Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
701 PM EDT Sat Sep 20 2014

Synopsis...
scattered showers and perhaps a thunderstorm or two are expected
very late tonight into Sun morning...with most areas dry and rather
mild by Sun afternoon. A cold front crosses the region by daybreak
Monday with high pressure bringing dry weather for the rest of next
week. Temperatures start off cool early next week...but moderate to above
normal by the end of the week.

&&

Near term /until midnight tonight/...
7 PM update...no significant changes to the forecast. Made some
adjustments to the sky cover forecast to account for the latest
trends. Otherwise...the forecast is on track.

This evening will feature dry weather with pleasant temperatures.
Most locations will see temperatures in the upper 50s and 60s this
evening...as surface winds will not decouple with a developing low
level jet.

&&

Short term /midnight tonight through Sunday night/...
overnight into Sunday afternoon...

A shortwave/low pressure system off the Carolina coast will be
lifting northeast overnight. This will induce a 30 to 40 knot
southerly low level jet...transporting deeper moisture northward
into southern New England. This will result in a burst of
instability and a band of scattered showers moving from west to
east. Timing looks to be after midnight into Sunday
morning...approximately between 6z and 15z. Instability parameters
are marginal...but there might be enough elevated cape for a few
embedded thunderstorms. This risk for severe weather is extremely
low Sunday morning. However...with 0 to 6 km shear of 25 to 30
knots/0 to 1 km helicity around 100...and a few hundred j/kg of
cape....can/T completely rule out an isolated strong storm near the
South Coast Sunday morning. Again though...that appears to be a
very low probability but something will need to watch.

The steady and heaviest rain will likely remain off the coast with
the surface low pressure system given limited
baroclinicity/frontogenesis. However...it still may scrape the far
southeast New England coast for a time where precipitable waters approach 2 inches
so brief heavy rain is possible.

In a nutshell...scattered showers and perhaps a few thunderstorms
are expected mainly Sunday morning. We are not expecting a soaking
rain by any means...but a period of heavy rain is possible near the
far southeast New England coast if the plume of tropical moisture
can make it that far northwest. Otherwise...most of Sunday
afternoon should be dry with just a lot of clouds around the
region...except perhaps the far southeast New England coast where
showers may linger a bit longer.

Low temperatures overnight will be much milder than last night with a
developing low level jet not allowing winds to decouple. Lows will
mainly be in the upper 50s and lower 60s by daybreak Sunday. Highs
on Sunday will be in the middle to upper 70s and it will feel a bit
humid with dewpoints well into the 60s!

Late Sunday afternoon and Sunday night...

A cold front will approach from the west late Sunday afternoon and
slowly make its way through the region by daybreak Monday. A lot of
middle level dry air and a lack of instability will limit precipitation
potential along the front. There still may be a few spot showers
and perhaps a thunderstorm across the interior late Sun afternoon
into the evening across the interior...but that is about it. Most
of Sunday night should be dry in a given location. Low temperatures will
mainly in the upper 50s and lower 60s by daybreak Monday.

&&

Long term /Monday through Saturday/...
headlines...

*low chance showers Monday. Then dry the rest of the week
*cool temperatures during the week...moderating by weekend

Overview...

The period starts off with good agreement on a trough draped over
the East Coast and a ridge over the West Coast. The ridge slowly
moves east as time GOES on and the upper level moves north into
Canada. Good agreement in the ensemble nao and ao teleconnection
forecasts to stay in the positive phases. The pna looks to start
positive and go neutral as time GOES on...but then models diverge in
their outputs as a weakness in the ridge allows for some spread.

Dailies...

Monday...the morning begins with a departing cold front that is
fighting with some dry midlevels allowing some of the precipitation to dry
up. Most of the precipitation should fall in northwestern portions
of the area and offshore. Behind the front...strong mixing occurs up
to around 850 mb where 20 to 25 knots winds exist so gusts up to 25 kts
is not out of the question. 850 mb temperatures ahead of the front are
around 10c so middle to upper 70s are not out of the question there.
Northwestern sections won't get that high as the front cross through
too early.

Tuesday through Friday...behind the cold front...a strong 1035 mb
high pressure system comes into the region with cooler
temperatures...but not as cold as the previous cold front. Dry
weather will also prevail during this period. Some model
disagreement as to the placement of the upper level jet approaching
the region from the south towards the end of the period. Think the
European model (ecmwf) may have it too far north while the 6z GFS has it further
south. The exact placement of this jet will give US when cirrus
clouds move north from moisture off of the southeast US. Winds will
be light during this period with the upper jet up north so better
chance for cooling temperatures at night especially with the recent
dry spell we/ve been having. In periods of onshore winds...fog could
occur.

Saturday...a cold front approaches the region from southeast Canada

&&

Aviation /00z Sunday through Thursday/...
forecaster confidence levels...

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

Short term /tonight through Sunday night/...

Tonight...moderate confidence. VFR conditions through 04z.
Increasing low level moisture should allow MVFR-IFR ceilings/visibilities to
over spread the region very late this evening and into the overnight
hours. Scattered showers and perhaps a few embedded thunderstorms
may work into the region after 06z.

Sunday...moderate confidence. Mainly IFR conditions expected in the
morning with scattered showers and perhaps an isolated thunderstorm
or two. Conditions should improve to MVFR and perhaps even VFR
during the afternoon across the interior. However...lower
ceilings/visibilities may hang tough though along the southeast New England
coast.

Sunday night...low to moderate confidence. Scattered low clouds and
fog patches are possible especially across eastern New England.

Kbos terminal...high confidence in taf through 06z. Moderate
confidence after 06z.

Kbdl terminal...moderate to high confidence in taf through 04z.
Moderate confidence after 04z.

Outlook...Monday through Thursday...

High confidence throughout the period

VFR through the period except for any showers on Monday in the
northwestern areas where MVFR could be needed. Winds around 10kts
during the day Monday with gusts to 20 or 25kts in places. These
winds die off on Tuesday and for the rest of the week.

&&

Marine...
forecaster confidence levels...

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

Short term /tonight through Sunday night/...

Tonight...moderate confidence. South winds of 10 to 15 knots with
high pressure well off the coast. There is a low probability that 5
foot seas could develop for a time across the outer-waters...but
decided to keep waves just below for now. Later shifts will have to
keep a close eye on this potential and perhaps the need for a short
fused Small Craft Advisory advisory.

Sunday and Sunday night...moderate confidence. Southeast winds at
10 to 15 knots on Sunday will shift to the southwest Sunday night
ahead of a cold front. However...swell from low pressure near the
benchmark will likely result in Small Craft Advisory seas across the outer-waters by
Sunday night. Since its mainly 3rd period...will defer the issuance
of headlines to the next shift. Scattered showers and perhaps a
thunderstorm may affect the waters this morning. There may also be
some visibility restrictions in patchy fog...especially during the
Sunday morning.

Outlook...Monday through Thursday...

High confidence...

Seas from 5 to 8ft on Monday and into Tuesday with the passage of a
low to the southeast of the region. Even with winds below Small Craft Advisory
thresholds...an Small Craft Advisory may be needed for this period for the swells.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...none.

&&

$$
Synopsis...Frank/99
near term...Frank/rlg
short term...Frank
long term...99
aviation...Frank/99
marine...Frank/99

National Weather Service Glossary of Abbreviations