Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 031336
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
936 AM EDT Fri Jul 3 2020

.SYNOPSIS...

A backdoor cold front will progress southwestward across Southern 
New England today, bringing clouds and cooler onshore flow. 
Showers and thunderstorms that could produce localized heavy 
rains are possible from midday through evening across western 
Massachusetts and western Connecticut. Mostly cloudy conditions 
early on the Fourth of July will trend mostly clear to partly 
cloudy by the afternoon, with temperatures still a bit cooler 
than average. Progressively warmer and more humid air invades 
the area next week with chances of afternoon/evening showers or 
thunderstorms each day. The best chance for storms looks to be 
Wednesday and Thursday.

&&

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

935 AM Update...

* Showers & Embedded T-storms with localized street flooding 
  possible near and west of the CT River Valley this afternoon

Backdoor cold front continued to push southwest and has reached
west central MA and the CT/RI border at mid morning. This 
backdoor front will cross most of our region by afternoon, 
although lose a bit of its punch near and west of the CT River 
Valley. NE flow and low clouds should keep afternoon temps along
the coast in the upper 60s to lower 70s. Meanwhile...near and 
west of the CT River Valley temps were already near or above 80 
and some of these locations may briefly peak in the middle 80s 
despite and abundance of clouds.

The main focus of the forecast will be pockets of heavy rainfall
and localized street flooding risk this afternoon near and west
of the CT River Valley. Enough heating will occur in this region
to result in 1000 to 1500 J/KG of Cape. This will combined with
shortwave energy in west to northwest flow aloft to trigger an
abundance of showers and embedded t-storms this afternoon.
Relatively weak wind fields and limited lapse rates will keep
the severe weather threat low. That being said, the backdoor 
cold front will provide a focus for training of showers and
embedded t-storms with heavy rainfall. This will result in the 
potential for some localized street flooding especially near and
west of the CT River valley.

Meanwhile, across eastern MA and most of RI instability will be
quite limited given maritime airmass. That coupled with the 
fact that the best forcing will also be further west will result
in mainly dry weather other than a few brief showers. It will 
be rather cloudy though through the afternoon.

&&

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

Tonight:

Models show lighter rains shifting southeastward into the more 
stable air later tonight across RI and into eastern/southeast MA and 
the Cape as the surface low shifts further southward into the 
offshore waters. Winds should take a turn towards the north as this 
occurs, with still considerable cloud cover across most of the area. 
Enough breaks in the clouds could lead to areas of mist or fog, with 
somewhat better chances across western locales with the wetter 
ground. A cool and cloudy evening with lows in the mid 50s north 
and west, to lower-mid 60s south and east.

Saturday/Independence Day:

Surface low will continue to pivot southeast of 40N/70W on 
Independence Day. More breaks in the clouds earliest in western MA 
and western CT, with considerable clouds across eastern MA and much 
of RI. However, sky cover even in eastern areas should start to 
trend more optimistically by the afternoon. Though most of the 
holiday should be dry, there is a limited risk for isolated showers 
or even a t-storm east of the Berkshires. As today, warmer temps to 
exist in the west where more sun supports highs in the mid 80s. 
Across central MA and RI looking at mid to upper 70s, and in the low 
to mid 70s across eastern MA and the Cape.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...

Big Picture...

Northwest flow over the weekend with a shortwave moving through on 
Sunday. The pattern then becomes more zonal, with additional 
shortwaves moving through the flow. Consensus favors moving these 
shortwaves through on Monday and Thursday, but this timing can and 
probably will change with each model run. 

Contour values at 500-mb are forecast near normal over the weekend, 
but trend above normal during the week. This indicates the deep-
layer warms and supports a warming trend through the week except for 
any days we experience a cooling easterly flow at the surface.

Moderate confidence in the forecast.  

Daily details...

Saturday night and Sunday...

Axis of high pressure from Nova Scotia settles south Saturday night, 
turning our low-level flow from easterly to southwest. Shortwave 
moves through the flow, supported by an upper jet from Canada that 
moves across Northern New England. The GFS is faster in moving this 
shortwave/upper jet through the northern areas, the ECMWF slower. 
The airmass over Southern New England will be unstable Sunday, with 
CAPE of 500-1000 J/Kg, LI of -2 to -3. PW values will be around 
1.25, which is moist but not as high as can be this time of year.

Expect a quiet Saturday night with areas of fog. Weather remains 
rain-free Sunday morning with fog/stratus burning off to sun. The 
pressure pattern looks weak, so expect sea breezes to develop along 
the eastern coast. 

Showers/scattered thunder will initially develop north of 
Massachusetts during the midday/afternoon, with the northwest flow 
pushing the convection southeast. Best chance in Srn New England 
will be in Northern and Northeast MA, but convection may also 
develop in the low-level convergence along sea breeze boundaries 
near the coast. Winds 30-40 kt at 500-mb may add some strong wind 
gusts to the convection.

Dew points around 60 suggest Sat night min temps in the 60s. 
Guidance values for max temps were about 3F lower than observed 
values on Thursday, so this forecast will add 3F to guidance values 
for Sunday...85 to 90 inland and 80 to 85 in the coastal plain.

Monday through Thursday... 

Monday features a cold front moving through...the GFS moves it 
through early while the ECMWF waits til afternoon/evening. The 
airmass remains unstable with right entrance region of the upper jet 
over the region. The front stalls to our south, then sweeps north 
across Srn New England later Tuesday, which may generate some more 
convection. Another cold front approaches through Canada Wednesday 
and may stall over Northern New England Thursday. The airmass 
remains unstable through the period. PW values climb to 1.5 inches 
Monday, and then to near 2 inches Wednesday/Thursday. 

Bottom line...expect a chance of showers/thunder each day. High PW 
values Wed/Thu suggest local downpours. 

Mixing reaches to 850 mb or a little higher Monday, 925 mb Tuesday, 
and 850 mb Wed/Thursday. Temps aloft support mid to upper 80s 
Monday, low to mid 80s Tuesday, and lower 90s Wednesday/Thursday. 
Dew points are expected in the mid 60s early week, then upper 60s 
and around 70 Wed/Thursday. Heat index values will reach the mid
90s in some areas Wednesday and Thursday.

&&

.AVIATION /13Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Today: High confidence in categories/trends, moderate on 
timing.

Backdoor front advances MVFR-IFR stratus ceilings westward. The
exception to this is at western terminals (BAF-CEF-BDL-HFD, 
possibly ORH?) with heavy SHRA/TSRA developing near and east of 
the Berkshires, on the western edge of the stratus. Timing of 
TSRA looks to be between 16-23z, with brief MVFR to LIFR visby 
in heavy downpours. Will carry VCTS in the TAF for these western
terminals. Elsewhere mainly Light unrestricted -RA or even -DZ 
in stratus. NE winds 5-12 kt, strongest near the coast with 
gusts to 20 kt at times.

Tonight: Moderate confidence.

MVFR-IFR conditions at most terminals with continued stratus
eastern half of the airspace. Early showers spread SE into the
southern waters but nil threat of thunder. Could see possible 
mist or fog develop anywhere, but think western terminals have a
slightly better shot with any breaks in clouds. As sfc low sags
south late, winds to back a bit to N/NNE around 5-9 kt. 

Saturday/Independence Day: High confidence. 

Any fog in the west dissipates early to VFR. MVFR-IFR stratus
continues central/eastern airports, but gradual improvement mid
to late afternoon toward VFR. Isolated SHRA/TSRA near the 
Berkshires but coverage too limited to include in TAFs. N/NE 
winds become ENE/E 4-8 kt.

KBOS Terminal...High confidence in TAF thru 11z, then trends 
moderate. VFR thru most of the AM push, then sub-VFR stratus 
developing around/after 12z.

KBDL Terminal...High confidence in TAF thru 14z, then trends 
moderate. VCTS/TSRA developing ~15-17z, some question on how 
long thunder lasts.

Outlook /Saturday Night through Tuesday/...

Saturday Night: Mainly VFR, with areas IFR possible. Patchy BR.

Sunday: VFR. Slight chance SHRA, isolated TSRA.

Sunday Night: Mainly VFR, with local IFR possible. Patchy BR.

Monday: VFR. Chance SHRA, slight chance TSRA.

Monday Night: Mainly VFR, with local IFR possible. Chance SHRA,
slight chance TSRA, patchy BR.

Tuesday: VFR. Chance SHRA, slight chance TSRA.

&&

.MARINE...
SCAs remain in effect for most of the eastern waters through early 
this evening. Increasing northeast winds (gusts 25-30 kt) and 
building seas (3-6') associated with a backdoor front will lead to 
small craft advisory conditions on the eastern waters, though will 
get marginally close on southern waters later today into tonight. 
Areas of low clouds and fog tonight on most waters with periods of 
showers. 

While still cloudy, easing winds and lowering seas should be the 
rule for Independence Day. 

Outlook /Saturday Night through Tuesday/...

Saturday Night: Winds less than 25 kt. Patchy fog. Visibility
1 to 3 nm.

Sunday: Winds less than 25 kt. Local visibility 1 to 3 nm.

Sunday Night through Monday: Winds less than 25 kt. 

Monday Night: Winds less than 25 kt. Chance of rain showers,
slight chance of thunderstorms, patchy fog. Visibility 1 to
3 nm.

Tuesday: Winds less than 25 kt.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ250-
     254.
     Small Craft Advisory until 6 PM EDT this evening for ANZ231-
     251.

&&

$$
SYNOPSIS...WTB/Loconto
NEAR TERM...Frank
SHORT TERM...Loconto
LONG TERM...WTB
AVIATION...WTB/Loconto
MARINE...WTB/Loconto