Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Taunton Massachusetts
738 PM EDT Thursday Apr 17 2014

Synopsis...
unseasonably cool weather will prevail through Friday as high
pressure builds over the region. Low clouds and patchy drizzle
will affect coastal areas due to the onshore flow. A weakening front
will push across the region on Saturday. High pressure returns
into Monday with moderating temperatures. Low pressure and a cold
front will cross the region Tuesday into Wednesday with the chance
for showers. Dry and seasonable conditions are forecast into late
next week.

&&

Near term /until 6 am Friday morning/...
725 PM update...
low stratus clouds tending to break up with sunset...though some
still linger from S of Boston down to southeast Massachusetts at 23z.
Otherwise...noting high cirrus clouds streaming across the region
from the SW as seen on latest infrared satellite loop.

Can see some low clouds moving underneath the thin cirrus east of the
Outer Cape and Nantucket heading back to the west...so feel that
current forecast is on track to bring back low clouds and possibly
some pocket of drizzle overnight into early Friday morning across east
coastal areas into Rhode Island/north CT.

Updated near term grids to bring conditions current and
incorporated into overnight trends.

Previous discussion...
model cross sections show a rather deep layer of low level
moisture returning to coastal areas later tonight...so we
continued theme of low clouds and patchy drizzle overnight.
Farther inland...some of this moisture should also be prevalent
for a time overnight. Expect to see patchy low clouds centered
around daybreak Sat... but low levels should be dry enough to
prevent any fog/drizzle.

Temperatures will drop off quickly early tonight...especially
across interior...before clouds arrive late. Lows are a blend of
GFS/NAM MOS which yields 20s to lower 30s.

&&

Short term /6 am Friday morning through Friday night/...
high pressure builds over southern New England Friday. Low level
moisture should be easily trapped beneath subsidence inversion...
especially near coast per model cross sections. Expect early
morning clouds to scour out across interior...but hold firm closer
to coastline where marine layer should also be deep enough to
support patchy morning drizzle.

Continued east/NE winds will keep temperatures below average but a
little milder than today. Used blend of bias-corrected GFS/NAM MOS
since they have been running a little too cool which gives highs
in 40s coast to 50s interior. Could come close to 60 across CT
valley.

Flow turns more west/SW Friday night as high moves offshore. Models
maintain low level moisture early on near coast but erode it
overnight as middle/high level moisture streams into region ahead of
weakening cold front. Expect to have low clouds/patchy fog early
especially near Cape Cod and islands. This may thin out as
thicker middle/high clouds arrive but confidence is low. Should be a
bit milder with lows mainly in 30s.

&&

Long term /Saturday through Thursday/...
highlights...

* dry and seasonable through Monday
* showers forecast for Tuesday into Wednesday

Overview and model preferences/confidence...

Noting medium range models continue to signal a flat...split flow
surface and upper level regime across the lower 48 into early next
week...keeping fast moving weak systems across the northern tier
states. By around Sunday...models are signaling an 500 mb cutoff low
developing off the southeast u... could bring a change in the
overall upper level flow around the middle of next week. Some
questions as to the timing of this feature and when it will move
offshore. By the middle of next week...looks like a more amplified
long wave pattern develops as an 500 mb short wave digs out of central
Canada and the Great Lakes. This looks to arrive around the Tuesday-
Wednesday timeframe...which signals a period of unsettled weather for
the region. Another cutoff low also looks to develop across the
northern rockies late in the period...which may lead to a slowing
of the progression of systems.

Used a blend of the ensemble means and previous forecast...all of
which are in good agreement for most of this forecast period.

Details...

Saturday...
expect a dry...weak front to cross the region during the day.
Went with a dry forecast...as there is little if any moisture
associated with this feature embedded in the dry westerly upper
flow. Winds will shift to northwest during the afternoon but...with a
light pressure gradient...not expect them to become too gusty. Highs
will range from the middle 50s to lower 60s.

With mostly clear skies and light north winds...expect overnight lows
will drop to the upper 20s across SW New Hampshire into north central and west Massachusetts...
possibly as far S as the Springfield area...ranging to the upper 30s
to around 40 along the immediate S coast. These colder interior
readings will run around 5 degrees below seasonal normals.

Sunday and Monday...
continued high confidence for dry and seasonable conditions
across the region as high pressure becomes centered across New
England. Will be somewhat cooler along east coastal areas as the
orientation of the north-S ridge axis will keep an east-NE wind flow in
place. It will remain rather chilly along the coast on Sunday with
highs only in the Lower- Middle 50s...even some upper 40s on the
Outer Cape and Nantucket...but will moderate somewhat by Monday as
the ridge Narrows as it settles S. Highs Monday will remain cooler
across the Outer Cape and Nantucket to the immediate S
coast...topping off in the Lower-Middle 50s...but ranging to the 60s
elsewhere. Some high clouds may begin to filter in later Sun night
into Monday across central and northern areas.

Monday night through Wednesday...
models continue to signal this to be an unsettled period as broad
low pressure and its associated cold front work out of the Great
Lakes. Some question as to the arrival of precipitation...looks to be a
bit slower than previously forecast. May see scattered showers move into
S New Hampshire/northwest Massachusetts well after midnight Monday night.

Best shot of precipitation will be the Tuesday into Wednesday morning
timeframe as the front tends to slow down as it moves across the
region...in response to cutoff low pressure trying to exit off the southeast
U.S. Coast. Might see best quantitative precipitation forecast values into the Tuesday night timeframe
as models are trying to signal a weak wave moving along the
front... possibly bringing a decent shot of precipitation into central
and eastern areas. Some disparity as to where this will occur.
Kept probability of precipitation on high end chance category...but feel highest will be
mainly across central and eastern Massachusetts into Rhode Island for now.

Expect highs on Tuesday to be mainly in the 60s...except 50s
along immediate S coast...but only in the 50s to lower 60s during
Wednesday with cloud cover and precipitation. Current timing suggests back edge
of precipitation should start to shift into western areas during Wednesday
afternoon.

Wednesday night-Thursday...
some timing issues on the operational model runs...but looks to
be drying out from west-east early Wednesday night...then high pressure builds in
out of central Canada for Thursday. Somewhat lower confidence
during this timeframe due to timing.

&&

Aviation /00z Friday through Tuesday/...
low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.

High confidence through Friday...moderate confidence Friday night.

Through this evening...mainly VFR ceilings across the region with
isolated MVFR ceilings /around 3kft/ across east and southeast Massachusetts at 23z. These
will break up briefly by 02z. Expect low clouds to return around
or after 04z.

Overnight...continued NE flow should bring lower MVFR ceilings into
much of region tonight...especially near coast. Areas of IFR ceilings
across Cape Cod and islands as well as higher terrain inland
overnight. Patchy drizzle also possible but mainly near coast.

Friday and Friday night...ceilings slowly lift Friday but coastal areas should
remain MVFR for much of day. Lower confidence Friday night as west/SW
flow should erode lower cloud cover...but MVFR/IFR ceilings could once
again impact coastal sites.

Kbos terminal...high confidence. Period of MVFR ceilings /035/ south
of Airport this afternoon.

Kbdl terminal...high confidence.

Outlook...Saturday through Tuesday...

Saturday through Monday...high confidence. VFR.

Monday night and Tuesday...moderate confidence. VFR to start. May
see local MVFR in scattered showers moving into S New Hampshire and central-west Massachusetts
after midnight. Will see areas of low end MVFR ceilings/visibilities through
Tuesday as front slowly crosses the region.

&&

Marine...
forecaster confidence levels...

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

Short term /through Friday night/...high confidence.

NE winds will diminish through this evening. Seas will take their
time to subside however given persistent NE flow through Friday.
Extended Small Craft Advisory primarily for seas mainly on outer waters. This will
also result in low clouds/patchy drizzle on waters later tonight
into Friday morning.

Should see more in way of fog development Friday night...especially
on S coastal waters as flow becomes more west/SW but is light. Seas
should remain above 5 feet on outer waters.

Outlook...Saturday through Tuesday...

Saturday...expect west-northwest winds at 15 knots or less. Seas remain at or above 5
feet mainly over the open waters.

Sunday...NE winds return across the waters as high pressure builds to
the north. Gusts up to 20 knots early. Seas linger around 5 feet on the
eastern outer waters...though may subside Sun night.

Monday and Tuesday...expect winds to shift from southeast to S-SW but
remain light...generally 15 knots or less through this timeframe. May
see some gusts up to 20 knots later Tuesday. Seas below small craft
criteria.

&&

Hydrology...
minor flooding continues along portions of the main Stem CT River.
Crest has reached Northampton and is forecast to reach
Thompsonville this afternoon and Hartford tonight. Moderate
flooding is forecast on lower reaches near Middle Haddam.

Flood warnings remain posted along CT river at...

Montague
Northampton
Thompsonville
Hartford
Middle Haddam

The stages on the Pawcatuck River were nearing crest. The stage
at westerly appears to be cresting late this afternoon...while
close to cresting at Wood River Junction.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory until 8 PM EDT this evening for
anz231>234.
Small Craft Advisory until 6 am EDT Friday for anz235-237-251.
Small Craft Advisory until 6 am EDT Saturday for anz250-254-
255.
Small Craft Advisory until 2 PM EDT Friday for anz256.

&&

$$
Synopsis...evt/jwd
near term...evt/jwd
short term...jwd
long term...evt
aviation...evt/jwd
marine...evt/jwd
hydrology...staff