Lexington Weather

Lexington, Massachusetts, USA

70°F

7/27/2024 12:15am

Lexington Conditions: Clear
Temperature: 69.6°F / 20.9°C
Dew Point: 57.4°F
Relative Humidity: 65%
Wind: Calm, 10-min avg: Calm, gust: None
Barometer: 30.03 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 231901
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
301 PM EDT Tue Jul 23 2024

.SYNOPSIS...
Cloudy and generally dry weather prevails this afternoon,
although a weak area of low pressure to spread rain showers
northward into southeastern New England tonight. A stronger 
front will move across the region Thursday with more showers and
thunderstorms, followed by dry conditions Friday into early 
next week with increasing heat by Sun and Mon

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
200 PM Update:

Quasi-stationary frontal boundary remains anchored south the 
southern coastal waters, its position being reinforced 
southward by the passage of a weak area of low pressure now 
lifting NE from the waters NE of Nantucket. Although some 
shallow mixing has taken placed and allowed from limited breaks 
in overcast, weak ridge of high pressure extending southward 
from ME/NH has helped maintain considerable cloudiness for most 
of Southern New England. That's led to temperatures not moving 
very far with current readings in the upper 60s to lower to mid 
70s, and also has led to tempered instability values. So despite
elevated humidity levels, the lack of much sunshine has taken 
the edge off that humidity.

Pop-up showers may still develop in interior MA, CT into
northwest RI in widely scattered coverage thru sundown, but the
muted instability (about 500-800 J/kg of surface based CAPE) 
should mitigate the potential for thunderstorms. 

Into tonight, modest cooling of the boundary layer likely to
lead to re-developing and expanding low cloud cover from SE to
NW. This occurs ahead of a weak area of low pressure now over
the TN Valley region, which is forecast to move NE toward the
lower Hudson Valley by early Wed AM. Associated showers tied to
this feature to overspread our southern coastal waters and
northward toward the Hartford-Providence-Plymouth corridor.
Looking overcast but generally dry elsewhere. Lows mid 60s to
around 70.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
3PM Update

Highlights: 

* Periods of rain Wednesday morning will taper off late morning into 
  early afternoon. Muggy conditions with temperatures in the middle 
  and upper 70s.

* Spot shower and or thunderstorm possible during the afternoon and 
  early evening, mainly across western Massachusetts.

In general, Wednesday is expected to be fairly cloudy with continued 
unsettled conditions, which will lead to below normal afternoon 
temperatures across most of southern New England. The day's weather 
is driven by cyclonic flow aloft and shortwave energy ejecting off 
the Mid Atlantic coast and weak surface low moving northeast along a 
quasi-stationary boundary. This renews a surge of showers generally 
along the southern coast of Rhode Island and Massachusetts between 
12z and 18z. The caveat, position of the boundary. Current thinking 
it is extended south of Long Island and eastward to Nantucket, with 
the heaviest rains at the immediate shore and over the southern 
waters. But, if the boundary is further north so will be the heavier 
rains. There remains a low threat for flash flooding given the 
anomolously high PWATs and deep warm cloud layer. As result, the ERO 
for Wednesday remains 'Marginal'. 

Similar to Tuesday, showers ending by late morning/early afternoon 
with a few breaks of sunshine. Perhaps a brief afternoon shower and 
or thunderstorm, although there is little CAPE and a poorly forced 
environment. There area with the best chance would be far western 
Massachusetts, here forecast CAPE are between 500 and 1000 J/kg. 
And, as with most cases, there will be poor mid-level lapse rates 
and lack of effective shear, do not expect any organized strong to 
severe thunderstorms. 

Afternoon temperatures are cool, with many locations not getting out 
of the 70s. The lower Connecticut River Valley would be the spot to 
hit 80 degrees, essentially Hartford to Springfield. While it is 
cool it will also be muggy/soupy with dew points in the low 70s.

Wednesday evening through the predawn hours of Thursday another hit 
of rain is possible with a cold front. While not delivering any 
relief from the humidity, more of a wind shift from the southeast to 
the southwest. Overnight temperatures settle into the upper 60s and 
low 70s with similar dew points. Given winds are light, there could 
be areas of fog develop during the second-half of the night and 
sticks around for the start of Thursday morning commute. 

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
3PM Update

Highlights...

* A few severe storms possible Thu afternoon/evening.
* Drier and seasonable conditions Fri into the weekend.
* Heat and humidity begins to return early next week.

Details...

Thu and Thu evening... Moist airmass with PWATs hovering around 2.0 
inches will have been in place since yesterday by the time we reach 
Thu.  However, noting just a hair bit of mid-lvl drying coming out 
of the mid-Canadian mid-lvl ridge that supports a slight uptick in 
H7-H5 lapse rates to above 6.0 C/km during the afternoon and evening 
as a relatively progressive cold front shifts SE across the region.

With this, models supporting ML CAPE values approaching 1000 j/kg 
with peak heating and thanks to the approaching front, have most 
ingredients to reinforce convective activity from W to E through the 
afternoon/evening.

Regarding and severe potential...with these parameters in place, 
combined with sfc dewpoints in the lower-mid 70s as noted by the 
previous forecaster and modest low-mid lvl shear, would support some 
disorganized cells.  Tail end of current CAMs also support this at 
this time. Noting low lvl v-profiles, so with the moist airmass in 
place could be a wet downburst/damaging wind threat with convection 
that gets going.  This will be the most likely risk.

Otherwise, with the upper lvl trof still in the tilt process, the 
front remains fairly progressive, and so most of the remaining 
convection should be done by evening.

Fri into the weekend... Cold front will usher in a drier/cooler 
airmass with originations from as far north as the Yukon.  Noting 
gradual ridge building to 1-2 std deviations by Sun, associated with 
some fold-over ridging from the central US/Canada.  Temps and 
dewpoints closer to seasonal normals through the weekend.

Early next week... With the continued aforementioned ridging, 
modified central US airmass combined with a bit of a feed of the 
Gulf of Mexico developing, will see increasing heat and humidity 
into the middle portion of next week.  Typical model divergence with 
mid-week shortwave likely fed by convective feedback issues as 
guidance suggests MCS development upstream.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:

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

18Z TAF Update:

Through 00z Wed: High confidence. 

Most persistent IFR-LIFR cigs for the Cape and Islands, with 
periods of mist and light NE winds at times. Elsewhere, mix of 
generally VFR-MVFR ceilings; -SHRA develops for TAFs N/W of I-95
thru 00z Wed, some potl for TS BDL-BAF-ORH too, but is remote a
chance to include in TAFs. Light NE to E winds, trending N for 
western airports, speeds under 5 kt.

Tonight and Wednesday: High confidence in trends, moderate on 
timing.

IFR-LIFR conditions continue for the Cape and Islands tonight.
Mix of VFR-MVFR elsewhere should degrade to widespread MVFR
tonight from SE to NW, with periods IFR and mist developing
overnight. Mainly dry thru 04z, but risk increases for SHRA
after 04z mainly south of HFD-PVD-PYM line, which gradually
spreads northeast towards ORH and BOS by daybreak Wed. Timing of
the onset of widespread MVFR/IFR tonight and when (if?) it 
starts to improve on Wed is uncertain; there could be some 
optimism/1-category improvement by 14-16z Wed but that timing is
subject to adjustments. E to SE winds, speeds 4-7 kt.

Wednesday Night: Moderate confidence. 

Deterioration likely Wed night into the MVFR-IFR range in all 
areas on NE to N winds, though timing uncertain. 

KBOS TAF... Moderate confidence in TAF. OVC VFR (FEW-SCT 
015-025 bases lurking around) to continue until at least 02z. 
Deterioration likely thereafter to MVFR after 04z, with possible
window of IFR after 08z. Unclear exactly when improvement may 
occur but opted for 14z Wed for 1 category improvement. ENE/ESE 
winds 4-7 kt thru the period.

KBDL TAF... Moderate confidence in TAF. OVC VFR bases with
possible SHRA thru 00z. Conditions then deteriorate to MVFR
overnight with possible IFR after 07z associated with light
SHRA and mist. Light N winds thru tonight, trending light SE
Wed.

Outlook /Thursday through Sunday/...

Wednesday Night: Mainly VFR, with local IFR possible. Chance
SHRA, isolated TSRA.

Thursday: Mainly VFR, with local MVFR possible. Chance SHRA,
isolated TSRA.

Thursday Night: VFR. Chance SHRA, isolated TSRA.

Friday through Saturday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

Tonight through Wednesday Night... High Confidence. 

Stationary boundary draped across the southern waters will provide a 
source for shower and storms to develop tonight into the first-half 
of Wednesday along with periods of poor visibility. Overnight and 
Wednesday winds are from the ESE to SE with speeds 10 to 15 knots 
and seas between 2 and 4 feet. Wind shift occurs late Wednesday 
evening into Thursday morning as a weak front moves across the 
waters, wind shift from SE to SW, wind speeds less than 10 knots 
with seas 3 to 4 feet. 

Outlook /Thursday through Sunday/...

Wednesday Night through Thursday: Winds less than 25 kt. Seas
locally approaching 5 ft. Chance of rain showers, isolated
thunderstorms. 

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

Friday: Winds less than 25 kt. Seas locally approaching 5 ft. 

Friday Night through Saturday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

SYNOPSIS...KJC/KS
NEAR TERM...Loconto/Hrencecin
SHORT TERM...Dooley
LONG TERM...MD
AVIATION...Loconto/Dooley/Hrencecin
MARINE...Dooley