LexMAWeather

Area Forecast Discussion
for Lexington MA

        
700 
FXUS61 KBOX 191857
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
257 PM EDT Sun Aug 19 2018

.SYNOPSIS...
High pressure over the Maritimes and a stalled front to our
south will combine to bring a northeast flow to Southern New
England through Monday night. A cold front will bring showers 
and thunderstorms Tuesday night into Wednesday. High pressure 
will build over the region Thursday, bringing fair weather and 
comfortable humidity through the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...

High pressure centered over the Maritimes will bring subsidence
much of the time, trapping low level marine clouds over NE
CT/RI/Eastern Mass. A weak shortwave moves overhead during the
night and could generate a few light showers/sprinkles in this
area, but confidence on that happening is low. Fewer clouds 
over the rest of Srn New England. 

With dew points 55 NW to 62 SE, expect min temps to range from
the high 50s in Western/Central Mass to the mid 60s along the
coast.

&&

.SHORT TERM /MONDAY THROUGH MONDAY NIGHT/...

High pressure remains in place over the Maritimes with a
resulting northeast low level flow over Southern New England. 
Low level moisture will diminish through the day as the sun
heats the air, so morning clouds should give way to more sun in
the afternoon. Cape Cod and Southeast Mass in general may have
the toughest road to sun. Another shortwave moves overhead and
may generate a light shower or sprinkle mainly along the Cape
and Islands.

Sea temps around the coast are in the upper 60s and 70s, which
will influence values along the coast, with temps in the lower
70s. Inland temps will be more subjet to mixing, with mixing to
900 mb suggested by the forecast data. Temps at that level are 
equiv to 9-10C at 850 mb...so max sfc temp potential would be in
the mid to upper 70s. If we were to mix up to 850 mb directly, 
slightly warmer temperatures would support max sfc temps around 
80F.

Similar air mass Monday night, with dew points a degree or two
lower. Expect min sfc temps roughly 55-65.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Highlights...

* Strong/severe storms possible Tuesday night and Wednesday.
* Fair weather Thursday into the weekend with comfortable humidity.

Overview...

Large scale pattern features uper trough over Northeast through end 
of week before we see re-emergence of upper ridge which begins to 
establish itself closer to SNE by end of weekend. Aside from brief 
return of tropical humidity Tuesday night and Wednesday ahead of 
cold front, this setup favors near average temperatures but more 
importantly brings our first stretch of more comfortable humidity 
(dewpoints in 50s and 60s) later this week and into weekend. It does 
appear that heat and humidity may very well return beyond Sunday as 
upper ridge becomes more dominant.

Details...

Main concern deals with potential for strong or even severe storms 
Tuesday night and Wednesday. Localized wind damage/flash flooding 
are main threats but an isolated short-lived tornado is also 
possible.

Low CAPE/high shear environment in place ahead of cold front should 
be favorable for strong or even severe thunderstorms late Tuesday 
night (after midnight) into Wednesday morning. SPC SREF shows decent 
probabilities of more than 500 J/kg of MLCAPE and 0-1km helicity of 
more than 100. Tropical airmass returns as GEFS shows precipitable 
water values climbing above 2 inches (+2-3 SD) and dewpoints rise 
back into 70s. 12z NAM/GFS support this thinking and show 30-40kt of 
0-6km shear collocated with best instability.

Linear convection looks to be favored but typical of these 
environments, we also need to be concerned with possibility of an 
isolated, short-lived tornado developing along line due to low LCLs 
and high 0-500m helicity of over 100. 

For now, this is something to be aware of which will be refined in 
later forecasts.

Front moves offshore around midday Wednesday. Drying and subsidence 
behind short wave will bring an end to showers/storms by early 
Wednesday afternoon. A prolonged stretch of dry and more comfortable 
weather follows Thursday and into weekend as high pressure builds 
over region.

&&

.AVIATION /18Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...Moderate confidence

Tonight...

Marine clouds in Eastern Mass expand west tonight, but in a
northeast flow it is questionable that the lower ceilings would
reach past the I-84 corridor...Worcester to Hartford. The
forecast brings cigs to 3000-4000 north and west of that line,
with MVFR cigs south and east and potential for IFR cigs over
Cape Cod and Islands. Low chance of a shower, with little
influence on visibility. Northeast wind maintain during the 
night but diminish in speed.

Monday...

VFR northwest and MVFR-IFR cigs southeast to start the day.
Expect sun heating to eat away at the sky cover, bringing VFR
conditions to all areas. Exception to this could be Nantucket
and parts of Cape Cod, where 2000-3000 foot cigs may linger
through the day. Northeast gusts to 20 knots mainly along the
coastline.

Monday night...

VFR lingers in the west. MVFR/IFR cigs expands over 
Eastern/Central Mass and RI/Northeast CT due to marine flow.

KBOS Terminal...Moderate confidence in TAF. 

MVFR ceilings, roughly 2000-2500 feet, build over the airport
and linger through most of the night. Conditions improve to VFR
around 8-9 AM.

KBDL Terminal...High confidence in TAF. VFR.

Outlook /Tuesday through Friday/...

Tuesday: Mainly VFR, with local IFR possible. 

Tuesday Night: Mainly IFR, with areas MVFR possible. Breezy.
Chance SHRA, slight chance TSRA, patchy BR.

Wednesday: MVFR/IFR conditions possible. Breezy. Chance SHRA,
slight chance TSRA.

Wednesday Night: Mainly VFR, with local IFR possible. 

Thursday through Thursday Night: VFR. 

Friday: 

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...High confidence. 

Northeast wind through the period. Gusty winds near 25 knots
slowly diminish tonight but 5-6 foot seas linger on the outer
waters. A combination of wind and seas will keep Small Craft
Advisory in effect through early tonight, with areal coverage
slowly diminishing through Monday and Monday night.

Spotty light showers/sprinkles are possible tonight and Monday,
but coverage will be very limited. Mainly good visibility. 

Outlook /Tuesday through Friday/...

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

Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers, slight chance of thunderstorms. 

Wednesday: Winds less than 25 kt. Seas up to 5 ft. Chance of
rain showers, slight chance of thunderstorms. 

Wednesday Night: Winds less than 25 kt. Local rough seas. 

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

Thursday Night through Friday: Winds less than 25 kt. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 7 PM EDT this evening for ANZ232.
     Small Craft Advisory until 6 PM EDT this evening for ANZ233-
     234.
     Small Craft Advisory until 3 PM EDT this afternoon for ANZ230.
     Small Craft Advisory until 4 PM EDT this afternoon for ANZ231.
     Small Craft Advisory until 3 AM EDT Monday for ANZ235-237.
     Small Craft Advisory until 2 AM EDT Monday for ANZ250.
     Small Craft Advisory until 8 PM EDT this evening for ANZ251.
     Small Craft Advisory until 6 PM EDT Monday for ANZ254.
     Small Craft Advisory until 4 AM EDT Monday for ANZ255.
     Small Craft Advisory until 9 AM EDT Monday for ANZ256.

&&

$$
SYNOPSIS...WTB/JWD
NEAR TERM...WTB
SHORT TERM...WTB
LONG TERM...JWD
AVIATION...WTB/JWD
MARINE...WTB/JWD