Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 290211
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1011 PM EDT Fri Apr 28 2017

.SYNOPSIS...
Another mild night tonight with a risk for showers and
thunderstorms for some in the morning. Warm conditions follow
once again for the daytime Saturday. After a cold frontal
passage Saturday night high pressure builds over New England
Sunday with dry but cooler air. Sprawling low pressure in the
Plains will push a warm front through our area Monday, then
swing a cold front through on Tuesday. Another storm moves up
from the Gulf of Mexico late Thursday or Friday.

&&

.NEAR TERM /THROUGH SATURDAY/...

1020 PM Update...

Overall trend in the forecast remains on track for tonight.
Areas of fog continue to impact the Cape and the Islands per
latest observations and webcams. Will hold off on any dense fog
advisory at this time, but something to watch over the coming
hours.

The focus then turns to the potential showers and thunderstorms
during the early morning hours. Still quite mild across the
region and with guidance showing elevated instability
increasing overnight, confidence is high that thunder will
occur. Main focus still appears south of the Pike, but still
cannot rule out a few rumbles of thunder across northern MA.
Will continue with current forecast and will evaluate the 00z
guidance at the next update.

Previous discussion...

Not confident enough to hoist an advisory given that the W
component of the flow is typically not conducive to marine fog
over land, but it will need to be watched. Otherwise, increasing
mid-upper clouds and high dwpts from the washed out front this
afternoon will limit overnight mins to the upper mid and upper
50s.

The one key to the forecast for the overnight and early tomorrow
is a burst of elevated instability associated with the remnant
cold pool from a developing MCS across the lower OH valley late
this evening. Noting an injection of moisture into the column
from the remnants of the MCS, and with steady lapse rates
leftover from this afternoon/evening near 6.0-6.5C/km, this will
yield MU/elevated CAPE profiles near 500j/kg between 09Z (5AM
local) and 15Z (11am local). This is also associated with a
modest LLJ around 40 kt yielding about 45-50 kt of 0-6km shear.
Therefore, the ingredients are available for a second round of
early AM elevated convection.

One caveat as is often the case with upstream MCS development is
steering of the convection. MCS is likely to turn somewhat S,
following the gradient of highest sfc based instability, but
given the increase in elevated stability further N (over SNE),
the threat risk remains. Could be a two part type of event, one
watching the actual remnants of the MCS relative to its cold
pool, and the second further N along a weak shortwave/LLJ. Given
these factors, have focused the highest POPS/chances for TSRA
across CT/RI and SE MA similar to this morning, however the risk
is definitely non-zero further N. Severe risk is low given the
bulk of the instability is elevated but the influence of the
LLJ does suggest a few gusty winds are possible if stronger TSRA
are observed.j

Otherwise, after the shortwave/instability shifts E in the late
morning, expect another gradual clearing with only diurnal SC
through the afternoon. Airmass similar to today suggests another
round of highs in the upper 70s to low 80s, warmest away from
the S coast. A bit more breezy however, as the lingering LLJ
could yield some wind gusts to around 25 mph.

&&

.SHORT TERM /SATURDAY NIGHT/...
More typical setup for spring overnight tomorrow. Lingering
clouds but more room for cooling as drier dwpts begin to filter
in through the afternoon and evening tomorrow. However,
expecting slow backdoor cold front progression during the late
night into Sun timeframe. Therefore, looking at mostly lows in
the upper 40s and low 50s. Which is closer to seasonable normals
than we have experienced with the moisture of the past few
days.

&&

.LONG TERM /SUNDAY THROUGH FRIDAY/...
Big Picture...

Longwave pattern shows ridges off each coast with a trough over the
Midwest. This pattern starts shifting east late next week.

Shortwave pattern shows two systems of interest in the long term.
The first is a closed low over the Southern Rockies during the
weekend that ejects through the Great Lakes Tuesday and is absorbed
into the Hudson Bay low thereafter. This draws a strong upper jet
across New England Monday night and Tuesday. The second system is a
shortwave south of the Aleutians this weekend that reaches the
Wash/BC coast Monday and digs a closed low over the Southern states
by Friday.

Model mass fields are similar through Wednesday then disagree on the
late week system...the GFS shows a coastal storm while the ECMWF
shows it tracking north through Central NY.  The GGEM shows a little
of both. Also large run-to-run changes. As the system is well out
over the North Pacific, we also note time and sampling issues. This
means at least moderate confidence in the early week system but low
confidence in the late week system.

Details...

Sunday...

High pressure builds over the region with a light east to southeast
wind.  Warm front stalled over the Mid Atlantic could combine with
the east flow to spread some clouds into the area, but airmass looks
otherwise dry. Expect coolest temps at the East MA shore, warmest in
the CT River Valley.

The high shifts offshore Sunday night and the warm front shifts
north. This should bring an increase in sky cover. Can/t rule out
scattered showers at this time from the onshore flow, but these
should be locally generated as the southwest low level jet will be
too far away to generate lift over the front.

Monday-Tuesday...

Low pressure in the Great Lakes lifts northeast, pulling the warm
front north through New England Monday or early Monday night.
Questions remain regarding timing of the warm front and associated
wind shift. Because of this we have nudged guidance temps about 2
degrees cooler. Some lingering showers especially north of the Mass
Pike. But expect most of this to move off to the north. Low level
southwest jet moves up the coast with 40 knot winds at 2000 feet,
helping generate gusts to 20 knots.

Cold front sweeps east from New York Monday night. Second low level
southwest jet moves in with 50-60 knot winds at 2000 feet. Upper jet
with 110 knots moves over New England after midnight and provides
upper venting as the front moves through and provides low level
convergence. Stability parameters look favorable for thunder with
forecasts of Totals 48-50 and LI 0 to -2.  Precip water forecast at
1.7 inches. Expect a period of showers and scattered thunder with
the front Monday night and early Tuesday. Locally heavy showers
possible.

Drier air moves in on Tuesday.  Some uncertainty as to how deep the
mixing will be behind the cold front, although model cross sections
show it as high as 800 mb. Temps in this layer will support max
surface temps to at least 70F and possibly to the mid 70s. Winds in
the layer reach as high as 30 knots, so a gusty afternoon in store.

Wednesday...

Surface low over the Maritimes and cloud shield into Northern New
England.  Upper shortwave lingers over Northern Maine along with
cold pool over Northern New England. This should generate some
diurnal clouds, especially north of the Mass Pike. Temps aloft
support max sfc temps in the 60s.

Thursday-Friday...

As noted above, long range models seem to agree that a storm will
affect the Northeast USA but disagree on how it will do so.  This
and the fact that this is a Day 6/7 forecast will limit forecast
pops to 30-40 pct. Model consensus on timing would bring rain into
our area Thursday mid to late afternoon, but we do note the ECMWF is
six hours faster than consensus. Best chance looks to be Thursday
night and Friday.

&&

.AVIATION /02Z SATURDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short term...through Saturday night.

Through 04Z (midnight local)...Moderate confidence.
VFR for all TAF sites except ACK where LIFR in fog expected.
This fog may expand into FMH and HYA...while becoming IFR these
sites will likely have better vsbys than ACK.

After 04Z through tomorrow...Moderate confidence.
As noted, IFR likely along or close to coastal RI and S coastal
MA in low CIGS/fog. Otherwise VFR mainly inland but with a risk
for some SHRA/TSRA. Best chance for these would be across
CT/RI/SE MA between 09Z and 14Z from W-E. Lower risk further N.
After these move through expect SCT-BKN low end VFR clouds
through the day. Winds shift from SW to W in the morning with a
few gusts around 20 kt during the afternoon.

Tomorrow night...High confidence.
W flow and mainly VFR.

KBOS TAF...Generally high confidence in TAF. If sea breeze
occurs this afternoon/evening it would be relatively short
lived.

KBDL TAF...High confidence in TAF.

Outlook /Sunday through Wednesday/... Moderate confidence.

Sunday... High confidence.

VFR.

Sunday night to Tuesday... Moderate confidence.

Ceilings lower to MVFR/IFR Sunday night and the start of Monday in
low clouds and fog/drizzle. Brief improvement to VFR possible
Monday, then a cold front moves through Monday night and early
Tuesday with showers and scattered thunder.  Low level wind shear
leading the cold front with southwest winds of 50-60 knots at 2000
feet AGL. The cold front moves offshore Tuesday morning followed by
clearing skies and southwest winds gusting 25-30 knots.

Wednesday... Moderate confidence.

VFR. West winds gusting to 20 knots.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Lingering offshore swell continues on the waters, holding at 5-6
ft. Therefore, offshore small craft advisories will continue
until this swell subsides a bit more. Otherwise, another round
of fog tonight especially across the southern waters and then
another risk for showers or a thunderstorm on the waters during
the early morning hours.

After the showers/storms and fog dissipate during the morning
hours tomorrow, W winds become a bit more breezy, especially
near shore with gusts to 20 kt, however feel these will remain
below small craft advisory thresholds throughout the day. There
is a low risk for slightly stronger W gusts during the overnight
hours along with seas reaching near 5 ft, but this is very
marginal and doesn`t warrant another advisory at this time.
Later shifts can re-evaluate.

Outlook /Sunday through Wednesday/... Moderate confidence.

Sunday...

East winds less than 20 knots and seas less than 5 feet.

Monday...

Warm front moves north of the waters late Monday. East winds turn
from the southeast and eventually from the south but remain less
than 25 knots.  Seas less than 5 feet most of Monday, then build 5-8
feet Monday night on the exposed southern waters. Winds 2000 feet
above the surface will increase to 50-60 knots, so potential for
gusts to 30 knots Monday night. Showers and possibly thunder Monday
night.  A Small Craft Advisory will probably be needed.

Tuesday...

Cold front moves across the waters early Tuesday. Winds will shift
from the southwest after the front moves through. Showers/thunder
end after the wind shift. Southwest winds may gust 25-30 knots
Tuesday. Seas remain 5 to 8 feet on the southern waters exposed to a
southwest flow.  A Small Craft Advisory will probably be needed.

Wednesday...

West winds will be 20 knots or less. Seas of 5-6 feet linger on the
southern outer waters.

&&

.TIDES/COASTAL FLOODING...
* Isolated minor coastal flooding impacts during astronomically
  high tide tonight along the east coast

The astronomical high tides remain very elevated into this
weekend.

Given a lingering swell offshore and its remnant surge, with high
tide values only a few tenths shy of typical minor flooding levels,
a Coastal Flood Statement continues for the high tide near 2 AM
tonight for east coastal locations. At this time, we anticipate
only minimal impacts at typically prone locations.

Also, due to the breadth of the fetch, we may encounter enough
wave action with lengthening periods on top of an elevated tide
to cause pockets of erosion tonight along the Martha`s Vineyard
south coast, Nantucket south and east coasts, and the outer
Cape ocean side from Truro to Chatham.

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

12.14 feet / Saturday 12:56 am

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 11 PM EDT this evening for ANZ250.
     Small Craft Advisory until 2 AM EDT Saturday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Doody
NEAR TERM...WTB/Doody/Dunten
SHORT TERM...Doody
LONG TERM...WTB
AVIATION...WTB/Doody
MARINE...WTB/Doody
TIDES/COASTAL FLOODING...staff


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