Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 210737
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
337 AM EDT Sat Apr 21 2018

.SYNOPSIS...
High pressure building over the Great Lakes today moves east
over New England for Sunday and Monday, then off to the east
Tuesday. This will bring our area dry weather and gradually
rising temperatures. Wet and breezy weather forecast midweek.
Perhaps a late week lull before becoming dreary again next
weekend.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
High pressure over the Great Lakes with a diminishing pressure
gradient over Southern New England. Sunshine will generate some
mixing, reaching to about 850 mb. Also today the models show a
weak shortwave moving through the Northeast and supported by a
120 knot upper jet.

Morning temps at or below freezing will quickly recover this
morning. Winds in the mixed layer will support developing gusts
around 20 knots midday and afternoon. There is enough of a
gradient to maintain a west to northwest wind. Temperatures at
850 mb reach -2C to -4C, and when mixed to the surface this
would support max temps in the 50s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
As the high pressure moves overhead, winds will become light and
subsidence should make our skies clear. With dew points in the
20s, except 30-35 Cape and Islands, expect some areas away from
the coast will dip below freezing, the rest will be in the 30s.

The high maintains clear skies and light winds Sunday. This will
allow sea breezes along the coast late morning through
afternoon. Mixing will support max temps in the mid 50s to
around 60.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
*/ Highlights...

 - Warm up early week, dry, watching for seabreezes
 - Wet, breezy midweek
 - Perhaps a reprieve Friday, possibly Saturday
 - Return of wet, dreary weather for the following weekend

Overview and model preferences...
Defined ridging is expected to define the sensible wx early
this coming week, with height anomalies finally becoming
positive by early Mon. H85 temps respond as well, reaching above
0C and likewise, positive anomalies. However, will need to
watch a developing trof just W of the continental divide, a
response to enhanced ridging along the W coast of the PACNW and
BC. This wave shifts rapidly E through the early portion of the
week and looks to phase with a slowly meandering S stream
cutoff. This phasing will bring about a return to longwave trof
across the E, and yield a return to unsettled and wetter wx
reinforced yet again by a secondary shortwave out of the NW
territories late week. Recent runs are struggling somewhat with
the sfc low pres development/track with the leading wave. This
is likely due to the fact it will not be fully developed or
sampled until Sun. Given this, will lean most heavily on
ensemble means to account for differences in final outcomes.

Details...

Mon and Tue...
As mentioned above, ridging remains in control. H85 temps rise
above 0C, to an average near +3C Mon, then +5C on Tue. This
should allow several locations to reach the 60s. Can`t rule out
a stray 70F, but noting strong subsidence inversion which may
limit mixing. Also, with weak flow under cresting high pres and
SSTs still in the low-mid 40s, sea breezes are likely especially
Mon, but also possible Tue. This will lead to strong coastal
temp gradient. Cool overnights with radiational cooling.

Wed...
Low pres develops across out of the cold pool from remnant
convection across the SE CONUS. Depending on how quickly the
wave aloft deepens, the low will either track more N, or E with
time. This is where the uncertainty in rainfall totals on Wed
reside. The E track would usurp the available moisture and shift
strongest LLJ to the S while a deeper trof would draw the
parent low/moisture/dynamics N toward New England. Ensemble
means still favor the former over the latter, so this update
will feature this solution, but this will need to be watched. In
either case, PWATs and LLJ nearly 2 std deviation above normal
as well as a conditionally unstable profiles suggest at least a
risk for periods of moderate to heavy rain unless the E track
verifies. Low probs of QPF near 1.00 suggested.

Thu and Fri...
The depth of the wave will also define sensible wx details on
Thu and Fri. If the trof deepens later (the low pres E track
suggested above), then Thu could feature continued wet and
unsettled wx. Otherwise a brief period of meso-ridging will
follow. Whether this occurs on Thu and Fri or Fri only is still
somewhat uncertain. With a developing longwave trof and some
moisture clouds/SHRA possible either day, but POPs will be
generally lower than Wed.

Next weekend...
As mentioned in the overview, a secondary trof will be moving
across the CONUS and merging with the longwave trof, deepening
it across the E. This may lead to another round of wetter and
cooler than normal conditions, but this is of course toward the
end of most deterministic/ensemble skill set.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

Today... VFR. Patchy daytime clouds this afternoon, but with
bases above 4500 feet. Northwest winds with gusts around 20
knots midday and afternoon, diminishing by evening.

Tonight and Sunday... VFR. Mainly clear skies and winds less
than 20 knots. Developing sea breezes Sunday late morning and
afternoon.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

Outlook /Sunday Night through Wednesday/...High confidence.

Sunday Night: VFR.

Monday through Tuesday: VFR.

Tuesday Night: Mainly VFR. Chance RA late.

Wednesday: Mainly MVFR, with local IFR possible. Breezy. RA
likely.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Sunday/...High confidence.

Northwest winds gusting to 20 knots today, then lighter winds
tonight and Sunday. Seas remain below 5 feet through the
weekend.

Outlook /Sunday Night through Wednesday/...High confidence.

Sunday Night through Tuesday: Winds less than 25 kt and weak
seas.

Tuesday Night: Winds less than 25 kt and weak seas. Slight
chance of rain.


Wednesday: Winds less than 25 kt. Seas locally approaching
5 ft. Rain likely, rain showers likely.

&&

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

&&

$$
SYNOPSIS...WTB/Doody
NEAR TERM...WTB
SHORT TERM...WTB
LONG TERM...Doody
AVIATION...WTB/Doody
MARINE...WTB/Doody



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