Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 071344
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
944 AM EDT Fri Aug 7 2020

.SYNOPSIS...
Surface low pressure passing offshore and upper low pressure
crossing New England tonight and Saturday may bring a few
showers/thunderstorms through Saturday. However, dry weather
will dominate much of the time. Very warm temperatures and 
rising humidity levels will be the story for early to middle of next 
week. Isolated to widely scattered showers or storms possible each 
day, though better chances on Wednesday and Thursday associated with 
a slow-moving cold front.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
930 am update...

an area of light rain is trekking across eastern CT as I type.
none of the hi-res models had this feature. The rain is pretty
light, seeing mesonet obs of .01 to .04 inches. Expect the
preciptation to make across RI and into SE MA over the next few
hours, but given that there is minimal deep forcing and the 
atmosphere below 600mb is on the dry side per the 12z Chatham 
sounding, would think it will ultimately diminish to sprinkles. 
The other current radar echos across the Cape and Islands are 
really just mid level clouds with little more than a sprinkle 
likely making it to the ground (again, per the CHH sounding). 
For the rest of the afternoon, looks like a good deal of clouds.
Will continue the idea of low PoPs across the south coast/Cape
due to the weak frontal remnants south of Long Island, and
across far western sections due to being on the edge of the best
instability across NY State. 

7 AM Update: 

Regional radar mosaic shows light showers mainly across the 
Berkshires and moving into parts of NW CT. Additional somewhat 
steadier rains were moving northward from the waters south of
Long Island and should begin to affect the South Coast and the
Cape and Islands around mid-morning. Radar looks worse than
what's actually falling though, as much of these echoes are 
falling from fairly high-based cloud cover (7000' or higher). 
Best chance for rains thru this afternoon is south of the Mass 
Pike, and especially central RI into the South Coast, Cape and 
the Islands closer to the stalled front. Thinking for most 
areas away from the South Coast, conditions should be more 
cloudy than wet. Clouds and the onshore NE flow will keep 
temperatures somewhat cooler today. 

Previous discussion...

A shortwave from the Great Lakes affects our weather today and 
Saturday. For today, the trough is digging as it moves through the 
Eastern Lakes, with the supporting upper jet aligned with the 
southwest upper flow over New England. The right entrance region of 
the jet is over Southern New England, supporting dynamic lift over 
the region. Surface low pressure attempting to develop over
Virginia is a sight on this supporting lift.

High moisture remains offshore well south of the region, but 
PWs do reach 1.5 inches so moisture is not completely absent. 
Stability parameters are mediocre or lower. Early morning radars
do show showers over the ocean well south of Long Island moving
northeast. 

With showers existing south of the southern waters, there is a 
chance of showers reaching Srn New England aided by the
developing surface low as it passes offshore. But mostly along 
the South Coast and Islands, possibly a few miles farther north.
Low-end pops will be used for today. Observations and satellite
show lots of clouds over New Jersey and PA moving northeast, so
sky cover will trend toward mostly cloudy skies where that is 
not already the case.

Mixing will reach to 850 mb where temps are 12-13C, supporting highs 
in the lower 80s. An east wind into the Eastern Mass coast may keep 
that area in the 70s. An east-southeast wind along the South Coast 
and Islands will do the same for that coast as well.

&&

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

The Great Lakes shortwave moves into New England tonight, with weak 
cold advection aloft from the accompanying cold pool. Stability 
parameters trend to unstable. For example, the total-totals rise to 
around 50 in Wrn Mass/CT after midnight. Expect scattered showers 
and possible thunder during this time. Dew points in the 60s, so 
expect similar values for early morning min temps. 

Saturday...

The upper trough and cold pool remain overhead much of the day, 
moving offshore late. This will generate scattered showers/thunder 
for at least a part of the day. Expect much of the day to be dry, 
but punctuated by these scattered showers.  Continued east wind will 
keep coastal areas in the 70s. The CT Valley, being farther west, 
should reach the lower and possibly mid 80s.

&&

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

* Drying out with more sun and warming temps for rest of the 
  weekend. 

* Heat/high humidity then return Mon into Wed. 'Heat Wave' criteria 
  could be met at a few sites. 

* Daily thunder chances thru midweek, greatest now on Wed-Thu with 
  a slow moving cold front.

Details... 

Saturday Night into Sunday Night: 

Overall not too bad for the remainder of the weekend. Clearing and 
drying trend Sat night into Sunday under modest shortwave ridging. 
For late Sunday and moreso Sunday night, a few recent model runs 
have offered a suble and weakening vort max progress ESE somewhere 
near or around our area from the convectively-active Midwest. Lot of 
spread in the guidance on timing and placement of this vort max, 
many of which are more into northern New England. Something to watch 
but will side toward a dry forecast for now. 

850 temps rise into the upper teens Celsius range by Sunday with 
most areas well into the 80s, though downsloping off the Berkshires 
could bring a few locations to near 90. Though dewpoints will be on 
the rise, humidity levels should be still fairly tolerable. 

Monday through Thursday: 

Typical mid-summer pattern then develops for early next week, one 
that will bring with it building heat and humidity levels toward 
uncomfortable/oppressive levels. 850 mb temps are well into the 
upper teens (around +17 to +19C) with dewpoints mid 60s to lower/mid 
70s. It's a generally warm pattern, but prolonged SW flow may offer 
near daily chances at evening lower clouds and fog and briefly delay 
stronger heating. Specific to temperatures, that's really the only 
significant source of uncertainty at this point and those details 
not likely to be resolved prior to a couple days before. A few sites 
stand a chance at seeing a heat wave in this period, and elevated 
heat indices could reach levels where Heat Advisories could garner 
consideration. Expect several days in the upper 80s to low 90s with 
little relief at night (low to mid 70s lows Mon/Tues and Wed nites).

A cold front will be slow to move from the Great Lakes into the 
Northeast states, and it will serve as a focus for greater t-storm 
chances toward midweek. Greater anticipated net coverage of clouds 
may also help reduce temps Wed-Thu a bit, but it still will remain 
quite humid. Otherwise, thunder chances look on the low side and 
mainly near the terrain in western MA/western CT.

&&

.AVIATION /14Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

12z TAF Update:

VFR most areas today, though MVFR conditions may affect the 
Islands in scattered showers and an embedded thunderstorm.
Better chance at deteriorated conditions set in tonight as MVFR
becomes more widespread across Srn New England with a few 
showers or a thunderstorm. IFR ceilings possible along the 
South Coast and Islands. Conditions improve to VFR Saturday, 
mainly in the afternoon. 

Light winds trend from the east today at 10kt or less, mostly in
Eastern MA and RI with winds more light/variable in the CT
Valley. Continued light east wind Saturday.

KBOS Terminal...High confidence in TAF. 

KBDL Terminal...High confidence in TAF.

Outlook /Saturday Night through Tuesday/...

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

Saturday Night through Sunday Night: VFR. 

Monday: VFR. Slight chance SHRA.

Monday Night: VFR. 

Tuesday: VFR. Breezy. Chance SHRA, chance TSRA.

&&

.MARINE...
Winds/seas will remain below Small Craft thresholds through
Saturday. Wind directions trend from the North early today, then
from the east late morning. Winds remain from the east through
Saturday. Main concern for mariners will be the risk for a few 
thunderstorms, especially tonight and early Saturday. 

Outlook /Saturday Night through Tuesday/...

Saturday: Winds less than 25 kt. Slight chance of rain showers,
slight chance of thunderstorms. 

Saturday Night through Monday Night: Winds less than 25 kt. 

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

&&

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

&&

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