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Area forecast discussion 
National Weather Service Taunton Massachusetts
1046 PM EDT Tuesday Jun 30 2015

Synopsis...
a frontal system will bring a chance of strong to severe showers and
thunderstorms to the region early Wednesday through the afternoon.
The front stalls near the South Coast into Thursday before moving
offshore Thursday night. Weak high pressure will bring mainly dry
and seasonably warm weather Friday into early next week...but
there is a low risk for a few showers across the cape/islands
Thursday night into Friday night as weak waves of low pressure
track to the south.

&&

Near term /until 4 am Wednesday morning/...
1045 PM update...

Continued quiet weather across southern New England late this
evening. Regional radar data showing it will likely not stay that
way all night. Decent convection already across northeast PA into
New York state. That should be our air mass later tonight. Will need to
continue to monitor instability overnight. Middle level lapse rates
are about 6c/km..,indicating some elevated instability. Low level
lapse rates are poor. At this point...not as confident we will
see strong thunderstorms late tonight. The main risk would be
locally heavy rainfall.

Previous timing of precipitation still looks reasonable...but do
expect some tweaks will be made later tonight.

&&

Short term /4 am Wednesday morning through 6 PM Wednesday/...
a potent middle level shortwave and its associated cold pool aloft will
approach southern New England early Wednesday morning. This will
likely be enough to initiate some showers during the early morning
hours. While surface instability is lacking during this time...there
is some elevated instability which will provide for some scattered
elevated thunder within the broader area of showers. While there
is still some question with the exact timing of this
occurring...models do seem to be converging on the 09-12z time
frame.

Precipitable water values near 2.0 inches Wednesday morning into
early afternoon. Coupled with the strong low level jet...this is a
recipe for heavy rainfall within the showers and thunderstorms. A
quick inch of rain in less than an hour is not unreasonable. This
could result in poor drainage and urban flooding. The benefit of
the strong low level jet is storms should move along at a fairly
good speed. This will reduce the likelihood of flooding somewhat.
Main flooding threat will be with any training thunderstorms.

Later into the morning Wednesday...the threat turns more to severe
weather and potential heavy rain. As this shortwave approaches...the
upper level cold pool will incite steeper middle level lapse rates.
This increases the instability towards the surface to go along
with the elevated instability. This is evident in sbcapes forecast
over 1000 j/kg. A decent low level jet has provided plenty of
shear...0- 6 km values are 30-50kts and 0-1 km helicity values are
over 100 m2/s2.

The main question surrounding this forecast is whether the
instability increases before the shear diminishes in the afternoon.
If everything lines up...strong to severe thunderstorms with
damaging winds and large hail are likely. Depending on the timing
of the warm front moving northward through the region /expected to
be early Wednesday afternoon at this time/ and the helicity
values...there is also potential for an isolated tornado or two.

&&

Long term /Wednesday night through Tuesday/...
highlights...

* mainly dry and seasonably warm Thursday through early next week but
low risk of a few showers cape/islands Thursday night into Friday night

Overview...
extended period starts with rather amplified pattern for early July
with ridging along the West Coast and across the central Atlantic
northward to Greenland which results in trough from Great Lakes into new
eng lifting to the north by Friday. However...shortwave energy moving
through west Canada will serve to deamplify the pattern this weekend
with weak polar jet setting up across northern Continental U.S.. weak
shortwaves moving through the flow may bring a weak moisture starved
cold front on Sat. Then some ridging aloft may develop across the
NE early next week...but timing of next shortwave is uncertain.
Overall it looks mostly dry and seasonably warm this weekend into
early next week with heat and humidity remaining well to the south.

Details...

Wednesday night into Thursday...a few leftover showers or perhaps a thunderstorm
possible early Wednesday evening...otherwise a dry night expected. The
cold will be slowly crossing the region Wednesday night...stalling in vicinity of
of the S coast on Thursday as it becomes parallel to the upper level
flow. However...latest guidance indicates a rather dry column Thursday
with very low ki values and surface instability is about none. So
expecting a mainly dry day with pt-mosunny skies and seasonable
temperatures.

Thursday night into Friday night...there is still a level of uncertainty
for this period...mainly with respect to how far north moisture to
the south gets. Cold front moves to the S Thursday night and weak high
pressure settles into the region which should bring dry weather and
seasonably warm temperatures Friday. However...there will be a series of
weak waves riding along the boundary to the south which will have
to be watched. It is possible a few showers could flirt with the
immediate S coast and cape/islands Thursday night into Friday night. At
the very least...expect more cloud cover during this period. But
any north adjustment could bring a steadier rain to cape/islands
Thursday night into Friday.

Sat through Tuesday...
a weak front moves into the region sometime Sat into Sat night
associated with a northern stream shortwave to the north...but expect
dry conditions. Otherwise high pressure builds into the region behind
the front sun into early next week. There is uncertainty with timing
of next impulse which will determine whether any showers move in
next Tuesday. Temperatures moderate sun/Monday based on warming middle level temperatures.
Low confidence on temperatures next Tuesday.

&&

Aviation /03z Wednesday through Sunday/...
forecaster confidence levels...

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

Tonight...moderate confidence. VFR this evening...but IFR stratus
and patchy fog likely developing along the southeast New England coast
late tonight. Also...an area of showers and thunderstorms move into
western New England toward daybreak with conditions lowering to
MVFR. Confidence in the timing at a particular location is the
lowest confidence part.

Wednesday...high probability for widespread rain showers and scattered thunderstorms and rain.
Risk for a few strong storms and heavy rainfall. IFR and MVFR
conditions likely in shra/tsra. Conditions improving to VFR during
the afternoon...but scattered T-storms may redevelop.

Kbos terminal...high confidence in taf.

Kbdl terminal...high confidence in taf.

Outlook...Wednesday night through Sunday...

Wednesday night...moderate confidence. Patchy MVFR/IFR stratus and fog
developing...otherwise VFR.

Thursday...high confidence. VFR.

Friday through sun...moderate confidence. VFR. But still a bit of
uncertainty regarding a shower risk along the S coast and
cape/islands Thursday night into Friday night.

&&

Marine...
forecaster confidence levels...

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

Mainly quiet boating weather this afternoon and tonight. Winds and
seas will increase somewhat late tonight into Wednesday in response
to a frontal system approaching the waters. Small craft advisories
have been issued for the southern outer waters and Rhode Island and
Block Island sounds. These may need to be expanded as we move into
Thursday.

Outlook...Wednesday night through Sunday...

Wednesday night...prefrontal SW gusts to 20 knots with Small Craft Advisory seas over
the southern waters.

Thursday...light SW winds...but lingering southerly swell yielding Small Craft Advisory
seas over the southern waters.

Friday through sun...high pressure over the waters leads to a period of
mainly quiet boating weather with light winds and seas below Small Craft Advisory
thresholds.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from noon Wednesday to 8 am EDT Thursday
for anz254>256.
Small Craft Advisory from 8 PM Wednesday to 8 am EDT Thursday
for anz235-237.

&&

$$

Synopsis...kjc/rlg
near term...Belk
short term...rlg
long term...kjc
aviation...kjc/rlg
marine...kjc/rlg

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