Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 240212
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1012 PM EDT Tue May 23 2017

.SYNOPSIS...
Dry conditions prevail early tonight. Another wave of low
pressure tracks south of New England late tonight into early
Wed, bringing a chance for rainfall to the south coast. This
will be followed by dry and seasonable conditions Wed afternoon
and night. Low pressure brings showers and cooler temperatures
Thursday and Friday. Weak high pressure will bring a dry start
to the holiday weekend Saturday. Another coastal low brings late
Sunday into early Monday. Leftover showers are possible Monday
and Tuesday.

&&

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

1030 PM Update...

18Z RGEM and 00Z NAM has come in quite dry for tonight which
cause a bit of a pause for the near term forecast. Comparing the
latest Hi-res guidance, will begin a trend in that direction.
Coastal low is staying a bit further offshore with a lot of dry
air in the mid to upper levels. Right now, confidence is that
the immediate south coast, Cape and Islands have the best
potential for rainfall very late tonight into tomorrow morning.

Otherwise high level clouds aloft will keep temperatures from
falling tonight. Anticipate overnight lows remaining in the
50s.

Cannot rule out some patchy fog across the Islands and the south
coastal plain as better low level moisture advects into the
area. Still a good temp/dewpoint spread so am not expecting
dense fog tonight.

Aside from the adjustments mentioned above, the overall trend in
the forecast generally is on track.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Wednesday...

Low pressure tracks further northeastward during Wednesday, with the
low center likely passing near 40N/70W in the morning. Precip
chances are at their highest early in the morning, with chance pops
IJD-SFZ-near OWD and southeast, then high chance to likely pops Cape
and Islands. Where it does rain, QPF should be limited to around
0.25 inch or less.

Otherwise, chances for rain diminish for late morning, with a dry
afternoon, as the low moves further offshore and away from our
region. Northeast winds will gust during the morning hours across
the Cape and Islands due to tight pressure gradient from building
high pressure to our west, and passing coastal low to the east.
Gusts will be near 15 to 25 MPH.

As a surface high pressure builds into the area, should see
increasing sunshine, partly sunny skies. The east/northeast onshore
winds will limit high temps to some extent across the coastal plain.
In addition there is a chance of sea breeze development across south
coastal MA/RI. Forecasting high temps in the mid 60s to mid 70s,
except lower highs in the upper 50s to mid 60s along east and south
coastal MA/coastal RI.

Wednesday Night...

Increasing clouds expected in anticipation of another low pressure
system to affect our region. The low emerges from the vicinity of
Virginia and North Carolina, starting to make its way to the Mid
Atlantic Coastline during the overnight. 12Z model runs look to
provide a slower approach of this low, so Wed night should be
predominantly dry. However as we advance towards daybreak Thu, have
slight chance pops for northern CT/southern RI. In addition, could
see some low clouds and patchy fog within the coastal plain as a
light east wind continues.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Big Picture...

The longwave scale retains cyclonic flow over the Eastern USA much
of the period while a ridge builds over the Western USA.

Deep trough over the Eastern USA evolves into a closed low that
sweeps across New England Friday.  A weak ridge brings drier weather
Saturday, then a shortwave races through on Sunday. A northern
stream closed low over Canada drops south over the Great Lakes, then
guides a southern stream shortwave across New England early in the
week.

Upper mass fields are similar through Saturday.  The fields
thereafter show similar projections but with different timing
through the early week.  We used a blend of model data for the
extended range grids with low-moderate confidence for the Sunday
night-Monday system.

Details...

Thursday-Friday...

Shortwave coming around the upper low will generate a coastal low
near Chesapeake Bay Thursday. That low will then move up the coast
Thursday night and cross our area Friday. The 12Z models show a 40-
50 knot south jet between 850 and 1000 mb moving up the coast
Thursday. This shifts well offshore over Thursday night as the upper
shortwave kicks east to the coast. Meanwhile a 20-30 knot east jet
at the surface will feed surface level moisture into our area. The
presence of these low level jets suggest a good inflow of moisture
into our area, while the shortwave and upper jet will provide
sufficient dynamics for lift.

Two questions in the data are the jet shifting well offshore as it
does and a mid-level dry zone overhead Thursday afternoon which
fills in Thursday night. Either of these could interfere with the
expected rainfall. For now, we will slow the onset of rain into
Thursday morning...which may still be too soon if that dry layer has
anything to say about it...with the major portion of rain taking
place Thursday night and Friday morning.

The coastal low and upper low move past Southern New England Friday
morning. This will bring winds around out of the north-northwest and
cut off the inflow of moisture. Expect the leading precip to taper
off, but with leftover showers through the day Friday.

Low level flow will be from the east Thursday, coming over water
temps in the 50s.  Thursday max temps in eastern Mass will be
buffered by this water with max values upper 50s and lower 60s.
Farther inland, low level temps suggest a max of 60-65.  With dew
points in the upper 40s to mid 50s and rain, expect min temps to be
close to dew point. Temps aloft on Friday suggest max temps around
60 or low 60s in the east and mid to upper 60s in the CT Valley.

Saturday...

High pressure brings a day of rest from the wet weather. Temps of 8C
to 9C at 850 mb suggests max temps in the low to mid 70s. If the
surface flow is light enough, a sea breeze may occur along the east
Mass coast and hold max temps there in the 60s.

Sunday through Tuesday...

The models show a dance of sorts in the atmosphere. Closed low from
Canada dips south into the Great Lakes area. In doing this, it pulls
a shortwave in the southern stream farther northeast. This will push
surface low pressure through the Ohio Valley and to the New Jersey
coast on Sunday. The ECMWF is faster with the surface low, with less
resulting interaction between upper systems. The GFS is farther
north and shows more interaction.  In any case, expect showers for
our area for Sunday afternoon and night. For Monday and Tuesday the
northern closed low shifts closer to us and should provide a
continued chance of clouds and showers.

&&

.AVIATION /02Z WEDNESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

Tonight...Moderate confidence. Mainly VFR with possible MVFR/IFR
developing after 2AM across the south coast, Cape and Islands.
Expect low clouds and fog, as well as passing showers from
coastal low later tonight. Also areas of fog south coastal
MA/Cape/Islands. Gusty NE winds to near 20 kts possible near
daybreak. Chance for showers mainly Cape/Islands later tonight.

Wednesday...High confidence. Improving trend to VFR, with the
exception of Cape/Islands, where IFR/vsbys cigs may linger thru Wed
am, and IFR cigs may last into the afternoon. East wind
persists. Passing showers Wed am, mainly across Cape/Islands.

Wed night...Across interior southern New England, VFR likely and
dry. Along east coastal and south coastal MA including Cape/Islands,
IFR possible in low clouds and fog. Light NE/E wind persists thru
the night.

KBOS TAF...High confidence in TAF.

KBDL TAF...High confidence in TAF.

Outlook /Thursday through Sunday/...moderate to high forecast
confidence.

Thursday... IFR cigs and vsbys in rain and fog with east wind
gusting to 20 knots along the shoreline.

Friday... Conditions improve to mixed VFR/MVFR in showers as winds
shift from the northwest.

Saturday... VFR. Diminishing northwest wind.

Sunday... VFR early, trending to MVFR/IFR in showers and fog by
Sunday night. Winds from the southeast.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Tonight and Wednesday...

Another coastal low will develop across the Mid-Atlantic and move
northeastward towards the waters tonight, then pass northeast of the
waters on Wednesday. Visibilities could drop in patchy fog and light
to moderate rainfall late tonight and Wed morning. Improving
visibility and rainfall ending by midday Wednesday as high
pressure builds in the area. However, due to the pressure
gradient between the offshore low and high pressure, a period of
gusty northeast winds is expected during Wednesday and
Wednesday evening. Seas will begin to build during Wed as low
moves further offshore.

Highest confidence of SCA thresholds for winds/seas being met during
Wed/Wed night are along the southern outer coastal waters, and RI/BI
Sounds. A Small Craft Advisory has been issued for this area.

Outlook /Thursday through Sunday/...

Thursday-Friday...

Poor vsbys in rain and fog Thursday and Thursday night. Easterly
winds around 25 knots with strongest winds during the afternoon and
evening. These winds then lift north of the waters tonight. Seas 5
to 6 feet on the outer waters Thursday and Thursday night. Low
pressure moves past the waters Friday morning, at which point the
winds shift from the NNW with speeds 20 knots or less. Expect seas 5-
7 feet on the outer waters and RI Sound.  Small Craft Advisory may
be needed on some of the waters both days.

Saturday-Sunday...

Winds remain below 25 knots through the period.  Seas subside below
5 feet early Saturday. Seas may return to 5 feet on the outer waters
late Sunday night.  Poor vsbys possible in showers and fog late
Sunday and Sunday night.

&&

.TIDES/COASTAL FLOODING...
High astronomical tides will occur over several tide cycles along
eastern Massachusetts coast from Thursday into Memorial Day, with
tides around 12 feet in Boston and 4 feet on Nantucket.

At the very least, minor nuisance flooding will occur in the most
vulnerable locations such as Morrissey Blvd in Boston. Any surge on
top of these tides would lead to more widespread, but minor,
coastal flooding, including on Nantucket. Right now, this looks
to be a possibility Thursday into Friday due to expected onshore
winds. Model surge guidance (ESTOFS) shows a potential 0.6 ft
surge, which would give a storm tide near 13 feet Thursday
night in Boston and just under 5 ft on Nantucket.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 10 AM Wednesday to 6 AM EDT Thursday
     for ANZ254>256.
     Small Craft Advisory from 2 PM Wednesday to 6 AM EDT Thursday
     for ANZ235-237.

&&

$$
SYNOPSIS...WTB/NMB
NEAR TERM...Dunten/NMB
SHORT TERM...NMB
LONG TERM...WTB
AVIATION...WTB/NMB
MARINE...WTB/NMB
TIDES/COASTAL FLOODING...staff



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