Area Forecast Discussion
Issued by NWS Boston, MA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS61 KBOX 240752
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
352 AM EDT Sat Jun 24 2017

.SYNOPSIS...

Potential flooding and severe weather threats through the early
half of the day, especially across Southeast New England, as the
remnants of Tropical Storm Cindy clash with a sweeping cold front
that will push offshore during the later half of the day, improving
quickly and continuing overnight. Seasonably warm and less humid
conditions Sunday will be followed by cooler weather and few
diurnally driven showers and isolated thunderstorms Monday into
Wednesday. A warming trend begins for the latter half of the
week with a return to summer heat and humidity by Friday.

&&

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

*/ Highlights...

 - Potential flooding threats across Southeast New England
 - 1 to 2 inch rainfall rates per hour possible
 - Additional threats of gusty winds and possible brief tornado

*/ Discussion...

Scattered showers and thunderstorms becoming more focused along and
ahead of a sweeping well-defined cool frontal boundary. Collision
with the remnants of Cindy and associated tropical plume, there is
the threat of heavy rain and flash flooding, especially across SE
New England, including the possibility of gusty winds and perhaps a
brief tornado. Yet, there remains about a N-S spread of the axis of
heaviest rain that leads to a level of uncertainty. This has been a
tricky forecast. Let`s start with a broad overview.

Ahead of the front, strong, convergent H925-7 moisture transport
pushes precipitable waters to 2.5 inches, all in association with
Cindy remnants becoming focused along a well-defined cool frontal
boundary. Elevated instability up to 1k j/kg, but all below the
freezing level of around 15 kft within an atmospheric column that is
saturated up to around H3. Deep layer forcing ahead of the mid-level
long-wave trough axis and cool front, expecting efficient warm-rain
processes with rainfall rates of 1-2"/hr that usher a threat for
localized flash flooding. Secondary threats of gusty winds, but not
thinking wet microburst owing to the lack of mid-level dry air.

Also, importantly, have to watch out for the potential of a brief
tornado given strong 0-1 km shear with helicity values modeled as
high as 250 m2/s2. Low LCLs with surface dewpoints well into the
70s, again along the well-defined cool frontal boundary, aside from
the weak instability, a brief tornado is possible and this was seen
yesterday across the Pittsburgh PA region. Very similar setup with a
few brief tornadoes observed. SPC, in agreement, has the S-coast in
a MARGINAL risk of severe weather with a 2-percent tornado risk
within 25 miles of a point.

So which guidance is preferred? Interrogating initialization, the
WRF and Canadian model solutions are out. The EC is the southern-
most solution. And a consensus of HRRR/NAM/GFS/UKMET/RAP/SREF focus
mainly on SE New England. But even within consensus there is some
wavering N to S, however it is focused on SE New England overall.

Looking at upstream trends and considering above discussion, will go
with definite PoPs over SE New England. Expecting a roughly SW to NE
band of storm total rainfall within a 6-hour timeframe, possibly
less, of 1 to 2 inches. Collaborating with WPC will highlight the
region for a MARGINAL risk of EXCESSIVE RAINFALL. What is keeping
rainfall amounts from being higher is the progressive flow over the
region. So in thinking of isolated to scattered impacts, not wide-
spread, and with 1 and 3 hour flash flood guidance at 2 to 4 inches,
will hold off on a FLASH FLOOD WATCH. Thinking nuisance urban, poor-
drainage flooding. Will have to keep an eye on coastal communities
given coincident timing of high tide, however a tide lower than
astronomically high tides that have occurred as of late. With the
high tide, potential for drainage issues. Will issue a SPECIAL
WEATHER STATEMENT to highlight the potential flooding concerns,
as well as severe weather threats, more importantly the possibility
of a brief tornado, which was discussed above.

Behind the front, incredible drying discerned from model forecast
soundings. The airmass aloft relatively unchanged with respect to
temperature, notched downward slightly from +18C to +15C at H85, the
sun immediately coming up, we will warm up quickly into the mid to
upper 80s with some locations in the low 90s. However much less
humidity as behind the cool front dewpoints drop gradually with dry
air advection, down around the mid to upper 50s given deep layer
mixing up to H8 of a much drier airmass building in aloft. So in
summary, the later half of the day should turn out fairly nice and
pleasant.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...

Tonight...

Overall becoming quiet. Some initial light shower activity across
N/W MA and CT, up against the high terrain, as mid-level energy
advects through the continued cyclonic flow promoting localized
theta-E / moisture convergence. But soon after weak height rises
overnight out ahead of a more stout mid-level feature through the
longwave trough over the Great Lakes Region. Surface high pressure
emerging, drying out and clearing out, W winds becoming light, there
is the opportunity for radiational cooling. Indications of low to
mid 50s in the coolest of guidance at typically prone locations. Dry
air advection with dewpoints fall, not anticipating fog threats.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...

Highlights...

* Seasonably warm/less humid Sun with a spot shower possible interior
* Cooler with a few showers/isold t-storm possible Mon into Wed
* Warming trend toward end of the week with return to summer
  heat/humidity by Fri

Details...

Sunday...

Fairly robust mid level shortwave moves NE from Gt Lakes. Brunt of
energy remains well to the north and west, but cooling aloft will
result in diurnal cu developing and can`t rule out a few afternoon
showers, mainly in the interior. Instability parameters are not
favorable so no mention of thunder. 850 mb temps near 12C and deep
mixing supports highs in the low to mid 80s but cooler south coast
with SW flow. Dewpoints in the 50s will make it feel much more
comfortable.

Monday into Wednesday...

Anomalous mid level trof moves eastward from the Gt Lakes Mon
pushing into New Eng late Tue and may linger into early Wed. Temps
continue to cool aloft Mon which may trigger a few showers or an
isold t-storm in the interior, but best chance of showers/t-storms
will likely be on Tue as the trof approaches and core of coldest air
aloft moves overhead. Timing of when the trof exits is somewhat
uncertain but GFS and ECMWF still have -21C at 500 mb 12z Wed before
temps aloft warm in the afternoon. So expect isold showers and
t-storms again on Wed with focus more across eastern half New Eng.
Temps through Wed will average near or slightly below normal with
highs mainly 75-80 and lows in the 50s.

Thursday and Friday...

Pattern change signaled as upper trof moves out with rising heights
and zonal flow across New Eng. This will result in a warming trend
with temps into the 80s Thu and possibly 90+ on Fri. Humidity levels
should still be comfortable Thu as dry air remains in the lower
levels, but higher dewpoints will be moving in by Fri. Mainly dry
weather, but developing warm advection may bring a few showers Thu
afternoon to northern areas, then chance of afternoon showers/t-
storms Fri as atmosphere destabilizes.

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...

Forecaster Confidence Levels...

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

Short Term /through Tonight/...Moderate confidence.

Today...
MVFR to LIFR mainly across SE New England with accompanying
lower VSBYs as RA/+RA spreads across the region this morning
with the threat of TSRA and SW gusty winds up around 30 kts. In
addition there is the threat of LLWS as winds 2 kft agl will be
out of the SW around 50 kts. All of this clears out during the
later half of the day, becoming SKC, with winds shifting W.
Quickly becoming VFR with Cape and Islands holding out longest,
21z.

Tonight...
W winds becoming light. Mainly SKC. VFR.

KBOS Terminal...LLWS threats should remain S/E of the terminal.
Looking at RA/+RA threats around 12z, yet the bulk of those
threats look to remain S/E. Will hint for a brief 3-hour period
in the TAF. Breezy SW winds with gusts up around 25 kts possible
towards noon, then winds become light and W.

KBDL Terminal...RA/+RA potential moving in around 9z. TSRA is
possible. TEMPO IFR VSBYs with +RA. Could see some gusty winds.

Outlook /Sunday through Wednesday/...

High confidence. Mainly VFR. A few diurnally driven showers and
perhaps an isold t-storm each day.

&&

.MARINE...

Low clouds and fog across the SE waters especially as the remnants
of Cindy advect from the SW out ahead of a sweeping cool front,
most of the activity mainly through the afternoon hours. Moderate
to heavy rain, along with the threat of thunderstorms, some of
which could be strong to severe with gale force wind gusts.
Mariners may also need to be on the lookout for possible water
spots as the environment is conducive for brief tornadic spin-ups.

Improvement towards the evening hours as the cool front sweeps
the waters. Quickly clearing out as winds shift to the W and
become light. The 6 to 8 foot wave action on all outer waters
and S inner waters should begin to diminish towards Sunday morning.

Outlook /Sunday through Wednesday/...

Sunday...High confidence. Some lingering 5 ft seas possible over the
southern waters, otherwise winds will remain below SCA.

Monday through Wednesday...High confidence. Quiet boating weather
with winds and seas below SCA. A few hours of nearshore lower 20
knot wind gusts are possible each afternoon.

&&

.TIDES/COASTAL FLOODING...

Astronomical tides are rather high through this weekend during the
night time cycles. Boston has a 12.4 ft high tide around midnight
tonight and 12.2 ft just after midnight Sunday night.

While, offshore winds are forecast current conditions suggest a
0.4 surge which will result in minor splashover. Thus will go
ahead an issue a coastal flood statement for tonight`s high tide.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for
     ANZ231>234.
     Small Craft Advisory until 11 AM EDT Sunday for ANZ235-237.
     Small Craft Advisory until 2 AM EDT Sunday for ANZ250.
     Small Craft Advisory until 8 PM EDT Sunday for ANZ254>256.

&&

$$

SYNOPSIS...KJC/Sipprell
NEAR TERM...Sipprell
SHORT TERM...Sipprell
LONG TERM...KJC
AVIATION...KJC/Sipprell
MARINE...KJC/Sipprell
TIDES/COASTAL FLOODING...WFO BOX Staff



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.