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Area forecast discussion
National Weather Service Taunton Massachusetts
950 am EDT Thu Mar 30 2017

Synopsis...
mainly dry and seasonable conditions continue today followed by
the first impacts of low pressure passing south of the region
tomorrow. Low pressure from the Ohio Valley passes south of New
England Saturday. This brings a mix of rain and snow Friday and
Saturday with several inches possible in northern
Massachusetts. High pressure brings dry weather Sunday and
Monday, followed by another approaching storm on Tuesday.

&&

Near term /until 6 PM this evening/...

950 am update...

Plenty of strong late March sunshine should result in a nice
early Spring day. Good mixing should yield high temps in the
upper 40s to the lower 50s across most of the region. Northerly
flow though will hold high temps mainly in the middle 40s across
the cape/islands. Sea breezes may also develop this afternoon
along the very immediate eastern Massachusetts coast, but overall a very
nice afternoon.

&&

Short term /6 PM this evening through 6 PM Friday/...
through 06z (2am local)... mainly dry with increasing clouds
expected ahead of an approaching warm front and with moisture
loading of the column from the top-down. Initial cooling still
expected in spite of the increasing clouds with temps dropping
back into the low 30s (maybe a few upper 20s) before precip
shield begins to move in from the west.

Early tomorrow morning and the remainder of the day...
the first of a pseudo-two phased system begins during the early
am hours tomorrow. Weakening warm front from initial parent low
pres in the Ohio Valley region will bring an area of modest warm
advective precip into the region from west-east through the morning
hours. Given anticyclonic flow aloft combined with a fairly deep
layer of dwpt depressions near or exceeding 10c initially,
latest trends have slowed the precip shield somewhat as it
enters the west reaches of S New England. Latest pops will reflect
this, in fact it's possible some locations particularly across east
Massachusetts and Rhode Island may not see precip with this feature at all, and may
require the added dynamic lift of the developing low pres Fri
evening/overnight before any significant precip falls. This
trend will have to be watched as it will be one of many factors
that will impact initial snow/sleet totals. Even though we start
as snow most places as explained below, this timing may
contribute to more/less impacts for Fri am commute.

In any case gradual transition to a widespread precip shield
through the late morning into the evening hours. The next issue
will be p-type. Latest trends not too much help given that GFS
remains the weaker outlier with north stream phasing, suggesting a
weaker/colder solution overall with European model (ecmwf) slightly colder to the
N, but allowing S low level jet further inland along the lines of the NAM
but not as robust. Taking a blend approach but with slight
weight on the European model (ecmwf) given it's only slight shift this update,
with some of the original forecast still in play. With
precipitation onset it looks to be mostly all snow save for
maybe within about 20nm of the S coastline thanks to wet-bulbing
through the column below 850 mb where temps are already mainly
below 0c. Initially totals will still be highest north of the Massachusetts
Pike predominantly due to higher slrs and a longer period of
lift. Looking at a daytime totals of 3-7inches within north extent
of the current Winter Storm Watch, (ie along the New Hampshire/Vermont border).
Lower amounts toward the S, mainly 1-4 along the Pike within 30
miles or so with a T or less closest to the S coast. The real
layer to watch is the 850-700mb layer for the low level jet and how
efficiently it will bring it warmer air. Have a gradual S-north
transition to a mix of sleet and rain through the daylight hours
but this will also be dependent on precipitation rates driven by
the overrunning. Therefore, for more on the overnight portions
of the storm, see below, but for now will be maintaining current
Winter Storm Watch given continued uncertainty in p-type and
timing, but this is again based on the overruning remaining as
robust as currently forecast. The recent trend to lessen
somewhat the initial impact could lead to lower totals of both
snow/sleet with the initial warm advective precip. In essence,
still uncertainty in p-types and timing.

&&

Long term /Friday night through Wednesday/...
big picture...

Longwave flow is nearly west-east with a slight trough over the
northeast USA. Shortwave scale flow shows northern and southern jet
streams directing at least three and possibly four shortwaves
through New England through the middle of next week. Approximate
timing would bring one southern shortwave through on Saturday, a
northern shortwave on Sunday, and another southern shortwave on
Tuesday. Another shortwave crosses the nation on Wednesday but
remains west of New England during the forecast period.

Differences in model mass fields with the GFS consistantly faster by
6 to 12 hours. The general pattern is similar but with a wide range
of different details either side of that basic pattern. Pattern
confidence is moderate through the weekend, diminishing to low by
Tuesday-Wednesday.

Details...

Friday night and Saturday...

Midwest weather system redevelops along the mid Atlantic coast early
Friday night. The NAM was farthest north of the group Friday
night...the European model (ecmwf) farthest north on Saturday. All positions are
favorable for a slug of measureable precip as suggested by the
precipitable water values which top out between 0.75 inches and 1.10
inches Friday night. The daily mean value for April 1 is about 0.5
inches and the 90 percent exceed value is 0.93, so these values are
well above average.

Precip type will be quite varied but trending through sleet and
freezing rain to rain through the period. Even southern areas could
see some sleet at times Friday night. But those southern areas
should change to rain, while northern areas start as snow and change
to sleet/freezing rain Friday night...then to rain Saturday.

Winter Storm Watch continues north of the Mass Pike. Potential
remains for 6 inches or more along the northern tier of Massachusetts with
lesser amounts farther south. Additional concern is the water
content of the snow, which is expected to be high. Looking at a
sharp gradient in expected snow amounts between northern Massachusetts and the
Mass Pike corridor, in part due to the expected change to
sleet/rain. This has limited our forecast confidence both in and
adjacent to the watch area. So no changes in headlines planned at
this time.

Low level southerly jet starts feeding the best moisture into our
area starting Friday evening in CT and expanding east by midnight.
We have 100 pct pops during this time frame. The moisture inflow
diminishes on Saturday. Models show a band of strong low level winds
around the coastal low as it passes southern New England Saturday
with speeds of 45 to 55 knots at 2000 feet above the South Coast and
islands. The ocean induced temperature inversion may keep the
highest winds aloft, but potential for 30-35 knot gusts along the
South Coast and islands Saturday.

The low moves east of CT by afternoon and east of Nantucket by
Saturday evening. Expect pcpn to diminish west to east during this
time. It is possible the rain may change back to snow before ending.

Sunday and Monday...

High pressure builds in from the west bringing fair weather Sunday.
Sufficient mixing during the afternoon from cold air aloft and
daytime sunshine...gusty afternoon north-northwest winds. Temps aloft suggest
Max sfc temps in the mid 40s to lower 50s.

Continued dry weather and lighter winds Sunday night as the high
builds overhead. Dew points in the 20s suggest min temps in the 20s
and lower 30s.

Some increase in clouds Monday as the next weather system
approaches, but otherwise a dry day under the high.

Tuesday...

Low pressure ejecting from the southwest USA is projected to
approach our area on Tuesday. This system bears some similarity to
the Friday-Saturday system, including a weak cold air damming signal
and a coastal redevelopment. This could be another coastal
rain/inland mix situation, but currently expected temperatures
support mostly rain.

Wednesday...

High pressure builds in with dry weather and near seasonable
temperatures.

&&

Aviation /14z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through Friday/...

Today...high confidence.
VFR with light winds. Mid to high level cigs increasing late.

Tonight...moderate confidence.
Cigs lower and thickening towards morning, but mainly after
06z and especially N/W. -Sn develops mainly after 06z as well
and slowly spreads west-east across portions of MA/CT. This will lead
to a reduction rapidly to IFR/LIFR vsbys before ceilings drop below
MVFR. Light NE winds.

Fri...low confidence.
High confidence in a trend toward IFR/LIFR everywhere as both
ceilings and vsby lower. Confidence is low mainly a transition from
snow to a mix of -sn/pl/fzra and pure rain through the day. The
heaviest precip waits until Fri evening, but this mix could
still occur during the daylight hours Fri.

Kbos taf...moderate confidence that a sea breeze will develop
this afternoon, otherwise high confidence in VFR through 6z
tonight.

Kbdl taf...high confidence in taf through 06z tonight.

Outlook /Friday night through Monday/...

Friday night... moderate confidence.

Widespread IFR/LIFR expected into Saturday morning, with some
improvement west to east Saturday afternoon. Precip types will be
mostly rain along the coast. Inland areas will start as snow or a
rain/sleet mix and then change to rain during Saturday. Gusty
easterly winds near the coast Friday night and Saturday morning with
gusts 30 to 35 knots possible. Winds become north-northeast Saturday afternoon.

Sunday-Monday...moderate confidence.

VFR. Northwest winds Sunday become variable Monday and east Monday
night.

&&

Marine...
forecaster confidence levels...

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

Short term /through Friday/...

7 am update...

Winds and seas have diminished enough to drop small craft
headlines on Rhode Island Sound and Block Island Sound as well as Boston
Harbor and Narragansett Bay. Otherwise no changes planned.

Previous discussion...

Winds and seas diminish through the day today. First ground dropping
below small craft thresholds this morning for bays/sounds and
then through the afternoon elsewhere across the waters. After
small craft advisories are dropped a period of mainly quiet
boating weather is expected through tomorrow evening as east winds
remain light and seas take time to increase. Only caveat will be
for rainfall across the waters beginning Fri morning.

Outlook /Friday night through Monday/...

Friday night-Saturday...moderate confidence.

Increasing east winds with potential gusts to 35 knots on the waters
south of Provincetown and the South Coast. Sesa will build to 5
to 10 feet on the exposed waters. The coastal weather system causing
this will move east of the waters Saturday night. A gale watch has
been posted for the southern and southeast waters and includes Cape
Cod Bay. Winds turn from the north or northeast as the weather
system moves off Saturday night.

Sunday-Monday...moderate confidence.

High pressure builds over the waters. This will bring diminishing
winds and seas. Small Craft Advisory may linger on the outer waters
Sunday.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...Winter Storm Watch from Friday morning through Saturday
afternoon for maz002>006-008-010-026.
Rhode Island...none.
Marine...gale watch from Friday evening through Saturday evening for
anz232>235-237.
Small Craft Advisory until 2 PM EDT this afternoon for
anz231>234-251-255-256.
Gale watch from late Friday night through Saturday evening for
anz231.
Gale watch from late Friday night through Saturday evening for
anz250-254.
Small Craft Advisory until 11 PM EDT this evening for anz250-
254.
Gale watch from Friday evening through Saturday evening for
anz255-256.

&&

$$
Synopsis...wtb/doody
near term...Frank

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