Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 232317
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
717 PM EDT Mon Apr 23 2018

.SYNOPSIS...
High pressure remains in control through Tuesday bringing dry
weather with mild daytime temperatures. Low pressure approaches
and then passes through New England Tuesday night into
Wednesday night, bringing rain to the area. Another weak low
pressure center may bring another round of showers to the area
Friday night.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

Quiet weather continues. Seabreezes will dissipate this evening.
Guidance still struggling with the very low dewpoints. Thinking
dewpoints will rise as daytime mixing ends. Adjusted the
forecast to account for observed trends.

Previous discussion...

The high pressure that was centered overhead today begins to
shift to the southeast of our area tonight. Clear skies continue.
With winds expected to be light, expect good radiational
cooling. Forecasting lows in the 30s for much of the area,
except for low 40s in some urban centers and portions of the
Cape and Islands. Patchy/areas of frost are expected to develop.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Tuesday...

As the surface high moves further offshore, Tuesday will bring
developing S to SW wind. A sea breeze is possible along east coastal
MA for part of the day, however as the SW flow increases some during
the afternoon, the east coastal sea breeze should come to an end.
Mostly sunny skies expected for much of the day, with increasing
clouds during the afternoon. A slightly warmer day with highs in the
60s across much of the area, some locales may reach 70 degrees. The
cooler area will be the immediate south coast of MA/RI, where the
persistent S wind will keep highs in the upper 50s to near 60.

Tuesday night...

Models show mid level ridging moving east of our area while deep
trough currently over the lower Ohio/Mississippi Valleys works
its way eastward. This pattern shift prompts a moisture-rich
low pressure system to track eastward towards the Delmarva
coastline. Moisture well ahead of this system will result in
skies becoming cloudy during the evening, but it will take some
time to moisten the column with respect to any precipitation.
Expecting rain to become likely after midnight. PWATs during the
overnight increase to 1 to 1.3 inches, so some moderate
rainfall possible but generally thinking rainfall of a tenth to
half inch prior to 12Z Wed.

Cloud cover and increasing dew points will keep overnight lows
somewhat milder, lows in the low to mid 40s for much of the area.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights...

* Wet weather Wednesday, diminishing Wednesday night
* Another round of showers possible Friday night and late Saturday

Discussion...

The 23/12Z operational guidance has come into better agreement
with the details into Thursday. Fairly good overall agreement
thereafter, although the Canadian model really goes off on its
own later this weekend. Preferred a solution more like the GFS
and ECMWF camp. Further prefer an ensemble mean approach,
especially after Thursday.

Wednesday and Wednesday night...

Essentially the same story from the past few days. Multiple
waves phase in the vicinity of the central Appalachians, or
perhaps into the Mid Atlantic. Coastal low develops and tracks
north through southern New England Tuesday night into Wednesday
night. This will mean a period of wet weather for our region.
Modest elevated instability still present, so cannot dismiss the
possibility of isolated thunderstorms.

Still looking like a general one half to one inch of rainfall
when all is done. Locally heavier amounts possible in any
persistent/slow-moving thunderstorms.

Thursday through Friday...

Except for possibly widely scattered late Thursday afternoon or
early evening instability showers over the NW higher terrain, it
looks dry Thursday afternoon through at least most of Friday.
Confidence has increased of at least most of daytime Friday
remaining dry.

Friday night...

A weak coastal low pressure may pass by just close enough for a
risk of showers late Friday into Friday night, mainly across
southeast MA and RI. Also will have to monitor the progress of a
cold front associated with a low pressure in the St. Lawrence
valley, which could also bring a chance for showers across the
western half of southern New England late Friday night. Confidence
remains rather low on these details, so later forecasts may
change.

Rest of this weekend...

A cold front associated with a northern stream trough may
result in scattered showers Saturday, but confidence in details
remains low.

Monday...High pressure expected to build in with drier weather.

&&

.AVIATION /23Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

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

Tonight...VFR with clear skies and excellent visibility.

Tuesday...VFR. Prevailing wind out of the SSW, initially light
enough where a sea breeze may develop along east coastal MA for
a time. Then as winds aloft strengthen somewhat during the
afternoon, the winds along east coastal MA should shift back to
the SSW.

Tuesday night...VFR to start. Becoming mainly MVFR after
midnight, with local IFR possible towards daybreak. SHRA
spreading into the area mainly after midnight.

KBOS Terminal...High confidence in TAF. Sea breeze this afternoon,
with a sea breeze probable for late morning into early afternoon
Tue.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...High confidence.

Wednesday: Mainly IFR, with local MVFR possible. Windy with
local gusts to 30 kt. RA with isolated TSRA. Patchy FG.

Wednesday Night: Mainly IFR, with local MVFR possible. Breezy.
RA likely, tapering off to SHRA. Isolated TSRA possible. Patchy
FG.

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

Thursday Night through Friday: VFR.

Friday Night through Saturday: VFR. 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 Tuesday Night/...High confidence.

Tonight...
High pressure over the waters tonight with South winds 10 kt or
less, seas 1 to 2 feet on the outer coastal waters. Good
visibility. Only concern for recreational boaters continues into
early this evening is for sea breezes along the coastline today
with 15-20kt gusts and choppy seas near shore, especially when
opposing outgoing tide.

Tuesday...
South wind increasing over the course of the day to 10 to 20 kts.
Sea breeze along east coastal MA possible late morning/midday, then
S winds should prevail as they strengthen. Seas 1 to 3 feet.

Tuesday Night...SE winds less than 25 kt. Seas locally approaching
5 ft, especially on the southern outer coastal waters. SCA
headlines may be needed. Rain showers becoming likely towards
daybreak.

Outlook /Wednesday through Saturday/...High confidence.

Wednesday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Areas of rough seas. Rain, isolated
thunderstorms, patchy fog. Areas of visibility 1 nm or less.

Wednesday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 13 ft. Rain likely, chance
of rain showers, isolated thunderstorms, patchy fog. Local
visibility 1 nm or less.

Thursday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Rough seas up to 10 ft. Slight chance of rain
showers.

Thursday Night: Winds less than 25 kt. Areas of rough seas.

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

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers.

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

&&

.EQUIPMENT...
The NOAA Weather Radio transmitter serving Providence is out of
service due to phone line problems. The phone company has
prioritized the issues and is working to get the weather
broadcast back on the air as soon as possible. The transmitter
serving Hyannis is back in service.

&&

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

&&

$$
SYNOPSIS...Belk/NMB
NEAR TERM...Belk/NMB
SHORT TERM...NMB
LONG TERM...Belk
AVIATION...Belk/NMB
MARINE...Belk/NMB
EQUIPMENT...



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