Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 100538
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
138 AM EDT Fri May 10 2024

.SYNOPSIS...
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 THIS MORNING/...
10 PM update...

Scattered light showers spreading east into western SNE as area
of deep moisture moves into the region ahead of mid level low 
across eastern Gt Lakes. Focus for showers tonight will be
across SW half of SNE from western MA into RI and south coast 
where best moisture is present. Moisture decreases to the north
and east so any showers will be spotty across north central and
especially NE MA. 

Previous discussion...

Caught under cyclonic flow with a shortwave trough lifting from
the OH Valley into southern New England late tonight. Low 
pressure slides into the Mid Atlantic.

Could see isolated to scattered showers and perhaps brief
periods of steady rain to the north/northeast of the low sliding
south of our region. NBM probabilities appeared much too high,
especially compared to latest obs. Leaned heavily on latest HREF
guidance, but in particular the NSSL WRF. This is doing well
compared to obs so far. Anticipate showers spreading in from
west to east. Best shot for 0.10" or greater is across western
MA/CT. Elsewhere totals should remain below 0.10". The area of
highest precip also coincides with where PWATs approach 1 inch.

Prolonged onshore flow will bring us cooler temps tonight. Lows
will be in the 40s.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/...
745 PM Update...

Highlights 

* Showers during the AM, especially for the MA-Pike south.
  Should gradually diminish as high pressure builds in.

* Dry and quiet weather Friday night, though a few spotty
  showers/drizzle possible over portions of the immediate south
  coast.

Still in cyclonic flow through this timeframe. A broad trough in
place over the central/eastern Great Lakes early on Fri. This
digs into the Mid Atlantic/Appalachians by late in the day.
Behind the trough a shortwave ridge builds into the eastern
Great Lakes. Low pressure remains south of our region on Fri and
slides further offshore on Fri as high pressure nudges in from
the north.

Main change in the latest update was to deviate from the NBM and
show decreasing chances of showers, especially across much of MA
as high pressure builds in. Consensus of guidance showing a dry
forecast especially with the upper/lower forcing more to our
S/SE. Showers could linger across portions of the south coast
late in the afternoon/Fri night, but wanted to show more of a
downward trend. Not out of the question there is some spotty
drizzle given the onshore flow into early Sat.

On top of the shower chances have also deviated from the NBM on
wind speeds/gusts. We've got a fairly decent 20-30 kt ENE/NE low
level jet developing near Long Island and perhaps into
Nantucket. Should not be terribly difficult to mix down per
Bufkit soundings, so have bumped up our wind speeds/gusts. Think
there could be some 20-25 mph gusts during Fri afternoon,
especially across RI and SE MA. Temperatures looked fine in
previous update, so have stuck with widespread 50s and perhaps
some low 60s in the CT River Valley.

Bit of a right turn on the forecast for Friday night. High
pressure building in, so have gone with a dry forecast. There
could be some spotty drizzle, but the NBM was way too high with
PoPs for eastern MA heading into early Sat. Should have a fair
amount of cloud cover until toward daybreak Sat. Lows in the
40s.

&&

.LONG TERM /SATURDAY NIGHT 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 /06Z FRIDAY THROUGH TUESDAY/...
Forecast Confidence Levels:

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

Through 12z...

Mix of VFR-MVFR cigs with occasional showers mostly south of 
BAF-TAN-PYM.

Today...High confidence.

MVFR-VFR cigs trending to VFR from NE to SW. A few showers  
south of the MA Pike into early afternoon. NE wind 10-20 kt with
higher gusts over the Islands. 

Tonight...High confidence.

VFR, except for ACK where MVFR cigs possible with low chance of
a few showers. NE wind 5-15 kt becoming N overnight. 

Saturday...High confidence.

VFR, but areas of MVFR cigs developing across portions of eastern
MA and Cape/Islands where a spot shower possible. NE wind 10-20
kt.

KBOS TAF...High confidence in TAF. 

KBDL TAF...High confidence in TAF. MVFR cigs this morning with
scattered showers will improve this afternoon. 

Outlook /Saturday Night through Tuesday/... 

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 5 ft over the southern
and eastern outer waters Friday AM into Friday night. Due to
this have hoisted a SCA in the latest update. 

Outlook /Saturday Night 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...Small Craft Advisory from 8 AM this morning to 8 AM EDT 
     Saturday for ANZ254>256.

&&

$$

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