Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 170709
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
309 AM EDT Thu Aug 17 2017

.SYNOPSIS...
Dry and pleasant conditions continue into tomorrow. A weather
system moving through the Great Lakes Region towards the end of
the workweek will bring showers and scattered thunderstorms to
southern New England during Friday and Saturday. High pressure
returns with fair seasonable weather Sunday through Tuesday. A
cold front approaches from the west Tuesday night and
Wednesday, bringing our next chance of showers.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
200 AM Update...

Forecast remains on track for the overnight hours. Weak wave moving
through the northwest flow this morning has trigger some strato cu
per latest IR imagery. Dewpoints continue to drop with a few
locations across northeast MA in the 40s. Surface temps are
radiating out with some locations in the low to mid 60s. Metro
regions are hanging tough with temps in the 70s, but with the cooler
dewpoints anticipate them to drop over the next few hours. OVerall
a very pleasant night across southern New England.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY NIGHT/...

Thursday...

High pres crests over the region providing stout
subsidence. Despite the mostly sunny conditions this suggests,
the combination of suppressed mixing and a cooler start to the
day will lead to overall high temps a few degrees cooler than
yesterday. This is also in spite of generally W flow. Mainly
upper 70s and low 80s in the warmest spots where the downsloping
can be maximized. Overall a pleasant day especially with dwpts
remaining in the 50s.

Will still need to watch mainly exposed beaches across Cape Cod
and Nantucket, as the remnants of Gert continue to shift east, a
lingering swell could maintain a moderate risk for rip currents
through the day in these areas.

Thursday night...

A pleasant evening to start, however upper lvl wave will be
tilting and lifting through Ontario and Quebec with sfc wave
pushing a warm front toward New England. PWATs increase through
the overnight hours, such that by 12Z Fri, they near 2.00 inches
(or nearly 2 std deviations above normal). Initial overruning
is weak and this moisture load is from the top of the column to
the sfc. Therefore, expecting only building clouds through the
evening, with the risk for any precipitation holding off until
the early morning hours. Indeed, timing of measurable QPF has
slowed on regional models due to this initial lack of lift and
dry column. Still could see some wetting rainfall by sunrise
mainly W of the Worcester hills in MA and Thames river region of
CT. Noting some QPF bullseyes, but these are mostly likely some
convective feedback. Column remains relatively stable through
12Z.

The increased moisture from W-E will limit diurnal cooling in
spite of the weak pres gradient to start. Coolest areas will
likely be across E MA/RI, where some radiational cooling can
occur initially. Overnight mins range from the upper 50s in
these cooler spots to the low 60s elsewhere due to the increased
cloud cover.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...

Big Picture...

Subtropical high remains in control across the Southern USA while a
general zonal flow continues across Canada and the Northern USA. The
high actually builds over the Western USA early next week,
supporting a deepening of the wave pattern during next week. One
shortwave moves through New England over the weekend. A second
shortwave moves across from the Gulf of Alaska, forms a closed low
over northern Canada, and then digs over Quebec midweek.

Contour heights are normal to a little higher than normal through
early next week.  As the Quebec closed low deepens, heights over New
England dip below normal midweek next.

Model mass and thermal fields from the 00Z suite are in general
agreement through Wednesday, building forecast confidence.

Details...

Saturday... Moderate confidence.

Models hold the the cold front over western and central sections of
Southern New England while moisture fields race east and drier air
moves in during the afternoon. PW fields show 1.5 inch values move
east of BOS-PVD by Saturday afternoon. CAPE fields show 1000-1500
J/Kg east of a ASH-ORH-IJD line in the afternoon. Winds aloft are
marginally significant, with 25-30 knots at 850 mb and 35-40 knots
at 500 mb. Stability parameters are mixed, with Totals in the mid
40s, except upper 40s ECMWF, and LI values subzero.

Expect the main area of showers/heavy downpours to move east of
Massachusetts during the morning, but with lingering scatterd
showers/thunder possible in the afternoon until the cold front moves
past.

With the flow becoming more southwest, expect deeper mixing than on
Friday...and so a warmer airmass. Temps of 15-17C at 850 mb would
support mid-upper 80s. Depending on the degree of clearing, temps
could be a little lower than this. Expect max temps in the 80s.
Surface dew points in the southerly flow will linger in the upper
60s and low 70s, so expect a warm and very humid day.

Saturday night...

Cold front moves offshore, and surface winds become northwest.
Expect any showers to taper off. Skies will either clear or
partially clear overnight. Dew points will linger in the 60s, with
min temps close to that level.

Sunday...

Upper shortwave and the coldest of the cold mid-level temps will be
over New England. The more significant cold temps as well as cloud-
level moisture will be over NH-VT. Cross-sections show Southern New
England will have a moist layer based at 850 mb, with drier air
closer to the surface. Expect diurnal clouds, probably a bit more
than guidance is suggesting, but otherwise a fair day with less
noticeable humidity.

Surface high pressure builds in with light variable wind Sunday
night. With dew points in the upper 50s and around 60, expect min
temps around this same range. Guidance temps were nudged a couple of
degrees colder.

Monday-Tuesday...

High pressure builds over the region Monday and shifts offshore
south and east by Tuesday. Cross sections show dry air in place at
most levels through Tuesday evening, so expect dry weather both
days. Increasing mid and high clouds Tuesday ahead of the next
weather system. Temps aloft 14-15C Monday and 15-17C Tuesday,
supporting low-mid 80s Monday and mid to upper 80s Tuesday.

Tuesday night-Wednesday...

Closed low digs over Quebec and the Northeast USA with shortwave
driving a cold front into/through New England. Expect
showers/scatterd tstms with the front Tuesday night/Wednesday.

&&

.AVIATION /07Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...

Before 00z...High confidence. VFR except some very patchy
ground fog early AM mainly at typically prone terminals.

Today...High confidence. VFR. Winds shift lightly W during the
day except sea breezes along coast.

Tonight and tomorrow...Moderate confidence in trends, lower
confidence on timing. VFR to start with deteriorating conditions
after 09z as widespread rain approaches. Conditions will fall to
MVFR/IFR through the day as rain moves from west to east and
could be heavy at times restricting vsbys. Iso -TSRA is also
possible on Friday. Southerly winds with gusts near 15 kts by
the afternoon.

KBOS Terminal...High confidence in TAF. Sea breeze development
expected for Thu.

KBDL Terminal...High confidence in TAF.

Outlook /Saturday through Monday/...

Saturday... Moderate confidence.

MVFR/IFR cigs and vsbys in showers and fog Saturday morning will
improve to predominant VFR during the day. Continued MVFR
ub scattered showers/thunder through afternoon especially in
RI/Eastern Mass. Southerly winds during the day will shift from the
west/northwest Saturday night after a cold front moves through.

Areas of IFR cigs/vsbys in fog and low clouds later Saturday night.

Sunday and Monday...Moderate-High confidence.

VFR. Areas of IFR in early morning fog/low clouds, then improving
to VFR by each mid morning and continuing the remainder of each day.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Friday Night/...High confidence.

Noting seas have reached 8-9 ft along the open ocean waters S of
S New England as hurricane Gert begins its E shift. This will
allow these seas to gradually recede through the overnight
hours. Small craft advisories continue, but will be able to be
dropped as the night progresses. Otherwise, mainly quiet
boating weather continued into tomorrow night after these seas
recede.

Outlook /Saturday through Monday/...

Saturday...Moderate confidence.

Lingering potential for scattered showers/tstms through the day,
diminishing west to east Saturday night. Diminishing wind through
the day. Some gusts 20-25 knots in the morning.  Seas near 5 feet on
the outer waters and on RI Sound, but trending lower later in the
day.

Sunday and Monday...Moderate-High confidence.

High pressure builds over the waters. Winds remain 20 knots or less,
and seas 4 feet or less. Leftover showers early, but these move off
to the east during the morning.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 8 AM EDT this
     morning for ANZ254-255.

&&

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


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