NWS Technical Forecast Discussion


000
FXUS61 KBOX 281735
AFDBOX

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

.SYNOPSIS...
A mild start this morning with some more patchy dense fog will
lead to a mix of showers and isolated thunderstorms. Warmer
highs today will give way to another mild night. A cold front
crosses the region Saturday night, bringing much cooler weather
for Sunday, especially along the coast. Another cold front will
likely bring a period of showers sometime Monday into Tuesday.
Another storm may approach New England late Thursday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
10AM update...
Area of convection continues to move across the Islands this
morning along the nose of an LLJ within an area of slightly
higher elevated instability. Both the LLJ and the unstable
airmass will continue to shift E through late morning.

Otherwise, already noticing clearing in the SC across interior
SNE which, given the timing should still allow for plenty of
warming as the downsloping W flow develops. Still looking at
highs in the mid 70s to low 80s, coolest along the S coast
thanks to a slight onshore component to the flow.

Not as concerned about afternoon convection in spite of the
front slowing to a stall across S portions of the forecast area,
mainly due to drier air entraining through the column and
capping the airmass. However, will need to watch for continued
cloud cover along the S coast S of the front, as it may struggle
to erode through the afternoon.

Previous discussion...

Forecast remains on track for this morning. Approaching frontal
system has resulted in an a surge of moisture thanks to southerly
winds. This has resulted in a advection fog developing and moving
onshore. Went ahead and expanded the dense fog advisory into RI as
webcams show vsbys dropping from fog. Also expanded the timing, to
get through the morning commute.

Aside from the patchy dense fog, we continue to watch showers out
west from approaching cold front. These showers are quite fragment
due to the dry air aloft from weak mid-level ridge. However as they
continue to move eastward, feel they will tap more into the low
level moisture resulting in better coverage over the next few hours.
Have noticed a few stronger showers with embedded thunder off the
coast of NJ and south of Long Island. Latest SPC meso analysis shows
some elevated instability across the area as well as steepening mid-
level lapse rates. This well help support any thunder chances across
the area this morning. Therefore will continue the mention of
thunder.

Showers and embedded thunder will move from west to east early this
morning and should be offshore by the early afternoon hours.

This afternoon...

The front will swing through and stall/washout across the south
coast by the afternoon. Anticipate clearing and the return of
sunshine for today. Despite being behind the front, anticipate a
mild day with highs in the in the upper 70s with cooler conditions
along the south coast.

Model guidance is hinting at some surface CAPE, good shear and steep
lapse rates during the afternoon hours. However, subsidence behind
the morning shortwave and very dry mid levels of the atmosphere
should prevent convection from developing. As prev forecaster noted,
still have the front lingering near by, so if there is any moisture
we cant rule out the potential for an iso thunder. Something to keep
an eye on later today.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Dry air aloft as mid-level ridge moves over the region.
Continuous southerly flow however will limit overnight temps
from cooling. In fact, anticipate another mild night as today`s
frontal system is draped across the south coast. Dewpoints will
also be on the rise, so cannot rule out another round of patchy
fog, with the south coast begin the best spot for this to occur.


For the second half of the night, moisture aloft will begin to
increase in the mid-levels as an upper level shortwave and
associated system approaches from the west. Guidance has speed
up the timing, bringing showers through the area by dawn on
Saturday morning. Am concern that the NAM appears to indicate a
weaken MCS with the Hi-res guidance also showing a few stronger
showers. This is plausible as guidance continues to indicated
elevated instability overnight and the stalled front could be
the focus for come convection as the wave pushes through.

An area of high pressure to the north will slowly extend
eastward during the later half of the day. This will push a low
level boundary across the region during the afternoon. Will
keep a low chance of showers and possibly a thunderstorm
Saturday afternoon with some lingering instability, if the
atmosphere can recover from morning convection. Temperatures on
Saturday will be mild with highs in the mid to upper 70s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Overview...

28/00Z guidance is in reasonable agreement with the overall
evolution of the synoptic pattern through Wednesday. Details
become more problematic during the second half of next week.
Monitoring two primary periods of precipitation: one early next
week, and the other later next week. Will continue to favor a
consensus blend to smooth over the less predictable details.

Details...

Saturday night through Sunday...

High pressure noses into our region from southeast Canada. This
will push a cont front to our south, and bring drier weather
with near normal temperatures.

Sunday night...

A warm front start to approach our region. Scattered showers are
possible north of this front.

Monday-Tuesday...

This is one of the more likely periods for precipitation. A low
pressure moves through the Great Lakes, driving a warm front
through southern New England. Some showers, and perhaps a few
thunderstorms, are expected. A strong southwest low level jet
should maximize moisture transport Monday night, resulting in
periods of moderate to briefly heavy rainfall. Still need to
refine the timing details.

Wednesday-Thursday...

This is probably going to be mainly a lull between two larger
systems. Once the Monday/Tuesday system departs, looking at only
a couple of weakening boundaries to contend with. A shortwave
aloft may provide just enough lift to generate some showers
into Thursday morning. After that time, expecting an increasing
risk for showers ahead of another approaching low Thursday
evening and night. This timing remains low confidence, and will
need to be monitored for later forecast changes.

&&

.AVIATION /17Z FRIDAY THROUGH TUESDAY/...
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 now through this evening for all TAF sites except ACK, which
may linger in the low CIGS for a bit longer but transitioning to
a risk for some MVFR/IFR CIGS with SHRA/TSRA (see below). Terminals
from WST east the Cape/Islands may once again see a period of
late evening low CIGS and LIFR conditions, most likely on
ACK/MVY. Southerly sea breezes likely this afternoon, lower risk
for E coast.

After 04Z through tomorrow...Moderate confidence.
Still have to watch LIFR/IFR conditions mainly 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 /Saturday Night through Tuesday/...Moderate confidence.

Saturday night and Sunday...High confidence. VFR.

Sunday night to Tuesday...Moderate confidence.

MVFR/IFR cigs/vsbys in low clouds/drizzle and light showers Sunday
night. VFR and areas of MVFR in showers Monday and Tuesday.
Possible low level wind shear especially over RI and SE Mass
Monday night and Tuesday morning.

&&

.MARINE...
Forecaster Confidence Levels...

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

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

Small Craft Advisories will remain in effect mainly over the
offshore waters into tonight as a lingering 5-7 ft swell
remains. Winds shifting to the west during the day will back to
the SW by tonight and should remain below the 25 kt threshold
through the period.

The risk for a few thunderstorm offshore this morning continues
with another risk tomorrow morning.

Small Craft Advisories may be expanded into Saturday, but has
low confidence on wave forecast at this time. Could also see
25 kt gusts across the near shore waters.

Outlook /Saturday Night through Tuesday/...Moderate confidence.

Winds expected to remain less than 20 kt through Monday.
Southwest winds increasing ahead of a cold front Tuesday may
result in gusts up to 30 kt Monday night into Tuesday, mainly
across the southern coastal waters.

Seas should remain less than 5 ft into Monday morning, then
build across the outer coastal waters due to the increasing
southwest flow Monday afternoon into Tuesday.

&&

.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 8 PM EDT this evening for ANZ235-
     237.
     Small Craft Advisory until 11 PM EDT this evening for ANZ250.
     Small Craft Advisory until 2 AM EDT Saturday for ANZ254>256.

&&

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

NWS BOX Office Area Forecast Discussion



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