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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 230237

Area Forecast Discussion
National Weather Service Boston/Norton MA
1037 PM EDT Sun Sep 22 2019

Southwest flow ahead of an advancing cold front will make for a
rather warm night tonight, and one more very warm and breezy 
day Monday. Areas of dense fog are again likely tonight along 
the south coast of RI and MA. Showers and a few thunderstorms 
will accompany this front late Monday afternoon and Monday 
night. High pressure settles in Tuesday and Wednesday with mild 
days and cool nights. Swells from Jerry will cause increased rip
current risk along the coast, especially Monday through
Wednesday. Another front approaches Thursday into Thursday 
evening, with dry conditions returning Friday.



Main concern overnight will be the development of low clouds and
fog. SW winds will be a bit stronger tonight than last night,
which will be a limiting factor for radiation fog. However,
onshore winds and increasing dewpoints should lead to advection
fog towards the south coast of New England. Already have seen
signs of this at Nantucket. It may take a couple more hours to
really get underway. Visibility could lower to 1/2 mile or less,
so fog may be locally dense especially across the nearshore 

Current conditions pretty close to the previous forecast, but
did update to make minor adjustments.

Previous Discussion...

There were just some high level cirrus clouds across the region
late this afternoon. The only exception continued to be along
the south coast from Buzzards Bay to Block Island, where a fog
bank persisted, but was beginning to thin a bit. Temperatures
were in the mid to upper 80s inland...with Boston Logan having
hit 89 degrees! 

Gusty winds to around 15-25 mph late this afternoon will
diminish to around 10 mph after sunset. Mainly clear skies are 
forecast through tonight, except low clouds and fog may spread
inland across southern RI and southeast MA, mainly after
midnight. It could become dense in some of these
will have to watch for a possible Dense Fog Advisory if it 
develops. With southwest winds remaining around 10 mph 
overnight, am not expecting much in the way of radiation fog 
farther inland. Dewpoints will be on the rise, reaching the 
lower to mid 60s across the area overnight, so overnight lows 
should be holding in the mid 60s.


With a warm start overnight and still some sunshine Monday
morning, temperatures will soar to the upper 80s again, except
cooler on the south coast. Cannot rule out a few 90 degree
readings. These will not be records, though, since the Boston
record was 96 set in 1895, BDL record was 93 set in 1914, and
the PVD record was 92 set in 1970. 

BUFKIT soundings show the potential for southwest winds to
increase to 20-30 mph for a few hours across the interior Monday
afternoon. Have increased winds in the grids to reflect this.

Models are a bit quicker with development of showers ahead of
the front. Expect a few to develop by 2 or 3 PM in western MA,
then increasing through the late afternoon hours. By 9 or 10 PM
Monday night, showers and a few thunderstorms will be ongoing 
across the entire region. K Indices reach 33 to 35 but with
little CAPE, believe that any thunder will be elevated. Locally
heavy downpours are possible. Models show the heaviest rain in
eastern MA and southern RI between midnight and 6 AM Tuesday.
Lows will range from the mid 50s in northwest MA behind the
front to the mid 60s in southeast sections.

Expecting increasing swell from the south Monday, this time from
Tropical Storm Jerry. Will issue a High Rip Current Risk
statement for the elevated risk for dangerous rip currents, 
mainly over the islands including Block Island, Marthas
Vineyard, and Nantucket Monday. This risk area will likely 
expand on Tuesday.



* Showers linger across eastern areas through mid morning 
* Dry conditions and mild temperatures through mid week
* A cold front crosses Thursday into Thursday evening
* Some showers may approach with another front sometime next 


Northern stream mid level steering flow remains progressive 
through most of this timeframe as broad mid level high pressure 
sprawls across the mid Atlc and SE U.S. eastward across the mid 
Atlc. Cutoff H5 low pres moves along in the northern stream 
flow, pushing across northern New England Tue/Tue night. The mid
level heights lower somewhat, but still expect temperatures 
through mid week to remain above seasonal levels. Should see a 
couple more fronts move along in the progressive flow aloft, but
should not see a lot of precip with either system.


Cold front clears the coast Tuesday morning, but is slow to 
exit as it hangs up in the SW steering flow aloft ahead of the 
approaching H5 cutoff low. Carried CHC POPs mainly across Cape 
Cod and the islands, with slight CHC from the BOS-PVD corridor S
and E through mid morning, then should push off the outer Cape 
and Nantucket by midday. 

However, there are a couple of features that need to be 
monitored. First, a few mainly diurnally driven showers may push
into N central/NW Mass as H5 cutoff low moves across northern 
New England into early Tuesday afternoon. Also need to keep an 
eye on TS Jerry as the system makes its closest westward 
approach well SE of Nantucket on Tuesday, so this may slow the 
progression of the back edge of the precip/clouds. At this 
point, the leftover showers should push offshore by midday or 
early afternoon. 

Winds to shift to W during the midday and afternoon hours with 
some gusts up to 20 kt possible along S coastal areas. Expect 
temps to reach the mid 60s to around 70 across the higher inland
terrain to the mid-upper 70s across the coastal plain and lower
CT valley. 

Tuesday night through Wednesday night...
High pressure takes over, with dry conditions during this 
timeframe. W-NW winds expected Tue night and Wed as the high 
slowly moves off the mid Atlc coast, then will become light W-SW
on the backside of the ridge during Wed. Expect high temps 
mainly in the upper 60s to mid 70s on Wed, up to 5 degrees above
seasonal normals. 

Clouds may begin to approach after dark across western areas 
Wed night, but expect dry conditions to continue. Temps will 
remain on the mild side.

Thursday and Thursday night...
SW winds in place ahead of the approaching cold front Thu 
morning through midday. Some questions on the timing and track 
of the front, which may tend to slow down while the western Atlc
ridge slows down as well. Rather wide model solution spreads as
to the arrival of showers, if they make it at all. At this 
point, have carried slight CHC to low CHC POPs, but could be 
quite a bit more depending upon which model one believes. Kept 
the chance for some showers into Thu night, which may weaken as 
the front pushes offshore and another high approaches from the 
Great Lakes. Will still see temps running slightly above normal.

Friday through Sunday...
Should see another high build across the eastern seaboard late 
this week into next weekend as another broad H5 ridge builds 
across. Noting large H5 high pres extending from the mid Atlc to
SE states eastward to the mid Atlc, though position of the high
as well as the cutoff H5 low over Nunavut and the NW 
Territories and trough extending down across the mid and 
northern Rockies, or possibly in the Pacific NW. 

At this point, have carried a dry forecast for Friday/Friday 
night, then yet another front may approach sometime next 
weekend. However, with the ridge to the SE, this front may stall
or even wash out as it approaches. Rather low confidence for 
this portion of the forecast.


Forecaster Confidence Levels...

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

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

Tonight...Mainly VFR, except areas of IFR-VLIFR FG may 
redevelop and then linger at times along the south coast. Fog
may extend inland to about a KPVD-KPYM line. S-SW winds gusting
up to 15-20 kt. 

Monday...VFR. Increasing risk for SHRA and isolated TSRA during
the afternoon from W to E. MVFR possible with any RA/+RA. SW
winds gusting to 25 kt in the afternoon, especially over the

Monday night...MVFR in SHRA and TSRA. Precipitation ending in
western and central sections after 06Z. Locally heavy downpours

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF. 

Outlook /Tuesday through Friday/...

Tuesday: VFR. Breezy. Patchy FG.

Tuesday Night through Wednesday Night: VFR. 

Thursday: VFR. Breezy. Slight chance SHRA.

Thursday Night: VFR. Slight chance SHRA.

Friday: VFR.


Forecaster Confidence Levels...

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

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

Through Tonight...Expect S-SW winds gusting up to 20 kt. Areas 
of dense fog possible across the southern coastal waters. Dry 
weather prevails. Seas build across the outer coastal waters. 
Small Craft Advisories posted, beginning late tonight for the 
outer waters as seas build to 5 ft. 

Monday...SW winds will increase to 20-25 kt with a few gusts to
30 kt not out of the question late. SCA in effect with 5 to 7 ft
seas over the southern and southeastern outer waters, as swells
enter the picture from distant Tropical Storm Jerry. Chance of
showers toward evening. Visibility locally below 1 mile along
the south coast, especially in the morning, in areas of fog.

Monday night...As a cold front moves through, widespread 
showers and thunderstorms. Seas up to 5 to 7 ft over the 
southern and southeastern waters. Winds still gusting up to 20 
to 25 kt. SCA continues for those areas. Visibilities below 3 
miles in locally heavy rain showers.

Outlook /Tuesday through Friday/...Moderate confidence.

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Slight chance of rain showers, patchy fog. 

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

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

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

Friday: Winds less than 25 kt.


MA...High Rip Current Risk from 6 AM EDT Monday through Monday 
     evening for MAZ023-024.
RI...High Rip Current Risk from 6 AM EDT Monday through Monday 
     evening for RIZ008.
MARINE...Small Craft Advisory until 8 PM EDT Monday for ANZ232>234.
     Small Craft Advisory from 6 AM to 8 PM EDT Monday for ANZ231.
     Small Craft Advisory from 6 AM Monday to 6 AM EDT Tuesday for 
     Small Craft Advisory until 6 AM EDT Tuesday for ANZ250.
     Small Craft Advisory until 6 PM EDT Tuesday for ANZ254>256.