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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 211102

Area Forecast Discussion
National Weather Service Boston/Norton MA
702 AM EDT Sat Sep 21 2019


High pressure remains in control into Monday with summer-like 
temperatures and dry weather. A cold front will bring a period
of showers late Monday and Monday night followed by cooler, but
seasonably warm, weather through the rest of next week.



7 am update (no major changes)...

Dry weather continues with high pressure centered to our S. Light
W winds will become light S/SW across the interior. A different
story towards the coast, where the combination of above normal 
temperatures and light winds will lead to seabreezes. Max 
temperatures today should be a few degrees higher than Friday, 
even towards the coast.

While the swell from Hurricane Humberto will be subsiding, it 
will not be entirely gone. Still an elevated risk for rip 
currents Saturday. If going to the beach this weekend, be 
especially cautious if there are no lifeguards present.



Dry weather tonight into Sunday. The warming trend continues
with a high pressure off to our southeast producing an
increasing southwest wind. Above normal temperatures through 
this period.



*/ Highlights...

 - Rain anticipated late Monday into Monday night  
 - Thereafter mild days, cool nights, dry the remainder of the week
 - Temperatures running above average throughout

*/ Overview... 

Amplifying pattern. Impressive N Pacific jet signaled by a +WPO/+EPO 
that cascades energy and mild air over the CONUS initially. Kicking 
downsheared energy across the 4-corners region, a closing mid-level 
low latched onto continental moisture, there's a chance for rainfall 
around Monday night discussed in more detail below. However by late 
September a pattern-amplification emerges. One-two punch, NW Pacific 
Tapah becoming post-tropical, ingested into the Bering straight low, 
subsequent latent heat release amplifies the longwave trof that re-
captures earlier cut-off N Pacific energy. Heightened AK ridge that 
ultimately downshears a deep trof across the W CONUS into the end of 
September, PNA becomes largely negative as does the EPO/WPO. With a 
continued active N Atlantic tropics emphasized by the phase 1 MJO, 
an impressive heat pump and anomalously higher heights emerge over 
the E CONUS. No surprise to see above average temperature trends and 
below average precipitation per CPC. However along the NE-periphery 
of the heat-bubble no-trouble, there are opportunities for cooler 
air and rainfall chances along SE-sweeping cold front boundaries. So 
not entirely convinced we'll see the brunt of the heat expected for 
the SE CONUS N into the mid-Atlantic. Perhaps just on the cusp yet 
however for late September is still above average when highs are 
normally around 70 and lows normally around 50. A pattern of mild 
days and cool nights, some shots of wet-weather in-between. Break 
down the details below. 

*/ Discussion...

Monday into Monday night...

Widespread showers and appreciable rainfall remains possible. Lean 
towards wetter solutions given consensus forecast mid-level closed-
low solution acting on a narrow-ribbon of continental moisture. Much 
of the same, precipitable waters up around 1.75-inches, warm-cloud 
layer up around 12 kft preceding the sweeping cold front. A moist-
adiabatic, conditionally-unstable profile, albeit brief and right 
along the front, expected lift, low-level convergent forcing, and 
synoptic ascent beneath cyclonic flow and diffluence aloft makes a 
case of decent upward motions that should promote collision as well 
as coalescence such that decent rains should develop. Positive to a 
neutral tilt trof with modest W/SW shear preceding, instability is 
in question given marginal, moist-adiabatic lapse rates. Rumbles of 
thunder not ruled out, nor heavier downpours. Chance to likely PoPs. 
There's increasing signal of the possibility of significant rainfall 
with amounts around 0.50 inches. 

Rest of the week...

Rain chances around Thursday night appear suppressed by the weight 
of increasing heights across the E CONUS. However with the prior 
mentioned amplified pattern tightening over the W and central CONUS 
that New England finds itself beneath NW flow aloft, the opportunity 
for some slightly cooler air and rainfall chances along SE sweeping 
cold fronts over the weekend into early next week. However overall 
temperatures still look to run above average. 



Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

12z update (no major changes)...

VFR. Generally light W winds today across the interior. Sea-
breezes developing by midday along the coasts, impacting coastal
terminals. Isolated IFR in low clouds and fog for portions of 
the CT River valley around KORE in the morning hours. 

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF. 

Outlook /Sunday Night through Wednesday/...

Sunday Night: VFR. Breezy. 

Monday: VFR. Breezy. Chance SHRA.

Monday Night: VFR. Breezy. SHRA likely, slight chance TSRA.

Tuesday: VFR. Slight chance SHRA.

Tuesday Night through Wednesday: VFR.



Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

7 am update (no major changes)...

Adjusted the timing of Small Craft Advisories for today. Based
on latest buoy reports, swell is diminishing slower than
previously thought. Rough seas continuing up to 8 feet across 
portions of the outer coastal waters. 

Swell from Hurricane Humberto will continue to diminish through
the day. Relatively light winds will mean smaller wind waves. 
Local seabreezes near the coast today. Relatively tranquil winds
and seas expected tonight into Sunday, with light S to SW 

Outlook /Sunday Night through Wednesday/...Moderate confidence.

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. 

Monday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Areas of rough seas. Chance of rain showers. 

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of rough seas. Rain showers likely,
slight chance of thunderstorms. 

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

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

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



MA...High Rip Current Risk until 6 PM EDT this evening for MAZ022-
MARINE...Small Craft Advisory until 11 AM EDT this morning for ANZ250-
     Small Craft Advisory until 7 AM EDT this morning for ANZ251-



NEAR TERM...Belk/Sipprell
LONG TERM...Sipprell