Lexington Weather

Lexington, Massachusetts, USA

53°F

5/11/2024 3:48pm

Lexington Conditions: Mostly cloudy
Temperature: 53.1°F / 11.7°C
Dew Point: 42.4°F
Relative Humidity: 67%
Wind: Calm, 10-min avg: Calm, gust: 1 mph
Barometer: 29.80 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 080556
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
156 AM EDT Wed May 8 2024

.SYNOPSIS...
A warm front will bring a round of showers and isolated 
thunderstorms very late tonight and Wednesday. Chances for a few
strong thunderstorms capable of producing hail in western MA/CT
Wednesday afternoon. Periods of unsettled weather continue late
this week into this weekend...but not expecting a washout 
either with extended periods of dry weather too. 
Overall...temperatures will average below normal late this week 
into the weekend. Milder/Seasonable temperatures should return 
early next week.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...
950 PM update...

Stratus and fog confined to Cape/Islands tonight, and as 
previous forecaster indicated will not likely push much further
N than the south coast given the WSW BL flow. No change in 
timing of warm frontal showers which will begin to move into 
western MA/CT toward daybreak and overspread rest of SNE after 
daybreak.

Previous discussion...

* Areas of low clouds/fog expand onto the south coast overnight
* Showers/embedded t-storms approach western MA/CT near 
  daybreak

Areas of low clouds and fog were currently impacting Nantucket
early this evening. Light southerly flow with the cooling
boundary layer should allow the low clouds and fog to 
overspread areas near the south coast overnight. The low level
flow will maintain a westerly component overnight...which should
keep the bulk of the low clouds/fog from expanding much further
north than a KPVD-KPYM line.

The other concern will be shortwave energy/mid level warm front
approaching from the west near daybreak Wednesday. This will be
associated with a modest southwest LLJ and a pretty good 
instability burst. Showalter indices drop from nearly 10 to 
below zero Wednesday morning. There also a band of good 
lift/deeper moisture associated with the shortwave/mid level 
warm front. Therefore...while dry weather generally prevails 
overnight expect showers and possibly embedded t-storms with 
brief heavy rain to begin to impact western MA/CT near daybreak.
This activity will then move across the region Wednesday 
morning which is discussed in the short term section. 

Overnight low temps will bottom out in the upper 40s to the
middle 50s.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/...
350 PM Update:

Wednesday: 

Key Points...

* Overcast with warm frontal showers spreading eastward during the 
  morning to early afternoon hrs. Low chance of a rumble of thunder 
  with this activity. 

* Better chance at a few strong t-storms later in afternoon (as
  soon as 2 PM, but more probable ~ 4-7 PM) from Worcester/NW 
  RI westward into western MA/CT. Hail possible in the strongest
  of storms, which could reach up to 1 inch in diameter. 
  
Warm front then spreads showers eastward across the remainder of 
Southern New England during the morning hours, with 
continuing/residual showers around in eastern MA into the early 
afternoon. There is a modest decrease in Showalter indices to around 
0 with this morning round of showers, and though I can't rule out a 
rumble of thunder with the morning warm frontal showers, in many 
cases it would be the exception vs the rule. This will lock in cloud 
cover and lead to cooler temperatures with little optimism for 
breaks in the cloud cover. With overcast here, expect highs to reach 
into the mid 50s to lower/mid 60s.

For the mid to late afternoon...there are indications in a majority 
of guidance of some breaks in the cloud cover over western and 
central MA, northern CT and into portions of NW RI. As that occurs, 
a surge in 700-500 mb lapse rates 7.0-7.5 C/km advects over the 
aforementioned area, with a pocket of mid 50s to around 60 degree 
dewpoints in the lower Hudson Valley into western MA and portions of 
northern CT. With effective shear magnitudes around 40-50 kt, and 
with a narrow zone of frontal convergence with weak low pressure 
tracking up the CT Valley, we should see scattered t-storms develop 
in the lower Hudson Valley/Berkshires, moving ESE during the late-
aftn hours. MUCAPE values for the aforementioned area are 
progged around 1000-1200 J/kg per the HREF, with a bullseye in 
max updraft progs over western and central MA, northern CT into 
northwest RI. It isn't clear if these storms would be surface-
based here, but the steeper lapse rates aloft combined with 
effective shear magnitudes supporting organized updrafts could 
favor storms elevated above any surface stable layer/inversion. 
It's a setting which can yield storms capable of hail in strong 
storms, and per machine- learning progs from Colorado State and 
others, there is a lower probability that storms could produce 
hail satisfying severe criteria. SPC has a large portion of SNE 
in a Level 1 of 5/Marginal Risk for severe weather, though the 
best chance is for areas near and southwest of Worcester. Unless
storms can become surface based, the primary convective threat 
would be from hailstones. Opted to include enhanced wording for 
small hail in thunderstorms encompassing this general area. The 
biggest uncertainties here are on the spatial coverage and 
timing of any t- storms, and if storms could become surface- 
based. Temps in interior MA/CT and northwest RI have a good 
chance at reaching into the mid 60s to lower 70s with any cloudy
breaks. 

Wednesday Night: 

Weak low pressure initially near the Hudson Valley moves eastward 
early tomorrow night, with decreasing rain chances as it moves 
offshore. It's a pretty uncertain period though in the wake of this 
wave of low pressure; a number of model guidance wants to clear SNE 
out from cloud cover too, but with surface ridge nosing in from 
northern New England and light northerly winds cooling the PBL, I'm 
a little surprised the guidance is clearing things out to the degree 
it is. Pattern recognition seems to favor more cloud cover than
the guidance is offering. I sided the official forecast more 
pessimistically, with cloud cover hanging tough and/or filling 
back in as the PBL cools off. Will be generally dry, but I think
tomorrow night could feature a good amt of cloud cover and 
possible fog, too. Lows in the upper 40s to lower 50s.

&&

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

* Another period of showers sometime later Thu into early Fri
* Seasonable high temps away from coast Thu but quite cool on Fri
* Relatively cool this weekend with another period of showers by Sun
* Milder/more seasonable temps return early next week

Details...

Thursday and Thursday night...

Enough subsidence behind Wednesday/s shortwave to result in mainly 
dry weather to start off the day on Thursday. However...another 
piece of shortwave energy will be approaching from the west later 
Thu and Thu night. So we expect more showers to arrive...but 
specific timing is uncertain. Right now thinking later Thu into Thu 
night...but this will need to be refined. High temps are tricky 
too...but we might have enough of a dry window during the first half 
of the day to allow highs to reach the 60s to perhaps near 70 in CT 
River Valley. It probably will be tough though to break 60 in the 
immediate coast with onshore flow.

Friday...

A couple waves of low pressure pass to our south on Friday. This 
generates an easterly flow of cool air into southern New England. 
High temps will probably be held mainly in the 50s on Friday. 
Thinking the main shower threat will be during the first part of the 
day, but perhaps trending drier for the second half of the day as 
the low pressure system moves east of the region. 

This Weekend...

Upper trough sets up across our region this weekend. This will 
result in relatively cool/below normal high temps for most of the 
weekend along with onshore flow. Thinking is that highs will mainly 
be between 55 and 65...with the coolest of those readings on the 
immediate coast. It will also be a bit unsettled at times with the 
risk for some showers...but timing is uncertain and a washout is not 
expected. Current indications are that much of Sat may turn out dry 
with the better chance for some showers being sometime on Sun.

Early Next Week...

The upper trough will push east of our region early next week. This 
should allow for rising height fields and seasonable high 
temperatures.

&&

.AVIATION /06Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

06z TAF Update...

Through 12z...

IFR-LIFR fog and stratus for Cape/Islands which may expand along
the immediate south coast, otherwise VFR cigs. Showers and 
possible embedded t-storms moving into western MA/CT 09-12z. 

Today...Moderate confidence. 

MVFR-IFR conditions overspread the remainder of the region
during the morning as a cluster of showers and embedded t-storms
move through from W to E roughly 12-17z. A second round of 
scattered showers/t-storms is possible 18-23z, with best chance
from northern CT northward through interior MA. We included VCTS
in BDL/BAF/ORH/BED TAFS. Some hail is possible with the 
afternoon/early evening activity. A period of SW gusts to 20 kt
and pockets of LLWS possible this afternoon across SE MA and 
Cape/Islands as LLJ develops and moves across the region.

Tonight...Low to moderate confidence. 

SHRA/TS exits into the waters early this evening. Some clearing
is possible tonight for western MA/CT but stratus and patchy 
fog may linger across eastern MA and RI. Extent of IFR
conditions is uncertain. Light winds. 

Thursday...Moderate confidence.

Mix of VFR/MVFR conditions trending to widespread MVFR in the
afternoon as more showers develop. N wind 5-15 kt becoming NE. 

KBOS TAF...Moderate confidence in TAF. A brief period of showers
move through roughly 14-17z, with an embedded t-storm possible.
Cigs expected to lower to MVFR then IFR by this afternoon.
Scattered showers and possible t-storms redevelop to the north
and west this afternoon and may move across BOS mid-late 
afternoon.

KBDL TAF...Moderate confidence in TAF. A brief period of showers
move through roughly 11-14z, with an embedded t-storm possible.
Cigs expected to lower to MVFR then IFR later this morning.  
Another round of scattered showers or t-storm possible this 
afternoon and we have included VCTS in the TAF.

Outlook /Thursday Night through Sunday/... 

Thursday Night: MVFR/IFR conditions possible. Breezy. SHRA
likely, patchy BR.

Friday: Mainly MVFR, with areas IFR possible. Breezy. Chance
SHRA.

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

Saturday: Mainly MVFR, with local VFR possible. Breezy. Chance
SHRA.

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

Sunday: Mainly MVFR, with areas VFR possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Through Wednesday Night: High confidence.

Winds and seas below SCA thresholds through the period. 

Mainly easterly winds around 10-15 kt tonight into Wednesday; 
winds then turn WNW to NW at similar speeds for Wednesday night.
Seas mainly 4 ft or less all waters. Fog is likely tonight over
the southern waters. Periods of showers Wednesday, with 
possible embedded thunderstorms. Some storms could produce small
hail and locally rough seas.

Outlook /Thursday Night through Sunday/... 

Thursday: Winds less than 25 kt. Chance of rain showers, patchy
fog. Local visibility 1 nm or less.

Thursday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. Rain showers
likely, patchy fog. Local visibility 1 to 3 nm.

Friday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. Chance of rain showers. 

Friday Night through Saturday: Winds less than 25 kt. Seas up
to 5 ft. Chance of rain showers. 

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

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

&&

.TIDES/COASTAL FLOODING...
950 PM update...

We issued a coastal flood statement for the upcoming high tide
as a near 1 ft surge may lead to pockets of splashover given
high astronomical tides.

Previous discussion...

High astronomical tides coupled with onshore flow may result in
some splashover/very minor coastal flooding late this week. 
However...wind/waves do not appear high enough to result in a 
significant issues.

&&

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

&&

$$

SYNOPSIS...Frank/Loconto
NEAR TERM...KJC/Frank
SHORT TERM...Loconto
LONG TERM...Frank
AVIATION...KJC/Frank
MARINE...Frank/Loconto
TIDES/COASTAL FLOODING...Frank