LexMAWeather

Area Forecast Discussion
for Boston, MA

        
876 
FXUS61 KBOX 180737
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
337 AM EDT Wed Jul 18 2018

.SYNOPSIS...
Scattered showers linger along the S/SE coast into the morning
hours ahead of a sweeping cold front. Otherwise dry weather is
anticipated the rest of the work week into Saturday along with 
much lower humidity, including warm afternoons and cool nights. 
Unsettled weather in the form of showers return Saturday 
night/Sunday and continues into much of next week along with 
increasing humidity.

&&

.NEAR TERM /UNTIL 6 AM THIS MORNING/...

230 am update...

Scattered showers continue ahead of a cold front continuing to
push SE towards the coast, reaching by morning.

Additional wave activity through cyclonic flow associated with
an attendant H5 trof axis. Ascent acting upon available moisture
out ahead of the front, plenty of which, lending to shower activity.
Hardly any instability, have removed thunder, if any, isolated 
rumble. Expect shower activity to push SE towards the coast into
morning, lingering into the mid to late morning hours longest for
locations such as Nantucket. 

Biggest headaches revolves around cloud cover and fog. Forecasting
either has been a struggle with no one piece of forecast guidance
verifying. Obvious is scouring NW, however slow, lending to a later-
forecast process of partial clearing lending to radiational cooling
and potential valley fog development. So long as mid to high level
clouds linger, the opportunity for shallow ground fog is limited.
Noting fog outcomes out towards the Susquehanna and Delaware River
Valleys in NE PA and Mid-State NY. If clearing proceeds within the
CT River Valley, would expect not long after shallow ground fog
issues. 

As to SE New England, ahead of the cold front, moist, tropical air-
mass remains. Surprising light surface winds, however breezy NW per
vertical wind profilers at H925-85. A measure of mechanical mixing
must be ongoing preventing lower ceilings from developing keeping
them roughly 4-10 kft. Wedge of drier air noted in RAP RH fields at
such levels, also noting differences between ALY and CHH 0z soundings.
However model forecasts continue to signal low conditions enveloping
the Cape and Islands towards morning. Not an incredible amount 
of confidence, nor in rain chances as well. Have to simply watch,
wait and see. 

Today...

Clearing SE with a cold front pushing offshore, becoming dry and
pleasant. Surging lower theta-E / drier air with precipitable
waters dropping down around 0.50 - 0.75 inches. H85 cold air
advection as well, temperatures dropping from +14 to +10C. This
allowing a well-mixed boundary layer up to H8, mix down of faster
winds, drier air. Looking at N/NW winds around 10 to 15 mph with
dewpoints dropping around the low to mid 50s, some locations NW
in the upper 40s. Few to scattered clouds, strongest winds out
towards the Cape and Islands with potential gusts up to 20 mph.
Highs around the low to mid 80s.

&&

.SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/...

Tonight...

Perhaps some lingering clouds, otherwise mostly clear with light
winds allowing for radiational cooling. High pressure in control.
Looking at lows down around the low to mid 50s. Given antecedent
rains, dewpoints mixing out at max heating of the day near forecast
lows, can't rule out patchy, shallow ground fog, especially in 
sheltered interior valleys and notoriously prone areas. 

Thursday...

Slightly cooler and drier conditions compared to today. Mostly
clear with high pressure. Light winds. Altogether, highs around
the upper 70s to low 80s, few to scattered clouds, allowance of
sea-breezes along the shores towards mid to late morning.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...

330 AM update...

Highlights...

* Very pleasant weather w/warm afternoons & cool nights thru Sat 
* Showers/storms and humid conditions return Sat ngt into next week

Overview... 

18.00z ensembles (ECENS & GEFS) in good agreement on ridging 
developing over the northeast Fri and lingering into Sat. At the 
surface this translates to 1020 mb high over New England Fri and 
then drifting into the northwest Atlantic Sat, however ridge axis 
extends back into New England. This will provide very pleasant dry 
weather with comfortable humidity Thu night into Saturday. However 
by Sat night/Sunday ensembles and deterministic guidance in good 
agreement on a pattern change with mean mid level trough axis 
developing across the Great Lakes and OH Valley. This sets up deep 
moist S-SW flow along the eastern seaboard sending a tropical 
moisture stream from the lower latitudes into New England beginning 
Sat ngt/Sunday and continuing into much of next week. Thus a wet but 
warm and humid pattern for our region. 

Temperatures...

850 mb and 925 mb temperature anomalies are near normal Fri and Sat 
so expecting seasonably warm temperatures during the day. As for the 
nights, given the dry airmass in place with PWATs -1 to -2 standard 
deviations, light winds and mostly clear skies will provide 
radiational cooling conditions and support overnight lows in the 
50s. Given this setup will lean toward the cooler MOS guidance for 
min temps. Fri likely the warmer of the two days with highs well 
into the mid 80s. High pres overhead Fri will yield afternoon 
seabreezes keeping the coastline a bit cooler. Still warm Sat inland 
with highs 80-85 but 70s for the coastline given synoptic scale flow 
will support southeast winds. Not as warm Sunday with onshore flow 
and showers likely. However it won't feel that cool with dew pts 65-
70. As for next week seasonably warm temps expected (highs 80-85) 
but with humid airmass over the region overnight lows will be warmer 
than normal with lows 65-70. 

Precipitation...

Ridging provides dry weather Thu night thru Sat. Then late Sat night 
and especially by Sunday negative tilt mean trough sets up from the 
Great Lakes into the OH valley. This yields deep moist S-SW flow 
along the eastern seaboard into New England. This will result in 
showers from time to time Sat night/Sunday into much of next week. 
Heavy downpours possible too as PWATs climb to +1 to +2 standard 
deviations. 

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...6z update.  

Into morning...Moderate confidence.

NW LWM-ORH-HFD expecting partial clearing towards morning which
may contribute to shallow, patchy ground FG in interior, valley
terminals. 

SE IJD-PVD-BOS, especially closer to the coast, monitoring closely
the potential for IFR-LIFR CIGs BKN004-008 that linger as late as
the mid to late morning hours for ACK. 

Otherwise light winds increasing out of the N/NW towards morning.
Generally VFR. 

Today...High confidence. 

Last MVFR-IFR lingering along the Cape and Islands into the
morning hours as winds shift N/NW, becoming breezy. VFR.
Strongest winds over the Cape and Islands with potential gusts
up to 20 kts around 15-21z. 

Tonight...High confidence.

Light winds. Mainly VFR. Focus on interior, sheltered valleys
for potential shallow, MVFR-LIFR ground FG. 

Thursday...High confidence.

Light winds. VFR. Sea-breezes along the shores developing
towards mid to late morning. 

KBOS Terminal...

Hold with a mix of MVFR-VFR into the morning push, improving
thereafter. Lower confidence with respect to IFR. 

KBDL Terminal...

So long as mid-high level CIGs remain, lower chances of any
VSBY issues with respect to shallow, MVFR-IFR ground FG. Should
mid-high CIGs clear, greater concern with respect to VSBY issues.

Outlook /Thursday Night through Sunday/...

330 AM update...

Thursday Night: VFR. Patchy FG.

Friday through Friday Night: VFR. 

Saturday: Mainly VFR. 

Saturday Night: Mainly MVFR, with areas VFR possible. Breezy.
Chance SHRA.

Sunday: Mainly MVFR, with areas VFR possible. Chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Thursday/...High confidence.

230 am update...

Into today...

Cold front sweeping offshore towards morning with scattered
showers lingering. SW flow in advance, generally around 10 kts.
With passage, could see brief N gusty winds 10-15 kts with 
gusts up to 25 kts. Seas remaining below 5 feet.

Tonight into Thursday...

Beneath the influence of high pressure, generally light N winds,
5-10 kts. Onshore sea breezes developing during the mid to late
morning hours on Thursday along the near-shore. Seas remaining 
below 5 feet. 

Outlook /Thursday Night through Sunday/...

330 AM update...

Thursday Night through Friday Night: Winds less than 25 kt. 

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

Saturday Night: Winds less than 25 kt. Local rough seas. Chance
of rain showers. 

Sunday: Winds less than 25 kt. Areas of rough seas. Chance of
rain showers. 

&&

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

&&

$$
SYNOPSIS...Nocera/Sipprell
NEAR TERM...Sipprell
SHORT TERM...Sipprell
LONG TERM...Nocera
AVIATION...Nocera/Sipprell
MARINE...Nocera/Sipprell