Lexington Weather

Lexington, Massachusetts, USA

40°F

5/11/2024 6:26am

Lexington Conditions: Clear
Temperature: 39.7°F / 4.3°C
Dew Point: 36.5°F
Relative Humidity: 88%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 29.84 in Rising Slowly
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 110211
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1011 PM EDT Fri May 10 2024

.SYNOPSIS...
Low pressure tracks well south and east of New England through 
Saturday as high pressure builds in from the north. This will bring 
drier conditions but cool temperatures as a persistent NE wind 
prevails. Continued cooler than normal Sunday with hit or miss
showers as an upper low slides through. There could be some
small hail/graupel in any more vigorous showers. Milder next 
week but an unsettled pattern persists through midweek.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
1010 PM update...

Cloud cover starting to diminish across the interior late this
evening. Given somewhat drier air in mid levels moving in
overnight as mid level trough slides to the south and east, 
expect partly cloudy skies overnight for the aurora viewing, 
but low level moisture may lead to more cloud cover across SE MA
and the Cape/Islands. Period of radiational cooling should 
allow lows to drop to the upper 30s to lower 40s.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
The forecast for Saturday looks better than it did a few days ago as 
guidance has converged on a weak mid level ridge keeping things dry. 
The exception would be a spot shower over eastern MA thanks to 
moisture off the ocean and cooler temps aloft but vast majority will 
stay dry. It will be a similar setup to today in that we'll have 
cool NE flow at the surface bringing scattered to broken cloudcover 
moving in while mid and high clouds stream in aloft from the 
opposite direction. Thus, expect a mix of clouds and sun but 
continued northeast flow keeps highs cooler than normal for mid May, 
in the mid 50s (east) to mid 60s (CT valley). Overnight cloudcover 
increases ahead our next shortwave over NY. However, forcing is 
quite weak and so while cyclonic flow with the cold pool overhead 
will drive some showers, they will be widely scattered and should 
hold off until toward daybreak, mainly over the interior where 
orographic lift will provide some additional lift.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Highlights

* Hit or miss showers/storms on Sun. Graupel/small hail possible 
  with any stronger storms.

* Dry for much of Mon with a return of seasonable temps. 

* Unsettled late Mon into midweek with a frontal boundary sliding 
  through. Still have considerable uncertainty in how exactly things 
  evolve.

* Turning drier late in the week.

Sunday...

A trough/cutoff will be situated over the eastern Great Lakes/Mid 
Atlantic on early Sun. The cutoff rotates northeastward into New 
England by late on Sun. This slides into the Gulf of ME/Nova Scotia 
by early Mon. A broad low will be in place nearby or over the region 
on Sun before shifting offshore Sun Night.

This is our next shot for somewhat widespread hit/miss showers along 
with isolated thunderstorms. Do not have a whole lot of moisture to 
work with as EPS/NAEFS situational awareness tables indicating that 
PWATs are around -0.5 to -1 STD below model climo. This translates 
to PWATs roughly around 0.5". Though we do have anomalously low 500 
hPa height overhead (-2 STD per both SATA tables). The 500 hPa 
trough/cutoff overhead is also quite cold with temps of -20 to -25 
degrees Celsius. This in combination with diurnal heating should 
result in a fair amount of cloudiness. Given the cold core in place 
and up to 100 J/kg of MUCAPE could result in some graupel/small hail 
if any more vigorous activity can develop. The mid level lapse rates 
are around 5-7 degrees Celsius with low level lapse rates around 5-8 
degrees Celsius. Though these lapse rates really depend on the model 
being viewed, so there is considerable uncertainty given the weak 
forcing. Deep layer shear in the 0-6 km layer is quite low at this 
point, so not anticipating any strong/severe storms. Though again, 
given the environment think there could be some small hail pending 
how much instability is in place. 

High temperatures range from the mid 50s to the low 60s, with the 
coolest readings along the immediate coastline.

Monday through Wednesday...

Still caught under cyclonic flow through this timeframe with a 
cutoff in place over Quebec/Ontario/Hudson Bay. A ridge axis 
flattens out as it is building into the Mid Atlantic on Mon. A 
shortwave begins lifting in Tue and behind it a deeper trough may 
slide in for Wed. A weak high nudges in for much of Mon before a 
frontal boundary sneaks in late in the day. The front continues to 
slowly work its way in/through on Tue/Wed.

Not much of a change from 24 hours ago. Dry and quiet weather 
anticipated for much of Mon as high pressure nudges in from offshore 
of the Mid Atlantic. The high may shunt far enough southeast later 
in the day that spotty light showers begin leaking in from NW to SE 
Mon afternoon/evening. Though there is a lot of discrepancy here 
amongst guidance, so have just stuck with the NBM at this point. 
Temperatures will rebounding nicely (compared to the weekend) as we 
get southwesterly warm air advection with 7-9 degree Celsius 925 hPa 
temps. Highs will be around seasonable levels, ranging from the 
upper 50s across the Cape to the 60s elsewhere.

The front will still be overhead and perhaps working its way through 
late Tue/Wed. Though there is uncertainty in how things evolve as a 
deeper low rides along the front. This brings us shot for some 
heavier rains, especially late Tue and Wed. Ensemble guidance at 
this point indicating that there are low to moderate (10-40 percent) 
probs of 24 hr QPF AOA 1 inch. These probabilities are highest 
generally along the south coast with fairly good agreement from the 
GEPS/EPS guidance. The GEFS is more solidly in the low prob 
category. Both the GEPS/EPS also show low probs of 24 hr QPF AOA 2 
inches, while the GEFS shows none. Given there could be a few 
hundred J/kg of MUCAPE, so the thunder risk would bring the 
potential for locally heavier rainfall. A 1-1.5 inch PWAT plume 
impinges on the region, especially late Tue/Wed and given we've got 
3-3.5 km warm cloud layer depth there could be efficient warm rain 
processes going on. Something to monitor as we get closer, but the 
PWATs are only 1-1.5 STD above model climo at this point per the 
NAEFS/EPS. 

Should see temperatures continue to climb on Tue. May actually get 
another day of some dry weather if the front holds off coming in 
until late. Could see 925 hPa temps under S/SWly flow climb to 10-15 
degrees Celsius. The result will be readings in the low/mid 70s and 
could start feeling a bit more muggy out with dews in the mid to 
perhaps upper 50s. Temps return to more seasonable levels on Wed 
given the cloudiness in place.

Late in the week...

A ridge axis builds into the OH Valley/Great Lakes region early on 
Thu. The ridge builds into the Mid Atlantic/eastern Great Lakes by 
late in the day. The ridge may begin sliding offshore on Fri. High 
pressure builds into the region during this period.

Dry and quiet weather anticipated through this timeframe. Mass 
fields in decent agreement on Thu, but are more spread out by Fri. 
Have a bit more confidence in the temp forecast for Thu as opposed 
to Fri. There are some hints we could see more of an onshore wind 
component. For now the NBM guidance looks quite reasonable, so 
have stuck with it. Highs range from the 60s to mid 70s.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:

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

Tonight...High confidence.

VFR at all terminals except for ACK where MVFR ceilings linger.
Could see a spot shower through 03Z at ACK. Winds out of the
NE at 5-10 kts becoming N late. Could see some 15-20 kt gusts 
across the Cape/Islands this evening.

Saturday...High confidence.

VFR across the region, but MVFR persists at ACK. Could see some
of these ceilings also leaking into the central/outer Cape.
Confidence too low to include in HYA TAF at the moment. Winds
out of the NE/E at 5-15 kts. Will have gusts of 15-25 kts across
the Cape/Islands.

Saturday night...High confidence. 

VFR across southern New England. The only exception is across
the east slopes of the Berkshires where spotty rain showers
spread in toward daybreak. This could user in some MVFR ceilings
heading into the Sun AM push. Light and variable winds.

KBOS TAF...High confidence in TAF. 

KBDL TAF...High confidence in TAF. 

Outlook /Sunday through Wednesday/... 

Sunday: Mainly VFR, with local MVFR possible. Slight chance
SHRA, isolated TSRA.

Sunday Night: VFR. 

Monday: VFR. Breezy. Slight chance SHRA.

Monday Night: Mainly VFR, with local MVFR possible. Breezy.
Slight chance SHRA.

Tuesday: VFR. Breezy. Chance SHRA, isolated TSRA.

Tuesday Night: Mainly VFR, with local IFR possible. Breezy.
SHRA likely, isolated TSRA.

Wednesday: MVFR/IFR conditions possible. Breezy. Chance SHRA,
isolated TSRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Through Saturday night...

Low pres tracking south and east of the waters will bring NE 
winds 10-20 kt with occasional 25 kt gusts over southern waters
through Sat night with seas building to 5 ft. Extended the SCA 
for outer southern waters through Sat night for southeastern
waters. 

Outlook /Sunday through Wednesday/... 

Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers. 

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

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

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. Slight
chance of rain showers. 

Tuesday: Moderate risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain
showers. 

Tuesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 25 kt. Seas up to 5 ft. Rain showers likely,
isolated thunderstorms. 

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas up to 5 ft. Rain showers likely, isolated
thunderstorms.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 AM EDT Sunday for ANZ254-255.
     Small Craft Advisory until 8 PM EDT Saturday for ANZ256.

&&

$$

SYNOPSIS...BW/BL
NEAR TERM...KJC
SHORT TERM...BW
LONG TERM...BL
AVIATION...BW/BL
MARINE...BW/BL