Lexington Weather

Lexington, Massachusetts, USA

53°F

5/11/2024 1:14pm

Lexington Conditions: Overcast
Temperature: 53.2°F / 11.8°C
Dew Point: 44.1°F
Relative Humidity: 71%
Wind: Calm, 10-min avg: Calm, gust: 5 mph
Barometer: 29.82 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 092011
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
411 PM EDT Thu May 9 2024

.SYNOPSIS...

High pressure to the north brings mainly dry and mild conditions 
this afternoon. Some showers are possible this evening into early 
Friday as low pressure tracks to the south, otherwise drier and 
cooler for much of Friday as NE winds develop. Cooler than normal 
this weekend with rain chances increasing Saturday night and Sunday, 
then milder temperatures return early next week but an unsettled 
pattern persists.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

IR satellite imagery shows a band of moisture moving in which has 
moved over the southern half of SNE this afternoon increasing 
cloudcover and allowing some very light returns to bloom on the 
radar mainly off shore. Overall the dry day will continue for most 
of the region thanks to some subtle mid level ridging and a drier 
airmass further north, but we will see some scattered showers moving 
into northeast MA this evening on cool NE flow. Not everyone in 
northeast MA will see a shower, and they will be transient in 
nature. 

The more widespread showers tonight will occur over the southern and 
western parts of the region as a subtle shortwave passes overhead 
providing enough synoptic lift acting on the ~1" PWAT plume to 
produce rain showers overnight, generally light (a few tenths to a 
few hundredths).

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...

Tonight...

As mid level trough amplifies over the Gt Lakes, backing flow aloft 
will bring increasing moisture northward into SNE. However, there is 
uncertainty regarding how far north and east showers will get across 
SNE as best moisture will remain to the SW. Best chance for a period 
of rain tonight will be from western MA to RI closest to best 
moisture, with decreasing probs further NE into portions of eastern 
and NE MA.  

Lows will be in the 40s tonight, with increasing NE winds along the 
south coast and Cape/Islands as low pres moves off the mid Atlc 
coast. 

Friday...

Some lingering showers are possible Fri, especially across SW 
portions of SNE, but drier air will gradually move in from the north 
during the day as high pres builds south into eastern New Eng. This 
may lead to partial sunshine developing across portions of northern 
and eastern MA. NE flow will result in cooler temps Fri with highs 
mostly in the 50s, with some lower 60s possible in the CT valley.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...

Highlights

* Dry for much of Sat, but there could be some spotty showers across 
  the interior late.

* Hit or miss showers and perhaps a few rumbles of thunder on Sun. 
  Could have some graupel/small hail with any stronger storms.

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

* Unsettled late Mon into midweek as a frontal boundary slides 
  through. Though there is considerable uncertainty in how exactly 
  things will evolve. 

Saturday...

Stuck under cyclonic flow through this timeframe with a trough over 
the central Great Lakes early on Sat and a shortwave ridge over the 
eastern Great Lakes/Mid Atlantic. The shortwave ridge builds into 
southern New England by the afternoon, while the trough digs into 
the eastern Great Lakes/Mid Atlantic. The ridge axis slides offshore 
Sat eve. A weak high nudges into our region through much of this 
timeframe. A broad low slides into the eastern Great Lakes late in 
the day.

Overall anticipating that the weather will be dry and quiet for much 
of the day albeit still unseasonably cool due to onshore flow. The 
onshore flow coupled with height falls later on Sat and diurnal 
heating could bring some spotty showers during the afternoon. Not 
super confident on this as forcing is quite weak and AOA measurable 
24 hr QPF probs continue to diminish per EPS/GEFS and GEPS guidance 
from run to run. 

Should have fairly cool air in place with onshore flow. At this 
point 925 hPa temps range from roughly 1-8 degrees Celsius with the 
mildest temps across the Berkshires. The NBM temps actually seemed 
pretty dialed in at this point, so haven't made any changes. Highs 
range from the low 50s along the east coast to the low 60s across 
the CT River Valley.

Sunday...

The trough over the eastern Great Lakes late on Sat will lift into 
and perhaps through portions of New England during this period. 
There is potential that we get a cutoff beginning to set up over the 
region. A broad low will be in place nearby or over southern New 
England during this timeframe.

Think this is our next opportunity for hit/miss showers and perhaps 
a few thunderstorms. We really 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 hit/miss 
shower and thunderstorm activity. Given the cold core in place 
do think there could be some graupel/small hail if any more 
vigorous showers/storms can develop. Should note that 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 deterministic 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 given the 
environment think there could be some small hail pending how 
much instability is in place. 

The longer term convective allowing guidance, ie the NCAR MPAS and C-
SHiELD do show max vertically integrated graupel spottiness 
over the Northeast of 0.1 to roughly 0.5 inches. Though again 
appears very hit/miss in nature. Could have a bit of MUCAPE in 
place, but guidance all over the place. Most robust is with the 
NCAR MPAS with a few hundred J/kg over western MA/CT. Stay tuned
for future updates. 

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 on Tue and behind it a deeper trough may 
slide in for Wed. A weak high nudges in for a good portion of Monday 
before a frontal boundary slides in late in the day. The front may 
still be sliding in on Tue or overhead and into Wed.

Dry and quiet weather anticipated for much of Mon with high pressure 
nudging into our region from offshore of the Mid Atlantic. Late in 
the day some guidance begins sliding a front in, but lots of 
discrepancy here. So, have just stuck with the NBM at this point in 
time. We should rebound temp wise quite nicely to more seasonable 
values, ie readings in the 60s to low 70s. 

The front will still be overhead and perhaps working its way through 
on Tue/Wed. Though there is a lot of uncertainty in how things 
evolve as a deeper low could ride along the front bringing us 
heavier rains or kicking the front offshore. Ensemble guidance is 
also spread with the EPS a bit more amped up (like the ECWMF) with 
moderate/high probs (30-80 percent) of 24 hr QPF AOA 0.5 inches into 
Wed. The GEPS/GEFS show low probs (10-30 percent) of the same QPF 
binning. The EPS even showing some low probs of 24 hr QPF AOA 1 
inch. Something to keep an eye on as we get closer, but for now 
think that the NBM suffices. Temperatures trending near to above 
normal through this period.

&&

.AVIATION /20Z THURSDAY THROUGH TUESDAY/...

Forecast Confidence Levels:

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

18Z TAF Update:

This afternoon...Moderate confidence. 

VFR. N wind around 10 kt turning NE during the afternoon.

Tonight...Moderate confidence. 

MVFR cigs expanding westward across western and central SNE
terminals. Low prob for IFR. MVFR expected to stay out of
eastern MA terminals but confidence is only moderate. Scattered
showers developing, especially across western half New Eng. NE 
wind 5-15 kt.

Friday...Moderate confidence. 

Cigs improving to VFR from NE to SW as drier air works in from 
the N. A few showers possible, mainly from SW MA and CT to the 
south coast.

Friday night...moderate confidence. 

NE winds diminishing overnight. VFR except for marginal MVFR
cigs over the Cape and islands. 

KBOS TAF...Moderate confidence in timing of wind shift to NE.

KBDL TAF...High confidence in TAF today. 

Outlook /Saturday through Tuesday/... 

Saturday: VFR. Breezy. Slight chance SHRA.

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

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

Sunday Night: VFR. 

Monday through Monday Night: VFR. Breezy. Slight chance SHRA.

Tuesday: VFR. 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 Friday night...

Northeast winds below 20 kt may increase with gusts 20-25 kt 
tonight into Fri, especially over south coastal waters as low 
pres tracks to the south. Seas build to near 5 ft over the 
eastern outer waters late tonight into Friday night. A SCA may 
be needed for eastern MA waters toward Saturday but conditions 
are marginal.


Outlook /Saturday through Tuesday/... 

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

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

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 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, isolated thunderstorms. 

&&

.TIDES/COASTAL FLOODING...

High astronomical tides coupled with onshore flow may result in some 
splashover/very minor coastal flooding during the overnight high 
tide cycle tonight. However, wind/waves do not appear high enough to 
result in a significant issue. The daytime high tides are lower and 
do not pose a threat for coastal flooding.

&&

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

&&

$$

SYNOPSIS...BW/BL
NEAR TERM...BW
SHORT TERM...BW
LONG TERM...BL
AVIATION...BW/BL
MARINE...BW/BL
TIDES/COASTAL FLOODING...