Lexington Weather

Lexington, Massachusetts, USA

67°F

5/20/2024 2:30pm

Lexington Conditions: Few Clouds
Temperature: 66.7°F / 19.3°C
Dew Point: 57.7°F
Relative Humidity: 73%
Wind: SSW Calm, 10-min avg: 1 mph, gust: 9 mph
Barometer: 29.99 in Steady
Visibility: 10 miles
Rain Today: 0.00 in

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 152253
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
653 PM EDT Wed May 15 2024

.SYNOPSIS...

Steady rains spread in tonight as low pressure moves south of 
SNE. Rainfall could be heavy at times late tonight into early 
Thursday across CT, RI and perhaps into SE MA with some embedded
thunder. Rain tapering off Thursday afternoon into the evening 
gradually from north to south. High pressure nosing southward 
from the Canadian Maritimes will result in mainly dry weather 
Friday through the weekend with nothing more than a spot shower 
or two. It will be cool along the immediate coast with onshore 
flow through the weekend, but pleasant across the interior. 
Warmer temperatures are expected for the first half of next week
with mainly dry weather continuing.

&&

.NEAR TERM /THROUGH THURSDAY/...

Highlights...

650 PM Update...

* Steady rain spreads in late tonight into early Thu. Locally heavy 
  rain possible across CT/RI and perhaps into southeast MA. Risk of 
  poor drainage and nuisance flooding. Low prob of more significant 
  urban/small stream flooding if higher rainfall amounts can be 
  realized.

* Rain gradually tapering off on Thu from north to south as low
  pressure meanders further to the south/southeast

Previous forecast is on track. Showers...mainly light were
impacting parts of CT/RI and SE MA early this evening. The heavy
rain was now impacting Long Island and will slowly lift
northward impacting parts of CT/RI late this evening and
especially after midnight into Thu morning. Prior discussion
discusses this below.

Upper trough will lift toward southern New England overnight 
and cut off just south of southern New England into early Thu. 
The cutoff will gradually rotate south/southeastward later on 
Thu as a ridge builds into the eastern Great Lakes/Mid Atlantic.
A frontal boundary is parked nearby our region while low 
pressure meanders off the Mid Atlantic coast. The low will 
gradually lift toward Long Island tonight before sliding further
offshore on Thu. 

Guidance continues to handle the immediate near term quite poorly, 
so confidence in the near term a bit lower than would like. Biggest 
change to the forecast was to significantly increase QPF amounts and 
have also increased PoPs. Main concern is the potential for poor 
drainage/nuisance flooding across CT/RI and perhaps into SE MA late 
tonight into early Thu. Though if higher rainfall totals are 
realized than currently forecast and shown by CAMs then we could 
have a more significant Urban/Small Stream flooding event occur.

Overall there aren't any big glaring signals in the PWATs with both 
the NAEFS/EPS guidance only 1.5-2 STD above model climo per the 
situational awareness table. This translates to roughly 1-1.5 inch 
PWATs, which is elevated but not overly impressive for this time of 
year. Despite this our 925 hPa jet is anticipated to increase to 
roughly 25-50 kt E to SE oriented LLJ. The GEFS is indicating a 
roughly 3-4 STD E/W component of the wind, which impinges on the 
south coast late tonight into early Thu. Will have this on top of a 
relatively stagnant synoptic setup as a cutoff low at 700 hPa 
developing just to our south tonight. During the winter time these 
situations result in "mid level magic", which generates a heavier 
band of precipitation. This time of year it can occur as well, but 
the big question is the baroclinicity and where exactly does this 
heavy band set up? At this point signs point to CT/ RI and perhaps 
into southeastern MA per ensemble guidance. The HREF, NCAR MPAS and 
C-SHiELD all show low to high probs (10-70+ percent) of total QPF 
AOA 1 inch from roughly the MA Turnpike south with the highest probs 
across the immediate south coast of RI/CT. These pieces of guidance 
even are showing low to mod (10-50+ percent) of total QPF AOA 2 
inches, again with the best shots across southern RI/CT. This even 
matches up with the EPS and to some extent the GEFS/GEPS guidance 
though the probs are much more muted. Based on these signals blended 
the NERFC forecast with the latest NAM guidance, but if the LPMM per 
the HREF is correct then these totals may need to be increased 
further. For now am advertising 1-3 inches of rain across CT/RI with 
the highest amounts across southern RI. Elsewhere anywhere from 
roughly 0.25 to 1 inch is possible when precip is all said and done.

The big question will also be the convective influences. There isn't 
a whole lot of instability, but given the LLJ and forcing could see 
some embedded thunder within the rain shield. Could also have some 
efficient rain with warm cloud layer depths of 2.5-3.5 km. These 
factors would increase the heavy rain and flood threat. Despite 
these factors am not confident in the exact location and the 
heaviest rains could be quite localized, so have held off on a Flood 
Watch in the latest update. Though we will need to closely monitor 
things tonight into early Thu.

Should see the precipitation gradually tapering off during the 
afternoon as the low pulls away. So those precip chances will 
diminish from north to south, but it may not be until Thu evening 
where areas south of the MA Pike become dry. Given the stronger 
E/ENE LLJ in the latest update have increased wind speeds/gusts, so 
will be a bit breezy on Thu and feel quite cool given the onshore 
flow.

&&

.SHORT TERM /THURSDAY NIGHT/...

Highlights

* Any lingering rain showers across the south coast push offshore. 
  Cloud cover gradually decreasing with easing winds.

Ridge axis over the eastern Great Lakes/Mid Atlantic begins to build 
into western New England by early Fri. Caught between high pressure 
gradually building in from Upstate NY/northern New England and low 
pressure meandering SE of our region.

Low pressure continues to move further offshore, which will allow 
the high to gradually build in. This is anticipated to bring an end 
to the shower activity across the south coast Thu evening. Should 
see the cloud cover gradually diminishing, but there is some 
uncertainty here. Onshore flow could bring in a marine stratus 
layer, especially across eastern areas into early Fri. For now have 
kept the forecast more optimistic, but may need to be fine tuned in 
future updates especially if the high is weaker than currently 
forecast and isn't able to pump in the drier low level air.

Will still be a bit breezy during the evening, but should see those 
wind speeds/gusts diminishing as high pressure relaxes the pressure 
gradient.

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...

Key Points...

* Dry weather dominates Fri/Sat/Sun with perhaps a spot shower or two
* Highs Fri/Sat/Sun...55-65 immediate coast & 65-75 further inland
* Warmer the first half of next week with mainly dry weather

Details...

Fri/Sat/Sun...

High pressure over the Canadian Maritimes will nose southwestward 
into our region Fri into the weekend. This will combined with a low 
pressure system well to our south to generate onshore flow. This 
should hold high temperatures mainly between 55 and 65 along much of 
the immediate coast Fri and into the weekend. Temperatures inland 
are more tricky and will be dependent on the amount of solar 
insolation. There will be clouds at times with the onshore 
flow...but also may see drier air circulating around the high 
pressure system in the Maritimes. So away from the coast...current 
thinking is highs will run between 65 and 75 and whether we end up 
on the lower/higher side of those numbers will be dependent on the 
amount of sunshine received.  

There is not much synoptic scale forcing or deep moisture around to 
support much precipitation Fri into the weekend. Perhaps just a spot 
shower or two...but dry weather should dominate.

Mon/Tue/Wed...

It looks like the first half of next week should feature warmer 
temperatures as some upper level ridging builds into the region. The 
magnitude of the warmth remains uncertain as some shortwave energy 
will be moving across the mid-Atlantic states which may act to 
temper it. Nonetheless...temperatures probably average above normal 
with highs in the 70s to perhaps near 80 at times at least away from 
the immediate coast and any marine influences. Thinking dry weather 
continues to dominate given upper level ridging and lack of synoptic 
scale forcing.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...

Forecast Confidence Levels:

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

00z TAF Update...

Tonight...Moderate confidence.

Conditions deteriorate to MVFR/IFR from south to north and 
perhaps even some localized LIFR possible after 06Z as steadier 
rains push in from the south. Could see an embedded t-storm or
two overnight into Thu am push for portions of the south coast,
but given the limited instability the risk was too low to 
include in the TAF forecast. Heavier rains possible across CT, 
RI and perhaps into SE MA. Winds shift to the E and increase to 
around 10-15 kts. Will have some 20-25 kt gusts along the 
immediate south coast late.

Thursday...Moderate confidence.

Expecting to start the day with MVFR to IFR conditions and
perhaps some localized LIFR with a steady rain. Should see this
precip shield gradually collapsing as the day progresses, but
still could have some spotty showers linger into the afternoon.
Ceilings will gradually improve to MVFR/VFR by the afternoon
from north to south, but it will be a slow process given the
prolonged onshore E to ENE winds. Will have speeds of 10-15 kts
with gusts of 20-25 kts and the highest speeds along the
immediate south coast.

Thursday Night...Moderate confidence.

Should see conditions improve to VFR for most. Any lingering
showers across the south coast should push offshore before
midnight. Given the prolonged onshore flow there is potential
that eastern MA could see some MVFR/IFR stratus/fog. For now
have kept in VFR range, but something we will need to hone in on
as we get closer as it would likely impact the Fri AM push.
Winds ENE to NE at 5-10 kts.

KBOS TAF...Moderate confidence in TAF.

Conditions lowering to MVFR/IFR tonight as steadier rains move 
in and will impact the Thu AM push. Winds shift to the E/ENE 
tonight. Winds remaining out of the ENE on Thu with rain showers
gradually tapering off and ceilings slowly improving to 
MVFR/VFR.

KBDL TAF...Moderate confidence in TAF.

MVFR conditions will gradually lowering to IFR tonight and 
winds shift from the SE to the E/ENE. Winds becoming gusty Thu 
morning through the afternoon, but showers gradually tapering 
off and slowly improving from IFR to MVFR/VFR.

Outlook /Friday through Monday/... 

Friday: VFR. Breezy. 

Friday Night through Saturday: VFR. 

Saturday Night: Mainly VFR, with local IFR possible. Slight
chance SHRA.

Sunday: Mainly VFR, with local MVFR possible. Breezy. Slight
chance SHRA.

Sunday Night: Mainly VFR, with local IFR possible. Breezy.
Slight chance SHRA.

Monday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

Have expanded the SCA to include Buzzards Bay and Vineyard Sound
due to the strong E/ENE winds/gusts.

Tonight...High confidence.

Increasing E/ENE winds 15-25 kt toward Thu morning. Winds
gusting 15-30 kts with the highest gusts across the southern
waters. Could even have some brief gale force gusts across the
southern outer waters into Thu AM. Could also see some embedded
thunderstorms across the southern waters. Vsby lowers to 1-3 
miles in areas of fog and rain, especially southern MA and RI 
waters.

Thursday...High confidence. 

E/ENE winds 15-25 kts, gusts 20-30 kts possible. Areas of rough
seas with rain. Could still see some embedded thunderstorms
across the southern waters through the first half of the day.
Rain tapering off from north to south across the eastern waters
during the afternoon.

Thursday Night...High confidence.

Rain tapering off across the southern waters north to south as
the night progresses. Winds ENE to NE at 10-20 kts with gusts
15-20 kts gradually easing as the night progresses. 

Outlook /Friday through Monday/...

Friday through Saturday: Winds less than 25 kt. Seas up to
5 ft. 

Saturday Night through Sunday Night: Winds less than 25 kt.
Seas up to 5 ft. Slight chance of rain showers. 

Monday: Winds less than 25 kt. Seas up to 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Thursday for 
     ANZ231>234.
     Small Craft Advisory from 2 AM to 11 PM EDT Thursday for 
     ANZ235-237-254.
     Small Craft Advisory from 11 PM this evening to 11 PM EDT 
     Thursday for ANZ255-256.

&&

$$

SYNOPSIS...BL/Frank
NEAR TERM...Frank/BL
SHORT TERM...BL
LONG TERM...Frank
AVIATION...BL/Frank
MARINE...BL/Frank