Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 301955
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
355 PM EDT Fri Sep 30 2016

.SYNOPSIS...
Low pres moves South of the region through the weekend, yielding
cool and damp conditions. Low pressure over the Ohio Valley lifts
to the Great Lakes, then moves east across New England Monday and
Tuesday. High pressure builds over New England Wednesday and
Thursday. Still watching Hurricane Matthew, but uncertain
information whether it affects New England or not.

&&

.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
Initial band of precip associated with a combination of
overrunning from stalled frontal boundary about 100 sm offshore of
the S coast and robust 35-40 kt H92 LLJ. In fact, the band is
pretty much perfectly colocated with this LLJ. Only meso-scale
guidance and 12Z ECMWF have had this timing correctly through the
day, as most other OP guidance is far wet/fast, so with this
forecast many of the parameters were updated with a blend of the
ECMWF/HRRR/RAP.

This band will pivot S-N and likely fill in somewhat given the
increasing moisture through the column (dry mid-layer from this
mornings CHH/ALB soundings continues to erode) through midnight.
This is thanks to the inverted-ridging still evidence in the
lowest lvls. Even through the LLJ lifts N of the region and
dissipated in- situ, the increased moisture and strong overruning
signal (relatively deep inversion from sfc-H85, with conditional
instability aloft) expect continued on- off shra activity through
the overnight, so have POPs peaking during the initial band
passage, then very slowly diminishing into tomorrow morning.

Overall QPF values have been lowered a bit, as ECENS probabilities
are quite low for values greater than 0.5 inches in 24 hours and
agree now with it`s operational run. Therefore, given hourly
precip values with the banding currently shifting onshore have
been 0.05-0.20, totals through the overnight hours generally rest
at about a half inch.

One other issue tonight and tomorrow is winds. Although near 40 kt
LLJ is expected, the very strong inversion will limit mixing to
the sfc. However, can`t rule out some gusts approaching 25-35 mph
and although this is below the threshold for wind advisories,
stressed trees may yield a bit more damage than we would
otherwise forecast.

Min temps not expected to drop too much from current values even
in spite of dwpts in the 40s, so suspect mainly widespread low-mid
50s.

&&

.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Tomorrow and tomorrow night...
Although not a washout, expect continued risk for shra across the
region thanks to PWATs increasing to nearly 2.0 inches. This is
combined with upper lvl difluence on the periphery of the warm-
conveyor/wrapped occlusion attendant to cutoff low pres in the OH
valley. The reason a washout is not expected, however, is that
overall dynamics driving the rainfall potential is very low and
forced only by weak mid lvl overruning from stalled frontal
boundary to the S. Latest ECMWF highlights much lower QPF, and
will be lowering total QPF values across the region as well.
Inverted ridging, although weakening is likely just strong enough
to counteract the weak forcing. Final totals (including this
evening/overnight) range from about 0.5 inches in NW MA to about
1.5 inches across the Cape/Islands. Although some locations could
over-achieve.

Temps still below normal thanks to flow off the Gulf of Maine and
continued cloud cover. Looking at highs in the mid to upper 50s,
and lows mainly in the upper 40s and low 50s.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...

Big Picture...

Big Picture...

Longwave pattern continues to show West Atlantic high in place, but
shifting south over the weekend. Closed Upper Low over the Ohio
Valley shifts north, then moves east as the Atlantic high moves out
of the way. This upper low moves over New England Monday and
Tuesday.  High pressure then moves over the Northeast USA Wednesday
and Thursday.  Upper low along the British Columbia coast swings
across the USA during the week, then lifts northeast across Eastern
Canada Friday.

And then there is Matthew. There is a great difference in how the
models handle the storm, especially north of the Bahamas. The GFS
continues to outrun the ECMWF in bringing the storm up the coast,
but reacts to the building midweek upper high by dancing and
spinning east of our area. The difference continues to be the GFS
showing a greater phasing of Matthew with a southern stream
shortwave, while the ECMWF shows either no phasing or greatly
delayed phasing...and thus a much slower movement north.

We favored a model blend for most of the long term. The GFS has been
trending slower on Matthew, so a movement away from either extreme
seems a good thing.

Details...

Sunday...

Low level east winds are overlaid with 20 knot southwest winds at
4000-5000 feet. Meanwhile the 90-knot upper jet associated with the
closed low...this jet approaches Southern New England and brings
some meager upper venting. We will maintain clouds and chance pops
through the day and through Sunday night.  Low level northeast flow
will keep us in a small diurnal range, with 50s and 60s during the
day and 50s at night.

Monday-Tuesday...

Upper low and cold pool move overhead Monday and Tuesday. Monday
will feature the most instability with GFS-ECMWF-NAM-GGEM showing
totals in the upper 40s and lower 50s. This looks mainly to be
scattered showers Monday but with enough instability to mention a
widely scattered tstm.  Less coverage Tuesday, but enough for
scattered or widely scattered showers.  Surface wind flow will
continue from the northeast keeping temps below normal especially in
Eastern MA and RI. Mildest temps will be in the CT Valley.

Wednesday through Friday...

High pressure actually starts building south from Canada on Tuesday,
but is most notable starting Tuesday night with temps and dew points
cooling into the 40s. The high brings fair and dry weather Wednesday
and Thursday with partly sunny skies. Northeast flow continues.

Friday currently looks to maintain the dry weather as high pressure
moves off through the Maritimes.  If Matthew moves close enough,
then we could see some showers well out ahead of the system. But
very low confidence in such a scenario so we have indicated only
slight chance pops in eastern and central sections. Otherwise dry
weather.

&&

.AVIATION /20Z 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/...

Through 00Z...High confidence in trends, lower confidence in exact
timing.

Expect MVFR/VFR conditions to taper toward MVFR all TAF sites
through about 21Z. Also, band of rain overspreads the region with
lowering VSBYs from S-N 18Z through 22Z and continues into the
overnight hours.  Winds remain ENE 15-25 kt.

Tonight into tomorrow...Moderate confidence.
ENE winds increase overnight just off the deck, could see some
minimal LLWS as wind speeds reach 30-40 kt at 2kft. Otherwise,
MVFR continued to lower to IFR CIGS into the early morning hours,
then linger at low end MVFR/IFR through the day tomorrow with on
and off rain showers lowering vsbys.

Tomorrow night...Moderate confidence.
May see some minimal improvement during the overnight hours,
however low CIGS remain an issue, at least at MVFR.

KBOS TAF...High confidence in trends, moderate confidence in
timing.

KBDL TAF...High confidence in trends, moderate confidence in
timing.

Outlook /Sunday through Wednesday/...

Sunday...Moderate confidence. MVFR/IFR cigs and vsbys in showers and
fog. Diminishing northeast flow.

Monday-Tuesday... Low confidence. VFR with areas of MVFR in
scattered showers.

Wednesday...Moderate 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 Night/...

High confidence.

No major changes to current marine headlines. Gale warnings
continue through evening for the southern waters, with small craft
advisories for all waters now and thereafter. It will likely take
until into Sunday for seas to drop below 5 ft as they peak near 11
ft on the SE waters this evening.

Outlook /Sunday through Wednesday/...

Diminishing Northeast wind Sunday and Monday with gusts 20 knots or
less. Northeast winds increase again Tuesday and Wednesday on the
Southern and Southeast waters with frequent gusts 20-25 knots.
Seas 5-6 feet on the eastern waters Sunday but diminishing. Seas
build again Tuesday and Wednesday with 5-7 foot seas on the outer
waters. Small Craft Advisory will be needed for much of this time
period.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 5 PM EDT Saturday for ANZ232.
     Small Craft Advisory until noon EDT Saturday for ANZ233-234.
     Small Craft Advisory from 10 PM this evening to 8 PM EDT
     Saturday for ANZ230.
     Small Craft Advisory until 8 PM EDT Saturday for ANZ231.
     Small Craft Advisory until 11 AM EDT Saturday for ANZ236.
     Gale Warning until 2 AM EDT Saturday for ANZ235-237.
     Small Craft Advisory until 8 AM EDT Sunday for ANZ250-251-254.
     Gale Warning until midnight EDT tonight for ANZ255.
     Gale Warning until 5 AM EDT Saturday for ANZ256.

&&

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



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