Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 281113
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
713 AM EDT Wed Oct 28 2020

.SYNOPSIS...
Weak low pressure tracking south of New England will bring a period
of rain this morning before exiting this afternoon. High pressure
briefly moves in tonight. The remnants of Zeta will bring a soaking
windswept rain Thursday into Friday morning, with rain changing to
snow late Thursday night before ending across the interior,
especially in the hills. Behind this departing system, dry but
blustery and cold weather follows later Friday. High pressure
returns this weekend with dry but chilly weather Saturday followed
by milder temperatures Sunday and next Monday.

&&

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

7 am update...

Rain has overspread the region this morning as the shortwaves
moves through. The area of heaviest rain has been across MA from
the Berkshires to the Boston area where several hundredths to
several tenths have fallen. The back edge of the precip should
move into western MA by late morning.

Previous Discussion...

Low amplitude shortwave in progressive pattern will result in a fast
moving surface wave which tracks south of New Eng this morning. Deep
moisture and modest forcing for ascent will bring a period of rain
this morning to much of SNE but focused across the coastal plain.
Given the quick movement of this system, expect rain to exit the CT
valley by late morning, ending in RI and eastern MA between noon and
2 pm. Rainfall will average 0.10-0.25" with locally higher amounts
south of the Mass Pike. Plenty of low level moisture lingers behind
the system so low clouds will persist through the afternoon,
although some breaks of sun are possible late in the day in the CT
valley. Chilly morning with temps in the 40s with rain, with highs
recovering into the 50s south of the Mass Pike in the afternoon.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
Tonight...

Mid level confluent flow with surface ridging will result in dry
conditions tonight. Moisture from Zeta should remain to the south
through tonight. Low clouds will erode this evening, but mid/high
clouds will be overspreading the region during the night leading to
mostly cloudy skies. Lows range from the upper 30s in interior
northern and western MA to mid 40s near the south coast.

Thursday...

Low pres assocd with remnants of Zeta will move NE to mid Atlc by
Thu evening, while deep moisture plume lifts north across SNE.
Soundings shows some drier air in the low levels which may delay
onset of the rainfall so we opted to slow down timing of the rain
more in line with the ECMWF. Rain should be overspreading the
south coast in the morning but likely holding off until mid
afternoon north of the Mass Pike. Highs will be mostly 45-50
with low/mid 50s possible along the south coast and
Cape/Islands, but falling temps later in the afternoon as cooler
low level temps begin to drain southward.

&&

.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Highlights...

* Soaking rain Thu night/Fri from remnants of Zeta, likely
  ending as wet snow inland, especially in the hills.

* Blustery/cold Fri and Sat, then milder Sun

* Rain chances return Sunday, drying out Monday and Tuesday

Details...

All eyes remain on a wet and blustery period late Thursday into
Friday as a couple of disturbances bring widespread rain, gusty
winds, and even snow to southern New England. The two systems in
question are, first, the remnants of Hurricane Zeta which should
make landfall in the Gulf of Mexico Wednesday night before lifting
into the mid-Atlantic within 24 hours; second is a mid-latitude
upper level disturbance that tracks eastward, just behind the
post tropical system.

This series of systems will provide a good soaking to the drought
stricken area thanks in part to a generous tap of tropical moisture
(PWATs 1 to 3 standard deviations above normal for late October)
beneath the right rear quadrant of a sprawling 165 kt 300 mb jet.
This will provide plenty of forcing for ascent, together with the
strong low level jet. All indications are that we could see a good 1
to 2 inches of rain before all is said and done late Friday.

The biggest question concerns precip type, namely when/where rain
switches over to snow as winds begin to pull in subfreezing air
through the column. Ensemble guidance continues to indicate a good
chance of a switch over to snow during the overnight hours Thursday
into early Friday...the best chance of accumulation being in the
high terrain of the Berkshires and Worcester Hills. This remains a
low confidence forecast when it comes to snow amounts given that we
remain 48 hours out...there are still some significant questions.
Exact timing will make a big difference as snow will of course stand
a better chance of accumulating during the overnight hours. After
quite a warm stretch the ground will struggle to accumulate snow,
but lack of solar insolation and intensity can help to overcome
that. This system is on the weaker end, remaining an open wave at
700 mb (weaker than the ECMWF was showing yesterday). However, with
strong dynamic forcing and frontogenesis we could see our first few
inches of snow. A much colder airmass arrives behind the system
Friday, and as this cold air filters in there`s a decent chance
we`ll see snowflakes closer to the coast. Still, would not expect
any accumulation.

This system will also bring some strong winds with it. A very strong
low level jet develops which will bring gusty winds on Friday,
especially for our marine zones. A Gale Watch is in effect, and seas
of 10 to 14 ft will be possible on the outer waters. Northeast winds
on Friday may lead to some minor coastal flooding on the
northeast/east facing shores, but very low astronomical tides will
help keep coastal flooding from being a bigger issue.

A much colder airmass overhead will lead to likely our coldest night
of the season so far on Friday night, down into the 20s (teens in
the highest elevations). Should see some improvement by Saturday
night but still quite cold, at the freezing mark or below away from
the coast. This is thanks to high pressure which also keeps things
dry for the first half of the weekend. Sunday unsettled weather
returns ahead of a mid level trough. Southerly flow, though, will
help to moderate temperatures. By next week things turn dry and
cooler.

&&

.AVIATION /11Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

12z update...

Through 12z...
VFR cigs trending to MVFR toward daybreak as steadier rain moves
in from the west.

Today...Moderate confidence.
VFR in eastern MA and RI this morning with MVFR/IFR further west
in heavier rain. Lowering to IFR this morning as rain moves
across the region. Rain exits from west to east 15-18z with
gradual improvement to VFR in the interior after 18z. MVFR/IFR
persists through the day eastern MA.

Tonight...High confidence.
Lingering MVFR cigs possible early over Cape/Islands, otherwise
VFR. West wind gusts to 20 kt over ACK in the evening, then
diminishing and becoming NW overnight.

Thursday...High confidence in trends. Lower confidence in timing.
Conditions gradually lowering to MVFR then IFR from south to
north as rain overspreads the region. Rain and lower cigs/vsbys
may be delayed until late afternoon north of the Mass Pike.

KBOS Terminal...Moderate confidence in TAF. Exact timing
uncertain. Cigs lowering to MVFR/IFR with a period of rain
12-18z. Improvement to VFR toward 00z.

KBDL Terminal...Moderate confidence in TAF. Exact timing
uncertain. Cigs lowering to MVFR/IFR with a period of rain
10-16z. Improvement to VFR mid/late afternoon.

Outlook /Thursday Night through Sunday/...

Thursday Night: MVFR/IFR conditions possible. Strong winds with
areas of gusts up to 40 kt. SHRA.

Friday: Mainly VFR, with local IFR possible. Strong winds with
gusts up to 40 kt. Chance SHRA, chance SHSN.

Friday Night: VFR. Windy with local gusts up to 30 kt.

Saturday: VFR.

Saturday Night: VFR. Breezy. Slight chance SHRA, slight chance
FZRA.

Sunday: Mainly VFR, with areas MVFR possible. Windy with local
gusts up to 30 kt. Chance SHRA.

&&

.MARINE...
Today...Southerly winds becoming SW/W during the afternoon over
south coastal waters. Gusts to 20 kt developing in the afternoon.
Seas below SCA. Vsbys reduced to 1-3 miles in rain and fog,
improving from west to east during the afternoon.

Tonight...W/NW winds with gusts to 20 kt at times. Good vsbys.

Thursday...Light N/NW winds turning NE and increasing in the
afternoon. Gusts to 25 kt over south coastal waters by late in the
day. Vsbys lowering to 1-3 miles in developing rain and fog.

Outlook /Thursday Night through Sunday/...

Thursday Night: Moderate risk for gale force winds with gusts
up to 40 kt. Areas of rough seas. Rain showers. Local
visibility 1 to 3 nm.

Friday: Moderate risk for gale force winds with gusts up to
40 kt. Rough seas up to 12 ft. Chance of rain showers.

Friday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 12 ft.

Saturday: Winds less than 25 kt. Areas of rough seas.

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

Sunday: Moderate risk for Small Craft Advisory winds with gusts
up to 30 kt. Seas up to 5 ft. Chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Gale Watch from Thursday evening through Friday evening for
     ANZ231>235-237-254>256.
     Gale Watch from late Thursday night through Friday evening for
     ANZ250-251.

&&

$$
SYNOPSIS...KJC/BW
NEAR TERM...KJC/BW
SHORT TERM...KJC
LONG TERM...BW
AVIATION...KJC/BW
MARINE...KJC/BW


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