Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 251710
AFDBOX

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

.SYNOPSIS...
This afternoon will remain cooler than normal and cloudy with
thinning clouds late in the afternoon. High pressure will bring
a return to dry and milder weather for Wednesday and most of
Thursday. Cold front with a surface low pressure wave impacts
southern New England Thursday night and exits the area late
Friday. Dry weather and seasonable temperatures expected next
weekend.

&&

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

1 PM Update...
No significant changes to the forecasts. Dropped POPs just a tad
but still expect widely scattered showers to cross MA this
afternoon. Temperatures have been running a little cooler than
forecast in a number of spots but think the high temperatures
still look reasonable especially with a wdge of thinning clouds
making its way south along the north shore toward greater
Boston. Still will be mostly cloudy but should begin to
experience some thinning in the overcast as the afternoon
progresses. No significant changes made to the winds.

10 AM Update...
Short wave trough moving across southern New England is
anticipated to result in widely scattered showers later this
morning into mid afternoon. Short term high resolution guidance
dries up the band of showers approaching Berkshire County at 10
AM, and it`s possible they will struggle to get over the
Berkshires. Nevertheless, water vapor imagery depicts a rather
sharp short wave trough so have opted to have a period of POPs
in the 20 to 25 percent range through mid afternoon across MA
and adjacent northern areas of CT and RI.

Temperatures and winds appear to be on track at this time with
few changes needed.

7am update...
No significant changes to previous forecast. Weak wave just now
pushing into NW zones and will gradually rotate through the
region today, yielding a few showers and low clouds. Could begin
to see breaks by late afternoon as drier air works its way back
into the column and is allowed to mix with lower levels.

Previous discussion follows...

Weak surface wave moving off the Mid Atlc coast with ridging
nosing down from the north will maintain a moist NE flow across
SNE. Cross sections show considerable moisture in the lower
levels with moisture deepening in the column as mid level trof
axis and shortwave approaches from the NW. This will keep lots
of clouds across the region along with a risk of a brief shower
although most of the time it will be dry. While it will be
milder than yesterday, temps will still be unseasonably cool
with readings in the mid/upper 60s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
Tonight...
Mid level trof axis exits the region this evening followed by
rising heights and high pres building in overnight. Models show
good drying moving in from the N and NE which will lead to
clearing skies overnight. Good radiational cooling with light
winds will result in a chilly night with lows ranging through
the 50s with potential for upper 40s in the colder locations in
the CT valley and interior E MA. Anomalous temp regime for late
July.

Wednesday...
High pres in control with dry air in the column will lead to a
pleasant day with lots of sunshine. Temps will recover into the
upper 70s and lower 80s but developing seabreezes will keep it a
bit cooler along the immediate coast.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Highlights...
* Dry most of the day Thu.
* Showers and possible thunderstorms late Thu with showers
  likely lingering into Fri
* Dry weather and seasonable temperatures likely for the weekend

Overview and model preferences...
Although weak ridging will begin this long-term period, it will
be followed by an unseasonably strong shortwave which will
begin to carve out long-wave trof across the northeast CONUS and
much of the E seaboard late in the week, and into the weekend.
Although the initial wave is likely to cutoff per latest
ensemble and operational model runs, the damage will have
largely been done, leaving the longwave trof in its wake as it
shifts E as early as the late weekend. Given this, periods of
unsettled wx can be expected, along with generally cooler than
normal temperatures through much of the latter half of the week.
Generally, will use a consensus blend of guidance for this
update. However, will opt to lean away from 24.12Z ECMWF which
generates an anomalously deep low pres crossing the region Thu
night night into Fri. This seems a bit overdone and is a bit of
an outlier.

Details...

Wed night into Thu...
Weakening ridge will shift E through the period, but remains
the dominant player for sensible wx across New England through
most of the day on Thu. Cold front/low pres development
associated with a robust wave will slow the overall progression
of any precip/lift these features can generate. Therefore, will
continue to suggest mainly NIL pops with only slight chance in
the W/NW by 00Z Fri. Wed night mins comfortably cool for
sleeping, mainly in the 50s to low 60s in the heat islands.
Daytime highs will be dictated by speed of incoming cloud cover
but with H85 temps rebounding to near +14C, could see a few
spots in the upper 70s and low 80s.

Thu night into Fri...
Potentially most unsettled period of the forecast. Very robust
shortwave, especially by July standards, will be carving out a
trof across the NE. Exactly how this plays out at the sfc
remains somewhat uncertain, as both ensembles and operational
guidance suggest anywhere from a standard cold frontal passage,
to as deep as a 992mb low pres passage directly over New
England. The latter may be a result of convective feedback,
however if true it would certainly suggest a period of moderate
to heavy rainfall, as lift would be much more significant, and
able to fully realize PWATS nearing 2.00 inches. In any case,
destabilized soundings and modest shear will be in place,
allowing for some convective elements to accompany the
widespread SHRA/RA. Will continue to suggest some TS and
continue the likely POPs put forth from the previous forecaster.
Will need to watch this period, because if the more robust low
pres is the ultimate answer, heavy rain could yield some
localized urban/street flooding potentially more widespread than
more localized convection. The increased cloud cover and cool
air associated with the digging trof will keep highs mainly
below normal, suggesting highs may once again struggle to reach
the low-mid 70s, but may be cooler if the more amplified
solution.

Sat and Sun...
Although continued digging of the longwave trof occurs, a
cutoff forms in the Canadian Maritimes which allows for the more
anticyclonic sheared (W half) of the trof to slide across New
England. This should allow for drier NW flow, limiting any
additional SHRA/RA and drying the column enough for mainly dry
wx as well. Still a few clouds, particularly diurnally driven
can`t be ruled out given the trof base is just downstream. Temps
remain cooler than seasonal normals. H85 +10C to +11C would
yield highs mainly in the mid 70s, potentially 5-8 degrees
colder than normal.

Early next week...
Although longwave trof continues to dominate across the E, the
weakening cutoff/vortex in the Maritimes will give way to brief
ridging as secondary wave moves out of the Canadian prairies.
Therefore, although cooler temperatures are likely to continue,
exactly how this complex interaction of trof plays out could
allow for some unsettled wet wx at times, but definitely not a
washout, as it keeps the deep layer moisture well to the S.

&&

.AVIATION /17Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Wednesday/...

This afternoon...Moderate confidence. Mostly MVFR ceilings
through mid afternoon with gradually lifting to VFR ceilings
during the late afternoon into the evening.

Tonight...High confidence. VFR with clearing skies with the
exception of patchy overnight fog at interior airports.


Wednesday...High confidence. VFR.

KBOS Terminal...Moderate confidence. MVFR ceilings lifting to
VFR by late afternoon or early evening.

KBDL Terminal...Moderate confidence. MVFR cigs lifting to VFR
during late afternoon.

Outlook /Wednesday Night through Saturday/...

Wednesday night through Thursday...High confidence.
VFR. Mainly S to SW winds.

Thursday evening through Friday...Moderate confidence.
Mix of MVFR/IFR conditions with widespread SHRA and risk for
occasional TSRA mixing with fog. Winds will be shifting through
the period, but cannot rule out some gusts to around 20 kt as
these shifts occur.

Saturday...Moderate confidence.
Mainly VFR especially during the day on Sat.

&&

.MARINE...
FORECASTER CONFIDENCE LEVELS...

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

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

Through tonight...Diminishing NE winds with speeds less than 20
kt. Hazardous seas will result in small craft advisories over
the outer waters today but seas gradually subsiding below 5 ft
tonight.

Wednesday...High pres over the waters will result in light
easterly winds with seabreezes developing nearshore waters. Seas
below SCA.

Outlook /Wednesday Night through Saturday/...

Conditions during most of the period will likely remain below
Small Craft Advisory thresholds. However, this is a risk that
winds and seas may approach 25 knots and 5 feet, respectively, in
an increasing SW flow Thursday afternoon and a northerly flow
Friday afternoon and night, depending upon the strength of a low
pressure area that passes along the southern waters Friday. Rain
and fog will lead to visibility restrictions especially Thu
night into Fri.

&&

.CLIMATE...
Record low maximum temp of 66 degrees set at PVD yesterday.
Previous record was 67 set in 1997.

&&

.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>256.
     Small Craft Advisory for hazardous seas until 6 PM EDT this
     evening for ANZ251.

&&

$$
SYNOPSIS...KJC/Doody/Thompson
NEAR TERM...KJC/Doody/Thompson
SHORT TERM...KJC
LONG TERM...Doody
AVIATION...KJC/Doody/Thompson
MARINE...KJC/Doody
CLIMATE...



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