Lexington Weather

Lexington, Massachusetts, USA

51°F

4/19/2024 9:51am

Lexington Conditions: Overcast
Temperature: 50.5°F / 10.3°C
Dew Point: 43.9°F
Relative Humidity: 78%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 30.14 in Falling Slowly
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 141717
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
117 PM EDT Sun Apr 14 2024

.SYNOPSIS...
Brief period of sunshine early this morning but will see
increasing clouds as a frontal system moves in this afternoon
and tonight. This frontal system to bring a risk for showers and
embedded thunderstorms over western Massachusetts, western and
southern Connecticut and southern Rhode Island later this
afternoon into early tonight. Only light rains are expected 
elsewhere in Southern New England. Showers then diminish 
late tonight. Dry and seasonably mild Monday through Wednesday.
Unsettled pattern develops late week with near normal 
temperatures a few chances for rain Thursday into the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
1 PM Update

Forecast is on track. Remnants of MCS discussed earlier is now
over southern New England in the form of light showers. These
will continue east over the next couple of hours while gradually
fizzling out at the approach the coast. After a brief lull in
precip this afternoon, we should have another round of light to
moderate showers across the region during late afternoon and
continuing into the evening hours. A rumble of thunder will be
possible across western MA and CT and possibly over portions of
the south coast later this evening. Additional details to follow
when the afternoon forecast package is published in a couple of
hours. 

Previous Discussion

310 AM Update: 

The deep mid/upper level trough responsible for scattered to 
overcast cloud cover and blustery westerly winds yesterday is slowly 
but surely exiting into the eastern outer waters early this morning. 
Starting to see decreased cloud cover over western New England, with 
still considerable cloud cover east, and with west breezes around 10-
15 mph, current temps are still mainly in the 40s (lower/mid 40s 
interior MA/northern CT, and mid/upper 40s RI and eastern MA). 
Taking a wider view, well to the northwest over the Upper Midwest is 
a noticeable but weak-amplitude shortwave disturbance embedded in 
rather strong NW flow aloft, with weak sfc low near northern WI. 
This feature separates the exiting cooler airmass over the Northeast 
CONUS with warmer, conditionally unstable air over the Midwest and 
OH Valley. This latter frontal system will be affecting our area 
later today and tonight. 

Should see skies trend mostly clear for a brief period of time this 
morning, allowing for some sunshine for a short period of time. 
However mid to high clouds race in pretty quickly by mid to late 
this morning from W to E, which will gradually lower and thicken as 
we move into the afternoon with the approach of weak low pressure 
that's now over the western Great Lakes. Some models generate light 
showers later this morning/early afternoon, but RH profiles are 
still somewhat dry, so I'd expect it to be a generally dry day 
through the early afternoon. 

Better chances for at least showers develop around mid-afternoon 
today, but not expecting any washout. However a portion of the 
low pressure's warm sector still looks to spread into western 
MA, western/southwestern CT and perhaps into southern RI later 
this afternoon, and that could spark thunderstorms across a 
portion of that area later this afternoon. It is still a bit 
unclear how far north and east the thunder potential would 
extend. Flow fields are quite strong through a deep depth of 
atmosphere, which should favor enough convergence for t-storms 
to fire on. Advection of steeper mid- level lapse rates takes 
place too, around 6.5-7.5 C/km between 700- 500 mb. In addition,
there is a rise in dewpoints into the upper 40s. There isn't 
much of any surface-based CAPE to speak of, but there is about 
200-600 J/kg of most-unstable CAPE associated with the steeper 
lapse rates in western CT, parts of western MA and into 
central/southern RI. The various convective-permitting models 
and HREF key on areas well to our southwest in PA/far southern 
NY as having the greatest severe-storm potential, and I can't 
really disagree with that evolution. But I do think there is the
potential for t-storms from a rough Springfield to Providence 
line SW later this afternoon into tonight; a couple of the 
strongest cells could be strong enough to produce small hail 
given the mid-level lapse rates and stronger flow in mid-levels.
Didn't go with any extra wording as yet though because I think 
the risk is more isolated, and SPC also has backed off its level
1 of 5 severe-thunderstorm risk from yesterday, restricting it 
to the Berkshires and Hartford County with its update this 
morning. PoP was raised into the Likely/low Categorical range, 
with a 10-30% chance for thunderstorms. 

Should be warmer than yesterday too, with highs in the 50s to lower 
60s. Should be fairly breezy from the SW today, around 10-15 mph.

For tonight...the frontal system from this afternoon producing 
showers and a few thunderstorms is expected to shift ESE into 
the waters early tonight. Rain chances end around midnight 
toward dry weather, and we should see a gradual decrease in 
cloud cover later tonight/overnight as high pressure ridges in. 
Lows in the 40s with light NW winds.

&&

.SHORT TERM /MONDAY/...
Showers and a few rumbles of thunder come to and end well before 
daybreak Monday. Mid level trough persists over southern New England 
as weak surface low moves northeast into the Canadian Maritimes. 

The forecast for Monday has trended a bit cloudier compared to 
previous updates. Morning clouds will mix out quickly as drying W/NW 
flow develops, but residual moisture at the top of the mixed layer 
will yield diurnal cumulus development at or around 5,000ft. While 
mainly dry, cannot rule out a stray shower or two, particularly 
during peak diurnal cu coverage in the late afternoon across eastern 
MA. All in all, the vast majority of localities will remain dry for 
the duration of Patriots Day with a mix of clouds and sun. 

With daytime mixing, winds atop the boundary layer of around 20kt 
will mix gusts of 10 to perhaps 20kt to the surface, highest across 
the terrain of central and western MA. 

Aforementioned drying northwest flow will result in a bit of 
downsloping that will influence temperatures with 925mb temperatures 
ranging between 6-8C and 850mb temperatures as high as 5C by 
afternoon. Given foliage is beginning to green up, especially in 
urban centers and south of the MA Turnpike, combined with dewpoints 
in the 40s, downsloping influence will not be as significant as we 
experienced during our last "warm spell" a week or so ago. Still, 
highs should climb into the low to mid 60s for much of the region.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Highlights:

* Broad trough settled over the region for much of the week with a 
  brief period of mid level ridging in the Tuesday-Wednesday 
  timeframe

* Above normal temperatures give way to seasonable conditions as an 
  unsettled pattern develops mid to late week

Tuesday and Wednesday...

Pesky broad trough finally relinquishes its grasp on southern New 
England Tuesday afternoon as mid level ridging builds in from the 
west; the ridge cresting over our region late Wednesday. 

Efficient mixing to ~775mb Tuesday with downsloping flow will set up 
another above average day with temperatures expected to tack a few 
degrees onto Monday's highs; generally in the mid 60s with a few 70s 
possible in the CT River Valley. Persistent NW flow will limit 
diurnal cumulus development, with Tuesday looking to be our sunniest 
day of the week. Surface winds shift to the north and eventually 
east on Wednesday. With still cool SSTs, in the 40s, and increasing 
cloud cover ahead of our next potential precip maker, expecting a 
cooler day Wednesday even with stronger ridging aloft. Highs will 
range from the low 50s on the outer Cape to low and mid 60s inland. 

Wednesday Night and Beyond...

Cutoff low over the Great Lakes attempts to make a run for New 
England late Wednesday night and Thursday, but surface high centered 
just east of Nantucket looks to eat away, at least initially, at the 
precipitation shield of the advancing front. By Thursday, cutoff 
weakens significantly and moves north into Ontario as mid level 
ridging redevelops. Anticipating unsettled conditions Thursday with 
near normal temperatures, but trends towards a drier day have 
started to emerge compared to previous forecasts. Will note, that 
there is significant uncertainty regarding the timeframe with 
temperatures directly influenced by how far east the precipitation 
shield can advance, so populated both the PoPs and temps with NBM; 
placing a cap on PoPs at "40" to highlight the drier trend. 

More defined trough drops south into the Great Lakes on Friday as 
mid level ridging moves seaward. Another chance for rain is possible 
to round out the work week/start next weekend, but uncertainty again 
remains high at this time.

&&

.AVIATION /17Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

18Z TAF Update

Through 00Z... Moderate Confidence

VFR with scattered light rain showers mainly away from The
Cape/Islands terminals. Low chance for TSRA at BAF/BDL after
21Z. Moderate southwest winds from 10 to 15 knots with gusts
from 20 to 25 knots.

Tonight...High Confidence.

VFR. Any remaining SHRA/TSRA diminishes after 03Z. This will be
followed by some gradual clearing from west to east through 12Z.
Southwest winds becoming northwest from 5 to 10 knots.

Tomorrow...High Confidence.

VFR. W/WNW winds around 10 knots. Some gusts up to 20 knots
possible.

Tomorrow Night...High Confidence.

VFR. Steady WNW continue. 

KBOS TAF...Moderate to high confidence in TAF.

High confidence in VFR and SW winds this afternoon. Lower
confidence with respect to duration of -RA. Thinking best chance
for steady -RA will be between 21 and 00Z with more intermittent
showers thereafter.

KBDL TAF...Moderate to high confidence in TAF.

High confidence in VFR and SW winds this afternoon. Lower
confidence with respect to potential TSRA. The window would be
between 22Z and 00Z, but latest model guidance has trended
toward thunderstorm activity staying further to the south and
west. Nonetheless, TSRA remains a possibility and has been
included in a TEMPO group in the 18Z TAF. 

Outlook /Monday Night through Thursday/... 

Monday through Tuesday: VFR. Breezy. 

Tuesday Night: VFR. 

Wednesday: VFR. Slight chance RA.

Wednesday Night: Mainly VFR, with local MVFR possible. Breezy.
Chance RA.

Thursday: VFR. Breezy. Chance RA, chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Overall high confidence. 

SCAs are in effect for most waters through today, then continue
into Monday over the southern waters. 

SW gusts today start around 15-25 kt, but then increase over the
southern waters to around 25-30 kt this afternoon. Winds then
shift to W/NW and decrease tonight to around 10-15 kt by
daybreak Monday, continuing at these levels through Monday. 

Seas around 5-8 ft over the outer waters this afternoon
continuing into tonight, then a slower decrease in seas to
around 3-5 ft through Monday.

There is a risk for thunderstorms later this afternoon into 
tonight over the southern waters, with light showers over the 
eastern waters. Otherwise, dry weather to prevail for Monday.

Outlook /Monday Night through Thursday/... 

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. 

Wednesday Night: Winds less than 25 kt. Chance of rain. 

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft. Chance of rain,
chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ231-
     251.
     Small Craft Advisory until 2 PM EDT Monday for ANZ232>235-237-
     250.
     Small Craft Advisory until 8 PM EDT Monday for ANZ254>256.

&&

$$

SYNOPSIS...Loconto
NEAR TERM...RM/KS
SHORT TERM...KS
LONG TERM...KS
AVIATION...Belk/RM
MARINE...Loconto/KS