Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 252320
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
720 PM EDT Tue Jul 25 2017

.SYNOPSIS...
Skies will clear tonight. High pressure will bring a return to
dry and milder weather for Wednesday and into Thursday. A low
pressure wave will move along an approaching cold front late
Thursday, passing across the region during Friday. Some leftover
showers may linger across southern areas for part of Saturday
along with cool northeast winds. Dry, seasonable conditions
return late next weekend into early next week.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...

705 PM Update...
Clearing skies moving southwestward on the NE wind around a
bubble of high pressure across VT and upstate NY at 22Z. Visible
satellite imagery shows the clearing quite nicely. Expect the
clearing to continue to push across the remainder of the region
this evening. Some mid and high clouds may linger near the CT
valley through around midnight or so. N-NE winds continue,
highest across Cape Cod and the islands, which will become
mainly N this evening there. Further inland, light/variable or
calm winds will be in place.

Have updated near term grids to bring conditions current,
especially the sky cover, T, Td and winds.

Previous Discussion...

Through Tonight...Upper trough heads east of New England, and
drier air filters into the region. Anticipate gradual clearing
this evening. This should allow enough radiational cooling to
allow interior temperatures to drop to the lower to mid 50s.
There could be isolated pockets in the interior NW valleys that
dip into the upper 40s, rather notable for late July. Winds will
be light with a drift from the north.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...

Wednesday...
Surface high pressure and upper ridging will dominate across a
dry air mass. Anticipate plenty of sunshine with just some
afternoon cumulus and temperatures generally in the upper 70s
and perhaps near 80 in the lower portions of the Merrimack and
Connecticut Rivers. Temperatures may be capped in the mid 70s
along coastal areas due to developing seabreezes.

Wednesday night...
Expect dry conditions to prevail but will probably experience
increasing high clouds late ahead of a shortwave trough dropping
southeast from central Canada. A weak SW gradient will become
established.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights...

* Dry early Thursday
* Showers and isolated thunderstorms move into northern and western
  areas Thursday afternoon/evening
* Best chance for showers/scattered thunderstorms Thursday night
  through Friday night
* A few showers linger into Sat, then dry but cool conditions
* Dry and seasonable weather likely Sunday into Monday

Overview and model preferences...

Weak H5 ridge pushes E to the Maritimes early in this period,
while digging H5 trough moves E-SE out of the Great Lakes.
However, individual model solutions vary on the intensity of the
trough as it shifts E. This is giving widening surface solution
spread as the surface front and possible wave development
during the Thu night-Fri timeframe.

12Z GFS appears to be an outlier as it pushes this system
further S late this week and keeps the wave weaker/further S.
Also, GFS blasts the H5 short wave offshore by late Thu/Thu
night, while most of the remaining suite is slower. A few
members even try to develop H5 bubble cutoff in the NW flow over
the western Great Lakes Thu night/Fri and move it eastward.
While the GFS moves the surface front offshore, the 12Z GGEM and
00Z ECMWF keep the front a bit further N but also have low pres
moving along it. The 12Z NAM is a bit more toward the GGEM/EC
camp, though a bit further S with the developing low over the
lower Appalachians. By Friday night, most models have the low
passing S of New England, but northern fringe of the precip may
reach into the region though each solution varies on how far N
the precip shield extends. Have low to moderate confidence
during the late week timeframe. Am leaning toward a non-GFS
blend especially for the Thu night to Fri night timeframe.

Beyond this, should see the H5 trough move offshore during the
upcoming weekend, but noting broad long wave troughing setting
up across the eastern half of the U.S. while high amplitude
ridge builds from the west coast into British Columbia. Should
see generally dry but cooler than seasonal norm temps with
continued NE wind flow as surface high builds across the western
Great Lakes/northern Plains. Overall, will use blend of
models/ensembles for this timeframe.

Details...

Thursday...
High pressure ridge shifts S Thursday with SW winds in place.
Leading edge of clouds/scattered showers look to push into
northern areas from NE through central-W Mass into N central CT
during Thu afternoon. Marginal instability, but could see a few
isolated thunderstorms through 00Z or so. Highs will be mainly
in the 70s, ranging to around 80 across the lower CT valley. SW
winds may gust up to 15-20 kt along S coastal areas, Cape Cod
and the islands.

Thursday night through Friday night...
Models continue to signal low pressure wave shifting E from the
Ohio valley along the passing cold front. Some model
differences in frontal placement and how quickly the low moves
E. At this point, looks like best chance for showers/scattered
thunderstorms will occur during this timeframe. Decent
instability with this front. Best shot for precip will occur
mainly near and S of the Mass Pike where likely POPs are in
place. Noting good slug of QPF pushing through, with 1-1.5
inches possible across S coastal RI and SE Mass during this
timeframe.

Winds shift to N-NE as the low passes during Fri, then will pick
up late Fri/Fri night mainly along the S coast as H925 jet up to
40-45 kt passes. Winds may gust up to 20-25 kt along the
immediate S coast/Cape Cod and the islands for a time mainly
around or after midnight Fri night.

Saturday...
Showers may linger along the S coast through at least midday
Sat, depending upon how fast the low pushes offshore, allowing
the front to move further S. Conditions should improve during
the afternoon but it will be cool with NE winds in place for
most of the day. Highs will only be in the 70s.

Sunday-Monday...
Looks dry as high pressure to the W holds in place. Light winds
will be in place as well. Temps will rebound close to seasonal
levels.

Tuesday...
Low pressure may develop on the front S of New England, while
another cold front approaches from the NW with its own wave. Big
question will be how quickly the northern front approaches.
Kept only slight chance POPs for the entire area for now. Temps
continue close to seasonal norms. Rather low confidence.

&&

.AVIATION /23Z TUESDAY THROUGH SUNDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday Night/...

Through Tonight...High confidence.
VFR with clearing skies moving SW this evening. Patchy late
night fog may occur at some interior airports for a few hours.

Wednesday...High confidence.
VFR. Any patchy interior valley fog should burn off by 12Z-13Z.

Wednesday night...High confidence. VFR.

KBOS Terminal...High confidence. VFR. Sea breeze expected to
develop by mid to late Wednesday morning.

KBDL Terminal...High confidence. MVFR cigs lifting to VFR by
around 22Z. VFR for rest of the period.

Outlook /Thursday through Sunday/...

Thursday...Moderate confidence.
Mainly VFR. May see local MVFR VSBYS in scattered
showers/isolated thunderstorms from about a KBVY-KORH-KIJD line
westward Thu afternoon. SW winds may gust to 20 kt along S
coastal terminals.

Thursday night through Friday night...Moderate confidence.
Mainly VFR CIGS, but pockets of MVFR-IFR in heavier
SHRA/isolated TSRA. Areas of widespread SHRA mainly from the
Mass Pike S late Thu night and Fri, with sct SHRA N of the Pike.
Patchy late night/early morning fog with local IFR VSBYS. Winds
will be shifting through the period, but cannot rule out some
gusts to around 20 kt as these shifts occur.

Saturday...Moderate confidence.
May see pockets of MVFR conditions along S coast in lingering
SHRA and patchy fog. Otherwise mainly VFR. NE winds gusting to
around 20 kt from Plymouth county southward through midday then
diminish.

Sunday...Moderate to high confidence.
VFR. Light N-NE winds with local sea breezes.

&&

.MARINE...
FORECASTER CONFIDENCE LEVELS...

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

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

Tonight...Seas will gradually subside overnight. Small craft
advisories for hazardous seas continues through 11 PM for Mass
Bay and Ipswich Bay, as even a light N-NE flow was keeping seas
elevated to 5 or 6 feet there. Seas will remain higher through
most of the night across the eastern outer waters to E and S of
Nantucket, where small crafts remain in place as well.

Wednesday...High pres over the waters will result in light
winds with near shore seabreezes. Seas will be below SCA
thresholds.

Wednesday night...Winds shift to S-SE as high moves off the S
coast, then will shift to SW and pick up on the eastern outer
waters after midnight. Seas 4 ft or less.

Outlook /Thursday through Sunday/...

Conditions should be mainly below small craft criteria for most
of this period. May see N-NE winds gusting up to 25 kt on the
southern open waters late Fri night through Sat as front
passes. Seas also build up to 5-6 ft on the southern outer
waters for a time Sat into Sat evening. Rain and fog will lead
to visibility restrictions especially Thu night into Fri.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 4 AM EDT
     Wednesday for ANZ250-254-255.
     Small Craft Advisory for hazardous seas until 11 PM EDT this
     evening for ANZ251.

&&

$$
SYNOPSIS...EVT/Thompson
NEAR TERM...EVT/Thompson
SHORT TERM...Thompson
LONG TERM...EVT
AVIATION...EVT/Thompson
MARINE...EVT/Thompson


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