Lexington Weather

Lexington, Massachusetts, USA

66°F

5/20/2024 3:00pm

Lexington Conditions: Clear
Temperature: 65.8°F / 18.8°C
Dew Point: 56.5°F
Relative Humidity: 72%
Wind: Calm, 10-min avg: Calm, gust: 3 mph
Barometer: 29.99 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 200707
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
307 AM EDT Mon May 20 2024

.SYNOPSIS...

Today is the cornerstone of a pattern change towards more mild 
conditions though clouds and cool temperatures will still hang 
across the Cape. Much milder conditions Tuesday with heat peaking on 
Wednesday. Frontal passage Thursday, with increased thunderstorm 
chances, will usher in more seasonable temperatures for the holiday 
weekend. 

&&

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

Today is the cornerstone of a pattern shift towards summer like 
conditions across southern New England with surface high pressure 
centered over southern NH/ME and mid level ridging continuing to 
build into southern New England.  Greatest forecast challenge today 
will be cloud cover, and by proxy, high temperatures with high 
resolution guidance again struggling overnight to accurately depict 
clearing across wide swaths of the area. As of this writing, very 
little fog has developed across the region away from the coast while 
light drizzle continues to advect over SE MA and the Cape. 

Most abundant sunshine will be across the CT River Valley where 
clear conditions developed overnight. While dry air moves in with 
force above 850mb today, and RH falls to as low as ~25% between 850-
300mb this afternoon, model soundings show the surface inversion/low 
level moisture driven by onshore flow will hold steady through at 
least mid morning across eastern MA/RI, with the NAM suggesting the 
inversion won't mix out until early afternoon. With high to our 
north, a gradual wind shift from NE to S/SW through late evening 
should kickstart clearing this afternoon. So, while gradual clearing 
is expected for portions of the CWA from Worcester, east, clouds 
will really struggle to mix out across the Cape/Islands/South Coast 
today where PWATs remain above 1.1". With a high degree of 
uncertainty in the cloud cover forecast, particularly for eastern 
MA/RI, there is potential for a bit of a bust in temperatures,
particularly in eastern MA. Generally trended towards a middle 
of the road solution with highs around 80 across CT and western 
MA, in the low 70s across the coastal plain, and as cool as the 
upper 50s on Nantucket!, more clearing and deeper mixing will 
result in much warmer temperatures. &&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...

Patchy fog development is possible again tonight though developing 
SW winds around 5-10kt, and dewpoint depressions around 4F should 
preclude widespread fog across the interior. With that said, the 
same mechanism of SW flow should encourage some fog/reduced vsbys 
along the South Coast and Islands where both temps and dewpoints 
hover around 50F and SSTs are in the low to mid 50s. 

With surface high still in place and mid level ridge growing 
stronger, guidance is still quite poor in diagnosing cloud cover for 
Tuesday, with several CAMs, like the HRRR, showing a mostly cloudy 
day. Not feeling particularly keen on the "cloudy all day" solutions 
with PWATs falling to as low as 0.7" and low RH above the mixed 
layer, but early morning fog/stratus, particularly along the south 
coast, may take a few hours to burn off. Decent boundary layer 
mixing with 850mb temperatures approaching 16C will support highs 
well into the 80s across much of the interior, though onshore flow 
across the south coast will keep temps much cooler there, in the 60s 
to perhaps 70s. Given SW flow, Boston MAY see their first 80F 
reading of the season, and while it's possible the ASOS may not hit 
80F, the western suburbs of the city have a high likelihood of doing 
so. 

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...

Highlights: 

* Peak of the midweek very warm weather is Wednesday - 80s highs 
  with a few locations in the CT and Merrimack Valleys seeing a few 
  90 degree readings. Tolerable humidity levels.

* Cold front Thursday afternoon to early evening brings a reprieve 
  to the warmth but also could fuel scattered t-storms. 

* Drier and with cooler, more seasonable temperatures Friday into 
  the Memorial Day Weekend.

Details: 

Still looking at Wednesday as the warmest day of the anticipated 
midweek spell of very warm temperatures reminiscent of summer. 850 
mb temps rising into the +14 to +16C range along with full mixing 
and southwest breezes should promote these very warm temperatures. 
NBM probs continue to show moderate to high (60 to 90 percent) probs 
of temps reaching 90+ degrees in the CT and Merrimack Valleys, with 
lower (no greater than 30%) probs of 90+ high temps in central MA, 
northeast CT and portions of Metrowest. These are a bit higher than 
the prior 13z NBM cycle. The one thing that is a little peculiar in 
the NWP is that modeled dewpoints were shown to be in the lower to 
mid 60s across a good majority of the guidance; I don't think values 
this high are necessarily realistic given the well mixed PBL and 
adiabatic downsloping that should be taking place per model 
soundings. That's also a concern/source of uncertainty as we move 
into Thursday as well. Advertised highs upper 80s to low 90s in the 
CT and Merrimack Valleys - although those could be somewhat 
conservative - and the mid 80s for most of northern RI, central and 
eastern MA. Cooler readings near the south coast (upper 60s-mid 70s) 
where SW flow will allow for a bit of a maritime breeze. Should be a 
pretty mild Wednesday evening too, mid 50s to lower 60s, but did 
note some of the guidance showing weakening convection moving in 
from eastern NY into our far western counties in MA later in the 
overnight. Left a slight chance PoP mention in the forecast for now 
for the eastern Berkshires but we'll have to see if this 
materializes in subsequent guidance. 

A cold front then trudges eastward through Southern New England on 
Thursday. It seems as though in the 00z guidance that the front is 
about 6 hours slower than prior model cycles; that may allow for 
another very warm day with a slower timing, but also could allow for 
slightly more convective instability to help spark showers/t-storms 
along the front, particularly near and west of I-95. Similar to 
Wednesday, models are now forecasting dewpoints in the lower to mid 
60s although those seem a bit high. If we can see dewpoints push 
into the lower 60s, we could see CAPE values around 800-1500 J/kg 
accompanied by deep-layer southwesterly shear magnitudes around 35 
kt (stronger flow mainly to our north). This is really on the 
borderline for storms becoming strong to severe and doesn't really 
scream "severe storms", but is something we'll be needing to monitor 
in the coming days. Kept highs in the upper 60s/around 70 near the 
south coast, and in the upper 70s to mid 80s further inland.

For Friday, the cold front's southeastward passage further offshore 
could be delayed a bit as the front's orientation becomes nearly 
parallel to the 700-300 mb flow, which could keep clouds and showers 
around the south coast for part of the day. However high pressure is 
progged to ridge into much of New England, with ridging continuing 
into a good part of the Memorial Day Weekend. Temperatures Friday 
into the weekend trend cooler and more seasonable than midweek, with 
highs in the 70s (60s near the coast). Thus so far, weather 
conditions for Memorial Day Weekend seem favorable.

&&

.AVIATION /07Z MONDAY THROUGH FRIDAY/...
Forecast Confidence Levels:

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

06z TAF Update:

Through Today... Low to Moderate Confidence in Vsby

Forecast continues to be complex with conditions ranging 
anywhere from LIFR to VFR across the region. Away from the Cape 
and Islands, terminals generally MVFR to VFR but patchy fog 
through 12/13Z this morning may degrade conditions to LIFR 
before widespread MVFR (on the coast) to VFR (elsewhere) 
develops late this afternoon, though, VFR may be delayed to as 
late as 18/20Z in places like Boston and Providence. On the 
Cape, IFR likely to persist for a good portion of the day, with 
a chance for MVFR very late this afternoon. Winds calm to 
slightly NE this morning will gradually swing around to the 
south, first across the CT River Valley around 16/18Z, and last 
for the Cape and Islands.


Monday night...Moderate confidence.

IFR to MVFR continues for the Cape and Islands, generally VFR
across the interior except IFR where patchy fog. 

Tuesday... Low cigs/vsby due to fog burn off quickly to VFR for
all terminals. Winds from the SW will be strong enough to stave
off a seabreeze. 

KBOS TAF...Moderate confidence in TAF trends 

MVFR to IFR this morning, MVFR occurring through early 
afternoon before clouds scatter out to VFR. VSBY/cig reductions 
possible again tonight. Winds NE this morning shifting to the 
south late this evening. 

KBDL TAF...High Confidence

VFR outside of some patchy fog both this morning and tomorrow
morning leading to brief periods of IFR. Winds calm becoming
S/SE this afternoon and SE tomorrow. 

Outlook /Tuesday Night through Friday/...

Tuesday Night: VFR. 

Wednesday through Wednesday Night: VFR. Breezy. 

Thursday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

Thursday Night: VFR. Breezy. Chance TSRA, slight chance SHRA.

Friday: VFR. 

&&

.MARINE...
Forecaster Confidence Levels:

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


Today... High confidence

Lows clouds/drizzle and patchy early morning fog with vsbys 1-3 
miles, slowly burns off with improving conditions mid to late 
afternoon. High pressure over SNE results in light NE winds become 
east and then southeast late in the day. 

Monday night...high confidence. 

Ridge of high pressure crest over the waters, yielding light SE
winds becoming SW. Patchy fog may limit vsby at times. 

Tuesday... high confidence 

AM fog will burn off to pleasant boating conditions. SW winds
with seas less than 5 ft. 

Outlook /Tuesday Night through Friday/... 

Tuesday Night through Wednesday Night: Winds less than 25 kt.
Seas locally approaching 5 ft. 

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

Thursday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers, slight chance of
thunderstorms. 

Friday: 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 2 PM EDT this afternoon for ANZ254.

&&

$$

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