NWS Forecast Discussion

000
FXUS61 KBOX 192345
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
745 PM EDT Sat Aug 19 2017

.SYNOPSIS...
A second cold front will cross the region overnight, accompanied by
a few showers and possibly an isolated thunderstorm. Behind the
front, warm but very pleasant late summer weather with lower
humidity Sunday. High pressure brings dry and seasonable weather
Monday. It will be hot and humid Tuesday. ahead of an approaching
cold front, which will bring a chance of showers and thunderstorms
from late Tuesday afternoon into early Wednesday afternoon. High
pressure will bring dry and cooler weather late in the week
into next weekend.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
740 PM update...

Light northeast winds have been persistent with some fog in
Boston Harbor. Forecasting winds to become light then southwest,
with fog dissipating there. With high dewpoints, starting to see
fog developing on Nantucket and parts of the south shore. Will
need to watch this closely for a possible Dense Fog Advisory.

Otherwise, strong thunderstorms stretched from northeast PA
through Binghamton and to just west of Albany. The NAM and HRRR
are handling this area quite well. The 18Z NAM has a coherent
band of 38K, 50 Total Totals, and -3 Showalter Indices in that
area now and progressing steadily across western MA around 03Z,
central MA around 06Z and exiting northeast MA around 09Z.
Somewhat unstable in northern CT as well. The NAM and HRRR have
60-80 percent PoPs in these areas tonight. As a result, have
updated the grids to include a 50 percent chance of showers and
thunderstorms in western sections tapering to a 30 percent
chance in eastern and northeastern sections overnight.

Previous discussion...

After sunset...

Mid/upper trough swings thru the region overnight. This feature is
already generating lots of convection across eastern OH and western
PA/NY. Impressive trough amplitude for late Aug however trough is
deamplifying as it moves into our region as wind fields aloft
decrease. This results in weak synoptic scale lift over our area. In
addition, despite a brief spike in K indices, mid level moisture is
limited along and ahead of short wave trough. So given limited mid
level lift and moisture combined with loss of daytime heating, most
of the guidance including the 00z NCAR ensembles weakens or
completely erodes convection as it approaches western New England
from NY state. Thus other than the risk for a few showers (perhaps
isolated T-storm) across western MA, expect mainly dry conditions
overnight.

Given the fropa is not until after 06z west and closer to sunrise
for eastern MA, expect another warm and muggy night ahead although
not as humid as last night. Given this could see some patchy fog
tonight.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Sunday...

Spectacular late summer weather tomorrow with mid level mean trough
axis pushing offshore resulting in height rises across the area.
Warming temps aloft and flow becoming less cyclonic will yield less
diurnal clouds/more sunshine than today. Model soundings show a deep
boundary layer with drier air aloft mixing to the surface. This
combined with downslope WNW winds will result in dew pts falling
into the upper 50s. So despite temps climbing well into the MU80s to
near 90, lowering dew pts will make it somewhat tolerable. Given the
deep blyr and downsloping winds, increased temps a few degs warmer
than guid.

Sunday night...

Any diurnal clouds erode with sunset giving way to mostly clear
skies. This combined with high pres building into the area will
promote radiational cooling and temps falling into the upper 50s
outside of the urban areas. Used MOS temps to derive mins tomorrow
night.

&&

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

At upper levels, closed low pressure will be moving slowly
across Canada, from west of James Bay Tuesday to eastern Quebec
by Saturday. The trough axis extending southward from the low
will remain to our west until Saturday. The cold pool of air
will arrive over our region around Thursday and remain in place
into next weekend.

At the surface, high pressure will pass southeast of the region
Monday. A cold front moves through Wednesday. Then strong high
pressure builds over the Great Lakes states, with its influence
extending eastward into New England.

Details...

Monday...

High pressure off the DelMarVa coast will provide sunny skies
for southern New England, warm temperatures, and relatively
light southwest winds. Forecasting highs iin the mid to upper
80s over most of the interior, with comfortable humidity.

Tuesday...

Southwest breezes behind departing high pressure and well in
advance of an approaching cold front will usher in very warm and
more humid air. With 925 mb temperatures of about 25C, am
expecting highs to reach the upper 80s and lower 90s. Winds
could gust to 15-20 mph during the afternoon. Dewpoints will be
rising to near 70 by late afternoon and heat indices will be in
90-94 range, just shy of heat advisory criteria. With the
afternoon heating will come a chance of a few showers and
thunderstorms late in the day, especially in western MA and
northern CT, closer to the approaching front.

Tuesday night-Wednesday...

A cold front will slowly work its way across the region.
Showers and thunderstorm probabilities increase to 40-60 percent
Tuesday night and mainly in eastern sections on Wednesday. The
ECMWF is slower with the frontal progression than the GFS. Wind
fields are somewhat strong, with 35-40 kts at 925 mb and 850 mb
Tuesday night. Cannot rule out a few strong thunderstorms
Tuesday night, even though they would not be occurring during
the maximum heating. Skies will be clearing from west to east
during the day. West to northwest winds could gust to 20 to 25
mph behind the front. Highs mainly 80 to 85 Wednesday.

Thursday-Saturday...

Strong high pressure building into the Great Lakes states will
exert its influence across New England. At the same time, an
upper level trough will be moving overhead, along with it
associated cold pool with 500 mb temperatures reaching -18C
Thursday and continuing into Saturday. It also will be cooler in
the lower levels. Forecasting high temperatures in the 70s each
day, possibly near 80 in spots. It should be dry for the most
part, with scattered diurnal clouds.

&&

.AVIATION /00Z SUNDAY THROUGH THURSDAY/...
Forecaster Confidence Levels...

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

Short Term /through Sunday Night/...

2340Z update...

Thru sunset...

VFR most locations. The exception will be along the immediate
eastern MA coastline with MVFR/IFR conditions in low clouds and
fog.

After sunset...

Onshore winds along the eastern MA coastline shift to S and then
SW which will give way to improving conditions, likely VFR by
03z or so. However, could see development of IFR in fog along
the south coast, including the islands due to high dewpoints.
There is a chance of showers and thunderstorms spreading into
western sections around 03Z, central sections by 06Z, and
weakening and exiting northeastern sections by 09Z. Could have
MVFR CIGS and VSBYS in any showers/thunderstorms.

Sunday and Sunday night...

VFR, dry weather and modest west wind.

KBOS Terminal...Moderate confidence.

Low clouds and fog will dissipate around 02-03Z as winds become
light southwest.

KBDL Terminal...High confidence in TAF. Chance of a shower or
thunderstorm for a few hours from roughly 04-07Z.

Outlook /Monday through Thursday/...

Monday through Tuesday...High confidence. VFR. Only exception
will be IFR in patchy late night fog, mainly Monday night, then
improving to VFR by mid morning Tuesday.

Tuesday night-Wednesday...Moderate confidence. VFR Tuesday but
areas of MVFR cigs/vsbys in showers and thunderstorms late
Tuesday afternoon in western sections, across the region Tuesday
night and Wednesday morning, and in eastern sections Wednesday
afternoon. IFR cigs possible in early morning fog/low clouds
especially in areas that have had rain. Clearing in west half
Wed. afternoon. Clear Wed night.

Thursday... 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 Sunday Night/...Moderate confidence.

Tonight...

Onshore winds across the eastern MA waters veer to the SW after
sunset and then west toward daybreak. Patchy low clouds/fog will
also move offshore with the wind shift this evening and especially
toward daybreak. Elsewhere, SSW winds continue with areas of fog
possibly redeveloping along the south coast. Chance of a shower
late tonight, especially along the north coast.

Sunday...

Modest WNW winds will promote dry weather and good vsby.

Sunday night...

WNW winds become very light as high pres build in from the west. Dry
weather and good vsby prevail.

Outlook /Monday through Thursday/...

Monday through Tuesday...High confidence.

High pressure south of the waters. Southwest winds remain less
than 15 knots Monday but may increase to near 20 knots late
Tuesday. Seas less than 3 ft, but increasing to 3 to 4 ft over
the outer waters late Tuesday.

Tuesday night-Wednesday...Moderate confidence.

A cold front approaches from the Great Lakes, crossing the
waters late Wednesday or Wednesday night. Southwest winds
increase with gusts 20 to 25 knots during Wednesday. Winds shift
to the northwest Wednesday night. Seas build Tuesday night and
Wednesday reaching 5 to 6 ft on the outer waters. The southwest
flow may also nudge those higher seas into RI Sound as well. A
Small Craft Advisory may be needed on some of the waters during
this time.

Thursday... High confidence.

As high pressure to our north and west builds over the northeast
states, winds will turn more to the north and northeast. But
they will be diminishing to less than 10-15 knots and seas will
subside to 3 ft or less.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Nocera/Field
NEAR TERM...Field/Nocera
SHORT TERM...Nocera
LONG TERM...Field
AVIATION...Field/Nocera
MARINE...Nocera/Field

NWS BOX Office Area Forecast Discussion