Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 200735
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
335 AM EDT Fri Apr 20 2018

.SYNOPSIS...
An upper level disturbance over New England moves east to the
Maritimes today. This may generate isolated light rain/snow
showers, mainly over the higher hills. High pressure builds
just west of New England today and Saturday band will bring
gradually warming temperatures and dry weather into early next
week. Storm system expected for mid to late week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Broad trough across the Northeast USA maintains cyclonic flow
over Souther New England. A shortwave embedded in this flow
moves south from the St Lawrence Valley, crossing our area
during the morning. Similar timing on this feature on both the
ECMWF and GFS.

Moisture cross-sections show decreasing values and depth this
morning and through the day. The initial lift from the shortwave
may be enough to generate clouds, but less confidence on
precipitation. Best chance for precip would be over the
Berkshire East Slopes and possibly to the Connecticut River.
Farther east, sprinkles if anything reaches the ground.
Otherwise a dry day.

Cold advection and cold temperatures aloft will allow mixing to
around 850 mb, possibly a little higher. Winds in this layer
will be 25-30 mph, so expect similar gusts developing late
morning through afternoon. Temperatures at the top of the mixed
layer will be -4C to -6C, suggesting max temps in the upper 40s
and lower 50s. Clouds may keep the hills in the mid 40s, and if
the coastal plain mixes a little deeper then temps there could
reach 53F or 54F.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Tonight...

High pressure builds to our west, maintaining a light pressure
gradient over us and building subsidence over the region. Expect
mostly clear skies and diminishing northwest winds. Dew points
will be in the mid 20s to low 30s, so expect min temps in the
low to mid 30s in Eastern Mass and Eastern RI, with 25-30
farther inland.

Saturday...

High pressure continues to build over the region. Skies should
remain mostly clear. Mixing may bring afternoon wind gusts of
15-20 mph. Temperatures in the mixed layer may be a degree or
two warmer aloft, so expect max temps in the upper 40s northwest
and low to mid 50s most places.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Highlights...

* Moderating temperatures and dry weather will prevail through the
  weekend and into next week
* Next batch of rainfall will occur during the mid-week

Overview and model preferences...
Under the influence of eastward meandering cutoff through the
weekend, the combination of below normal height, H85 temp and
H92 temp anomalies suggest that, in spite of dry wx, temps will
remain near or below normal until early next week. By Mon,
ensembles support height anomalies above normal, and temps do
respond, finally shifting to near/above normal as well as
ridging follows in the wake of the exiting cutoff. By the middle
portion of next week, a second cutoff slowly moving E in weak
flow on the N periphery of the subtropical ridge looks to merge
with some combination of both Pacific and arctic energy,
allowing for the redevelopment of a longwave trof across the E,
and especially the NE, CONUS. Ensembles/deterministic guidance
in fair agreement, so will continue to baseline the forecast
with a blend of these.

Details...

Sun...
The remnants of cyclonic curvature and H85 temps averaging near
-5C (about 1 std deviation below climatological normal values)
finally begin to shift E, but the effects will continue to be
felt. Even with full mixing and enough sunshine, temps are
likely to be held mainly in the mid and upper 50s. In spite of
the cyclonic flow, soundings show that subsidence is already in
place, likely limiting any cloud cover.

Mon and Tue...
Moderation finally occurs as the height/temp anomalies finally
shift to the E. In fact, H85 temps finally shift above 0C, to an
average of +2C on Mon and +4C by Tue. This should allow temps
to easily make it into the 60s each day (low risk for 70s with
some downsloping). The only caveat will be sea breezes, flow
will be weak with high pres cresting. Given SSTs in the 40s,
coastal areas will definitely feel the difference. Ridging and
strong high pres in control suggest mainly dry conditions
linger.

Wed and Thu...
The redevelopment of a longwave trof is looking more likely by
the mid-late week time frame. PWATs increase closer to 1.00
inches (per ensemble means) and slow occluding low pres
approaches from the SW. Guidance in fairly good agreement that
precip develops during the daylight hours Wed, and as previous
forecaster noted ensemble probabilities continue to suggest high
probs of at least 0.50in. Thermal profiles look warm, in spite
of the risk for secondary coastal development. In any case, a
transition back to wetter and unsettled wx is looking more and
more likely each run.

Late week...
The redevelopment of the longwave trof looks to maintain the
wet/unsettled conditions beyond mid week while also allowing
cooler air to filter back into the region.

&&

.AVIATION /08Z FRIDAY THROUGH TUESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Saturday/... High confidence.

Today...VFR with northwest winds gusting 25-30 knots from late
morning through afternoon. Isolated light rain/snow showers
possible, primarily over the Berkshire east slopes. Sprinkles
possible elsewhere.

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

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

KBOS Terminal...High confidence in TAF. VFR. Gusty NW winds
around 25 knots.

KBDL Terminal...High confidence in TAF. VFR.

Outlook /Saturday Night through Tuesday/...High confidence.

VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

Today...Northwest winds gusting 25-30 knots with seas 5 to 7
feet, mainly on the outer coastal waters. Small Craft Advisories
are in effect.

Tonight...Northwest winds continue to gust around 25 knots early
in the night, then diminish after midnight. Seas will subside
through the night, generally less than 5 feet. Small Craft
Advisory continues through the early part of the night.

Saturday... West to northwest winds will be 20 knots or less.
Seas will be less than 5 feet.

Outlook /Saturday Night through Tuesday/...High confidence.

Winds less than 25 kt and weak seas.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ231>234-
     250-251-254>256.
     Small Craft Advisory until midnight EDT tonight for ANZ230.
     Small Craft Advisory until 10 PM EDT this evening for ANZ236.
     Small Craft Advisory until midnight EDT tonight for ANZ235-237.

&&

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



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