Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 131348
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
948 AM EDT Sat Apr 13 2024

.SYNOPSIS...
An upper level low pressure area will bring increasing clouds
and blustery conditions today across Southern New England, along
with hit or miss showers. Clouds briefly decrease tonight, but
increase again on Sunday as a fast-moving frontal system moves
through. This should bring another round of light rain showers,
but there is a chance for isolated thunderstorms in southwest
Connecticut Sunday afternoon. Showers diminish overnight Sunday
setting the stage for a mild and dry start to the work week. 
More unsettled by mid week with a large degree of uncertainty 
revolving around temperatures.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

10AM Update: 

The previously mentioned cloudcover has increased in the last
several hours, as has the shower coverage. Radar returns
continue to move east over central MA/eastern CT while moving
northeast from the ocean toward Nantucket. This is due to the
placement of the low to our north and cyclonic flow around it.
Forecast remains on track. 

Previous Discussion...

Clouds have begun to overspread the region but overall seem a 
bit slower to progress eastward, with widespread sunshine across
much of metro-Boston. Do anticipate clouds will pick up in 
coverage pretty quickly over the next two hours as daytime 
heating picks up and wave to our south moves east. Additionally,
a few light showers have begun to develop across the upslopes 
of the Berkshires. These showers too will pick up in coverage 
over the next few hours. 

330 AM Update: 

Another seasonably mild early morning across Southern New England, 
with much of the area still embedded in southerly flow both at sfc 
and aloft. Though we had started the night with generally clear 
skies, over the last couple hrs we've started to see skies fill back 
in with cloudiness, especially in interior MA, CT and northwest RI. 
The culprit responsible is a slow-moving upper low seen in 
early morning water vapor imagery over the Northern Adirondacks 
in NY, with a trough axis extending southward from it into 
northeast PA. Underneath this upper low is a rather chilly low- 
level airmass, with the 00z BUF RAOB sampling 850 mb temps of 
0C. Regional radar was showing scattered to numerous rain 
showers across NY and PA underneath this trough. This upper 
level feature will drive today's weather. Here in Southern New 
England, current temps were in the mid 40s to low 50s with 
southerly breezes around 10-15 mph. 

The upper level low moves NNE into northern Quebec today, in turn 
shifting its trough axis eastward through SNE. With a lot of 
lowlevel RH around, expect the increasing cloud cover trends to 
continue and spread gradually eastward through today. Thus skies 
should trend closer to mostly cloudy (some cloudy breaks at times) 
in eastern MA/RI but with full overcast from Worcester County 
westward into adjacent CT. Upper trough axis will advect the cool 
low-level airmass into our area, leading to periodic instability 
type showers; thinking more isolated shower coverage further east 
one goes, but dodging more scattered to numerous shower coverage in 
the interior and especially in/around the Berkshires. Either way, 
QPF is not substantial at all, even where showers trend greater in 
number and coverage.

The cold low level airmass will favor steeper lower level lapse 
rates leading to an increasingly blustery to gusty day even 
underneath the cloud-filled sky. WSWly breezes with gusts around 25 
to 30 mph should be common by late this morning into the afternoon 
associated with a 35 kt 925 mb jet. Gusts around the higher terrain 
in the Berkshires and northern Worcester Hills have the potential to 
knock on the door of Advisory levels with gusts 35-40 mph this 
afternoon, up to 45 mph at times. Opted to refrain from Wind 
Advisories though, since it looks borderline - I'd imagine it 
should gust pretty well with the shallow colder air mass, but 
this area is also where temperatures project to be the coolest, 
and not sure we'll be able to fully mix some of the stronger 
jetcore to the surface. 

All in all, a generally cloudy and blustery day across SNE, though 
parts of eastern MA should still see some early sunshine. Though 
there could be showers around at any time, none are necessarily of 
consequence. Temps probably struggle to get out of the mid 40s to 
lower 50s in the interior, and could rise into the mid 50s for the 
MA coastal plain and parts of RI where some early sun could allow 
for some limited warming.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
330 AM Update:

Tonight:

Upper level trough axis and its cool pocket of air rotates through 
SNE into the Maritimes through tonight. Should see a decrease in 
clouds/shower chances tonight, although westerly winds will still be 
a bit breezy. With skies clearing out and turning drier, it could 
get pretty chilly tonight in portions of interior MA/CT where 
today's highs aren't likely to climb very much. Have lows here in 
the mid/upper 30s to around 40, with lows eastward into MA and RI 
generally in the lower 40s. 

Sunday: 

We start Sunday with low-amplitude ridging aloft embedded in a fast 
northwest flow. Quite a thermal contrast from the cooler airmass 
retreating into Canada with a warm and conditionally unstable air 
mass forecast to extend from the Ohio Valley and northern mid-
Atlantic, with its far northeastern fringe brushing into far 
southwestern portions of NY and CT. A progressive shortwave trough 
and weak sfc low spreads cloud cover and showers to SNE at least 
during the afternoon hrs Sunday. Mid-level lapse rates are pretty 
steep (around 6.5-7.5 C/km), and there are some CAM-based solutions 
which could favor possible thunderstorms in northern CT and perhaps 
into far western MA, but most instability parameters seem to 
key on areas further to our south and west in PA/far southern 
NY. This is also where SPC has outlined up to a level 2 out of 5
risk for severe weather in their Day-2 Severe Weather Outlook. 
The extent to which we can get into the unstable warm sector 
should be monitored but it has the look of one of those setups 
where SNE gets shut off from the unstable warm sector. I don't 
think the chances for thunder here are necessarily great, but I 
did include a slight chance mention of thunderstorms from a 
Springfield to Providence line south and west for now; the 
extent to which we can break out into the warm sector is a big 
question mark. 

Highs on Sunday should reach into the 50s to lower 60s.

&&

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

* Showers and perhaps a rumble of thunder persist overnight Sunday 
  before drying trend develops Monday and Tuesday

* Mild start to the week, uncertainty in temperatures the second 
 half of the work week

* Unsettled conditions return late Wednesday and beyond

Sunday night...

Broad trough drops south out of Quebec overnight as weak low moves 
north of our area across southern NH/VT before eventually moving out 
into the Gulf of Maine. Most precipitation associated with this low 
falls prior to 00Z Monday, but a few lingering showers will 
last through approximately 06Z. The threat for any rumbles of 
thunder diminishes very quickly after sunset across our region, 
though will remain active just to our west across NY and PA as 
cold front seeps south and west. 

Given we will be in the cool sector overnight behind the frontal 
passage, lows will fall back into the 40s for most localities. 

Monday and Tuesday...

Weak low bringing showers to SNE overnight will move northeast into 
the Canadian maritimes by 12Z Monday morning. Drying northwest 
surface flow develops behind the departing low as 925mb LLJ 
diminishes from ~40 to 50kt overnight to less than 20kt by 18Z. 
Expecting gusts of up to 20kt Monday morning falling off through the 
day to around 10kt by mid afternoon. Some downsloping, especially as 
mid level ridge begins to build in from the west, will enhance mixing 
and may allow temperatures to overachieve a few degrees, but with 
foliage starting to green up, guidance likely has a good handle on 
temps in the mid to perhaps upper 60s. 

Another beautiful, dry day on tap for Tuesday with temps perhaps a 
few degrees warmer, especially in the CT River Valley as mid level 
ridge continues build. Winds likely to be a bit breezier, with 925mb 
winds increasing to around 25kt.

Mid-week and Beyond...

Unsettled pattern redevelops late Wednesday through the end of the 
weak as a parade of lows/fronts sweep through the region. Mid level 
ridge crests over southern New England Wednesday as weak 
low/shortwave develops over the Ohio River Valley. This shortwave 
will then move northeast into New England in the Wednesday night to 
Thursday timeframe increasing rain chances. More potent trough dips 
south from Ontario into the Great Lakes to round out the work week, 
so while there is potential for a brief period of dry weather, rain 
chances look renewed Friday as cold front approaches from the west. 

Temperatures will be quite tricky to forecast late week. 
Aforementioned mid level ridge should sustain mild temperatures, 
generally in the 60s, through Wednesday. Significant uncertainty 
revolves around the late week high temperature forecast as low 
pressure/cyclonic flow influences the synoptic pattern. 50th 
percentile NBM suggests highs in the mid to upper 50s for most, 
while 75th to 90th percentile guidance is significantly warmer, in 
the 70s to near 80F, to close out the week! The whiskers of 
uncertainty below the 50th percentile are much shorter, with 10th 
and 25th percentile guidance within 5 degrees or so of ensemble 
averages. For now, used straight NBM to derive temperatures, as 
surface flow will be the primary factor in deriving temps late week 
given weak 500mb ridging persists through Friday.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:

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

12Z TAF Update:

Today: High confidence. 

Some clear skies across eastern MA this morning, otherwise BKN/OVC
VFR to MVFR cloud bases (bases 015-040) with hit or miss light 
diurnal showers. Strongest chance at MVFR bases along and west 
of PVD to BED line, though BOS could see periodic MVFR bases. 
Bases should trend toward SCT-BKN VFR/MVFR later in the day. 
Turning blustery with winds 25-35 kt from the SW to WSW/W, but 
gusts could be as much as 40 kt at BAF and ORH.

Tonight: High confidence. 

VFR with decreasing cloudiness. West winds around 10-13 kt with
easing gusts.

Sunday: High confidence in trends, though moderate in timing
showers. 

Quick-moving frontal system from the Great Lakes spreads
increasing/lowering cloud bases into the VFR/MVFR range. Best
chance of scattered showers during the afternoon after 18z;
there could be a rumble or two of thunder south and west of a
BDL-PVD line later in the day but this is not set in stone. W
winds shift to SW then S around 10-12 kt.

KBOS TAF...High confidence in TAF. Mainly BKN VFR to at-times
MVFR bases (thinking 025-040). Could be a hit or miss shower 
but not enough confidence in coverage to include as a TAF 
mention. Bases should trend VFR by late afternoon to tonight. SW
winds increase around 15 kt with gusts 25 kt, then becoming W 
and decreasing to around 10-12 kt tonight.

KBDL TAF...High confidence in TAF. BKN to OVC VFR bases trend
MVFR early this morning, with periodic P6SM -SHRA. Bases start
to lift towards VFR levels tonight. S winds around 10 kt to
start, then shift to SW/WSW and increase to around 10-14 kt with
gusts 25-28 kt late AM to aftn, easing to 10 kt tonight. 

Outlook /Sunday Night through Wednesday/... 

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

Monday: VFR. Breezy. 

Monday Night: VFR. 

Tuesday: VFR. Breezy. 

Tuesday Night: VFR. 

Wednesday: VFR. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

330 AM Update: 

Overall high confidence.

Colder air over milder water temps should favor a gusty day on 
the waters today. SW to W wind gusts increase to around 25-30 kt
on the eastern waters, and up to 35 kt on the southern nearshore
and offshore waters today. SCAs were issued for the eastern 
waters, and Gale Warnings on the southern waters. Gale Warnings
will need to be downshifted to SCAs as winds subside, not likely
until later tonight. Seas will remain elevated and rough, around
7-10 ft on the outer waters.  

Easing west wind gusts expected late tonight into early Sunday;
however SW gusts ramp up again to around 15-20 kt with seas
around 4-6 ft. There could be a round of t-storms on the
far southern outer waters very late Sunday afternoon, but at
least scattered showers develop during the afternoon on all
waters.

Outlook /Sunday Night through Wednesday/... 

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Chance of rain showers,
isolated thunderstorms. 

Monday: Winds less than 25 kt. Local rough seas. 

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

Tuesday through Tuesday Night: Winds less than 25 kt. 

Wednesday: Winds less than 25 kt. Slight chance of rain
showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for ANZ230-
     236.
     Small Craft Advisory until 8 AM EDT Sunday for ANZ231-250-251.
     Gale Warning until 2 AM EDT Sunday for ANZ232-256.
     Gale Warning until 8 PM EDT this evening for ANZ233>235-237.
     Gale Warning until 5 AM EDT Sunday for ANZ254-255.

&&

$$

SYNOPSIS...Loconto
NEAR TERM...Loconto/BW/KS
SHORT TERM...Loconto
LONG TERM...KS
AVIATION...Loconto/KS
MARINE...Loconto/KS