Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 271356
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
956 AM EDT Sun May 27 2018

.SYNOPSIS...
Much cooler today along with spotty light rain except a steadier
rain possible late in the day along the south coast. A cold
front stalls south of New England today, while areas of rain
move east along it. East winds keep cool marine air over us.
Scattered showers or areas of drizzle overnight prior to
possible improvement into Monday. High pressure brings mainly
dry and warmer weather Tuesday through Thursday. An approaching
weather system brings showers Friday.

&&

.NEAR TERM /UNTIL 8 PM THIS EVENING/...

10 AM Update...

What a difference a day makes - 1027 mb high over the maritimes
combining with low pressure forming near PHL providing a cool
maritime flow into New England on gusty ENE winds. So
temperature forecast remains unchanged with highs only in the
50s today, except low 60s in the CT river valley from Hartford
to Springfield area.

Forecast challenges today - how far north does steady rain
track?

SPC mesoanalysis shows pronounced air layer at 925 mb over ME
into NH nosing into Worcester county. This is causing leading
edge of rain shield to erode this morning. Also convection
firing over NJ/NYC area is back building into southeast PA as
surface low forms near PHL. This also the area of greatest
instability. So concern here is that back building convection
combined with the lack of trough amplitude to back mid level
flow, heaviest and steadiest rain may remain offshore, possibly
clipping the south coast including Cape Cod and the Islands.
Otherwise most of the region may just see light spotty rain much
of the day. So to reflect this trend in the forecast have
decreased pops slightly from north to south with the focus for
steadier rain this afternoon along the south coast of RI/MA
with lighter and spotty rain elsewhere.


&&

.SHORT TERM /8 PM THIS EVENING THROUGH 6 PM MONDAY/...

Tonight...

A mix of scattered showers OR areas of drizzle. Abundance of
low-level moisture, however crux of mid-level energy and trof
axis is offshore. Beginning to see subtle height rises with drier
air starting to work in within mid-levels. However, continued
onshore NE flow as temperatures remain cool, steady around the
low 50s. Onshore, upslope ascent beneath cyclonic flow aloft,
some mid-level energy lingering, and stout dry-inversion
pressing down through the column, leaned with scattered showers
OR areas of drizzle. Undoubtedly continued low clouds, perhaps
some fog issues for the high terrain with lower clouds, maybe
along the SE coast.

Monday...

Clearing out? Weak ridging ahead of another potent N-stream vort-
max over S Canada, kicking earlier synoptics E, out to sea, surface
winds begin to revert S. Some improvement is possible. However
it remains to be seen whether the combination of daytime heating
and boundary layer mixing is able to meet with the suppressing
dry-inversion such that the cloud layer between is mixed out. Am
somewhat hesitant on forecast model clearing given such weak
advective patterns, leaning that clouds hold in tight. However,
if dry-air subsidence prevails, lowering down through the column,
mechanical mixing could lead to low cloud erosion. Low confidence
forecast at this time, but kept with earlier forecast trends of
milder conditions W while cooler E remaining socked in longer.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Big Picture...

Zonal flow remains in place over Canada and the Northern tier of the
USA. One sharp shortwave moves through this flow across Maine and
the Maritimes Tuesday. A shortwave ridge then moves east from the
Great Lakes.  Closed low over the Western USA gets kicked northeast
as another low moves south from the coast of Alaska. Upper
circulation associated with Alberto will be channeled north
through the Southern USA and merges midweek with the ejecting
Western low somewhere over the Midwest. The combined system then
moves east and overcomes the shortwave ridge over New England
Thursday night.

Model solutions show above normal contour heights through the
forecast period, and thermal fields also indicate above normal
temperatures.

Model solutions remain similar through Thursday. The models
then develop differences in detail for Friday and Saturday,
while maintaining a similar overall feel for the pattern. This
gives us high confidence in the forecast through midweek and
moderate confidence late week.

Concerns...

Monday night through Thursday...

Lingering moisture below 700 mb Monday night but with a diminishing
trend. This may be enough for partial clearing or a temporary
clearing overnight.

Meanwhile a sharp northern stream shortwave dives through Maine and
the Maritimes and pushes another cold front through New England.
This may be enough for a zone of clouds along the front, but
instability along the front Tuesday in Srn New England is marginal
with mid level lapse rates mostly less than 6C/Km and totals in the
mid 40s. The GFS maintains a sub-zero lifted index across CT-RI in
the afternoon along with some low level convergence. This is not
supported by the other models.

Once the front moves through, high pressure from Northern Ontario
builds in with strong drying...dew points in the 40s and strong dew
point depressions. The high moves overhead Tuesday night and
Wednesday, then shifts over the West Atlantic Thursday. This will be
a dry period with developing south-southwest winds Thursday. Expect
clear/clearing skies and light winds through midweek. Possible sea
breeze on the Mass East Coast Tuesday and Wednesday.

Temps aloft...or rather the 850 mb equiv...will be in the low
to mid teens suggesting max sfc temps in the low to mid 80s each
day away from the coasts. Dew points in the mid 40s to mid 50s
suggest min temps generally in the 50s.

Thursday night through Saturday...

The ejecting upper low from the Western USA merges with the remnants
of Alberto over the Midwest midweek and then move east into New
England toward the end of the week. Precipitable water values climb
sharply to 2 inches or higher Thursday night and Friday. This points
to a wet period to end the week with high moisture content and
potential for heavy downpours.

&&

.AVIATION /14Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday/...Moderate confidence.

14z update...

Not much change from previous forecast. Steadiest rain this
afternoon likely confined to south coast of RI/MA with spotty
light rain elsewhere. Previous discussion below.

==============================================================

Today...
East-northeast winds continue through the day at 10-20 knots,
with gusts to 25 knots at the shore. Cigs started at IFR in
eastern Mass and Worcester, and at low-end MVFR in RI and CT
and Western Mass. Most vsbys are VFR this morning, but with
patches of IFR mixed in.

Expect cigs and vsbys to lower through the day with IFR most
places this afternoon. Best chance for showers will be south of
Boston and the Mass Pike. A threat of LLWS over the Cape and
Islands.

Tonight...
No let-up in CIGs nor winds. -RA/RA replaced by SCT SHRA and
DZ.

Monday...
Possible gradual improvement W to E as winds shift S. CIGs lifting
MVFR to low-end VFR, lingering longest for SE coastal terminals.


KBOS Terminal...
Confident in MVFR-IFR CIGs. Possible today that CIGs could lift
to low-end VFR mid to late in the day, lowering back into evening
with DZ. Majority of -RA/RA S of terminal today.

KBDL Terminal...
Mainly IFR today into tonight. N winds prevailing through the CT
River Valley.

Outlook /Monday Night through Thursday/...Moderate confidence.

Monday Night: Mainly VFR, with local IFR possible in patchy fog.

Tuesday through Thursday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

Through tonight...

High pressure situated over SE Canada with low pressure sweeping
E along a stalled frontal boundary S of New England, NE winds
persist with potential gusts up to 25 kts, especially through
tonight. Waves building 6 to 8 feet over the E/SE waters, trickling
into the S waters later tonight, beginning to dissipate after
midnight tonight. Small craft advisories continue.

Potential visibility restrictions today as rain is forecast,
especially for the S/SE waters, while drizzle will be an issue
overnight for a majority of waters. Patchy fog at times.

Monday...
Weak high pressure setting in, winds relax and begin to revert
S. Will continue to see wave action dissipate, as well as the
conclusion of small craft advisories.

Outlook /Monday Night through Thursday/...Moderate confidence.

Winds less than 25 kt. Seas less than 5 feet. Visibility locally
1 to 3 miles in fog Tuesday night.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until midnight EDT tonight for ANZ232>235-
     237.
     Small Craft Advisory until 6 PM EDT Monday for ANZ231-250-251-
     254-255.
     Small Craft Advisory until 6 AM EDT Monday for ANZ256.

&&

$$
SYNOPSIS...WTB/Sipprell
NEAR TERM...WTB/Nocera/Sipprell
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...WTB/Sipprell
MARINE...WTB/Sipprell



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