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Area forecast discussion
National Weather Service Taunton Massachusetts
403 PM EDT Fri may 26 2017

Synopsis...
low pressure moves offshore tonight followed by weak high
pressure this weekend which will bring mild days and cool
nights. Low pressure in the Great Lakes generates a coastal low
late Monday that passes off Nantucket on Tuesday. This will
bring wet weather on Memorial Day. Daytime clouds and scattered
showers will pop up each day Tuesday through Friday.

&&

Near term /until 6 am Saturday morning/...
comma head showers continue to rotate south into sne, focused
mainly across central/west Massachusetts into CT/Rhode Island within defined trowal
signature. These showers will decrease toward evening as the low
pulls away but a few showers will likely linger into the early
evening.

Flow gradually becomes anticyclonic tonight as weak high pres
builds in from the west so expecting a mainly dry night. Plenty
of clouds will persist given presence of low level moisture
which will limit temp drop. Mins will range from the upper 40s
to lower 50s.

&&

Short term /6 am Saturday morning through Saturday night/...
Saturday...
weak high pres builds across the region but low level moisture
remains abundant below the inversion which indicates clouds will
dominate with bkn-ovc cloud cover. Mid level shortwave
approaches from the west by late in the day which will also
bring increasing mid level clouds and can't rule out a brief
shower toward evening across CT. Soundings show mixing to about
925 mb with highs mostly in the 60s with a few lower 70s
possible lower CT valley. Seabreezes expected in the afternoon
which may hold temps in the upper 50s along east coastal Massachusetts.

Saturday night...
a brief shower possible from CT to southern Rhode Island in the evening
as shortwave passes to the south, otherwise dry weather as weak
high pres remains in control. Mostly cloudy skies persisting.
Mins mid 40s to lower 50s.

&&

Long term /Sunday through Friday/...
big picture...

Longwave scale maintains a ridge west/trough east pattern for much
of the long term period. Shortwave scale shows a closed low slowly
moving from Canada across the Great Lakes, eventually reaching New
England late in the week. Two jets in place for much of the period.
The northern jet sweeps around the closed low from western Canada
across the northeast USA. The southern jet swings from baja into the
Southern Plains and southeast USA. The two streams interact over
the Ohio Valley and mid Atlantic early next week.

Model mass fields are similar through Tuesday...the GFS and European model (ecmwf)
are similar through Thursday. GFS and European model (ecmwf) moisture and thermal
fields are similar during the same period while showing differences
in detail. This brings increased confidence in the overall pattern
through much of next week, although lingering uncertainty regarding
timing of individual shortwaves in the flow.

Details...

Sunday...

High pressure is in control Sunday with subsidence and light wind
flow. Moisture fields show an area of higher relative humidity below 800 mb and
suggests a cloud/sun mix with the fair weather. Temperatures in the
mixed layer are equiv to 5-8c, suggesting Max temps upper 60s to mid
70s. Sea breezes along the coast will hold temps there in the 60s.

Fair skies linger early in the night, but the next approaching
shortwave supports increasing clouds...expect this overnight. Dew
points will be roughly 45-50 which with light flow and initial fair
skies should allow temps to reach the lower 50s and possibly the mid
40s.

Monday-Tuesday...

Interaction between the northern and southern streams leads to a 120-
knot jet over the Ohio Valley which races east ahead of northern
stream shortwave during Monday. This crosses the surface cold front
and generates a triple-point low as it crosses the mid Atlantic
coast. This in turn spins up a weak southerly low level jet that
aims for srn New England during the afternoon/evening. Precipitable
water values reach 1.0-1.2 inches which is above normal but not
excessive.

Model consensus slows some slowing of the onset of measurable
pcpn in southern New England. The GFS holds off on measurable
until after 12z Monday while the ggem and European model (ecmwf) bring it to the
CT valley by 12z and possibly a little farther. We opted to slow
pcpn onset to a compromise time with chance pops to the CT
valley around 09z and to Worcester and the Rhode Island hills 12z...then
to most of eastern Massachusetts by 14z.

We will bring likely pops to the western sections for late
morning/early afternoon and for eastern sections for the afternoon.
Stability parameters are somewhat favorable for convection, with
totals around 50 while surface Li values will be near zero. We will
include scattered to isolated tstms Monday afternoon/evening.

The coastal low passes offshore Tuesday morning. Expect leftover
showers diminishing Tuesday morning, then expanding again as another
shortwave moves through.

Wednesday through Friday...

Upper low slowly sweeps across eastern Canada during the period,
with shortwaves moving through the flow across New England. Cold
pool advection aloft will destabilize the airmass and support
scattered daytime showers, especially in areas north of the Mass
Pike. Temperatures in the mixed layer will support Max temps upper
60s to mid 70s each day. Fair skies and dew points in the 50s will
support mins in the 50s each night...possibly cooling to mid 40s to
low 50s late in the week.

&&

Aviation /20z Friday through Wednesday/...
forecaster confidence levels...

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

Short term /through Saturday night/...moderate confidence.

Through 00z...mix of VFR/MVFR cigs will trend back to mostly
MVFR with pockets of IFR cape/islands. Showers continue across
central/west Massachusetts into CT/Rhode Island before diminishing toward evening. Northwest
gusts to 20-25 kt cape/islands.

Tonight...mostly MVFR cigs, improving to VFR in central and
western new eng. Diminishing northwest winds.

Saturday and Saturday night...mix of VFR/MVFR cigs. Seabreezes
developing late morning into the afternoon. Low risk for a brief
shower late Sat/Sat evening south of the Mass Pike.

Kbos taf...moderate confidence.

Kbdl taf...moderate confidence.

Outlook /Sunday through Wednesday/...moderate confidence.

Sunday... VFR. Light south wind becoming onshore on the mass East
Coast by midday.

Monday... VFR at first, lowering to MVFR in the afternoon and
IFR/LIFR at night. Southeast winds less than 20 knots. Rain/showers
developing during the morning and midday, with potential scattered
tstms in the afternoon/evening. Areas of fog at night.

Tuesday-Wednesday...

IFR Tuesday morning, improving to VFR Tuesday afternoon through
Wednesday. Scattered showers, mostly during the daytime. Light east
wind Tuesday becoming south in the afternoon, then west-southwest
Wednesday. Speeds less than 15 knots.Moderate confidence.

&&

Marine...
forecaster confidence levels...

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

Short term /through Saturday night/...high confidence.

Through early evening...N/NW gusts to 25 kt possible across
eastern Massachusetts waters through this afternoon.

Tonight...winds will diminish but southerly swell will maintain
5-7 ft seas over the south coastal waters with seas subsiding
below 5 ft over eastern waters.

Saturday...winds becoming east/southeast with speeds below 15 kt. Seas
will subside below 5 ft over southern waters.

Saturday night...quiet weather with light winds and seas below
Small Craft Advisory.

Outlook /Sunday through Wednesday/...moderate confidence.

Winds less than 20 knots and seas less than 5 feet through the
period. Showers and isolated tstms Monday, showers and fog Monday
night and early Tuesday.

&&

Tides/coastal flooding...
still observing a 0.5 to 1 ft surge along both coasts which
estofs and etss guidance is underestimating. Surge expected to
diminish to around 0.2 to 0.5 ft at the time of high tide later
tonight as winds and swell diminish. High tides this evening are
quite high (boston 12.41 feet around 1230 am, Providence 6.45
feet around 930 pm), even with minimal wave action, total water
level above 12.5 ft in Boston typically produces very minor
inundation for the notorious low-lying locations (i.E, morrissey
Boulevard in boston). In addition, minor inundation possible
along the South Coast if surge still around 0.5 ft. We will
issue a coastal flood statement to address these concerns.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 7 PM EDT this evening for anz231-
232.
Small Craft Advisory until 10 am EDT Saturday for anz235-237.
Small Craft Advisory until 6 am EDT Saturday for anz250.
Small Craft Advisory until 8 PM EDT this evening for anz251.
Small Craft Advisory until noon EDT Saturday for anz254>256.

&&

$$
Synopsis...wtb/kjc
near term...kjc

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