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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
000
FXUS61 KBOX 251743
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
143 PM EDT Sun Aug 25 2019

.SYNOPSIS...

Scattered to broken low clouds through the remainder of today into
this evening, dissipating into Monday as drier, seasonable conditions
work into the region through midweek. Possible showers Wednesday
night into Thursday, yet with an eye on the track and proximity
to a tropical system offshore late week which could produce high
surf and dangerous rip currents along the immediate shoreline.

&&

.NEAR TERM /THROUGH TONIGHT/...

2 pm update...

Cool Summer Day. At this point the boundary layer with the late
Summer sun has mixed upwards of 3-5 kft agl and thus entrained
drier air which is working S round high pressure situated over N
New England into S Canada. The challenge is along the E/SE shore
with the onshore flow component off the cool, moist maritimes.
And with the prevailing low-level flow becoming more E with time
we're seeing that broken to overcast lower cloud deck push further
W into the interior. So some headaches with the temperatures and
cloud cover, however feel the consensus of near-term high-res
guidance including the NBM has a good handle on it. Wherever
there is more of a N component of interior flow will see more 
scattered cloud decks, whereas with ocean-onshore flow clouds
will be more broken to overcast. 

Winds have come up with boundary layer mixing with upwards of 30
mph wind gusts being observed across the Cape and Islands. Will
see this wind persist into early evening and then taper slightly.
Should remain breezy along the E/SE shoreline into the overnight
hours. More below. 

Tonight...

Clouds versus fog. High pressure and drier air squeezes S as evident 
in latest RAP forecast guidance. Column drying, its parent inversion 
lowering towards the surface down to H975 to 950, N to S, moisture 
is trapped beneath which the boundary layer remains destabilized by 
the cool E/NE onshore flow. Multiple factors to this forecast: 1.) 
depth of the dry-inversion, and 2.) direction of surface-H95 winds. 
Determines whether low clouds or possible dense fog. 

The 12z Caribou, ME an indication of outcomes where a dry-inversion 
is lower. Looking upstream at overnight observations, dry advection 
took place with surface dewpoints lowering despite light winds. Only 
a few spots reported dense fog given a decoupled boundary layer. For 
us, not much HREF support, signals of dense fog showing up for the 
Worcester, MA region into northeast CT. HREF overall points towards 
an E steering wind of low clouds over a good chunk of S New England, 
especially SE, additional issues along E-slopes of higher terrain. 
NBM and consensus of high-res guidance supports a cloudy solution as 
well. 

If clouds become more widespread, broken to overcast overnight, as 
perhaps dewpoints slip with the building presence of high pressure 
and a drying column aloft, then leaning with mild lows, shouldn't 
see much in the way of dense fog say isolated wherever clearing is 
allowed to proceed, higher confidence wherever it rained earlier 
today. Will hint again at low clouds along E-slopes of high terrain. 

Clouds win. 

&&

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

2 pm update...

Monday...

Looking to clear out. A continued long fetch off across the Gulf of 
ME, two things: 1.) Sea-surface temperatures have lowered a bit with 
the turbidity and upwelling of the waters, and 2.) dry air advection 
will be quite strong and deeper within the column compared to today 
and sinking. So an expectation that low clouds will linger across 
New England during the morning hours, gradually dissipating S/W with 
drier, subsiding pushing S. But not ruling out scattered to broken 
cloud decks given boundary layer mixing of onshore cool, moist air. 
Expect cloudy issues to linger especially S-coastal New England but 
lesser drizzle issues. Late Summer sun still strong while mixing out 
to H85 undergoing cold air advection, around +8C, another day of 70s 
throughout the interior while 60s along the E/SE shore. 

Monday night...

Becoming clear. Daytime heating concludes, dry air and high pressure 
in place, looking at the boundary layer to decouple and light winds 
to prevail over most of S New England albeit the southeast. Warm air 
advection aloft, moisture is trapped beneath the dry inversion just 
above the surface. Dense fog not out of the question especially in 
areas of onshore flow and/or higher dewpoints (notably earlier in 
the day with the cross-over threshold). Lean with cooler guidance 
and evaluate fog based on winds (below 3 mph) along with temperature-
dewpoint thresholds of 2 degrees or less. This as a first guess, fog 
seemingly will be more prevalent 

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...

4 am update...

Highlights...

* Temperatures will remain cool into Tuesday, then will moderate
  to near and above normal by late this week
* Onshore winds will bring spotty light rain or showers at times
  into mid week, best chances on Wednesday
* Scattered showers will move across late Wednesday through
  Thursday

Details...

Tuesday...

High pressure will remain across the region, with the center
anchored across eastern Canada into northern Maine. This will
keep a cool E-NE wind flow in place. 00Z models indicating that
the high center may actually slip S across the region by
Tuesday, so the E-NE winds will diminish. This will allow good 
subsidence and dry conditions during this timeframe. May still 
see onshore winds along the immediate coast on Tuesday. 

Temperatures will moderate to near normal levels on Tuesday with
the lighter winds. Expect highs mainly in the 70s, but may hang
near 70 along the immediate E coast. 

Wednesday through Friday...

Big question at this point will be the passing of the potential
tropical system around the mid to late week period. 00Z models 
starting to show more spread as the low shifts NE from off the 
Carolina coast, which lends to lower confidence. In the 
meantime, a cold front tries to shift E out of the eastern Great
Lakes to western NY/PA. 

The western edge of the high will ever so slowly shift E Wed,
but the circulation around the offshore system will keep winds
from the E or SE. May also see some showers develop off the
ocean across E Mass into RI during the day. 

At this point, showers develop across western areas as the 
front approaches Wed night through Thu as the front passes. 
This could be slower, however, depending upon the timing and 
track of the passing offshore system. 

The early morning forecast still suggest that the tropical
system will pass well southeast of Nantucket. The most likely 
impact from this system may be a period of rough surf along 
ocean-exposed beaches.

There may be another front that might approach late this week,
but timing is very much in question.

&&

.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/...High confidence.

18z update...

Rest of today...
SCT-BKN low-end VFR over most of the terminals. MVFR lingering
at 020 over E-coastal and SE MA terminals, on and off with the
low-end VFR CIGs. E/NE winds 10 to 15 kts sustained, strongest
along the E/SE MA coast with gusts up around 25 kts, as high as
30 kts. 

Tonight...
SCT-BKN low-end VFR will lower into the overnight hours as E/NE
winds persist, light around 5 kts over the interior, around 10 
to 15 kts along the shore. Higher confidence of into-morning
MVFR for terminals along and S of the MA-pike, as well as along
the E-slopes of high terrain. Patchy IFR FG possible where SKC
prevails in the N-CT River Valley of MA. 

Monday...
Morning SCT-BKN MVFR mostly along and S of the MA-pike lifting 
and becoming FEW-SCT towards afternoon. E/NE winds continue 
around 10 to 15 kts with strongest winds along the SE coast with
gusts up to 20 kts. 

Monday night...
FEW-SCT CIGs may linger perhaps becoming BKN-OVC over southeast
MA, but a greater concern is interior IFR FG. Current confidence
is immediate W of the N/W 495 beltway and within sheltered
interior valleys through N CT and W MA. Light winds but a bit
more brisk E/NE flow 5-10 kts. 

KBOS Terminal...
Clear out the -DZ and hold BKN low-end VFR CIGs around 040. As
we go into evening, SCT-BKN CIGs prevail, lowering with time
down to 020 where more SCT conditions are favored. 

KBDL Terminal...
SCT-BKN low-end VFR around 040 into this evening. Can't rule out
MVFR going into Monday morning but will leave it SCT for now. 

Outlook /Tuesday through Friday/...Moderate confidence.

Tuesday through Tuesday Night: VFR. 

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

Wednesday Night: MVFR/IFR conditions possible. Chance SHRA,
patchy BR.

Thursday: Mainly VFR, with local IFR 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 Monday Night/...High confidence. 

4 am update...

Long-fetch E/NE winds will produce gusts 20 to 30 kt and seas 
building to around 5-6 feet. High pressure centered over the 
maritimes will maintain the ENE wind through Monday, maintaining
the rough seas on the outer waters through that time. 

Wind gusts of 20 to 25 kt are likely on the Southern waters with
25 to 30 kt possible on the Southern Outer waters. Wind gusts 
diminish Monday as the Maritime high pressure expands south.

Small Craft Advisory continues. We extended the advisory period
through the day Monday. No other significant changes were made.

Outlook /Tuesday through Friday/...Moderate confidence. 

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

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

Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Local visibility 1 to 3 nm.

Wednesday: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers. 

Wednesday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, patchy fog. Areas of
visibility 1 to 3 nm.

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 11 AM EDT Monday for ANZ232>234.
     Small Craft Advisory until 6 PM EDT this evening for ANZ231-
     236.
     Small Craft Advisory until 5 AM EDT Monday for ANZ235-237.
     Small Craft Advisory until 8 PM EDT Monday for ANZ250-254>256.

&&

$$

SYNOPSIS...Sipprell
NEAR TERM...Sipprell
SHORT TERM...Sipprell
LONG TERM...EVT
AVIATION...Sipprell/EVT
MARINE...WTB/EVT