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Area forecast discussion
National Weather Service Taunton Massachusetts
715 am EDT Thu Jul 20 2017

Synopsis...

Hot and humid weather persists today. A strong thunderstorms
with gusty winds are possible mainly south of the Massachusetts Turnpike
tonight. Dry and very warm afternoons expected Friday and
Saturday, but with lower humidity. Unsettled weather with
showers at times are expected Sunday through Tuesday with below
normal temperatures.

&&

Near term /until 6 PM this evening/...

710 am update...

Skies already mostly sunny except for lingering dense fog across
Nantucket, which should burn off over the next couple hours.

A very mild start early this morning will allow temperatures
to warm into the 90 to 95 degree range away from any localized
marine influence with 850t near +18c. Some dry air will push
into the region, which will help increase mixing potential. A
few sites may see westerly winds increase to 15-20 miles per hour. These
winds will allow for mixing especially across the western half
of the region. Thus have lowered dewpoints in that region.
However across Rhode Island and southeast MA, 70+ dews will pool during the
afternoon. This combined with the warm temperatures will push
heat indicies values to 95f. Went ahead an issued a heat
advisory for regions that have the best shot for 95f heat
indicies and that observed them yesterday.

Aside from the heat and humidity, falling heights through the day
may trigger a few isolated showers and/or thunderstorms. Best region
for this to occur will be across the Massachusetts east coastline where sea
breeze boundary will the area of focus. A strong storm cannot be
rule out as cape values will reach above 1000 j/kg and increasing
shear values. Gusty winds and heavy rain will be the main threat.

&&

Short term /6 PM this evening through 6 am Friday/...

Tonight...

Low confidence forecast for tonight as there is still some
model spread on the convection potential tonight. Models have
trended farther south with a potential cluster of showers and
thunderstorms propagating from the Great Lakes region. With an
upper level ridge firmly entrenched across the central conus,
thinking is that convection over the western Great Lakes region
will track more southeast than eastward, following the better
instability and flow.

Biggest question is where this flow will set-up. The latest ec and
UKMET continue to bring the cluster through the region, while the hi-
res guidance and NAM have trended south of the Pike. Will trend the
forecast towards the later as the bulk of the instability axis is
south as well as the nose of southerly low level jet and the flow of the
corfidi vectors. 850 mb Theta-E gradient also has trended a bit
further south than 24 hours ago.

Best timing for the cluster of thunderstorms to move through the
region will be after 7pm and exit around 2 or 3am. Modest lapse
rates above 6 c/km combined with 40-50 kts of 0-6 km bulk shear
values and falling heights ahead of the complex will lead to a
few strong to severe storms. Greatest risk will be gusty winds
and heavy rainfall as precipitable water are well above 1.5 inches. Agree with
Storm Prediction Center on the marginal risk as the storms could be strong to
perhaps severe. Again greatest potential will be south of the
Pike.

Will need to continue to monitor trends for the complex, especially
this afternoon once the storms eventually develop. This will help
determine southern New England actual risk for this evening and early tonight.

&&

Long term /Friday through Wednesday/...

Highlights...

* Fri & Sat: dry and very warm afternoons but with lower humidity
* sun into Tue: cooler than normal + unsettled with showers at times
* drying trend likely by the middle of next week

Details...

Friday and Saturday...

Anomalous closed upper level low across eastern Canada will result
in northwest flow aloft into southern New England Fri & Sat. The
result will be dry weather into at least Saturday afternoon. Plenty
of sunshine should push high temps well into the 80s on both days,
and a few locations may reach 90 on Friday. However, drier air will
mix down resulting in lower humidity than what we have experienced
the last few days.

Sunday through Tuesday...

The anomalous closed low across the Canadian Maritimes will help to
carve out an upper level trough across the northeast. The result
will be temps averaging below normal. Specific daily high temps are
uncertain and will depend on timing, cloud cover, and location of
any surface boundaries. Highs will mainly be in the 70s over this
time frame, to perhaps a few lower 80s. It is also possible that a
some locations have a day where it struggles to break 70 as a result
of a surface boundary to the south coupled with northeast winds/low
clouds.

Plenty of shortwave energy rotating around the upper trough will
result in periods of showers and perhaps isolated thunderstorms at
times. It does look rather unsettled, but the entire period will not
be a washout it is just impossible to try to pin down timing this
far out.

Wednesday...

Forecast highly uncertain in this time range, but guidance shows
upper level trough finally lifting out which would result in dry and
pleasant weather.

&&

Aviation /11z Thursday through Monday/...
forecaster confidence levels...

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

Short term /through tonight/...high confidence.

Today...high confidence. VFR across the region early this
morning, except dense fog which should burn off across
Nantucket over the next hour or two. Shallow sea breezes may
the focus for a isolated showers/-tsra during the afternoon.

Tonight...moderate confidence. Chance rain showers/thunderstorms and rain mainly south of
Massachusetts Pike. A few storms may be strong and contain gusty winds.
Majority of impacts evening and towards midnight, clearing out
during the early morning hours with SW winds becoming west. Great
risk is terminals south of the Pike.

Kbos terminal...high confidence in VFR conditions. Greater
uncertainty as to whether or not a sea breeze develops this
afternoon, but based on mesoscale models feel it probably
occurs. A spot rain showers/-tsra may also be in the vicinity of the
terminal this afternoon.

Kbdl terminal...

Outlook /Friday through Monday/...

Friday and Saturday...high confidence in VFR conditions.

Sunday and Monday...moderate confidence. MVFR to IFR conditions
likely at times in periods of showers, low clouds and fog patches.
VFR conditions will also likely occur, but timing of all this is
uncertain.

&&

Marine...
forecaster confidence levels...

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

Short term /through tonight/...

Today and tonight...high confidence.

SW flow becoming west towards the morning before returning SW.
During daytime periods and along the S/southeast near shore, gusts up
to 20 kts are expected.

Possibility of showers and thunderstorms sweeping the waters
tonight. Southern waters have the greatest chance for this to
occur. Seas remaining below 4 feet.

Outlook /Friday through Monday/...

Friday and Saturday...high confidence. Pleasant boating weather as
weak pressure gradient will keep winds/seas below small craft
thresholds.

Sunday and Monday...moderate to high confidence. East to northeast
winds may gust between 15 and 20 knots at times, but probably remain
below small craft thresholds for the most part. Areas of fog and
showers may reduce visibilities for mariners at times.

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...heat advisory from 1 PM this afternoon to 6 PM EDT this
evening for maz017>021.
Rhode Island...air quality alert from 11 am this morning to 11 PM EDT this
evening for riz001>008.
Heat advisory from 1 PM this afternoon to 6 PM EDT this
evening for riz001>004-006.
Marine...none.

&&

$$
Synopsis...dunten/Frank
near term...Frank/dunten

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