Preview of NWS' New Version of Forecast
This preview is not operational and should not be used for support decisions.

Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
000
FXUS61 KBOX 231112
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
712 AM EDT Fri Aug 23 2019

.SYNOPSIS...
A cold front along the South Coast this morning moves well
offshore during the day. Showers along the front will move
offshore with the front. Drier less humid air moves in from the
northwest. High pressure will linger across central and eastern
Canada through this weekend into early next week. Cool
temperatures both Sunday and Monday with the onshore wind flow
in place, along with spotty light rain or showers especially
near the coast. As the high slowly pushes east, temperatures
will moderate by mid week, but there could be a threat for
showers as a cold front approaches.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

650 AM Update...

Noting the cold front S of Long Island to near Nantucket at 09Z.
However, N of the front, a band of showers lying from SE NY
through most of CT into N RI and central to NE Mass on latest NE
regional 88D radar imagery. Only spotty light showers being
reported on latest observations, with the steadiest light rain
from around KHPN-KDXR-KIJD and KOWD southeastward at 10Z. While
the rain elements continue to shift NE, while the entire area 
is slowly shifting S-SE behind the front. Mainly dry conditions
noted across central and western Mass, but may see a few spotty
showers over the next hour or two. 

Back edge of the cloud deck, noted across SW ME to central NH
and southern VT at 1030Z as seen on latest GOES-East Nighttime 
Microphysics RGB satellite imagery. Big question will be how 
long will the clouds linger before slowly pushing southward. 
This may take a while as the H5 long wave trough will be slow to
swing across, so may see the W-SW mid level steering flow 
linger longer than currently forecast. 

Previous Discussion...

Once the flow shifts, expect clearing skies, earliest in 
northern MA and latest along the South Coast. Cross sections 
show drying aloft and at the surface, but a lingering moist 
layer around 850 mb. The mixed layer is expected to reach a 
little past 850 mb, so there should be some diurnal cu formation
over the interior. The dry air should limit the extent. 
Temperatures at 850 mb will be 10-12C, so we expect max sfc 
temps of 76F to 82F. Best of all, dew points will be in the 
upper 40s and 50s everywhere this afternoon/evening.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
A fair weather period, quite welcome to most after the recent
spell of humidity. High pressure over Northern Ontario builds 
east, reaching the Maritimes late Saturday. This brings a light
north flow to New England tonight, turning to a northeast flow 
by Saturday afternoon. 

With dew points upper 40s and 50s, expect min temps tonight in 
the 50s most places...with upper 40s in the cold spots and 
around 60 in large urban areas. The developing northeast flow
will play havoc with mixing, but daytime heating and 8C at 850
mb suggest sfc temperatures into the 70s. If anyone stands a 
chance at 80, it will be the CT Valley of Hartford-Springfield.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...

Highlights...

* Expect much cooler than normal temperatures Sunday and Monday
  especially along the east coast
* Onshore winds will bring spotty light rain or showers at 
  times into mid week
* Temperatures return to near normal levels around mid week
* Another chance for showers Wednesday into Thursday

Details...

Saturday night through Monday...

With a large high pressure area extending across Quebec and 
northern New England, a return northeasterly wind flow will 
bring cool temperatures across the region especially along east 
coastal areas into early next week. 

With the onshore wind flow, the long ocean fetch will bring 
increasing low level moisture westward. So, can not rule out 
periods of spotty light rain and/or showers at times mainly over
the weekend. By Monday, it appears the southern extend of the 
large ridge will start to shift into N Mass, which will bring 
drier conditions across the entire region with good subsidence. 
So, any spotty precip early should end, though clouds may linger
across eastern and S coastal areas through the day. 

Expect daytime highs mainly in the lower to mid 70s, but may 
not break 70 along the immediate E coast as well as outer Cape 
Cod and Nantucket. 

With the long fetch across the western Atlc, will see E to NE 
winds gusting up to 25-30 mph along the E coast both Sunday and 
Monday.

Monday night through Thursday...

The large high center across the Maritimes will begin to shift 
E during this timeframe as a cold front slowly shifts out of the
Great Lakes. Winds will diminish somewhat as the pressure 
gradient relaxes, then will veer to SE-S by later Wed into Thu. 

Increasing uncertainty as to how quickly the ridge will shift 
further offshore along with the timing of the approaching cold 
front. At this point, the ridge will keep dry conditions in 
place on Tuesday, except for spotty light showers mainly near 
the S coast. Deepening layer moisture as well as slowly but 
steadily increasing dewpoints will approach starting later 
Tuesday and continue through Thursday. Expect highs on Wed to be
close to seasonal levels, then upward to around 5 degrees above
normal on Thu. 

The approaching cold front will bring a more organized shot for
showers starting late Tuesday night across the E slopes of the 
Berkshires, then slowly shifting E Wed and Thu. Timing of the 
actual front still in question due to the exit of the Maritime 
high pressure.

&&

.AVIATION /12Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...Moderate confidence.

Through 14Z...

Mainly VFR. Mainly dry N central and W Mass. A band of showers 
from N CT through NE Mass will shift NE, but the entire zone 
will shift S-SE. May see brief, local MVFR conditions in any 
heavier showers or lower CIGS mainly across NE CT/RI/S coastal
Mass.

Rest of Today...

VFR most of the region. Brief MVFR cigs/vsbys in showers along 
the South Coast through around 16Z-18Z. Otherwise becoming
mostly sunny with dry conditions. A few puffy clouds form 
inland during the day, but with bases 4000-5000 feet. Northwest 
winds turning north by sunset.

Tonight...

VFR with mostly clear skies and light north winds.

Saturday...

VFR. North wind turns from the northeast during the afternoon.

KBOS Terminal...High confidence in TAF

KBDL Terminal...High confidence in TAF.

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

Saturday Night: VFR. Breezy. 

Sunday through Monday: Mainly VFR, with local MVFR possible.
Breezy. 

Monday Night: Mainly VFR, with local IFR possible. Breezy.
Patchy BR.

Tuesday: VFR. Breezy. 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/...High confidence. 

Today...Scattered showers this morning with brief poor vsbys. 
Conditions should improve from north to south through around
midday, with drying in the afternoon. Winds will be less than 
25 kts through the day. Seas 4 feet or less.

Tonight...Winds less than 20 kt and seas 4 feet or less.

Saturday...Winds less than 20 kt. Seas 4 feet or less. 

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

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

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas up to 5 ft. Slight chance of rain. 

Sunday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas up to 5 ft. Slight chance of rain. 

Monday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Seas up to 5 ft. Slight chance of rain. 

Monday Night: Winds less than 25 kt. Seas up to 5 ft. Slight
chance of rain. 

Tuesday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

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