Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 081120
AFDBOX

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

.SYNOPSIS...
Showers and isolated thunderstorms move through the region this 
morning ahead of a warm front. Then a second round of scattered 
showers and thunderstorms develop this afternoon with a few strong 
storms capable of producing hail. Periods of unsettled weather 
continue late this week into this weekend with showers at times. 
Temperatures will average below normal late this week into the 
weekend. Milder/Seasonable temperatures should return early next 
week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
7AM Update...

Current radar shows the first line of thunderstorms now entering
western MA and CT. Expecting those storms to quickly move east
this morning being offshore by the noon hour. As for the 
afternoon severe threat, no big changes at this time, however, 
guidance is trending afternoon temps up in the CT river valley 
this afternoon which will enhance the amount of instability 
available today. 


Key Points...

* Showers and isolated t-storms move through this morning

* A second round of scattered showers and t-storms develop this 
  afternoon. A few strong to marginally severe storms possible, with 
  large hail the primary threat

Decent instability burst and moisture transport ahead of a warm 
front will result in a period of showers and embedded t-storms 
moving W to E through SNE this morning. This occurs with a sharp 
increase in PWATs along with KI increasing from near zero to the 30s 
in a 6 hr period along the nose of a modest low level jet. The 
duration of showers will be relatively brief, not lasting more than 
2-3 hours in a given location and should be moving to the east this 
afternoon as dry slot moves in from the west.

Then we will have to watch for a second round of scattered showers 
and t-storm development this afternoon as a fairly robust mid level 
shortwave moves into New Eng. Good cooling aloft as 500 mb temps 
drop to -16 to -18C will result in rather steep mid level lapse 
rates 7-7.5 C/km overspreading the region. This will contribute to 
MUCAPES 500-1000+ J/kg developing across much of the region this 
afternoon. Expect scattered convection developing as forcing for 
ascent increases ahead of the cold front. HREF max updraft progs 
suggest convective initiation in eastern NY and the Berkshires then 
scattered storms moving ESE across the region. While storms may be 
mostly elevated, potential exists for marginally severe hail given 
impressive effective bulk shear values of 50+ kt along with steep 
mid level lapse rates. SPC has all of SNE within a marginal risk but 
it appears focus for strongest storms will be from northern CT 
through interior MA.

There is some uncertainty regarding the extent of surface based 
instability this afternoon due to presence of cooler low level 
airmass. Damaging wind threat will be dependent on surface based 
storms developing. Best chance of SBCAPES up to 1000 J/kg will be 
across western MA and northern CT as these areas may sneak into the 
warm sector ahead of a cold front, and decent low level lapse rates 
develop here. This is where strong to damaging wind threat will be 
highest with any strong storms. 

Highs may reach into the 70s across the Hartford to Springfield 
corridor with mostly 60s elsewhere. But temps may remain in the 50s 
across portions of NE MA.

&&

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

Any convection lingering into the early evening will weaken and move 
offshore as subsidence develops behind departing shortwave along 
with somewhat drier air moving in with decreasing PWATs. Expect 
partial clearing developing in the interior, but stratus and patchy 
fog may linger in the coastal plain. Lows upper 40s and lower 50s.

Thursday...

The day should start out dry as a wedge of dry air will be over SNE. 
But moisture increases from the SW during the afternoon ahead of 
next shortwave dropping through the Gt Lakes and OH valley, with low 
pres moving to mid Atlc region. Risk of showers will increase during 
the afternoon. Highs should be mostly in the 60s, mildest CT valley 
and coolest along the coast in eastern MA where developing NE flow 
will likely hold temps in the upper 50s here.

&&

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

* More widespread rain possible Friday into Saturday night

* Below average temperatures continue through the weekend

* More seasonable temps return next week

Friday

A shortwave trough exits out of the Ohio River Valley becoming 
negatively tilted as it moves over SNE. The surface low is initially 
vertically stacked with the shortwave aloft and begins to occlude as 
new low forms and intensifies offshore to the east. Although the 
best forcing will be to the south of SNE, the chances for a 
widespread rain are increasing. As a mid level low passes over and 
intensifies to the south of the region, this will drag down rather 
cold 850mb air around 0C to -2C. With ENE winds at the surface, 
along with cold 850mb temps, low clouds, and rain, high temps may 
struggle to even reach 50F across the region. Rain looks to continue 
into Friday night before tapering off Saturday morning. Overnight 
lows look to drop into the low 40s, with upper 30s possible in the 
high terrain. Not out of the question that snow mixes in with rain 
across the high terrain in the Berkshires.

Saturday

Surface and midlevel low move well out to sea bringing widespread 
rain to end in the morning. However, an upper level closed low drops 
south over the region leaving SNE under cyclonic flow aloft.  With 
weak forcing aloft and the mid level cold pool still in place, this 
will likely lead to a hit or miss light showers/drizzle. High temps 
on Saturday don't look to recover much with thick low level cloud 
layer still around. High temps should only reach the low to mid 50s 
with continued onshore flow.

Sunday

Another weak surface low exits the Ohio River Valley and passes just 
south of SNE.  This could bring another round of widespread light 
rain to the region. Uncertainty with this system remains high given 
the weak upper level forcing and not much run to run consistency on 
the track of the low. Highs remain below normal Sunday and could 
once again struggle to reach 50F.

Next week

The pesky upper level low and mid level cold pool finally move east 
as a midlevel shortwave brings WAA and 850mb temps above 0C to even 
+10C by mid week.  This will allow high temps to moderate back into 
the low to mid 70s. As for rain chances, high pressure to the south 
should keep things dry early in the week. Rain chances look to 
increase again mid to late week, but uncertainty is high at this 
time.

&&

.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

12z TAF Update...

Confidence continues to grow for either TSRA or SHRA with VCTS
for all terminals this morning. Low status and fog has cleared 
the Cape and Islands this morning. Expecting those terminals to 
remain VFR for a few hours before becoming IFR again later this 
morning as the line of storms rolls in.

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. Chance
SHRA.

Friday through Friday Night: MVFR/IFR conditions possible.
Chance RA.

Saturday: MVFR/IFR conditions possible. 

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

Sunday: Mainly VFR, with local MVFR possible. Chance RA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Through Thursday...High confidence.

Low level jet will bring a period of SW gusts to 20+ kt to south 
coastal waters this afternoon. Winds diminish tonight, becoming 
north by Thu morning then NE 10-20 kt during Thu afternoon. Seas 
remain below SCA thresholds. Areas of fog will reduce vsbys at times 
over south coastal waters through tonight. Scattered showers and 
embedded t-storms move through this morning into early afternoon. 

Outlook /Thursday Night through Sunday/... 

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

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

Saturday: Winds less than 25 kt. Seas up to 5 ft. 

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

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

&&

.TIDES/COASTAL FLOODING...
High astronomical tides coupled with onshore flow may result in some 
splashover/very minor coastal flooding during the overnight high 
tide cycles tonight and Thu night. 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...KJC/KP
NEAR TERM...KJC/KP
SHORT TERM...KJC
LONG TERM...KP
AVIATION...KJC/KP
MARINE...KJC/KP
TIDES/COASTAL FLOODING...