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 281340
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
940 AM EDT THU JUL 28 2016

.SYNOPSIS...
Offshore high pressure will bring warmth and humidity to Southern
New England today. A cold front over Northern New England drops
into our area late in the day and may bring scattered showers and
thunderstorms. Low pressure then moves up the front Friday and
brings more widespread rain and scattered thunder. Low pressure
exits Friday night, with dry and seasonable conditions on
Saturday. Another low may bring the chance for showers late
Saturday night into Monday. High pressure returns bringing dry but
warmer and muggy conditions into the middle of next week.

&&

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

Early morning fog/stratus has lifted near South Coast. Should have
a decent amount of sunshine through afternoon as high clouds
stream over region per satellite loop.

Concern for this afternoon is potential for showers/storms across
interior. Daytime heating will bring modest surface-based CAPE (up
to 1000 j/kg) away from coast but 0-6km shear is weak (less than
25kt) as are mid level lapse rates (under 5C/km). High-res models
and ensembles are pinpointing western and north central MA for
scattered showers/storms this afternoon, which is in line with
current forecast. Not expecting to see a lot of activity, but main
time frame looks to be 4 pm to 8 pm before loss of daytime heating
causes any activity to dissipate.

Gradient remains weak enough through most of afternoon to allow
for sea breezes to develop near both coasts by midday. However, we
do think S winds will begin to increase later this afternoon in
response to tightening pressure gradient, so many spots along E MA
coast should top out around 90 later in day. Otherwise, forecast
highs in 90s for many locations are on track. Dewpoints in 60s
will keep heat index no higher than 95 or so.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
Some leftover convection possible this evening/early night in
Northern/Western MA as the cold front settles through our area.
This should diminish with the setting sun and as a weak shortwave
ridge races overhead early at night.

Attention then turns to another shortwave, this one coming through
the Ohio Valley later tonight and across the Northeast on Friday.
The previous model suite gravitated to two scenarios. One
scenario showed the associated surface low passing close to New
England and bringing us a soaking rain, while the second scenario
kept the surface low and most of its rain offshore. The overnight
suite has converged on the first scenario, which we will follow
with this forecast.

Low level jet feeds high moisture content air starting around 12Z
Friday. This suggests timing the rain for late tonight through
Friday afternoon. Precipitable water values are forecast between 2
and 2.5 inches...very juicy air...so the potential will exist for
locally heavy downpours embedded in the rain area.

Aside from run-to-run consistency, the question remains as to
where the axis of heavy rain will set up. The stalled cold front
is the most likely feature to focus low level lift. This would be
somewhere near the South Coast. The projected tracks of the
surface low also favor the South Coast area with heaviest rain
along and just north of the low. We used a blend of RFC and WPC
rain data for our own forecast. Broad area averages will be an
inch to 1.25 inches. Within this, locally heavier amounts up to
2.5 inches is possible. We have considered the possibility of
issuing a flood watch but will hold off until the day shift to see
if the next model suite remains consistent with the overnight.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Highlights...

* Leftover showers will push offshore Friday night as low pressure
  exits
* Seasonable and dry for Saturday
* Another weak low will bring the chance of rain Saturday night into
  Monday

Overview and model preferences...

Medium range model suite showing better agreement early in this
forecast period, but still noting some model spread beyond Sunday
mainly with the 00Z Canadian GGEM which appears to be a fast
outlier with its development of surface and upper level feature
for another low approaching the region late this weekend into
early next week. Leaned toward a GFS/ECMWF model blend early in
the forecast /into Monday/ then transitioned over to the model
ensembles for the remainder of the forecast.

Continue to note a rather fast mid level flow in place in the
northern stream with developing broad H5 troughing across the
Great Lakes Friday night-Saturday, but the trough tends to sit
across the eastern Lakes through Saturday night before finally
starting to push E Sunday into Monday. With the stalled front S of
New England during this timeframe, looks like another wave of low
pressure moves along it. This will bring another shot of precip to
the region.

Once the H5 trough finally clears the region Monday afternoon and
evening, ridging builds across central Canada up to Hudson Bay.
With increasing H5 heights, will see temperatures rise to near or
a bit warmer than seasonal normals during Tuesday and Wednesday
with dry conditions.

Details...

Friday night...

As low pressure pushes offshore, will see precip heading E.
Conditions will improve from W-E, with skies becoming partly
cloudy away from the east coast. Some showers may linger across
Cape Cod and the islands through 2 or 3 AM. Patchy fog will likely
develop along the coast as well as the normally prone inland areas
as dewpts remain in the lower-mid 60s. Expect low temperatures in
the lower- mid 60s for most areas, except near 70 along the coast.

Saturday...

High pressure ridging extending from south central Quebec to off
the NJ coast will bring dry conditions to the region. Temps will
run close to or slightly above seasonal normals, with readings
topping off in the lower-mid 80s for most areas away from the S
coast. N-NW winds will start off at 10-15 mph early, but will
diminish as the high builds in. With the warm temperatures, sea
breezes will likely kick in from mid day through the afternoon.
May start to see mid level clouds increase across the CT valley
late in the day, with increasing clouds Saturday night.

Sunday through Monday...

Noting low pressure across PA/NJ region. With the W-SW wind flow
aloft, the moisture with this weak low will push slowly NE into
the region which is ahead of approaching H5 trough out of central
NY/PA. Big question is how quickly this feature will move across
the region, as it appears the H5 trough slows into Monday. Still
some timing issues in place, so held on to CHC POPs through this
timeframe. Looks like some decent instability moving in during
Sunday into Sunday night, so have also included a chance for
thunderstorms as well.

The H5 trough moves across the region during Monday. Will still see
some showers lingering, but should end late Monday or Monday night.

Tuesday-Wednesday...

High pressure builds slowly east during this timeframe, so should
see mainly dry conditions. Can not rule out some isolated diurnal
showers across far inland areas mainly on Wednesday. Expect
temperatures to rise up to around 5 degrees above seasonal normals
by mid week. Winds shift to S-SW, so will see dewpts rise as well.

&&

.AVIATION /14Z THURSDAY THROUGH MONDAY/...
Forecaster Confidence Levels.

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

High confidence through Fri.

Light W/SW winds will give way to sea breezes along both coasts
between 15z and 17z. Winds along E MA coast expected to shift to S
later this afternoon, probably around 21z. Scattered showers/storms
possible across western and north central MA after 21z and may
bring brief MVFR/IFR vsbys.

Tonight and Friday...

VFR before midnight. Rain and scattered thunderstorms developing
well after midnight. Ceilings/vsbys will lower to MVFR/IFR late
at night and Friday in this precipitation. Conditions may start
improving from west to east Friday evening.

KBOS TAF...High confidence. Sea breeze develops around 15z. Any TS
stays well W.

KBDL TAF...High confidence.

Outlook /Friday Night through Monday/...

Friday night...Low to moderate confidence. Patchy MVFR CIGS/VSBYS
mainly across central and eastern areas through midnight. Local
RA/+RA in any isolated thunderstorms. Patchy fog develops along
the coast and normally prone inland areas after midnight with
local MVFR- LIFR VSBYS. Light N-NW winds.

Saturday...Moderate confidence. Any leftover MVFR-IFR VSBYS in
patchy fog through 14Z, otherwise VFR. Light N-NW winds with sea
breezes developing along the coast.

Saturday night through Monday...Low confidence. Mainly VFR, but
local MVFR-IFR conditions in scattered -SHRA. Patchy fog late
Saturday and Sunday nights with local IFR-LIFR CIGS/VSBYS. A few
thunderstorms possible Sunday into Sunday night.

&&

.MARINE...
Forecaster Confidence Levels.

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

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

Winds remain below 25 knots and seas below 5 feet through the
period. There is a low confidence potential for 25 knot winds on
the southern outer waters Friday afternoon. Rain, heavy at times,
with possible thunderstorms. This will bring periods of poor
visibility.

Outlook /Friday Night through Monday/...

Friday night...Low to moderate confidence. Light E-SE winds shift
to N-NW overnight with gusts up to 20 kt over the open waters.
Rain likely with a few thunderstorms possible. Locally heavy
downpours. Seas 4 ft or less, though may reach 5 ft on the
southern outer waters. Reduced visibilities in areas of fog along
with the rain and any thunderstorms.

Saturday...Moderate confidence. N-NW winds gusting up to 20 kt
early, diminishing during the afternoon. Seas up to 5 ft on the
southern outer waters, subsiding during the afternoon. Locally
reduced visibilities early in patchy fog mainly near shore, but
improving by late morning. A few showers lingering near and E of
Cape Cod early.

Saturday night through Monday...Low confidence. Light SW winds
back to S-SE Sunday into Sunday night. Winds shift to NE late Mon-
Mon night with gusts up to 20 kt. Low pressure passes S of the
islands, bringing scattered showers. A few thunderstorms possible
mainly Sunday into early Sunday night. Seas should remain below 5
ft.

&&

.FIRE WEATHER...
Dry conditions will persist today, except possibly in western and
north central MA where scattered showers/storms may bring
localized rainfall Thu afternoon.

Minimum RH values of 30 to 40 percent are expected across much
of southern New England, except 40 to 60 percent on the South
Coast, Cape Cod, and the Islands. South to southwest winds will
stay under 20 mph.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Air Quality Alert until 11 PM EDT this evening for CTZ002.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...WTB/EVT
NEAR TERM...JWD
SHORT TERM...WTB
LONG TERM...EVT
AVIATION...WTB/EVT/JWD
MARINE...WTB/EVT
FIRE WEATHER...WTB/EVT



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