Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 242057
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
457 PM EDT Mon Apr 24 2017

.SYNOPSIS...
Low pressure will slowly track up the east coast Tuesday and
Wednesday, bringing periods of rain, gusty winds and the risk
of minor coastal flooding. Drier and milder conditions will
briefly return Thursday. A cold front may bring scattered
showers and possibly a few thunderstorms Friday into early this
weekend, but may waver across the region into early next week.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
Deep moisture plume will slowly lift northward into SNE tonight
resulting in lowering cigs. Band of light rain across northern
NJ and NYC area will struggle to move north through this evening
due to presence of dry layer in the low levels so expect dry
conditions into this evening. Eventually expect some spotty
light rain to move up from the south and west after midnight
as low level jet moves into the region along with increasing
low and mid level frontogenesis which induces modest forcing for
ascent. Best chance of rain will be from western New Eng to the
south coast region where low levels eventually moisten up.
However, it may remain dry through the night further north
across northern and especially NE MA where low level dry air
persists through 12z.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...
Tuesday into Tuesday night...
Closed/vertical stacked low off the SC coast lifts slowly
northward to the Mid Atlc region Tue night. With high pres
locked over the north Atlantic, this results in a deep moist
easterly flow into New England with low level wind and PWAT
anomalies increasing to 2-3SD above normal. This will bring
periods of rain, heavy at times through Tue night. The rain
will gradually spread north across SNE through the morning and
into the afternoon as low levels moisten and easterly low level
jet moves north across the region. Rainfall during Tue should
be mostly light to moderate. But a second period of heavier
rainfall is likely Tue night, as a SSE low level jet approaches
with increasing PWATS over 1.5". Expect a period of heavy rain
along with chc of a few thunderstorms along the coast after
midnight as elevated instability increases at the nose of the
low level jet. Rainfall of 0.75-1.5" is expected with locally up
to 2 inches in any heavier convective rain/t-storms. This will
not produce river flooding but minor urban and poor drainage
flooding is possible, especially Tue night.

It will be a chilly and raw day Tue with gusty NE winds and
temps in the mid 40s to lower 50s, mildest near the south
coast. NE winds will occasionally gust to 20-30 mph near the
coast with highest gusts over the Cape/Islands. Temps should
rise into the mid 50s Tue night across RI and SE MA, with slowly
rising temps overnight in the interior. Gusty easterly winds
will continue Tue night along the coast. 50+ kt low level jet
approaches the south coast late Tue night but inversion will
likely prevent these winds from mixing down.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...

Overview...

Interpreting the N Hemispheric pattern, the GFS / GEFS continues to
forecast a stronger MJO event in the Central Pacific (phase 8) while
the EC / ECens is more subdued. Aside, still think there is a sub-
tropical connection from the Pacific within the low-levels out ahead
of broader H5 troughing over the N Central Pacific that contributes
to a stronger H5 ridging pattern into the NE Pacific and subsequent
downstream amplified mid-latitude trough-ridge-trough pattern across
the CONUS into the Atlantic. A clockwise-gyre over the Atlantic with
a counter-clockwise across the CONUS, most obvious is the persistent
SW flow over the NE CONUS aloft, a pattern supporting an environment
across New England of warmer temperatures, higher dewpoints. Coupled
with any forcing mechanisms, i.e., trailing cold fronts ahead of lee-
cyclogenesis, would expect shots of showers and thunderstorms. Small-
scale details in the discussion below, most importantly the midweek
coastal low with sub-tropical connections.

Details...

Wednesday...

As weak low moves up the eastern seaboard, heaviest precip will
occur during the morning into midday hours.


Moderate to heavy rain forecast. Potential flood impacts, interior
and coastal which will be highlighted below. There is still an air
of uncertainty. The 24.0z NAM continues a slower trend while there
is spread in individual members of the 23.21z SREF. Global models
have slowed a bit though continue to exhibit consistency with over-
all trends. Their accompanying ensembles show decent clustering of
individual members. However, some concern expressed in earlier for-
casts that the building mid-level ridge ahead of deeper troughing
upstream may result in sub-tropical energy remaining cut-off from
the mid-latitude flow, that it hangs off the E CONUS rather than
lifting N as model solutions around late last week were suggesting
in their long-range. While all solutions are succinct that with the
building ridge the sub-tropical low deamplifies, it may be the case
that the low hangs out well through Thursday till it is kicked E by
stronger synoptics Friday. With this forecast, will go with a non-
NAM consensus.

Energy presently sweeping over the SE CONUS lifts N with the
mid-latitude flow along the eastern seaboard having latched
into deep subtropical moisture. GEFS / ECens / SREF continues
to signal around +2 SD anomalies of H925-85 convergent inflow
and precipitable waters as well as strong probabilities of a
1-2" rainfall event, surface dew points exceeding 60F, and E
onshore flow, mainly along the immediate coast, 15 to 20 mph
sustained with gusts 30 to 35 mph. Interrogating operational
models for more detail, still seeing that with low-mid level
ascent of the warm-moist conveyor belt ahead of the H5-7 vort-
max of rich theta-E sub-tropical air, H85 dewpoints exceed +10C
as freezing levels are pushed above 11 kft over much of S New
England beneath which the column is completely saturated. With
deep- layer forcing of precipitable waters exceeding 1.6"
(notably the daily max from SPC precipitable water climatology
for 0z and 12z on April 26th is around 1.2"), looking at an
efficient warm- rain process.

However, per consensus of forecast guidance, the low deamplifies as
it remains progressive. A good slug of rain is forecast, but perhaps
impacts will remain isolated.

While uncertainty of exact impacts remain, will continue with
high categorical PoPs. Negative showalters and a conditionally
unstable airmass aloft, can`t rule out a rumble of thunder, but
won`t prevail in the forecast. Feel such an environment supports
embedded heavier showers. Within likely to high categorical
PoPs will highlight a moderate rainfall event. Will go with a
widespread 1-2" event with particular focus S/E with convergence
of low level inflow a higher moisture content as well as along
the E-slopes of high terrain. Can not rule out higher amounts up
to 3".

Wednesday night through Thursday night...

Dreary, drizzly at times. Not an overwhelming amount of confidence.
While expecting lots of trapped low level moisture beneath a measure
of low level ascent, near-stable temperatures with areas of drizzle
and fog, there is some concern the sub-tropical low ends up slower,
lingering longer. Some of the heavier rain may fall early on with
the low drifting N/E later. Not entirely sure we`ll mix out during
the day Thursday, however if we do it should end up mild beneath
more than likely scattered to broken cloud decks.

Friday into Friday night...

A shot at warmer weather prior to a sweeping cold front with some
shower activity. Given strong synoptics pushing in from the W, S
flow should amplify and aid in mixing out the low-level environment.
Clearing out, we should warm into the 70s. The cold front as well as
showers later on dissipate in their progression E, thinking as a
result of more parallel low to mid level flow along the front and
beneath enhanced ridging out ahead of deeper upstream troughing.
Chance PoPs. Some elevated instability so will keep with the mention
of thunder, however confidence is low. If any, mainly over the CT
River Valley. Decent SW-NE unidirectional shear, a weak cap around
H85.

Saturday-Monday...

A busy pattern. A consensus of forecast guidance signals that the
pick of the weekend is Saturday. A warm front slowly lifting Sunday
possibly into Monday. Shower and thunderstorm chances appear
likely though in greater detail with respect to timing and
impacts remain uncertain. Will keep with chance PoPs for now.

&&

.AVIATION /21Z MONDAY THROUGH SATURDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

Through 00z...VFR with high clouds. Coastal seabreezes.

Tonight...Moderate confidence.
VFR cigs trending downward to MVFR/IFR along the south coast
towards daybreak with patchy -RA developing.

Tuesday and Tuesday night...Moderate confidence.
Conditions lowering to IFR/LIFR in the morning with widespread
rain, stratus and patchy fog, lingering through Tue night. E
wind gusts to 25 kt developing over Cape/Islands Tue with winds
veering to SE Tue night with similar gusts. LLWS developing
Cape/Islands after midnight as SE/S low level jet at 50+ kt
develops.

KBOS TAF...Moderate confidence.

KBDL TAF...Moderate confidence.

Outlook /Wednesday through Saturday/...

Wednesday...Moderate confidence.
MVFR-IFR CIGS with -RA/RA and patchy fog. Reduced VSBYS likely,
down to IFR in spots. May see areas of LIFR CIGS across the
higher inland terrain and along the coast. SE winds gusting up
to around 25 kt mainly across Cape Cod and the islands Wed,
should diminish Wed night. Low risk of TSRA along S coastal
terminals. LLWS impacts for coastal terminals with 40-50 kt S
winds 2 kft agl, possibly approaching 60 kt on the outer Cape
and Nantucket, shifts offshore late in the day.

Wednesday night...Moderate confidence.
Mainly IFR in areas of -RA, -DZ and FG. Areas of VLIFR along
the coast and across higher terrain. Light/variable winds,
except SE-S up at 5-10 kt across Cape Cod and the islands.

Thursday-Thursday night...Low to moderate confidence.
Drizzle and fog move offshore early Thu, but isolated showers
linger with local MVFR VSBYS. CIGS should improve to VFR across
western terminals, but will likely remain MVFR-IFR for central
and eastern areas. IFR-LIFR conditions may return in patchy fog
and low clouds Thu night with light winds.

Friday-Saturday...Low confidence.
Brief MVFR-IFR VSBYS early Fri, then improving. May see
scattered -SHRA across N central and W Mass early Fri. CIGS
should be mainly VFR, though areas of MVFR return Fri night and
early Sat. SW winds shift to W late Fri or Sat, though timing
very much in question.

&&

.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.

Tonight and Tuesday...
Increasing pressure gradient between low pres to the S and high
pressure to the N will lead to increasing easterly winds
spreading north across the waters late tonight and especially
Tue. Gusts to 25-30 kt expected with building seas. Reduced
vsbys in rain and fog Tue.

Tuesday night...
E winds veering to SE overnight. Wind gusts may briefly
diminish for a time Tue evening but increase again after
midnight with gusts 25-30 kt, especially south coastal waters
as SSE low level jet approaches. Reduced vsbys in rain and fog.

Outlook /Wednesday through Saturday/...

* Small craft advisories likely for a majority of the period.

Wednesday...Moderate to high confidence.
E-SE winds at 15-20 kt will continue, with gusts up to around
30 kt during the day. Seas continue to build up to 8-12 ft
during Wed, highest along the outer waters south and east of
Cape Cod and Nantucket. Winds and seas will slowly diminish Wed
night, though wind will remain onshore. Reduced visibilities
possible during the day, more likely at night in patchy rain,
drizzle and fog. Low risk of thunderstorms on the southern
waters through midnight Wed.

Thursday-Thursday night...Moderate confidence.
Light winds in place, mainly S across the southern waters and
E-SE on the eastern waters. Seas remain high, up to 6-8 ft on
the outer waters early and slowly subsiding but remain at or
above 5 ft over the open waters. May see local visibility
restrictions in patchy fog mainly after dark. Patchy rain and
drizzle may also linger.

Friday-Saturday...Low to moderate confidence.
Light S-SW winds early Fri will increase, gusting up to around
20 kt. Winds may shift to W as a cold front may approach
sometime Sat. Seas remain around 5 ft on the outer waters
Friday, though may subside Fri night or early Sat. Patchy fog
with locally lower visibilities early Fri. May see a few showers
late Fri into Sat if cold front approaches.

&&

.TIDES/COASTAL FLOODING...
* Potential minor flooding impacts during astronomically high
  tides Tuesday night through Wednesday along the east and south
  coasts

Not anticipating any flooding impacts for the Tue high tide as
winds, surge and seas will not have built up enough.

Minor flooding possible during the Tue night and Wed high tide
cycles along both coastlines. A storm surge around 1 ft with low
risk of 1.5 ft surge on top of high astronomical tides and
building seas may lead to minor inundation of vulnerable
shoreline roads. Some beach erosion is also possible, especially
for east and SE facing beaches.

Boston High Tides (flood stage 12.5 feet)...

11.47 feet / Tuesday 11:20 pm
11.15 feet / Wednesday 11:50 am

Providence High Tides (flood stage 7 feet)...

5.98 feet / Tuesday 8:11 pm
5.53 feet / Wednesday 841 am

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 4 AM Tuesday to 8 AM EDT Wednesday
     for ANZ232>235-237-255-256.
     Small Craft Advisory from 8 AM Tuesday to 8 AM EDT Wednesday
     for ANZ231-250-251-254.

&&

$$
SYNOPSIS...KJC/EVT
NEAR TERM...KJC
SHORT TERM...KJC
LONG TERM...EVT
AVIATION...KJC/EVT
MARINE...KJC/EVT
TIDES/COASTAL FLOODING...KJC



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