Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 231118
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
718 AM EDT Sun Apr 23 2017

.SYNOPSIS...

Weak high pressure builds across the area today and Monday providing
dry and warm weather. A coastal storm slowly tracks up the eastern
seaboard Tuesday and Wednesday bringing periods of moderate to heavy
rain along with gusty winds and the risk of coastal flooding to the
region. A drier trend along with warmer temperatures is forecast
Thursday into early next week with chances of showers and thunderstorms
at times.

&&

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

7 am update...

Updates to account for the latest trends. Otherwise the forecast
is pretty much on track and the discussion below is spot on.

Previous discussion...

Clearing trend continues from northwest to southeast. As mid level
trough moves offshore this morning, anticyclonic mid level flow over-
spreads the area resulting in strong subsidence and mid level drying.
This will promote abundant sunshine today and blyr mixing above 850
mb. Given this leaned toward the warmer MOS guid and even added a
few degs. This brings highs this afternoon to 65-70 away from the
coast. Will feel even warmer given light winds. Spectacular spring
weather! Enjoy.

Weak pres gradient results in seabreezes keeping the coastline in
the MU50s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...

Tonight...

High pres remains in control with dry/quiet weather continuing. Good
radiational cooling with high pres overhead. Thus leaned toward the
cool MOS temps. Could have some patchy fog late.

Monday...

Dry weather continues as mid level confluent flow provides subsidence
across the region. Morning sunshine will likely fade behind increasing
mid and high cloudiness on the southern horizon. Nonetheless still
very pleasant with highs in the MU60s inland and cooler along the
shore with seabreezes.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...

*/ Highlights...

 - Rain Monday night thru Wednesday night
 - Accompanying fresh water and coastal flooding concerns
 - Back and forth pattern late into early next week

*/ Discussion...

Monday night through Wednesday night...

Period of moderate, possibly heavy, rain forecast especially Tuesday
night into Wednesday morning. In brevity, energy presently sweeping
over the SE CONUS lifts back N into the mid-latitude flow ahead of
which there is a decent sub-tropical connection. Per GEFS/ECens/SREF
there remains anomalous signals on the order of +1-2 SD with respect
to precipitable waters and H925-85 winds converging into New England
along with increasing probabilities of a decent slug of rain within
a 6-12 hour window, again mainly around Tuesday night into Wednesday
morning. Subsequent low-mid level ascent ahead of main H5-7 vortmax
of rich theta-E sub-tropical air and H85 dewpoints exceeding +10C,
with freezing levels pushed above 11 kft making this practically a
warm-rain process, very atypical as noted per sounding climatology
for Chatham which has a record max of 1.42" 12z Wednesday April 26th
when the GFS is forecasting a 1.63" precipitable water. There are a
number of concerns and a couple of unknowns discussed below.

1.) Fresh water flooding. Potential 1.0 to 1.5 inches of rain in a 6-
12 hour period centered around Tuesday night into Wednesday morning.
Per USGS most streamflows in S New England are within their normal
capacity based on climatology. Latest ensemble river forecasts per
GEFS/SREF show slight rises but no indications of exceeding minor
flood stage. So present thinking is that rivers will remain within
their banks, yet there remains a risk of typical nuisance flooding
such as ponding of water on roadways, urban / poor drainage issues.
We`ve seen a fair amount of rain in the last 72-hours and prior to
the heavier rain, beginning Monday night there will be some initial
warm advective rains that`ll likely saturate soils leading to more
runoff. Focus of risk especially within the I-95 corridor and areas
S/E with the upslope onshore flow and signals discerned from CIPS
analogs. There is also some additional concern up against the E
slopes of higher terrain.

2.) Coastal flooding. Persistent E fetch beginning Monday night and
through Wednesday morning with the strongest winds mainly due to the
gradient flow between the N/E wedge of high pressure up against the
approaching low. Fairly stout inversion above the surface which will
limit mixing. Still can`t rule out 0.5-1.0 foot surge brought about
by the persistent E flow around 15 to 20 mph sustained with gusts up
to 30 mph during astronomically high tides Tuesday into Wednesday,
the Boston tides listed below. Keeping in mind of minor flood stage
in Boston is 12.5 feet and we`re dealing with multiple tide cycles,
with a 1.0 foot surge, considering minor coastal flood issues with
inundation of vulnerable shore roadways in addition to splashover,
perhaps even some beach erosion. Particular focus on Tuesday night
into Wednesday morning high tides.

Boston High Tides...

  10.88 feet / Monday 10:33 pm
  10.87 feet / Tuesday 10:59 am
  11.47 feet / Tuesday 11:20 pm
  11.15 feet / Wednesday 11:50 am

Please see the COASTAL FLOODING section below for additional details
which also highlights the threat of poor drainage flooding for coast-
line communities during periods of heavy rain coincident with high
tides.

3.) Uncertainty. Still some question on the strength and timing of
the low center as it lifts up the coast, perhaps complicated by the
wedge of 1035 high pressure N/E off of Newfoundland. There is some
spread beginning Tuesday night. With this forecast will go with a
non-NAM consensus. Still advertise categorically high PoPs as well
as coincident moderate rainfall. More confident of embedded heavier
showers, but still no indication of instability as we remain capped
and thus no mention of thunder. Heaviest rainfall amounts forecast
over S/E New England and along the E-slopes of high terrain.

Will continue to highlight potential threats within the hazardous
weather outlook.

As the low passes across S New England Wednesday - Wednesday night
there will be a lot of moisture associated trapped within the low
levels with a measure of ascent associated with the main mid-level
vortmax as winds become light. Practically textbook, expecting a
period of low clouds, drizzle, and fog, quite soupy. Gradually
improve going into Thursday, clearing.

Thursday and beyond...

Back and forth pattern. Sweeping cold frontal boundaries ahead of
which warm-moist, more unstable air surges N yielding the potential
for showers and thunderstorms. Immediately followed by cooler air
associated with high pressure out of Canada sliding E, winds turn
onshore ahead of a warm front approaching from the S/W lifting back
N before another surge of warm-moist, unstable air ahead of another
cold frontal boundary. Looking at an active weather pattern with
periods of wet weather and unseasonably warm temperatures followed
by cool, dry, more seasonable conditions. Going with a blend of the
global models, showers and thunderstorms possible Friday, turning
cooler and dry into Sunday, return of wet weather into early next
week.

&&

.AVIATION /11Z SUNDAY THROUGH THURSDAY/...

Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

12z update...

Any lingering IFR-MVFR over Cape Cod and Nantucket at 12z gradually
improves to VFR. Elsewhere, VFR with light NW winds becoming west.
However near the coastline light NNW winds become onshore around
midday. Dry weather prevails.

Tonight...VFR.

Monday...VFR and light winds.

KBOS TAF...High confidence in trends, moderate confidence in timing.
Seabreeze developing 15-16z.

KBDL TAF...High confidence in trends, moderate confidence in timing.

Outlook /Monday Night through Thursday/...Moderate confidence.

Monday night through Tuesday night...

Cigs lowering to a mix of MVFR / IFR with -RA/RA. Likely visibility
impacts with RA. Higher confidence of lower conditions over the high
terrain and across the S/E coastal plain terminals. Increasing E
flow with sustained winds around 15 to 20 kts along the coast with
gusts up to 30 kts possible. Height of the winds Tuesday - Tuesday
night.

Wednesday into Wednesday night...

IFR / LIFR cigs with -DZ/DZ. Light winds initially N/E becoming S/W.

Thursday into Thursday night...

Improving VFR during the day, lowering MVFR / low-end VFR evening
into overnight. Increasing SW winds.

&&

.MARINE...

Forecaster Confidence Levels...

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

Short Term /through Monday/...High confidence.

10 am update...No major changes to the forecast.

Today...

NNW winds 15 to 20 kt over the eastern MA waters relax later this
morning and become locally onshore by midday. Dry weather and good
vsby prevail. Only issue will be leftover east swells of 4 to 7 ft.

Tonight...

High pres over the region provides light winds and good vsby with
easterly swells continuing to erode.

Monday...

More of the same, weak high pres over the waters provides light
winds, dry weather and good vsby.

Outlook /Monday Night through Thursday/...Moderate confidence.

 * SMALL CRAFT ADVISORIES likely for a majority of the period.

Monday night through Tuesday night...

Coastal low slowly approaching from the S expected to sweep N/E over
the S/E waters Wednesday night into Thursday morning. It is ahead of
this low that E flow will become persistent, the height of the winds
Tuesday into Tuesday night with sustained flow around 10 to 15 kts
with gusts possibly as high as 30 kts. This will result in wave
action over the outer waters to build up around 8 to 10 feet into
Wednesday. Rain is also likely and there may be some visibility
impacts on the waters.

Wednesday into Wednesday Night...

While the low passes overhead Wednesday into Wednesday night and
winds become light, seas will be at their height Wednesday
diminishing into Wednesday night. It is also during the low passage
that there will likely be visibility impacts over the waters with
drizzle.

&&

.TIDES/COASTAL FLOODING...

Persistent E fetch along with astronomically high tide cycles from
Monday night through Wednesday, with the height of the winds Tuesday
into Tuesday night. Strongest sustained winds, mainly along the E MA
coast, of 15 to 20 mph, gusts up to 30 mph.

This presents the possibility of a 0.5 to 1.0 foot surge on top of
the high astronomical tides. Along with wave action, an increasing
confidence of minor flooding impacts, if not at least splashover.
Inundation of vulnerable shoreline roads is possible.

Wave action is forecast to build through Tuesday and continue into
Wednesday with waves offshore of 8-10 feet. Can not rule out some
beach erosion along E-facing Atlantic beaches.

Boston High Tides...

  10.88 feet / Monday 10:33 pm
  10.87 feet / Tuesday 10:59 am
  11.47 feet / Tuesday 11:20 pm
  11.15 feet / Wednesday 11:50 am

A final word, can`t rule out additional impacts as a result of heavy
rain. Combined with high tides there is the possibility of poor
drainage issues along all coastlines, including the S coast of MA.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory for hazardous seas until 6 AM EDT Monday
     for ANZ250-254.
     Small Craft Advisory for hazardous seas until 2 PM EDT this
     afternoon for ANZ251-255.

&&

$$

SYNOPSIS...Nocera/Sipprell
NEAR TERM...Nocera/Sipprell
SHORT TERM...Nocera
LONG TERM...Sipprell
AVIATION...Nocera/Sipprell
MARINE...Nocera/Sipprell
TIDES/COASTAL FLOODING...Sipprell



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