Area Forecast Discussion
Issued by NWS Boston, MA

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787
FXUS61 KBOX 240353
AFDBOX

Area Forecast Discussion
National Weather Service Taunton MA
1153 PM EDT MON MAY 23 2016

.SYNOPSIS...
An area of low pressure will wobble north along the coast into
this evening and through Tuesday bringing with it showers and the
chance of thunderstorms. Warmer weather follows beginning Wednesday.
A weak front will move south of the region Wednesday night with
dry, warm weather for Thursday. The front will move north toward
southern New England as a warm front on Friday bringing scattered
showers and thunderstorms. The front will continue to drift slowly
north over the Memorial Day weekend.

&&

.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...

1110 PM update...

Light showers rotating westward around cutoff H5 low pressure near
or just off the mid Atlc coast reaching the S coast of RI into N
central and NE CT at 03Z. Looks like most of the activity will
remain W of E MA and more of RI, but can not rule out widely sct
showers pretty much across the remainder of the region into the
early morning hours. Then, toward sunrise, will start to see sct
showers pretty much anywhere as weak short wave rotates westward
in the cyclonic upper flow to the south, as well as decreasing
temp/dewpoint spreads especially near the coast. With the onshore
wind, will see low clouds and patchy fog moving in. Would not be
surprised to see some spotty drizzle as well.

18Z GFS and new 00Z NAM both continue to signal increasing
instability, with total totals up to the lower 50s, surface based
LIs down to zero to -2 and K indices to the lower 30s around or
after 09Z. NAM appears too robust considering the lack of
convection into the region, so used a blend of the GFS/NAM with a
bit more weight to the GFS. Noting some CG and in cloud lightning
S of 40N late tonight, but appears this will rotate S of the
region for now. Have kept mention of thunder in the forecast
toward daybreak into Tuesday morning, though went with isolated
wording.

Remainder of forecast in pretty good shape, but have updated to
bring conditions current.

Previous discussion...

Low pressure wobbles N evolving into its dying phase undergoing
occlusion. During this morphology, focus is on the crux of mid-
level vortmax energy and parent weak low-level convergence
rounding W-NW round the low into S coastal New England. Should
see decent low to mid level forcing with some venting aloft, along
the leading edge of which a band of light to moderate rain should
emerge. Also isolated thunderstorms are possible given marginal
instability and weak shear parent to expected lift, but such
activity should be non severe with the main threats being
lightning and locally heavy rain.

A consensus of high-res and mos-guidance performed, confidence is
for a band of activity to pivot NW across S New England settling N
to S across W New England and along the high terrain. This keeps a
good chunk of E/SE MA in chance PoPs.

&&

.SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/...

Tuesday...

Breaking down. Low chokes itself off entraining drier air as it
stacks and undergoes occlusion within its dying phase. The trowaling
and comma shape to the low persists albeit weaker seemingly over W
portions of New England parent with low level convergent flow along
the E slopes of high terrain and forcing per rotating mid-level
vortex energy. But to the E we could see some improvement with
clouds becoming broken and perhaps some hints of sunshine. Thus can
not rule out partial clearing.

PoPs evolving with highest confidence over the Berkshires while
chance-scattered to the E/SE if forecast models are right in the
evolution of drier air wrapping into the system. A lot of wiggle
room and wobble with this system to which the high-res guidance is
not absolutely succinct but has a decent handle on the trend. Will
avoid high PoPs accordingly with the scattered to broken nature of
the low. Thunder does remain a possibility with continued marginal
instability and weak shear. Think isolated chances and will prevail
accordingly.

Cooler day with clouds in abundance and E/NE flow. Highs up into the
60s. An edge warmer to the W as conditions improve late in the day
with winds shifting out of the W/NW.

Tuesday Night...

The end. Low continues to undergo its dying phase choking upon drier
air as it peels off to the E. Comma head over the W sweeps E across
S New England per a consensus of high-res forecast trends, but by
this point the activity is all but scattered. Should see most of the
wet weather by consensus conclude by midnight behind which W/NW flow
prevails. Anticipated clearing and light winds, would expect lows to
drop into the 40s, and considering the recent rains, can not rule
out some dense fog development if conditions are right.

&&

.LONG TERM /WEDNESDAY THROUGH MONDAY/...
HIGHLIGHTS...

* Significantly warmer and dry Wednesday and Thursday
* Above average temperatures will continue Friday into Sat
* Hit or miss showers with thunderstorms are possible into the
  Holiday weekend.

OVERVIEW...

Large scale pattern is in agreement with both determinisic and
ensemble guidance. Upper level low will move towards the Maritimes
as Midwest ridge builds into the Northeast. This ridge will bring
summer-like warmth and humidity to the region as ensembles show both
the 700 and 500 heights nearly 1-2 standard deviations above normal.
Several waves will move through the flow keeping a chance of showers
and thunderstorms in the forecast for Late Thursday into Saturday.
Because of the mesoscale differences in the guidance trended the
forecast towards an overall blend.

DAILIES...

Wednesday into Thursday...Moderate Confidence.

Upper level low will push towards the Maritimes on Wednesday.
However, a quick moving shortwave will try to push through the
region during the day. This approaching frontal system will slow
down or stall as mid-level heights begin to build across the region.
Therefor, believe that an isolated shower across the higher terrain
is possible but most of the day appears to be dry. Good mixing
during the afternoon will help warm temperatures back to above
average with highs in the low to mid 80s. This may be the first
time that Boston Logan will hit 80F.

The cold front weakens and stalls over the region by Thursday as
ridging aloft continues. There is the potential for a spot
thunderstorm especially across the CT valley and points west where
the better instability will occur. So another mainly dry day is
anticipated on Thursday. Temperatures will be similar to that on
Wednesday with mixing up to 800mb. Will be cooler along the
coastlines as a sea breeze looks to develop during the afternoon.

Thursday night into Friday...Moderate Confidence.

Depending on where the front stalls, we could see scattered showers
and thunderstorms Thursday night into Friday. Guidance continues to
hint at the possibility of a decaying MCS moving from the Great
Lakes region towards southern New England. However latest trends is
that this will pass closer to NH than directly over southern New
England. Something to watch in the coming days.

Friday, it still remains unclear how far northward the front will
make it. If it stalls/remains over the region then showers and
thunderstorms may continue, keeping more clouds than sun which will
lower temperatures. Right now believe that the front will get
through a good portion of the area allowing for temps to reach into
the 80s with mid to upper 70s across the east coast.

Memorial Weekend...Moderate Confidence on Saturday. Low Confidence
on Sunday.

High pressure south of the region will begin to strengthen on
Saturday. This will continue to build the ridge to 2-3 standard
deviations above normal. Saturday looks quite balmy with 925 mb
temps warming close to 21-23C. This could result in highs in the mid
80s on Saturday, with a low probability of a 90F across the CT
valley. For now continue with a conservative route for temperatures.
These warm temperatures and lingering moisture may trigger off
afternoon convection on Saturday.

Low confidence on Sunday as EC wants to push a backdoor cold front
through the region where the GFS brings the front through on Monday.
Depending on the frontal timing, temperatures on Sunday could be in
the mid 80s with sun or the upper 60s with clouds and drizzle. Will
have to wait for the guidance to come in better agreement before
having a better idea on how the weekend will end for southern New
England.

&&

.AVIATION /03Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels.

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

Short Term /through Tuesday Night/...

Overnight...Moderate Confidence.

Mainly VFR through 04Z-06Z, except LIFR CIGS/MVFR-IFR VSBYS
moving onshore from Cape Cod to along E coastal Mass. Should see
MVFR- IFR CIGS/MVFR VSBYS move W to about KORH-KIJD with spotty
MVFR CIGS/VSBYS across the CT valley mainly after 06Z. Scattered
showers with MVFR conditions across N CT/W RI into CT valley.
Isolated thunder possible after 08Z mainly near S coast. Light
E-NE winds.

Tuesday...Moderate Confidence.

SHRA and chance TSRA continue with greatest confidence over W
terminals. Widespread MVFR/IFR impacts begin to shift E with the
usherance of winds backing out of the W/NW. Gusts of around 15 kts
possible.

Tuesday Night...Moderate confidence.

Continued clearing trend W to E with W/NW winds. Improving to VFR.
It is perhaps at the start of the period that MVFR/IFR will only
linger along the E coast of MA.

KBOS TAF...Sct showers will move across the terminal tonight and
into tomorrow. Low prob for some thunder. VFR will drop to
MVFR/IFR after 06z.

KBDL TAF...Confident the terminal will see a fair share of -RA
beginning around midnight and continuing through much of Tuesday
before clearing out late. Moderate prob for TSRA. MVFR conditions
after 06z.

Outlook /Wednesday through Saturday/...

Wednesday through Thursday...High Confidence. VFR.

Thursday night and Friday... Low to Moderate Confidence. Mainly
VFR...becoming MVFR. Showers and thunderstorms beginning late
Thursday night lasting through Friday.

Saturday...Moderate Confidence. Mainly VFR with some MVFR within
in scattered shower or thunderstorm.

&&

.MARINE...
Forecaster Confidence Levels.

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

Overnight and Tuesday...Moderate Confidence.

E-NE winds with gusts up to 15 kt mainly overnight, then
diminishing Tuesday. Seas 4 feet or less. Visibility reductions
in areas of fog moving W from Mass Bay and E of Cape Cod overnight,
local mist as well. Scattered showers along the S coast of RI,
then continue to develop across entire S coast, possibly reaching
to E coastal areas by daybreak. Isolated thunderstorms after 08Z
through mid morning Tuesday possible.

Tuesday Night...Moderate Confidence.

Winds back out of the W/NW with departure of the low. Change in
wind direction against a E/SE swell, could see some wave heights
up to 5 feet on the outer waters requiring small craft advisories.
Aside, rain comes to an end and visibilities improve if they had
not done so during the day Tuesday.

Outlook /Wednesday through Saturday/...Moderate confidence.

Wednesday...Seas subside into Wednesday afternoon. Light winds.

Wednesday night and Thursday...Light winds from the southwest
will shift to the northeast by Thu as a frontal system stalls
over the waters. Seas mainly below small craft criteria but cannot
rule out a few hours of 5 footers across the southeastern waters.

Thursday night and Friday...Southeast winds on the order of 10-15
kt and seas 1 to 3 feet. Just a slight chance of showers or a
thunderstorm.

Saturday...Southwest winds will prevail with seas less than 3
feet.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Dunten/Sipprell
NEAR TERM...Dunten/EVT
SHORT TERM...Sipprell
LONG TERM...Dunten
AVIATION...Dunten/Sipprell/EVT
MARINE...Dunten/Sipprell/EVT



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