Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 190808
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
408 AM EDT Fri Oct 19 2018

.SYNOPSIS...

After a cold start early this morning, much milder weather returns
this afternoon into Saturday along with gusty winds along the coast.
Temperature swings roll on through the following week. Warm ups out
ahead of sweeping cold fronts associated with light showery weather
and breezy conditions, frost / freeze conditions following as high
pressure builds across the region.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...

4 am update...

After a widespread freeze in many locations early this
morning...high pressure will move off the mid Atlantic coast today.
The result will be a milder southwest flow of air back into our
region. Despite the cold start...plenty of sunshine and rapidly
warming mid level temperatures will allow for a nice recovery by
afternoon. 850T actually rise to between +8C and +10C by late in the
day.  Given the cold start and lowering October sun angle this will
not be completely realized at the surface. Nonetheless...expect a
nice afternoon with many locations seeing highs recover into the
lower 60s.  The only issue will be gusty southwest winds of 20 to 30
mph that develop this afternoon along the coastal plain...strongest
of those across the Cape/Islands.

&&

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

Tonight...

Approaching shortwave will induce a fairly strong southwest low
level jet of 35 to 45 knots.  This will keep the boundary layer
mixed tonight...resulting in much milder temperatures.  Lows will
only drop into the upper 40s to the lower 50s in many locales this
evening, followed by rising temperatures along the coastal plain
with the gusty winds.  In fact...we did hoist a wind advisory for
the Cape/Islands.  SST still 60+ across much of our southern waters
which should allow the strong low level jet to mix down at
times...resulting in southwest wind gusts of 35 to 40 knots.

We also expect skies to become mostly cloudy overnight as low level
moisture increases ahead of the shortwave.  A few showers should
also develop...particularly across the far southeast New England
coast on the nose of the LLJ and where the moisture is deeper. Far
from a washout though as much of the night will feature dry weather.

Saturday...

Any left over showers should exit the region by mid to late morning
with the departing shortwave.  Will continue the wind advisory into
the late morning hours across the Cape/Islands...but should see wind
diminish behind shortwave.  In fact...with partial sunshine and a
mild start high temperatures should soar well into the 60s across
many locations.  Dry weather should dominate after any left over
showers exit the region by mid to late morning.  However...an
approaching strong cold front may result in a few more showers
developing late in the afternoon/early evening across our western
zones.

&&

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

*/ Highlights...

 - Rolling temperature trend through midweek
 - With any frontal passages, expect showery weather, breezy winds
 - In wake, frost / freeze conditions possible

*/ Overview...

Rolling temperature trend. NE Atlantic block downstream of a Eastern
N America H5 preferred trof and Western N America split-flow regime,
as N Pacific energy ejects E, +PNA reloads in a +AO environment, and
we see shots of cold, Canadian air at times through the weekend into
the early-half of next week. Cross-polar flow, greater
baroclinicity, local wind enhancement associated along sweeping
cold fronts. Mainly continental airmass association, expecting
some showery weather, a few wet snowflakes possible. Warmer
rebounds in-between thanks to the upstream S-branch of
aforementioned split-flow.

But a change is in the air. Most N hemisphere open, progressive, N
Pacific jet max rounds the E hemisphere into the W CONUS. Weakened
EPO, W CONUS split-flow and E CONUS H5 trof nudged downstream. The
pattern opens briefly, but a wary eye on energy ejecting E, riding
over W CONUS high terrain and obtaining lee cyclogenesis towards the
E CONUS into late October. Models continue to hint on possible storm
development, however an uncertain reality given deterministic and
ensemble model spread, low confidence forecast, unable to make any
outcome determination.

GFS / Canadian continues to be progressive over EC / ECens. UKmet
somewhere in-between. The split in forecast guidance mid to late
week. Consensus preference early, trend with greater EC weighting
late week.

*/ Discussion...

Saturday night into Sunday ... Cold air advection, steepening lapse
rates, mid-level trailing vortmax. With lingering column moisture,
in addition to breezy NW winds held below wind advisory criteria, 30
to 35 mph, associated with pressure rises, can`t rule out scattered
rain / snow showers. High outcome confidence over N/W high terrain.
Little to no accumulation.

Sunday night ... Stray from coldest MOS guidance but it`ll still be
chilly. Building surface ridge but still good NW flow aloft aligned
with prevailing H5 trof, energy continually streaming through. Can`t
rule out mid-level cloud decks and mix down of winds, so not looking
like an ideal cold environment. Dry.

Monday through Tuesday ... Chilly and dry Monday, return warmer air
and some possible showery weather courtesy of low-level isentropic
ascent along a lifting warm front. Rebound warmer temperatures with
chance PoPs. Light outcomes, hardly a washout given more continental
airmass association.

Wednesday into Thursday ... Low confidence forecast, but if leaning
with EC/ECens, linger a stalled frontal boundary longer in vicinity
while holding temperatures at or slightly below seasonable beneath a
near-zonal environment. Some clouds and light showers at times. A
sharper cold front not until late going into Friday.

Friday onward ... Little to no confidence. Continued exacerbated
spread among deterministic / ensemble models. Again, watching energy
ejecting E from H3 N Pacific speed max into W CONUS. How it evolves
E remains highly uncertain, outcomes widely unknown.

&&

.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/...

Today...High confidence.  VFR with southwest wind gusts of 20 to 30
knots developing this afternoon on the coastal plain.  Strongest of
the winds across the Cape/Islands.

Tonight...Moderate to high confidence.  VFR conditions continue for
the first part of the night...but may see more in the way of MVFR
cigs develop after midnight with increasing low level moisture.
Gusty southwest winds on the coastal plain with gusts between 35 and
40 knots at times on the Cape/Islands.  A few showers possible after
midnight...particularly across the Cape/Islands.

Saturday...Moderate to high confidence in TAF.  Any lower ceilings
should improve to mainly VFR by late morning/early afternoon.  Gusty
southwest winds along the coastal plain should diminish some during
the afternoon.

KBOS Terminal...Moderate to high confidence in TAF.

KBDL Terminal...Moderate to high confidence in TAF.

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

Saturday Night: VFR. Breezy. Slight chance SHRA.

Sunday: VFR. Windy with gusts to 30 kt. Slight chance SHRA.

Sunday Night: VFR. Windy with local gusts to 30 kt.

Monday: VFR. Breezy.

Monday Night through Tuesday: VFR. Breezy. Slight chance SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...

Today...High confidence.  Southwest wind gusts increase to between
20 and 30 knots by afternoon across the waters ahead of an
approaching shortwave.

Tonight and Saturday...High confidence. Strong southwest low level
jet developing coupled with the relatively mild ocean will allow 30
to 40 knot wind gusts over most waters tonight into Sat morning.
Gale Warnings posted for all waters except small craft headlines for
Boston Harbor/Narragansett Bay.  Long southwest fetch should allow
seas to build to between 6 and 9 feet across our southern waters.
The winds should begin to diminish Saturday afternoon as the core of
the low level jet lifts northeast of our region.

Outlook /Saturday Night through Tuesday/...Moderate to High

Saturday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Local rough seas. Slight chance of rain
showers.

Sunday: Strong winds with local gusts up to 30 kt. Areas of
rough seas. Slight chance of rain showers.

Sunday Night: Moderate risk for Small Craft Advisory winds with
areas of gusts up to 30 kt. Local rough seas.

Monday: Winds less than 25 kt. Seas locally approaching 5 ft.

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Slight chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Freeze Warning until 9 AM EDT this morning for CTZ002>004.
MA...Freeze Warning until 9 AM EDT this morning for MAZ005>007-
     009>013-017>021.
     Frost Advisory until 9 AM EDT this morning for MAZ014>016-022-
     023.
     Wind Advisory from 11 PM this evening to noon EDT Saturday for
     MAZ022>024.
RI...Freeze Warning until 9 AM EDT this morning for RIZ001>007.
MARINE...Gale Warning from 6 PM this evening to noon EDT Saturday for
     ANZ231>235-237-254>256.
     Small Craft Advisory from noon today to noon EDT Saturday for
     ANZ230-236.
     Gale Warning from 11 PM this evening to noon EDT Saturday for
     ANZ250-251.

&&

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


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