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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 220757

Area Forecast Discussion
National Weather Service Boston/Norton MA
357 AM EDT Sun Sep 22 2019

High pressure remains in control into Monday with summer-like 
temperatures and dry weather. Showers likely Monday night along 
a sweeping cold front. High pressure settles in through midweek 
with mild days and cool nights. 


Patchy fog did develop along portions of the south coast, as
well as across the usual suspects over the interior. Evaluating
the need for a dense fog Advisory, but will at least issue a
Special Weather Statement. 

Conditions will improve shortly after sunrise. Expecting plenty
of sunshine once the morning low clouds and fog dissipate.
Increasing SW wind, which should be enough to prevent the
seabreeze along the east coast of MA. Above normal temperatures
continue, and should be similar to Saturday.

Swell continues to diminish along the coast today. Much lower
risk for rip currents today. Caution is still advised if
entering the water at unguarded locations.


SW winds should help to maintain above normal temperatures
through Monday. Humidity will be increasing, too. Looking at 
patchy fog once more tonight into early Monday morning.

A mid level cutoff moves over the Great Lakes by late in the
day. This should produce an increase in clouds, and a risk for
showers late in the afternoon across western MA. Decent 
instability parameters, so cannot dismiss the possibility of a 
few thunderstorms ahead of a cold front. Locally heavy rainfall 
will be possible in any thunderstorms.



*/ Highlights...

 - Showers likely Monday night
 - Warm-up and dry through midweek
 - Another sweeping cold front around Thursday night
 - Low confidence forecast into the following weekend

*/ Overview... 

Tropics are active. Multiple cast members supported by a phase 
1 MJO crew. As members are drawn into the mid-latitude 
production they in part bring chaos to the show. From the NW 
Pacific to the NW Atlantic S equatorward, the multitude of 
systems, accumulated cyclone energy, latent heat release, all of
which has the opportunity to be drawn N as the pattern slows 
and buckles, the subsequent result of warm air advection across 
Alaska and further downstream over Greenland that yields 
subsequent blocks. A CONUS trof-ridge, near climatological- 
record percentiles by the weekend, the NE-CONUS in the NE-
periphery of the MS/OH Valley ridge with a +1-2 standard 
deviation temperature anomaly through the column, by weeks-end 
into the following weekend it'll be interesting as to the heat 
and humidity magnitude that'll be able to advect into the NE-
CONUS beneath prevailing NW-flow aloft likely prior to SE-
sweeping cold fronts. Rainfall opportunities that ensemble-means
continue to show synoptically remain uncertain as to outcomes 
as well as how warm it potentially could get. Any nudge in the 
ensemble mean longwave pattern could put us either in a warm-dry
pattern or a mild-wet setup. Nevertheless temperatures look to 
be above-average into early October as emphasized by CPC 
forecasts, but lower confidence with respect to precipitation 
outlooks. Dependence on the longwave pattern breakdown. Aside we
really need some rain. 

For now through the early half of the week, +WPO/+EPO signals 
with a robust N Pacific jet into the CONUS, energy cascades E. 
Grasping at continental-moist airmasses, there's opportunity for
some rainfall Monday night and again Thursday night (however 
the latter becoming weighed down by increasing heights as the 
pattern begins to buckle). Mild days and cool nights. Break down
details below. 

*/ Discussion...

Monday night...

Showers likely. Cold frontal lift, convergent low-level winds, 
all beneath a neutral-tilt trof with attendant vortmax, cyclonic
flow, and jet dynamics, certainly forcing will act upon a 
continental- moist airmass with precipitable waters close to 
2-inches. Narrow ribbon of moist-adiabatic, conditionally-
unstable profile with some decent W shear, can't rule out some 
heavy downpours, maybe a rumble of thunder. Forecast guidance 
has been all over the place with rain- fall. But given synoptics
would think something will squeeze out. Deep-layer moisture 
with the aforementioned ribbon well up to H5-3 and warm-cloud 
bases up to 14 kft, some areas could see that 0.50 if not more. 
Just a challenge to where exactly but there's indication of 
deep-layer lift will occur somewhere. Likely PoPs. Mild. 

Tuesday into Wednesday...

Clouds lingering. Light showers N beneath the closed mid-upper 
level low slowly working E. Downsloping W/NW flow. Gradual 
clearing, mild days, cool nights. Above-average temperatures 
when normally for late September we should see highs around 70, 
lows around 50. Expect it to be around +5 degrees with respect 
to the two. 

Thursday into Thursday night...

Sweeping cold front and a chance of showers. However heights on
the rise as the longwave pattern buckles, a trof digs over the 
W CONUS with the ridge building E. Rainfall outcomes could be 
suppressed prior to ending up on our doorstep. Remaining mild. 

Friday onward...

Low confidence forecast depending on the longwave CONUS setup. 
We could end up warm and dry, or rather mild with on and off 
rainfall chances along sweeping cold fronts. 


Forecaster Confidence Levels...

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

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

Today...VFR and dry weather. Patchy fog/IFR-MVFR along the 
south coast and in the CT River valley early this morning. 
Light/variable wind becoming S-SW and increasing 15-20 kt.

Tonight...VFR and dry weather. However, patchy IFR/MVFR in fog 
may linger at times along the south coast. Modest S-SW winds 
continue with strongest winds along the south coast and Islands.

Monday...VFR. Increasing risk for showers and isolated
thunderstorms during the afternoon from W to E. MVFR in any

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF. 

Outlook /Monday Night through Thursday/...

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

Tuesday: VFR. Breezy. Slight chance SHRA.

Tuesday Night through Wednesday Night: VFR. 

Thursday: VFR. Breezy. Slight chance SHRA.


Forecaster Confidence Levels...

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

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

Light winds this morning, becoming S-SW winds and increasing to 
15-20 kt this afternoon and evening with gusts up to 25 kt 
possible toward sunrise Monday. Patchy fog and low clouds may 
limit vsby this afternoon through through tonight. Dry weather 
prevails. Seas build across the outer coastal waters. Small
Craft Advisories posted for late tonight into Monday.

Outlook /Monday Night through Thursday/...Moderate confidence.

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Chance of rain showers,
slight chance of thunderstorms. 

Tuesday: Winds less than 25 kt. Local rough seas. Slight chance
of rain showers. 

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

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

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


MARINE...Small Craft Advisory from 1 AM to 8 PM EDT Monday for 
     Small Craft Advisory from 6 AM to 8 PM EDT Monday for ANZ231-


LONG TERM...Sipprell