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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
000
FXUS61 KBOX 210206
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
1006 PM EDT Fri Sep 20 2019

.SYNOPSIS...
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 the week.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
No major changes to the ongoing forecast this evening. Quiet
weather continues.

Previous Discussion...

Forecast remains in great shape with no significant changes 
needed attm. While current temps are outpacing forecast values 
by a few degrees, expectation of good radiational cooling
commencing with sundown should bring temperatures more in line.
Essentially a modified persistence forecast to last night with
high pressure in control; the main exception being that 
temperatures should be several degrees warmer than the last 
couple nights as low-level temps have moderated to some extent.
Still looking at development of radiation fog in the CT River 
Valley. Lows mainly upper 40s across western MA/Pioneer Valley 
and into the low-mid 50s elsewhere.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Saturday...
Mid level ridge builds across New Eng with warming temps aloft.
Expect lots of sunshine and warmer temps. 925 mb temps 20-21C 
so we should see highs into the low/mid 80s, except upper 70s 
along the immediate coast as sea breezes develop. 

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.

Saturday night...
Mainly clear skies with some high clouds spilling into the
region as mid level shortwave approaches. Lows ranging through
the 50s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Highlights...

* Continued dry weather with mild daytime temperatures Sunday.

* Rain/brief downpours associated with cold front Monday night-
  Tuesday. Possible embedded thunder.

* Brief cooldown Tuesday, turning warmer by late week/weekend.
  
Details...

Sunday...

Upper-level ridge crests and shifts eastward into the Gulf of Maine 
by early Sunday. Continued mild and dry weather looks to continue 
into Sunday under surface high pressure and highs into the 80s. Will 
also see some southwesterly breezes, strongest toward the south 
coast (nearing 25 mph). Later Sunday into the overnight, 
geopotential heights start to fall and ribbon of mid-high level 
moisture on increasing west-southwest flow should lead to an 
increase in clouds. Combination of clouds and southwesterly winds 
should keep temperatures from falling too far - into the 60s most 
areas. 

Monday - Tuesday...

First part of Monday now looking to be dry, though cloud cover will 
be steadily filling/lowering from west to east in advance of a cold 
front. Guidance still keying on frontal passing timing beginning 
mainly after sundown (12z NAM is a few hours earlier) through the 
first half of the evening across interior MA/CT and RI and not until 
the overnight toward the Boston-Providence/I-95 corridor, and into 
early Tuesday on the Cape, Islands and coastal waters. 

Ahead of and along the front, deep layer moisture quality improves 
with PWAT values rising to 1.5-1.75" and warm cloud depths are 
between 10-12kft. These values argue for brief downpours along what 
would likely be a strongly-forced band of frontal rain, perhaps 
embedded with rumbles of thunder driven mainly from favorable 
dynamics versus convective instability. Any embedded thunder would 
only serve to enhance rainfall rates, and while wind fields are 
strong with straight hodographs, strong storms aren't anticipated in 
light of the meager instability progged given unfavorable time of 
day and weak mid-level lapse rates. Even the faster NAM only 
generates about 500 J/kg or less of CAPE from a most-unstable parcel 
ascent. 

Still some leftover showers into Tuesday on the Cape, but across the 
interior looking at variable clouds with cold advection bringing 
temps closer to seasonal values. With cooler thermal profiles aloft, 
may still be enough moisture and shallow instability to produce some 
spotty daytime convective showers in northern MA mainly near and 
north of Route 2. Still some lingering clouds but dry 
weather anticipated by Tuesday night. 

Wednesday thru Friday: 

By Wednesday, the 500 mb pattern across the eastern roughly two 
thirds of CONUS transitions to one of quasi-zonal flow - governed by 
a prominent upper low over Hudson Bay with mean troughing into the 
northern Plains and northern Rockies, a closed low over the Desert 
Southwest, and a subtropical ridge over the Deep South into the 
northern Gulf of Mexico.

Wednesday's looking largely dry, with temperatures moderating amid 
850 mb temperatures rising to the 10-12C range supporting 70s to 
near 80s. Still some level of uncertainty on a potential frontal 
passage Thursday - with the GFS/Canadian indicating a wetter frontal 
passage than the ECMWF is, though limited moisture from the Gulf of 
Mexico could lend support to the drier FROPA indicated by the EC. 

Looking ahead, another period of above-normal temps looks to be in 
the offing for late next week into next weekend per most models and 
teleconnection forecasts centered in the 6-10 day period. It also 
jives with CPC's 6-10 day temperature outlook showing a significant 
tilt in the odds toward above-normal temperatures across southern 
New England.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

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

VFR to continue though Saturday Night, with generally light 
winds. Sea breezes developing by midday Saturday along the
coasts, impacting coastal terminals.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF. 

Outlook /Sunday through Wednesday/...

Saturday Night: VFR. 

Sunday through Sunday Night: VFR. Breezy. 

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

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

Tuesday: Mainly VFR, with areas MVFR possible. Slight chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Forecast still on track. Small Craft Advisories through 
Saturday due to rough seas and swell from Humberto. Should see 
the peak of the swell occur by this evening, with receding 
trends into Saturday night. Light winds through the period.

Outlook /Sunday through Wednesday/...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: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas. 

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 PM EDT Saturday for ANZ250-254.
     Small Craft Advisory until 8 AM EDT Saturday for ANZ251.
     Small Craft Advisory until 11 AM EDT Saturday for ANZ255.
     Small Craft Advisory until 6 AM EDT Saturday for ANZ256.

&&

$$
SYNOPSIS...KJC/Loconto
NEAR TERM...Belk/KJC/Loconto
SHORT TERM...KJC
LONG TERM...Loconto
AVIATION...Belk/KJC/Loconto
MARINE...KJC/Loconto