Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 221406
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1006 AM EDT Mon May 22 2017

.SYNOPSIS...
Cooler today. Some showers early this morning give way to a
lull from mid morning into at least early afternoon before more
widespread showers move in from the south later this afternoon
and evening. Mainly dry and seasonable weather is expected
Tuesday. A coastal wave of low pressure may bring a period of
rain to the south coast Tue night into early Wed. However by Wed
afternoon a brief drying trend develops behind the departing
wave along with pleasant temperatures. However the risk of more
rain returns Thu and Fri as low pressure approaches. The holiday
weekend begins with dry weather but there could be a period of
showers Sunday or Monday. Temperatures look to be near
seasonable through the period.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
10 am update...

Increased pops slightly for late am/midday based on radar
trends, to account for some lingering isolated/scattered showers
associated with lead short wave. Additional showers will be
possible later this afternoon/evening as a second short wave is
expected to cross our area, no changes in forecast for that
timeframe. Forecast temps look on track.

Early morning discussion follows...

==================================================================

Modest upper lvl shortwave linked to warm frontal movement,
increased PWATS around 1.25 inches and unorganized warm
advection is generating SHRA across the region this morning.
These will continue through about 8AM (12Z) until the wave moves
E of the region and allows for reinforced ridging and drying of
the lower lvls (which still have not completely recovered from
the low dwpts this past weekend).

Therefore, after this first round of wetting (although not
significant) rainfall, expecting a lull across the region
through the remainder of the morning hours as the synoptic
system reloads in favor of convection linked to the cold front
well S and a LLJ sliding up along S coastal locations by late
this afternoon and evening. Latest model runs support this,
showing much lower overall QPF totals, supported by a fairly dry
period in mesoscale model radar depictions through the day.

Therefore, have lowered POPs widespread until mid-late
afternoon, then raised them from SW-NE into the evening. In
fact, it now appears bulk of the rainfall will be focused across
SE MA/RI with lower totals further inland. This is along the
core of a modest LLJ around 30-40 kt which crosses the region
after 00Z. PWATs not overly impressive either, with highest
(1.5+) remaining well to our S, and more closely associated with
convection off the mid-Atlantic which is likely to usurp some of
the higher Gulf Moisture. Risk for TS is low, however noting
soundings supporting some modest conditional instability late
this evening/early overnight, so will continue a mention for an
isolated rumble of thunder.

Otherwise, although guidance shows upper lvl warm front lifting
across the region in advance of the weakened cold frontal
passage tonight, am concerned sfc warm front may struggle, as it
typically does through spring. Will be capping highs mainly in
the mid-upper 50s given that even with a break in showers, skies
should remain mostly cloudy through the day, with S to SE winds
carrying cooler onshore flow.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
Tonight...
Banded precipitation becomes more closely focused on a weakening
cold frontal passage through the late evening and early
overnight hours. Highest theta-e plume, along with modest
conditional instability becomes focused along the Sly LLJ mainly
across SE MA and extreme SE RI. Will be focusing POPS there.
Also final QPF totals above 0.5 inches (including any rainfall
today) will be focused on this area as well as precip is likely
to end across the interior late this evening.

Therefore, expect gradual improvement thanks to reinforced
subsidence from NW-SE through the overnight hours such that
precip should be ending even over the islands between 10Z and
12Z tomorrow morning. Some clearing inland is likely as well.
The one fly in the ointment here, will be that as winds shift to
the W-NW through the column and the subsidence inversion
develops, there is still plenty of low-lvl trapped moisture
especially areas E of the Worcester hills. This should leave,
even as rainfall ends. Fog and low clouds in its wake. May even
need to watch for some patchy dense fog in areas as precip ends.
Temps will be tricky thanks to the risk for clearing in the far
NW interior. Some cooling there could allow temps to drop back
into the 40s while areas further E remain in the mid to upper
50s with the low lvl moisture remaining in tact.

Tue...
Continued improvement through the morning thanks to reinforced
high pres building in from the NW. Most locations should see at
least a bit of sunshine through the day. The one caveat will be
S coastal locations as cold front will be settling to the S and
a secondary shortwave suggests the development of a coastal wave
off the mid-Atlantic. At the very least, this will reinforce
cloudiness across the S, limiting the sunshine and warming.
However, worst case scenario if the wave is strong enough it
will bring another round of light-mod rainfall mainly across the
immediate S coast/Cape/Islands by late afternoon. Latest trends
suggest a lower risk for this, as ECMWF, which was the most
robust has backed down somewhat on its QPF across the land.
Something to watch over the next 24 hours or so as mesoscale
guidance better captures this feature.

Otherwise, a much nicer day as H85 temps should remain averaging
+9C through the day. Inland areas that see the most sun will see
highs in the low-mid 70s. Coastal areas will be cooler due to
sea breezes, but as mentioned above, the increased risk for
clouds will limit S coastal locations even more.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Highlights...

* Dry and seasonable Wednesday Afternoon
* Showers likely Thu and Fri
* Holiday Weekend Likely begins Dry and Seasonable

Tuesday night...

Deterministic and ensemble guidance beginning to coverage on
next chance of rain Tue night into Wed morning across CT/RI and
southeast MA especially the immediate south coast and islands,
as next mid level wave approaches and spawns weak cyclogenesis
just south of New England. Remainder of the region especially
along and north of the MA pike likely remains dry. However these
small scale frontal waves can be tricky on the exact track of
its precip shield.

Wednesday...

Could be the pick of the week as any lingering rain early over
southeast MA scoots out to sea with good subsidence on backside of
departing wave. This should support at least partial sunshine during
the afternoon along with seasonable temps, mid 70s CT River Valley
to 60-65 across eastern MA coastline due to light onshore flow.

Thu & Fri...

Mid level trough and broad area of low pressure approach the region
from the southwest. Parent low over the eastern lakes gives way to
secondary low pres along the south coast of New England Fri. This
setup will support periods of rain regionwide & cool onshore winds
for eastern MA.

Holiday Weekend...

Rising heights and increasing anticyclonic flow will yield dry and
pleasant weather Saturday. Models then differ on timing of next
upstream trough with ensembles leaning toward Monday and GFS/EC more
toward Sat. Given the time range leaned toward the ensembles.
Temps looks seasonable with highs in the 70s.

&&

.AVIATION /14Z MONDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

10 am update...

Through today...Moderate confidence, mainly due to some
uncertainty in timing. Mainly VFR through midday today, with
isolated to scattered showers mainly affecting ORH/BED/BOS and
vicinity. Then, afternoon into evening CIGS gradually drop
through MVFR to IFR, along with more widespread SHRA/RA after
18Z. Winds remain mainly S, although with slight SE shift
possible. Minor LLWS possible mainly across the S coast late
this afternoon/evening.

Overnight...Moderate confidence.
IFR/MVFR with low CIGS/fog/SHRA mainly S and E of a line from
HFD-ORH-PSM, meanwhile some improvement is possible N and W of
this line especially during the early morning hours. Otherwise a
damp night. Some S-SW winds gust 20-25 kt mainly Cape/Islands.
Otherwise, winds shift to the NW by sunrise Tue.

Tue...Moderate confidence.
Improvement to VFR everywhere during the morning. Winds begin
out of the N-NW then back again to the S by late evening. Sea
breezes expected on coastlines. Will need to watch another weak
low pressure system south of the region which could maintain
wet weather and lower categories mainly for SE MA/RI terminals.
Right now will feature mainly an improvement all locations.

KBOS TAF...High confidence in TAF trends, lower confidence in
exact timing for the lower categories this afternoon which may
be off by a couple of hours. On and off light showers between 10
am and noon, with additional showers probable later this
afternoon and evening.

KBDL TAF...High confidence in TAF trends, lower confidence in
exact timing for the lower categories this afternoon which may
be off by a couple of hours. A few showers between 10 and 11 am may
lead to wet runways, but should be short lived. Additional
showers probable late this afternoon and evening.

Outlook /Tuesday Night through Friday/...

Moderate forecast confidence this period.

Tue night...VFR except MVFR possible south coast in risk of showers.

Wed...VFR except period of MVFR showers possible early over
southeast MA.

Thu & Fri...MVFR and/or IFR in periods of rain.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Tuesday/...

Late this morning into tonight...High confidence. Mainly quiet
to start although with isolated to scattered showers over the
waters. By afternoon, more widespread rainfall will combine
with stronger southerly winds reaching around 25 kt, which will
linger into the overnight hours. While seas should generally
remain at or below 5 ft, will be issuing a small craft advisory
for these winds beginning late this afternoon and continuing
until the winds shift to the W-NW early tomorrow morning. These
will mainly be fore the open waters, rather than the sheltered
bays/sounds. Fog/showers will reduce visibility mainly late this
afternoon through the overnight.

Tomorrow...High confidence.
Improvement across the waters expected. Winds and seas will
remain below small craft advisory thresholds. Winds will be
shifting from the W, back to the S by late in the day.

Outlook /Tuesday Night through Friday/...

Weak wave of low pres tracks just south of New England Tue night and
Wed morning. Then a stronger low may pass just south of the coast
Fri. Could be a brief burst of WNW winds up to 30 kt Fri night as
storm intensifies over Gulf of Maine Fri night.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 PM this evening to 4 AM EDT
     Tuesday for ANZ231>234.
     Small Craft Advisory from 2 PM this afternoon to 4 AM EDT
     Tuesday for ANZ235-237-256.
     Small Craft Advisory from 5 PM this afternoon to 4 AM EDT
     Tuesday for ANZ250.
     Small Craft Advisory from 5 PM this afternoon to 8 AM EDT
     Tuesday for ANZ254-255.

&&

$$
SYNOPSIS...Nocera/Doody
NEAR TERM...Nocera/Doody/NMB
SHORT TERM...Doody
LONG TERM...Nocera
AVIATION...Nocera/Doody/NMB
MARINE...Nocera/Doody/NMB



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