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Area forecast discussion 
National Weather Service Taunton Massachusetts
406 am EDT Thursday Jul 30 2015

Synopsis...
warm and humid conditions continue today. An approaching cold
front will trigger showers and embedded thunderstorms this
afternoon and evening with locally heavy rainfall...especially
across western portions of CT and Massachusetts. Not as warm and less humid
weather arrives Friday and continues into the weekend and probably
into much of next week. There is a low risk for a few showers and
an isolated thunderstorm this weekend into early next
week...however much of the time dry weather will prevail.

&&

Near term /through tonight/...

4 am update...

Patchy fog and stratus continues to develop early this morning.
Believe that the gusty SW winds across the cape and islands has kept
fog from going dense. Otherwise forecast remains on track.

Today and tonight...

**strong to possible severe thunderstorms will develop late this
afternoon bringing gusty winds and localized street flooding**

Strong upper level low will continue to spin across Hudson Bay.
Associated shortwave will move through the flow pushing a cold
front across the eastern Great Lakes into New England late this
afternoon. This front will bring a round of showers and
thunderstorms for western mass and Connecticut. Father east appears
the chance for a wetting rain seems to be lower as forcing along the
front weakens. 00z guidance also shows a wave moving up from the middle-
Atlantic coast which could slow down the passage front across the
cape and islands tonight. Regardless appears that the front will move
into western mass around 18-21z and offshore by 06z.

Details:

Ahead of the passage of the front...southwesterly flow will pull in
moisture today bringing dewpoints into the low 70s. Cloud cover will
also be on the increase within the warm air advection pattern. With this amount of
moisture...could see scattered showers and isolated thunderstorms
break out across the region between 15-21z. Locally heavy rainfall
will be possible with this activity.

As the front approaches the region...believe more of a convective
line will develop across the Hudson Valley and move into western
mass. Strong to possible severe thunderstorms are possible as the
system approaches southern New England. Poor middle level lapse rates...5 c/km and cloud
cover will limit instability. However...the region should be able to
develop around 1000-1500 MLCAPE across the interior given the 70+
dewpoints. 0 to 6 km shear is rather marginal...but does increase
to 30 knots across our northwest zones towards 00z. So although
the instability/shear is marginal...we are closer to the better
forcing as heights will be decreasing aloft. Therefore cant rule
out the possible severe thunderstorm as the system passes through.

Biggest threat with these storms appears to be heavy downpours
leading to nuisance flooding as precipitable water values are about 2+ inches.
Gusty winds/localized downburst also cannot be rule out due to the
inverted-v soundings. There is also the potential for a quick
spin-up due to the tropical airmass over the region leading to
low LCL/S...but overall confidence is low.

As the front passes...the potential for strong to severe storms
drops off east of the Worcester Hills. This is due to the loss of
instability from sunset as well as lack of better forcing. Still
cant rule out thunderstorms during the overnight hours due to
elevated instability.

Otherwise temperatures will be warm once again today. However not as
warm as yesterday as 925mb temperatures are cooler. So temperatures will
generally be in the upper 80s with a few spot 90s. Best location to
see 90+ will be across the Merrimack valley...which may see more sun
versus the rest of the area. Finally southwesterly low level jet will be on
the increase this afternoon allowing for wind gusts to reach between
20-30 miles per hour along the South Coast.

&&

Short term /Friday/...
Friday...

Front will be offshore by Friday morning leading to a descent day.
Although temperatures will be warm...humidity will be lower than the
previous days. Abundant sunshine will lead to mixing so winds could
get a tad gusty in the afternoon.

&&

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

* pattern change to more seasonable temperatures and lower humidity

* low risk of a few showers showers/isolated thunder each day but
much of the time remaining dry

Synoptic overview and model preferences...

Anomalous Canadian vortex over Hudson and James bays this period
results in a mean middle level trough developing across the Great Lakes
into the northeast and will be accompanied by below normal heights.
Numerous short waves will rotate through this broad trough into New
England this time period and will result in a reinforcing surge of
drier air with each passing jet impulse. These short wave troughs
may yield a few showers/isolated T-storm with each frontal passage.
However given the lack of moisture and instability much of the
period will feature dry weather. Deterministic guidance is
displaying large timing differences with these short waves given the
fast/broad flow aloft. Thus low forecast confidence on timing of
convective threat. Given timing and amplitude differences a model
blend provides the most skill and will use this as a basis for this
forecast.

Daily sensible weather details...

Friday night...dry weather and comfortable humidity /dew points in the
u50s to l60s/ with Post frontal airmass over the region.

Saturday...low amplitude short wave trough moves across New York state
into northern New England. Moisture and instability is lacking and
this should limit areal coverage of showers/T-storms to widely
scattered. Thus most locations remain dry. Seasonably warm with
highs in the 80s and lows in the 60s.

Sunday...should be a mainly dry day along with seasonable temperatures
/h85 +15/ with highs in the 80s and lows in the 60s.

Monday...another potential short wave trough approaches the region
from the west. Thus low risk of scattered showers/T-storms this
period. Seasonably warm with 500 mb temperatures around +15c.

Tuesday...Canadian vortex digs southward into Quebec with lowering
heights and cooler temperatures aloft into New England. This should yield a
greater chance for convection. Warmer and more humid conditions
likely ahead of middle level trough and attending cold front.

Wednesday...its day 7 but good agreement among deterministic and
ensemble guidance that closed low will be over eastern Quebec with
mean trough axis across New England. 925 mb and 850 mb temperatures are
about -1 Standard deviation cooler than climatology. Thus could have a period
of slightly cooler than normal weather this period.

&&

Aviation /08z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through Thursday night/...

Tonight...high confidence. Mainly VFR conditions except across the
cape...islands and South Coast where fog has developed. Conditions
could drop down to IFR. Also need to watch the CT valley.

Before 18z...high confidence. VFR to start with lingering fog
along southern terminals. Scattered showers and isolated thunder out ahead of
approaching cold front could drop conditions to MVFR. Winds across
the South Coast could gust out of the SW around 25-30 kts.

After 18z and tonight...moderate confidence. Winds ahead of an
approaching cold front will gust between 20-30 kts...especially across
the South Coast. Cold front will move across the region from west
to east beginning around 18-21z. This convective line could bring
strong to severe storms across western mass before fizzling out
across the eastern half of the region. Anticipate any lingering
showers to be out of the region by 06z. Patchy fog could develop
across the normal prone regions overnight.

Friday...high confidence. Lingering fog and stratus will
dissipate. Expect VFR with diurnal cumulus and gusty winds around 15-20
kts.

Kbos terminal...high confidence.

Kbdl terminal...high confidence in taf. There is a probability
that a strong storm could impact the terminal this afternoon.

Outlook /Friday night through Monday/...

VFR much of the time with a low risk for scattered showers Sat and
again Monday. Greater risk for thunder Monday. Moderate forecast
confidence.

&&

Marine...
forecaster confidence levels...

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

Short term /tonight through Thursday night/...

Today and tonight...moderate to high confidence.

Developing low level jet will result in near shore southwest wind
gusts of 25 to 30 knots developing this afternoon. In
addition...long southwest fetch will allow for seas to build to
between 3 to 5 feet across most open waters. Small Craft Advisory headlines will
continue today and tonight. Winds diminish this evening...but
will need to keep headlines up through tonight for the outer-
waters from the left over swell. High pressure will build on
Friday so Small Craft Advisory may be allowed to drop.

Outlook /Friday night through Monday/...

Fairly tranquil boating weather this period with modest SW
winds...good visibility and mainly dry weather.

Low risk of scattered showers and T-storms Monday.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory from noon today to 7 PM EDT this evening
for anz230>234-236-251.
Small Craft Advisory from noon today to 11 PM EDT this evening
for anz235-237.
Small Craft Advisory from noon today to 8 am EDT Friday for
anz250-254>256.

&&

$$
Synopsis...nocera/dunten
near term...dunten
short term...dunten
long term...nocera
aviation...nocera/dunten
marine...nocera/dunten

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