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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

                            
000
FXUS61 KBOX 212025
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
425 PM EDT Sat Sep 21 2019

.SYNOPSIS...
High pressure remains in control into Monday with summer-like 
temperatures and dry weather. Another round of showers are 
possible inland Tuesday afternoon and evening. Large high 
pressure follows with cooler but seasonably warm weather 
through the rest of next week.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
350 PM update ...

Anomalous (588 dam) deep layer ridge over the northeast late this 
afternoon providing warm but pleasant conditions with 3 pm temps 80-
85 inland, 75-80 elsewhere. Dew pts 55-60 so humidity fairly 
comfortable. Given this deep layer ridge moves across the area 
tonight dry weather will continue. 1025 mb surface high off the Mid 
Atlc coast but with pressure gradient weak across southern New 
England expecting most locations to decouple. Thus light winds, 
clear skies and dew pts in the 50s will support lows late tonight in 
the 50s as well. Therefore sided with the cooler MOS guidance 
for min temps. Patchy radiational fog inland expected. 

Only weather concern late today is leftover ESE swells of around 4 
ft with a period of every 11 seconds continues across the MA waters. 
Thus elevated risk of strong rips with modest surf across ocean 
facing beaches until sunset.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
350 PM update ...

Sunday ...

Even warmer than today away from the south coast with 925 mb temps 
pushing +21C across northern MA, that's up from +19C this afternoon. 
Thus mid 80s to perhaps upper 80s should be common across the 
Merrimack Valley of northeast MA into Boston as SW pressure gradient 
should be sufficient in the afternoon to preclude seabreeze 
development. Low 80s elsewhere except 70s along the south coast. It 
will feel warmer tomorrow than today as dew pts climb into the 60s 
especially south coast. In fact south coast beachgoers may 
experience some stratus and fog thru the day as this higher dew pt 
surges northward. Plymouth county northward should be stratus/fog 
free given more SW land trajectory. 

It will become breezy late in the day with SSW winds up to 20 mph. 

Upper level closed low currently over Lake Ontario this afternoon 
only generating some shallow CU/SCU given atmosphere is dry low to 
mid levels. Expect the same outcome as this feature moves across 
southern New England Sunday. Thus no sensible weather impacts. 

Sunday night ...

Warm night ahead of low level WAA increases with approaching front 
yielding tightening SW pressure gradient. Given dew pts in the 60s 
expect lows to be in the 60s as well along with patchy fog with dew 
pts higher than avg nighttime low. This will serve for a very warm 
start to the day Monday. Given SW gradient breezy conditions along 
the coast, Cape Cod, Islands and high terrain.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...

Highlights... 

* Area of showers Monday afternoon through Monday night, few 
  thunderstorms possible
* Another round of showers possible Tuesday across interior
* Dry conditions with mild days, cool nights, through remainder
  of week into next weekend

Overview... 
Mid level steering pattern generally westerly to start off 
Monday, but noting trough across the western/central Great Lakes
that will slowly progress E as cutoff H5 low forms and slowly 
crosses northern New England Tuesday/Tuesday night. As that 
cutoff low exits Wednesday, broad mid level ridge shifts east 
out of the eastern Great Lakes and NY state in response to 
digging H5 trough extending from southern Nunavut and the NW 
Territories into the northern Plains. As a cutoff but 
progressive H5 low forms across central Canada and pushes toward
Hudson Bay late next week, may see a digging short wave and 
associated frontal system push toward the region, though lower 
confidence with this feature. Leaned toward a non-GFS solution 
during this timeframe, as it appears the 12Z GFS was faster than
most of the remaining model suite.

Details...

Monday and Monday night...
As H5 cutoff low moves across the central Great Lakes, 
associated surface cold front swings across NY/PA toward the 
region. Good SW wind flow in place ahead of this system, pumping
warm, humid air across the region. May see gusts up to 20 to 
around 25 kt, highest along the S coast, Cape Cod and the 
islands. Temps forecast to max out in the lower-mid 80s away 
from the S coast, up to 10-15 degrees above seasonal normals. 
Clouds will increase across N central/NW Mass around midday, 
then across the remainder of the region during the afternoon. 
Will also see chances for showers move in during the afternoon 
as well.

Noting an increasing PWAT band with/ahead of the front in the 
1.8 to 2 inch range Monday night, so could see some brief, heavy
downpours with this feature. However, with the H5 trough moving
steadily E, the heavier rain should not last too long. Have 
forecasted QPF values up to up to 0.3 to 0.4 inches across 
central and eastern areas mainly Mon night. Have carried likely 
POPs across the region through the night. 

Also noting decent instability ahead of the front, with K 
indices in the lower-mid 30s and marginal TQ values for elevated
instability. May be enough to get isolated thunderstorms as 
this system passes Monday night, so have mentioned in the 
forecast. 

Back edge of precip should shift E out of the CT valley by 
around 06Z or so, but could linger across eastern areas through 
most of the night. 

Tuesday... 
Last of the showers should push E of most areas by 12Z Tuesday,
but may linger a few hours longer across the Cape and islands. 

Scattered showers may develop as H5 cutoff low pushes across 
northern New England. Looks like best shot for any precip will 
occur across N central/NW Mass, though can not rule out a few 
showers reaching further SE. These will be mainly diurnally 
driven, so the showers should weaken/dissipate Tuesday evening. 

Highs will run a good 10 degrees cooler than on Monday, mainly 
in the mid-upper 60s across the higher inland terrain up to the 
lower-mid 70s across the coastal plain and lower CT valley, but
will still run up to 5 degrees above normal. 

Tuesday night through Friday...
As mid level ridge builds across Tuesday night through 
Wednesday night, will see dry conditions as large surface high 
pres slowly moves across the northeast through at least Wed 
night. 

SW wind flow again in place as another front approaches late 
Wed night into Thu. Some question on timing, especially the GFS 
which appears faster than most of the guidance. Also, noting 
more moisture extending along the front on the GFS, while 
tending to dry out as the associated low lifts NE into Maine and
the Maritimes. So, have cut back on the POPs across most areas,
kept slight CHC POPs going across the Route 2 area of N Mass on
Thu. 

Another large high pushes E Thu night and Fri, bringing more 
dry and mild conditions. 

Saturday...
Some question on timing and track of approaching cold front out
of the Great Lakes late Sat/Sat night. H5 cutoff low pres 
crosses Hudson Bay, with cold front extending S-SW. Most model 
suite members keep precip out through Sat night, so went along 
with those for this forecast. 

&&

.AVIATION /21Z SATURDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

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

18z update (no major changes)...

Thru 00z ... VFR, dry weather and light winds with seabreezes 
at the coast

After 00z ... VFR, dry weather and light winds. Isolated 
MVFR/IFR inland due to patchy fog. 

Sunday ... VFR and dry weather. Could be some patchy fog/IFR-
MVFR along the south coast at times. Light/variable wind in the
morning becoming SSW and increasing 15-20 kt.

Sunday night ... VFR and dry weather. However patchy IFR/MVFR in
fog may linger at times along the south coast. Modest SSW winds
continue with strongest winds along the south coast and Islands.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF. 

Outlook /Monday through Thursday/...

Monday: VFR. Breezy. Slight chance SHRA.

Monday Night: Mainly VFR, with local IFR possible. Breezy.
Chance SHRA, slight chance TSRA, patchy BR.

Tuesday: VFR. Slight chance SHRA.

Tuesday Night through Wednesday Night: VFR. 

Thursday: VFR. Breezy. 

&&

.MARINE...
Forecaster Confidence Levels...

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

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

345 pm update ...

Easterly swells from Humberto have now diminished to 3 to 4 ft.
Thus have dropped the SCA. Otherwise light winds tonight into 
Sunday morning then SSW winds increase to 15-20 kt Sunday 
afternoon and evening with gusts up to 25 kt possible toward 
sunrise Monday. Patchy fog and low clouds may limit vsby Sunday 
afternoon thru Sunday night. Dry weather prevails. 

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

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

Monday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas. Chance of rain showers,
isolated thunderstorms, patchy fog. Areas of visibility 1 to
3 nm.

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

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

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Rip Current Risk until 8 PM EDT this evening for MAZ022-
     024.
RI...None.
MARINE...None.

&&

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