Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 281927
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
327 PM EDT THU JUL 28 2016

.SYNOPSIS...
A cold front over northern New England will drop south into our
area this evening and may bring a few showers or thunderstorms
across the interior. Low pressure will track south of New England
Friday and should bring heavy rain south of the Mass Pike. Drier
weather returns for Saturday, followed by another risk for showers
Sunday and Monday.

&&

.NEAR TERM /THROUGH TONIGHT/...
325 PM Update...

Showers/storms have fired across northern New England, and we may
see some scattered activity into parts of western/northern MA
before sunset as weak cold front drops south into our region.

Satellite loop shows bank of low clouds/fog entrenched from RI
Sound into Buzzards Bay where it is through late afternoon, before
starting to expand over South Coast, Cape, and Islands as we
approach sunset which will last through tonight.

Next area of showers/storms over mid Atlantic states will head our
way tonight as low pressure develops off NJ coast and tracks NE.
12z models have sped up timing of onset of showers overnight,
starting in lower CT Valley toward daybreak.

Warm/muggy night ahead with patchy fog due to light S flow. Lows
will drop back into the 60s and lower 70s.

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
 * Potential for flash flooding Fri, especially south of Mass Pike
 * Marginal threat for severe weather Fri near South Coast

Active day coming up Fri as low pressure tracks south of New
England along stalled front. Situation looks like a classic setup
for heavy rain and flash flooding south of Mass Pike, and also
potential for isolated severe weather near South Coast.

* Heavy Rain/Flash Flood Potential...

Classic flash flood pattern taking shape as anomalous 850 mb
southerly jet (+3-4 SD) brings tropical moisture (over 2"
precipitable water with surface dewpoints in 70s) over frontal
boundary sitting south of region. Forecast soundings show
potential for efficient warm rain processes with deep layer of
saturation up through high freezing level (near 15K ft). In
addition, model cross sections show strong low level frontogenetic
forcing (almost more typical of a wintertime scenario) with -EPV
above. Very good dynamics for the middle of summer!

While we are confident in excessive rainfall falling somewhere
south of Mass Pike, question is exactly where? Keep in mind there
is a lot of uncertainty as to where this stalled front ends up,
which will determine where axis of heaviest rain will be. Trend
in most of models has been to shift things south, more over
coastal waters south of New England. Model fields (lift/moisture)
suggest not totally jumping on board just yet and holding back,
keeping heavy rain/severe threat closer to South Coast. Model QPF
is one of the least reliable fields so it should be used with
caution. We may not really know for sure until radar trends begin
to tell story within 6 hours or so.

One thing we need to be aware of is if convection over mid
Atlantic ends up pushing south of region and essentially robs
available moisture. That would mean significantly less rainfall in
southern New England, and is one possible outcome.

Our rainfall forecast gives more weight to higher-res models in
order to better show sharper cutoff in heavier rainfall on
northern edge (north of Pike). We expect max 2-4" near South
Coast, Cape Cod, and Islands with about 1" as far north as Mass
Pike and up to 0.50" near NH border.

Flash Flood Watch has been issued for CT, RI, and SE MA where
higher rainfall rates (more than 1-2"/hr) would produce rapid,
significant urban flooding in some of the more flood prone
locations. Even though we are in drought conditions, extremely dry
soils can also lead to rapid runoff, which could result in rapid
flooding of some small streams and rivers, again primarily due to
urban runoff.

* Severe Potential...

In collaboration with SPC, the Marginal risk for severe weather
was expanded onto South Coast for Fri. Again, there is uncertainty
as to how everything will play out due to uncertainty with
position of front.

Models show plenty of instability (surface-based CAPE near 1000
j/kg) and strong 0-6km shear (more than 45kt) near front. Pattern
is favorable for short-lived waterspouts or an isolated tornado on
gradient between stable and more unstable airmass, which models
indicate will most likely be south of the islands and east of
Nantucket midday Fri into the afternoon. It is in this area where
0-500m helicity is forecast to exceed 90, low LCLs will be
present, and VGP (vorticity generation parameter) exceeds 25,
which past research has shown to be favorable.

Rain should fall most of day but taper off from W to E during
afternoon, with most of it done toward evening on E MA coast.
Highs will top out in 70s to near 80.

Everything clears out Fri night but lingering low level moisture
should lead to patchy fog, especially near South Coast.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Highlights...

 * High pressure Saturday
 * Showers and storms possible Sunday and Monday
 * Drier and seasonable weather mid-late next week

Overview and model preferences... Interesting weakly split flow
regime defines the mid-term period.  Weak ridge-over-trof split flow
develops and gradually moves across NE CONUS, finally giving way to
joined ridging toward the latter half of the forecast period.  The
trof is the primary wx-maker for late weekend and early next week,
providing a focal point for continued shra/tstm activity.  Timing
disagreements are minimal with the 12Z updates and even show
continuity with ECMWF EPS and GEFS means.  Therefore, will use a
consensus blend for this afternoon`s long-term update.

Details...

Sat... Confluence aloft following the initial convective low pres
passage Fri will allow for a period of quiet wx under weak high
pres.  H85 temps hover around +15C, a bit cooler than much of the
last week so expect mainly 80s for highs.  Sea breezes likely given
a weak sfc pres gradient, so expect cooler temps near shore.

Sun and Mon... Tricky forecast period as diffuse open wave and
modest sfc trof rest across much of the mid-Atlantic region and into
S New England.  This should provide a focal point for continued
shra/tstm development especially to the W and S, where highest
moisture lies.  PWATs linger around 2.0 inches to the SE but then
drop into New England.  Gradient winds mainly E-NE which, off the
cooler Gulf of Maine waters which is likely to limit convective
(particularly sfc based).  Therefore not confident in how much
thunder activity S New England will experience, however leftover
shra is possible.  CIPS analogs support this thinking, focusing the
highest risk for convective activity, SW of New England.

Will need to watch a secondary low pres wave development which could
provide better low lvl convergence and F-gen.  ECMWF is closest to
the S coast with this feature Sun night, but if it remains further
offshore, less rain will impact S New England.  Will need to monitor
as we approach as it has similar look to the low pres which could
bring heavy rain to the region tomorrow.

Tue through Thu... In spite of what occurs earlier in the week, the
split flow finally joins in the form of a ridge upstream which
gradually shifts to the E.  This should bring about a period of
mainly quiet wx with sfc high pres in place.  However, the
orientation of high pres brings in some cooler/drier air with H85
temps really only hovering around +12C, so temps should remain much
closer to seasonable levels than they have been for the last week or
so.

&&

.AVIATION /19Z THURSDAY THROUGH TUESDAY/...
Forecaster Confidence Levels.

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

Moderate confidence through Friday night.

E MA sea breeze has developed and finally made it into BOS. Winds
should turn more S along coast later this afternoon. Otherwise
scattered showers/storms possible across western and north central
MA after 21z and may bring brief MVFR/IFR vsbys.

Fog bank from RI Sound into Buzzards Bay should begin to expand
around sunset and should bring IFR/LIFR to much of South Coast,
Cape Cod, and Islands tonight into Fri. Elsewhere conditions lower
to MVFR or even IFR toward daybreak Fri and last through day in
showers/fog, especially across CT, RI, and SE MA.

Conditions improve Fri evening to VFR, but patchy fog may bring
local MVFR/IFR Fri night.

KBOS TAF...Moderate confidence. Sea breeze persists through late
afternoon before winds turn more to S.

KBDL TAF...High confidence.

Outlook /Saturday through Tuesday/...

Saturday...High confidence.
VFR.  Sea breezes likely.

Sunday and Monday...Moderate confidence.
Some showers and a few thunderstorms possible through the period.
Occasional MVFR conditions, but VFR likely dominates.

Tuesday...High confidence.
Mainly VFR.

&&

.MARINE...
Forecaster Confidence Levels.

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

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

Main concern is potential for strong or even severe storms over
south coastal waters Fri, mainly south of islands and east of
Nantucket where brief waterspouts are possible Fri afternoon.
Issuing a Marine Weather Statement to highlight this potential,
especially for recreational boaters.

Otherwise winds will remain below 25 knots and seas below 5 feet.
Patchy dense fog should expand over most of south coastal waters
from Buzzards Bay tonight into Fri, while rain and fog reduce
visibility Fri into Fri evening. Seas could approach 5 ft on outer
waters SE of Nantucket.

Outlook /Saturday through Tuesday...High confidence.

Mainly quiet boating weather as high pressure slowly builds over the
waters.  Winds and seas below small craft advisory thresholds.  Only
caveat could be an isolated shower or thunderstorm mainly Sunday or
Monday.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...Flash Flood Watch from late tonight through Friday evening for
     CTZ002>004.
     Air Quality Alert until 11 PM EDT this evening for CTZ002.
MA...Flash Flood Watch from late tonight through Friday evening for
     MAZ017>024.
RI...Flash Flood Watch from late tonight through Friday evening for
     RIZ001>008.
MARINE...None.

&&

$$
SYNOPSIS...Doody/JWD
NEAR TERM...JWD
SHORT TERM...JWD
LONG TERM...Doody
AVIATION...Doody/JWD
MARINE...Doody/JWD



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