Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 201907
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
307 PM EDT Fri Apr 20 2018

.SYNOPSIS...
High pressure builds just west of New England into Saturday and
will bring gradually rising temperatures and dry weather into
early next week. Wet and breezy weather forecast midweek.
Perhaps a late week lull before becoming dreary again next
weekend.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Minor adjustments to temperature and sky cover for this
afternoon. Otherwise, expecting dry weather with some clouds
from time to time.

High pressure remains to our west overnight, with a low pressure
moving east across the Maritimes. While this will maintain a NW
flow across our region, expecting the gustiness to diminish with
the loss of daytime heating.

Dry weather continue with lows at or below freezing across the
interior. Min temperatures higher across the coasts, Cape and
islands, mainly in the mid to upper 30s.

Expecting some frost to develop across the interior,
particularly at the more sheltered locations.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
High pressure moves closer to our region, but the core remains
to our west. Thus, general NW flow expected to continue through
Saturday night. Local seabreezes are possible, but remain a low
risk at this point in time. Dry weather continues beneath a mainly
clear sky.

Temperatures will remain below normal, still. More frost
expected Saturday night.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...

*/ Highlights...

 - Warm up thru 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...

Aleutian low focus. With maturation, immediate downstream
energy is cut off, westerlies dominate, polar air held N,
beginning influence via sub-tropical S-stream along isentropes.
However deepening toward occlusion, later death, strong warm SW
isentropic push into Canada, ridge dominance, energy sheared S,
N-stream captured, slip of cooler air late week. However depth
in question given progressive Atlantic plus S-stream influence.
Polar low dipping S following weekend, yet milder air infused
with transition into late-Spring. While the EPS progs anomalous
cool H85 temperatures into early May, outcomes may be dreary,
wet, cool, however not wintry.

Warming trend into early week, S-stream disturbance midweek, a
dip within preferred H5 trof pattern with sheared Pacific energy
swept E, brief cool down beneath clouds, rain, breezy via
clipping jet streak. Lull possible Friday prior to a stronger
system for the following weekend, potentially cooler. Seemingly
ebb and flow, yet appears Spring-like weather has finally
arrived. Forecast targets of opportunity discussed below.

*/ Discussion...

Sunday through Tuesday...

High pressure, light winds, monitoring: 1.) daytime sea-breezes
yielding sharp coastal-interior temperature gradients, 2.)
overnight radiational cooling, and 3.) when high pressure shifts
E allowing S/SW warmer air (mainly Tuesday). Suppressing ridge
remaining in place, ascending cloud cover per approaching
midweek storm holds off, Tuesday is the warmer of the two. Put a
mention of frost. Per MA agricultural extension, some farmers
have already planted.

Midweek...

Wet weather anticipated. S-stream clash with E-shear Pacific
energy. Tightening thermal gradient along 290-310K isentropes, S
low-level jet streak response. Precipitable water push to 1.25
inches, H85 dewpoints to 10C, high freezing level heights,
sneaking instability of a few hundred joules / kg. Sub-tropical
warm, moist conveyor belt with crux of front-end thump
Wednesday. S/SW low-level inflow below mid-upper level diffluent
SW. Jet coupling. Moderate, possibly heavy rain not out of the
question, however uncertain magnitude of faster winds aloft
transferring to surface. Likely PoPs. Sneak in thunder. Lean
breezier conditions across interior.

Following weekend...

Upstream ridge amplification, E CONUS trof reloads. Gyre of
cooler weather prevailing, cyclonic flow, moisture laden,
looking dreary, wet. Considerable spread in ensemble members /
meteograms, pass on any further discussion other than trending
broadscale pattern.

&&

.AVIATION /19Z 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/...High confidence.

Tonight...VFR with skies becoming mostly clear. Northwest winds
diminish.

Saturday and Saturday Night...VFR. Mostly clear skies and winds
less than 20 knots.

KBOS Terminal...High confidence in TAF.

KBDL Terminal...High confidence in TAF.

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

Sunday: VFR.

Sunday Night: VFR. Patchy frost.

Monday through Monday Night: VFR.

Tuesday: VFR. Breezy.

Tuesday Night: VFR. RA likely.

Wednesday: Mainly VFR, 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 Saturday Night/...High confidence.

Northwest winds will continue to diminish this evening and
overnight, as well as seas. Adjusted Small Craft Advisory
headlines this afternoon based on recent trends, mainly to
cancel the headlines. Left Boston Harbor for a little while
longer, where mixing was better with NW winds.

Expecting winds and seas to remain less than advisory levels
Saturday and Saturday night.

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

Sunday through Tuesday: Winds less than 25 kt.

Tuesday Night: Winds less than 25 kt. Chance of rain.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Local rough seas. Rain likely.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ250-
     254>256.
     Small Craft Advisory until 7 PM EDT this evening for ANZ230.

&&

$$
SYNOPSIS...Belk/Sipprell
NEAR TERM...Belk
SHORT TERM...Belk
LONG TERM...Sipprell
AVIATION...Belk/Sipprell
MARINE...Belk/Sipprell



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