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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 201350

Area Forecast Discussion
National Weather Service Boston/Norton MA
950 AM EDT Fri Sep 20 2019

High pressure remains in control through this weekend with 
summer-like temperatures returning. Sunday on into next week,
generally speaking warmer than average temperatures and drier 
than average conditions. Our next best chance of rainfall for 
the foreseeable future is around Monday night as a cold front 
sweeps through the region.


950 AM update...

Just a few high clouds spilling over the ridge, otherwise
lots of sunshine, light winds and warm temps this afternoon. 
Highs should reach well into the 70s across much of SNE. 

Swell from Humberto will also continue the threat for rough 
surf and dangerous rip currents. With warming conditions, will 
continue the High Surf Advisory through much of today along 
ocean-exposed portions of the coast.


Dry weather continues tonight into Saturday. A weak front pushes
through or dissipates across our region Saturday. There is only
enough moisture with this feature to generate a period of
clouds. The warmup will continue through this period. Above
normal temperatures expected. As a side-effect of the warmer
conditions and light winds, seabreezes develop Saturday, which
will make the coasts a bit cooler for a time.

While the swell from Hurricane Humberto will be subsiding, it
will not be entirely gone. Still an elevated risk for rip
currents Saturday. Will continue to evaluate the need for a High
Surf Advisory or Rip Current Risk statement through today. If
going to the beach this weekend, be especially cautious if there
are no lifeguards present.


*/ Highlights...

 - Mild days, cool nights 
 - Warmer than average temperatures, drier than average conditions 
 - Best chance of rain for the foreseeable future around Monday night

*/ Overview... 

Perhaps a cleansing rain around Monday night. Emphasized by a +WPO / 
+EPO, when the N Pacific mid-latitude pattern reloads does the PNA 
go largely negative with downsheared, deep troughing across the W 
CONUS. Subsequent downshear ridging neighbored by an active tropics 
courtesy of a phase 8 to 1 MJO do we see higher heights and warmer 
than average temperatures. Its only when the mid-latitude pattern 
releases that energy cascades downstream aided by a strong onshore N 
Pacific jet. An opportunity centered around early next week, could 
see some much needed rain that'll hopefully wash the allergens out 
of the air. Otherwise, as ensemble-mean total rainfall out through 
360 hours emphasizes, we're trapped between cyclonic flow well N/W 
and the tropical-axis / steering flow rounding persistent N Atlantic 
high pressure. Subsequent subsidence ridging that extends from the 
SE CONUS northeastward across our region towards Nova Scotia. The 
latest drought monitor indicating spotty abnormally dry conditions 
across the Northeast, precipitation the last 14- and 30-days having 
been only 50-percent of normal, any rainfall is welcoming. 

Mild days and cool nights, overall above-average for late September 
when highs are normally around 70 and lows around 50. How long this 
pattern lasts is met with both uncertainty and concern. CPC day 6-10 
and 8-14 day outlooks have been ratcheting up the confidence with 
respect to warmer than average and drier than average conditions 
across the E CONUS for late September into early October. Hit on the 
more significant details below. 

*/ Discussion...

Around Monday Night...

Widespread showers and appreciable rainfall possible. Latest suite 
of forecast guidance advertising a mid-level closed-low solution. A 
potential ribbon of deep-layer moisture with precipitable waters up 
around 1.75-inches, the warm-cloud layer up to around 12 kft agl, a 
promotion of a moist-adiabatic, conditionally-unstable environment 
with marginal lapse rates and some weak elevated instability. This 
combined with cyclonic flow, mid-level ascent, and diffluent motions 
aloft, lift along an associated cold front with convergent low-level 
winds, could see some decent rainfall, perhaps a rumble of thunder. 
A lot will ride on the amplification of synoptics. Both the 20.0z 
GFS and EC are in pretty good agreement of a neutral-tilt, closed 
mid-level low sweeping across the region Tuesday transitioning to a 
negative-tilt. Modest SW shear, it'll be interesting what upstream 
outcomes manifest with daytime-heating and cloud cover. Indications 
more robust thunderstorm activity could fire Monday across Upstate 
NY into W New England. If this activity could evolve further S into 
PA with the parent mid-level vortmax that we'd have a better shot of 
appreciable rainfall. Right now it looks as if N New England will 
see more significant rainfall. Chance to likely PoPs. A mention of 
thunder. And rainfall amounts upwards of around 0.25 inches. Still 
some uncertainty on timing, hopefully nailed down along with more 
specifics in the next day or two.


Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

VFR. Winds turning light W (under 10 kts) today, continuing 
tonight. A weak, dry front should push across our region 
Saturday. Winds remain generally from the W to NW, except along 
the coasts where seabreezes are expected to develop with the 
light flow. 

KBOS Terminal...High confidence in TAF. 

KBDL Terminal...High confidence in TAF. 

Outlook /Saturday Night through Tuesday/...

Saturday Night: VFR. 

Sunday through Sunday Night: VFR. Breezy. 

Monday through Monday Night: VFR. Breezy. Chance SHRA.

Tuesday: VFR. Slight chance SHRA.


Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

Seas continue to build as swell form Humberto impacts the coastal
waters today. This swell should subside some tonight into Saturday.
Winds generally remain less than 20 kt, but will be continuing 
the Small Craft Advisories due to rough seas across the outer 
coastal waters.

Outlook /Saturday Night through Tuesday/...Moderate confidence.

Saturday Night through Sunday: Winds less than 25 kt. 

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

Monday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas up to 5 ft. Chance of rain showers. 

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

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


MA...High Surf Advisory until 6 PM EDT this evening for MAZ007-019-
RI...High Surf Advisory until 6 PM EDT this evening for RIZ006>008.
MARINE...Small Craft Advisory until noon EDT Saturday for ANZ250-254-
     Small Craft Advisory until 8 AM EDT Saturday for ANZ251.
     Small Craft Advisory until 6 AM EDT Saturday for ANZ256.


LONG TERM...Sipprell