Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 150212
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1012 PM EDT Mon Aug 14 2017

.SYNOPSIS...
High pressure moves offshore overnight, allowing some scattered
showers to move into the region late, especially along the south
coast. The risk for showers lingers tomorrow, with an isolated
thunderstorm possible before improvement arrives again tomorrow
night. Dry conditions follow Wednesday and Thursday. A warm
front approaches Friday, bringing increasing clouds, and
scattered showers. Showers and Thunderstorms are possible
Saturday as a cold front enters the region. A drying trend
likely follows Sunday and Monday along with near seasonable
temperatures.

&&

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

1030 PM Update...

Overall trend in the forecast remains on track for this evening.
Did go ahead and increase precipitation chances across southern
New England after 2 AM. Appears higher moisture and in the
upper level jet dynamics will result in more widespread precip
chances across the interior. Several runs of the HRRR have
indicated this and it is shown in the 18z GFS as well as the 00z
NAM.

Otherwise dense fog remains on track for tonight across the CAPE
and the Islands. Will have to continue to watch its northward
progression to see if dense fog adv will need to be expanded.


================================================================

High pres remains in control this evening even as the column
moisture loads along a strengthening WSW-ENE upper lvl jet in
the form of CI. Filtered sunshine has still allowed temps to
reach near their seasonal normal values except where sea breezes
kicked in late this morning.

Weak S flow already developing and as such noticed an area of
marine fog/stratus already infiltrating Nantucket and sliding
NNE. Will watch this for mainly the remaining Islands and Cape
Cod for late this evening/overnight. Dense fog advisories may be
needed for these areas. Further inland, while some low clouds
are possible, less inclined to go with fog given the modest S
flow and slow rise of overnight dwpts in comparison to direct
marine influenced airmass over the Cape/Islands.

Otherwise, will continue to watch band of light-mod rainfall
attendant to the developing S shortwave/increasing jet sliding
along the stalled front. Heights do dip late tonight and
suggested modest overrunning N of the front attempts a lift N
itself late. PWAT plume reaches 1.50-2.00 inches mainly along
and S of the Mass Pike, but deeper moisture/instability
associated with Gert will remain well S of the front, closer to
the mid Atlantic states. Therefore, will cap POP increase to
chance overnight, but could see some SCT light SHRA develop
especially W MA/CT (closer to the right entrance region of the
upper lvl jet) and along the immediate S coast (closer to the
stalled frontal boundary. In any case only light totals of a
quarter inch or less are expected at this time.

Overnight mins will be milder than previous nights, mainly in
the 60s even in coolest spots thanks to an increase in moisture
and cloud cover preventing radiational cooling.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Tomorrow...
As upper lvl jet-streak becomes more refined it shifts its
equatorward entrance region mainly SSE of New England later in
the day even as secondary shortwave energy moves out of the
Great Lakes and into central and N New England. This should
allow initial round of SHRA to dissipate through the morning and
shift offshore, focused more intently on the strengthening Gert
and the front offshore which looks to capture it. Will maintain
slight chance POPs mainly along the immediate S coast with
particular focus on the Islands, closer to where the best
synoptic support will focus later in the day. PWATs remain in
the 1.50-2.00 inch range, supporting this area.

Otherwise, with the secondary shortwave moving in from the W
initially rising heights will give way to slight cooling aloft,
enhancing mid lvl lapse rates from around 5.0C/km to start,
closer to 6.0C/km by late afternoon mainly across NW MA. With
modest moisture plume (still focused only on the S coast), this
could lead to a period where CAPE values reach 700-1200j/kg but
with a very tight gradient toward the E due to influence of mid
lvl dry air and capping. This should limit focus for late day
convection to mainly the most extreme W/NW zones, and most
likely in N and central New England where diurnal heating will
be better maximized. Shear will be the once enhancing component,
as 0-6km shear values during the afternoon reach 45kt. Not
expecting much in the way of severe due to these limiting
factors, particularly the fact we remain split between two areas
of synoptic lift to the NW and S. This agrees with latest SWODY2
which keeps marginal risk focused mainly across N New England.

Highs a bit cooler than normal given a fair amount of early
cloudiness.

Tomorrow night...
Weakening shortwave/front and upper lvl jet axis shift further
offshore and become entangled in the future ET process of Gert.
This will allow W flow and drier air to entrain through the
remainder of the column even where mid lvl drying has not
already occurred. Although a mild/moist start, trend will be
toward clearing and gradual fog dissipation toward the early
morning hours. Although dwpts will begin dropping, they are
likely to remain in the 60s through sunrise, and with early
cloud cover and continually W flow, lows should not dip below
the low-mid 60s even in coldest spots.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Highlights...

* Pleasant summer weather Wednesday and Thursday
* Risk for showers/Thunderstorms Friday and Saturday
* Improving trend Sunday into Monday

Synoptic overview...

Persistent and anomalous northeast Pacific jet continues to eject
numerous short wave troughs across the Canadian/US border resulting
in the risk of showers/T-storms at times. In between systems, many
hours of dry weather.  As for temps, active northern stream jet
suppresses subtropical ridge and its associated excessive heat and
humidity across the southern states, resulting in temps here near
normal for mid Aug.

Temperatures...

As mentioned above, active northern stream jet suppresses
subtropical ridge and its excessive heat and humidity south of New
England. Overall this pattern supports temps near normal thru the
period. The warmest day in this stretch is likely Wednesday in the
post frontal airmass with WNW winds providing some additional
downslope adiabatic warming and keeping seabreeze offshore. Thus
most beaches will warm into the low 80s Wed with mid to upper 80s in
the coastal plain /including Boston-Providence corridor/ and the CT
river valley. The core of this dry airmass settles over the area Wed
ngt into early Thu, which will result in cooler but very pleasant
temps Wed ngt into Thu morning with lows in the 50s, possibly upper
40s across northwest MA. Given this airmass and potential
radiational cooling sided with the cooler MOS mins Wed ngt/Thu
morning.

Seabreeze and cooler airmass Thu will keep highs in the upper 70s
and low 80s. In fact dew pts could be in the 40s! Quite comfortable.
Cloud cover and convective chances Fri and Sat will also limit highs
to the upper 70s and low 80s. A drying trend along with post frontal
WNW flow Sun and Mon may result in slightly warmer temps.

Precipitation...

Post frontal airmass overspreads the region Wed and lingers into
Thu. Thus dry weather both days. Then next short wave approaches Fri
with increasing clouds and warm frontal rain/showers likely. Trough
is slow to exit so risk of convection Sat ahead of the approaching
cold front. PWATs climb to +1 to +2 standard deviations above climo
so some heavy downpours are possible. Ensembles and deterministic
guidance both suggest trough begins to exit around Sunday so a
drying trend is possible Sun/Mon.

Beach Forecast...

Swells from tropical storm Gert are expected to arrive into our
coastal waters Tue ngt and then peak Wed with southerly swells up to
7 ft possible. Therefore surf will become quite rough Wed along with
increasing rips. Beachgoers will definitely need to exercise caution
especially at south facing ocean beaches where surf and rips will be
strongest.

&&

.AVIATION /02Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

1030 PM update...

Dense fog continues across the Cape and the Islands. Will have
to watch to see if LIFR conditions impact SE MA and portions of
RI.
==================================================================

Short Term /through Tuesday Night/...

Tonight and tomorrow...Moderate confidence.
Mainly VFR except extreme SE MA/RI terminals which could
experience a period of IFR conditions in low CIGS with low vsby
fog possible especially Cape/Islands. This burns off after
sunrise. Some showers possible late tonight, mainly Islands.
Otherwise, a few showers and isolated T-storms mainly W MA
during the afternoon tomorrow.

Tomorrow night...Moderate confidence.
VFR gives way to another mix of IFR/MVFR mainly CT/RI and SE MA
while the remainder stays mostly VFR, then as winds shift around
to the W, transition back to VFR everywhere toward the morning.

KBOS Terminal...Moderate confidence in TAF. Low probability for
a few hours of MVFR conditions around 09z.

KBDL Terminal...High confidence in TAF.

Outlook /Wednesday through Saturday/...

Wed and Thu...VFR likely along with dry weather. Light WNW winds Wed
becoming onshore Thu. High forecast confidence.

Fri and Sat...marginal VFR-MVFR in scattered showers/T-storms both
days. Low risk for IFR conditions Fri night into early Sat. Forecast
confidence high on trends but lower on specific details.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Through tomorrow...
S winds develop with gusts to around 20-25 kt on the E waters
during the day tomorrow, but brief. Otherwise, they remain below
small craft thresholds through the day tomorrow. Seas will
increase late tomorrow, but should remain below 5 ft through
tomorrow evening. Otherwise, mix of fog and showers,
particularly on the S ocean waters and waters surrounding Cape
Cod during the overnight and early morning.

Tomorrow night...
Continued risk for showers early, but then dissipating late. The
main issue will be a gradually increasing southerly swell which
could reach as high as 7 ft on the southern open ocean waters by
daybreak on Wed. Therefore, will likely a round of small craft
advisories for mainly the southern waters.

Outlook /Wednesday through Saturday/...

Wed and Thu...fine boating weather both days, good vsby and dry
weather. Only issue will be across the ocean waters especially south
shore where large southerly swells up to 7 ft Wed, then eroding Thu.
Forecast confidence is moderate to high.

Fri and Sat...scattered showers/T-storms expected along with light
to modest southerly winds. Vsby may lower in precip and esp Fri
night in areas of fog. Mainly southerly wind waves with swells from
Gert likely well offshore.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...Dense Fog Advisory until 10 AM EDT Tuesday for MAZ022>024.
RI...None.
MARINE...None.

&&

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



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