Area Forecast Discussion
Issued by NWS Boston, MA

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FXUS61 KBOX 242300

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

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.



650 PM Update...

Northern fringe of high cloud deck moving slowly across southern
areas at 22Z as seen on latest IR satellite imagery as well as
OBS. Thin band of precipitation seen across S coastal areas on
NE regional 88D radar mosaic, but no rain is reaching the ground
due to continued very dry low level layer across the region.
Dewpts at 22Z were in the lower-mid 30s across N central and NW
Mass with temps hovering at 70-75 degrees. Very high dewpt
depressions indeed. With E-SE winds in place, T/Td spreads along
the S coast are much lower, though still dry enough to allow for
evaporation of the precip aloft before reaching the ground.

Have updated to bring conditions current and incorporated trends
into the evening forecast. This did lower dewpts a bit, but
should start to see them rise during the night.

Previous Discussion...

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.


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.


As cutoff H5 low pres and associated surface system moves up
the eastern seaboard Wed, it will slow and tend to weaken near
or just S of the region through Thu night. Band of heavy rain
moves across through midday Wednesday ahead of this system,
along with gusty E-SE winds that will slowly shift to S as
remnants of the surface low linger. Will likely see lingering
light showers, areas of drizzle and fog. Can not pinpoint when
or where the showers will occur, but suspect mainly across
central and eastern areas with the good low level moisture will
continue to feed into these areas with the onshore winds, albeit
diminishing as the low weakens.

Digging H5 trough across the Plains states will slowly shift E
as large subtropical high pres (Bermuda high) looks to set up
off the SE U.S. coast by the end of the week. This will make the
progression of the H5 trough difficult while mainly zonal flow
sets up across New England N of the high. A cold front may try
to make a run across the region, but appears will get hung up in
the W-E flow. Not seeing a whole lot of moisture or instability
with this front, especially as it wavers across the region by
this weekend. Will likely see S-SW winds late this week through
the weekend, which may shift to W at times depending where the
front lies. A lot of uncertainty with this portion of the


Wednesday...Moderate confidence.
As weak low moves up the eastern seaboard, heaviest precip will
occur during the morning into midday hours. Some question as to
how quickly the precip exits the region late Wed into Wed
night. 12z NAM/GFS tend to be quickly in pushing heavier precip
out, while the GGEM/ECWMF linger it a bit later as the low
lingers S of the region.

With the low lingering, could see some marginal instability work
into S coastal areas. Noted LIs around zero to -1 along the S
coast and over the southern waters, as well as K indices around
30. Have kept mention of isolated thunder across the immediate S
coast and adjacent coastal waters through midnight, but not

The more certain part of this forecast will be the lingering
showers, areas of drizzle and fog through Wed night. May see
visibilities locally reduced below 1 mile, less along the
immediate coast early Wed morning and again Wed night. Will also
see E-SE winds gusting up to 25-30 mph mainly across Cape Cod
and the islands where a strong low level jet will pass across.
Winds at 2Kft running in the 50-60 kt range, but not seeing a
lot of good mixing with an inverted layer aloft. Winds will
diminish as pres gradient slackens and winds start to shift to
more southerly during the afternoon or Wed night.

Thursday...Low to moderate confidence.
As weakening surface and upper low meander near or just off the
coast, will see some more scattered showers, patchy drizzle and
fog in place. The drizzle and fog will mainly occur during the
overnight and early morning hours, then return late Thu night.
With the lack of a strong onshore flow, though, looks like temps
should become milder as winds start to shift to a more
southerly direction, though they will remain light. Just seeing
a lot of low level moisture lingering through Thu night.

Friday...Low confidence.
Will start to see a more S-SW wind flow in place as surface and
upper low finally flattens out and a cold front starts to
approach. However, noting the SW H5 steering currents tend to
become more W-SW during the day and especially Fri night. So, as
the front approaches, will get caught in this flow and slow its
progression. Some showers may move into N central and western
areas ahead of this front as it tries to sag S across the region
Fri night or early Sat morning.

Saturday-Monday...Low confidence.
A lot of uncertainty with remnants of the front lingering
across the region. At this point, have kept slight chance to low
chance POPs across the region. Exact timing and position of the
precip still in question though. As the H5 long wave trough
tries to shift E, may see the front lift around the late Sunday
or Monday timeframe, but this timing is uncertain. Temperatures
should run warmer than seasonal normals for late April.


Forecaster Confidence Levels...

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

Short Term /through Tuesday Night/...

Through 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

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.


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.


* Potential minor flooding impacts during astronomically high
  tides Tuesday night through Wednesday along the east and south

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


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.


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