwhile, across eastern MA/RI mainly VFR conditions are 
expected. Light SE winds tonight becoming E between 5 and 10 
knots on Thursday. 

KBOS TAF...High confidence in TAF. 

Classic E/ESE sea breeze develops at the terminal by mid to 
late morning. 

KBDL TAF...High confidence in TAF.

Outlook /Thursday Night through Sunday/... 

Thursday Night: Mainly VFR, with local MVFR possible. Breezy.
Slight chance SHRA, patchy DZ.

Friday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.

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

Saturday: VFR. Breezy. Slight chance SHRA.

Saturday Night through Sunday: VFR. Breezy. 

&&

.MARINE...

Forecaster Confidence Levels:

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

Today...High Confidence.

A ridge of high pressure overhead will keep winds/seas below small 
craft advisory thresholds today. NE winds 5-15 knots will shift to 
the SE.

Tonight and Thursday...High Confidence.

A weak wave of low pressure will drop southeast across the mid-
Atlantic States with high pressure moving across the eastern 
Canadian Maritimes into Thu. This will result in SE winds this 
evening shifting to the E by Thu morning at speeds of 10 to 15 
knots. Some gusts up to 25 knots with marginal 5 foot seas may 
develop by Thursday across our southern waters. Later shifts may 
need to consider a small craft advisory...but it is marginal and 3rd 
period so will wait another cycle.

Outlook /Thursday Night through Sunday/... 

Thursday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers, patchy drizzle. 

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

Friday Night through Saturday: Winds less than 25 kt. Areas of
seas approaching 5 ft. Chance of rain showers. 

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. 

Sunday: Winds less than 25 kt. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

SYNOPSIS...Frank/BL
NEAR TERM...Frank/BL
SHORT TERM...Frank
LONG TERM...BL
AVIATION...Frank/BL
MARINE...Frank/BL