Lexington Weather

Lexington, Massachusetts, USA

46°F

4/18/2024 9:14am

Lexington Conditions: Overcast
Temperature: 46.0°F / 7.8°C
Dew Point: 35.2°F
Relative Humidity: 66%
Wind: Calm, 10-min avg: Calm, gust: 3 mph
Barometer: 30.12 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 141358
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
958 AM 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/...
10 AM Update:

MCS over western NY/northern PA if accellerating quickly east to
toward the Hudson River Valley. Latest short-term model guidance
suggests precip associated with this feature will reach the
forecast area sooner than originally anticipated. Thus, we now
have increased PoPs beginning around noon time for the western
areas and then increasing each hour toward the east. Given we
should see increasing cloudiness sooner as well, we have lowered
high temps down a couple of degrees since there should
theoretically be less time for diurnal heating. Beyond these
early afternoon showers, there is some uncertainty with respect
to coverage and intensity of additional precip expected
late afternoon/early evening with a passing frontal wave. We
will have more info with our next update when most of the 12Z
hi-res model guidance should be available. See the previous
discussion below for additional details.

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 midafternoon 
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 midlevels. 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 /14Z SUNDAY THROUGH THURSDAY/...
Forecast Confidence Levels:

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

12z TAF Update: 

Today and Tonight: High confidence in TAF/trends, moderate on 
t-storm coverage and timing. 

Categories should be primarily VFR. Clear skies this morning, 
increasing mid to high clouds late morning/early afternoon hrs 
from W to E. By afternoon, a frontal system arrives from the WNW
spreading at least light showers after 18z. Potential exists 
for thunder late in the day (after 22z) with the best chance at 
BAF and BDL this afternoon. Given the 9 hour limit on PROB30, 
transitioned PROB30 groups to TEMPO -TSRA late this afternoon 
and early evening. It is possible that PVD could see -TSRA too, 
but think the best chance is southwest of there. Winds to back 
from W to SW/S and increase in speed to around 10-15 kt; gusts 
to 20-25 kt over southeast MA.

Likely ongoing SHRA (possible TS for southern/southwestern 
airports) to then shift SE into the waters by 02-04z. SCT-BKN 
VFR ceilings to then predominate for the evening/overnight. SW 
winds shift to W/WNW and decrease to around 5-10 kt by daybreak.

Monday: High confidence. 

VFR. NW winds increase to around 8-12 kt with gusts 18-22 kt,
with strongest gusts over the terrain. Some spotty, no impact
showers possible from diurnal cu development Monday afternoon,
but will not have any impact to flight categories. 

KBOS TAF...High confidence in TAF. Mainly VFR through the
period. That said, risk of -SHRA could start as soon as 18z but
thinking best chance is after 22z. Showers end after 02z. W
winds around 10-13 kt this morning shift to SW at similar
speeds, with occasional gusts to 20 kt. Winds shift to W/WNW
tonight.

KBDL TAF...Moderate to high confidence in TAF. VFR most of
today, although could see temporary/brief MVFR/IFR visby if 
TSRA can develop around 22-02z. W winds become S today around
10-12 kt, then shifting to SW-W-WNW tonight.

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...Loconto/KS
MARINE...Loconto/KS