Area forecast discussion
National Weather Service Taunton Massachusetts
1100 PM EDT Tuesday may 21 2013
Synopsis...
a backdoor cold front will stall across southern New England tonight before lifting
to the north Wednesday. Warm and humid conditions continue into
Thursday and Friday with showers and scattered strong thunderstorms
possible. Cooler and less humid conditions are expected for the
Holiday weekend as high pressure builds across the northeast.
&&
Near term /until 6 am Wednesday morning/...
1045 PM update...
Still monitoring convection moving into southwest New Hampshire and western
Massachusetts. This convection appears to be moving into a less favorable
environment for it to hold together...at least at the point where
there would be significant concern for severe weather. Will still
need to monitor these storms as there are still fairly potent. The
expectation is these storms will be elevated above the fairly
stable marine layer which has entrenched itself across eastern Massachusetts
and Rhode Island. Thus the main threats would be heavy rain and possibly
some small hail.
Have updated grids to bring near-term forecast current with
observed trends.
Previous discussion...
Most of the overnight period should be dry. The backdoor front
may make it to the CT valley later tonight before stalling. Expect
widespread low clouds and fog to redevelop again tonight...
especially in the coastal plain and a dense fog advisory may be
needed again. Min temperatures will be mostly in the 50s with lower 60s
in the lower CT valley.
&&
Short term /6 am Wednesday morning through 6 PM Wednesday/...
backdoor front will move/jump to the north on Wednesday as low level SW
flow mixes down to the surface. Low clouds and fog will erode as
SW flow becomes established...but the low clouds/fog may linger
most of the day over the islands and especially ack. 850 mb temperatures
15-16c so it will be another warm and humid day with temperatures well
into the 80s away from the S coast and possibly a few locations
nearing 90 in the interior valleys.
Convection will be a concern once again Wednesday afternoon as
atmosphere destabilizes. Highest MLCAPES 1000-2000 j/kg will be
focused mostly west of i95 as SW flow will act to stabilize the
coastal plain. Deep layer shear is a bit weaker Wednesday and middle level
lapse rates are marginal...but magnitude of instability suggests
scattered storms will develop and cant rule out a few strong to severe
storms in the interior.
&&
Long term /Wednesday night through Tuesday/...
highlights...
* showers and scattered thunderstorms Thursday into Friday
* cooler and less humid for the Holiday weekend into early next
week
Overview and model preferences...
Medium range operational model suite signaling an amplified...
slowly progressive pattern across the lower 48. At the start...
noting long wave trough across the upper plains into the western
lakes though some differences in positioning...along with 500 mb
cutoff low moving into the West Coast. Lower than average
confidence on the progression of the upper level pattern
operational suite...though general surface system positions and
timing signals through the Holiday weekend seem fairly good.
Better continuity amongst the model ensembles...so leaned closer
to those but did incorporate a little of the 12z GFS/00z European model (ecmwf)
into the early portion of the long term /Wed night to Fri/.
Anticipate slow but steady change to the upper pattern late this
week as upper trough moves across the eastern U.S. Fri-Sat. This
will bring airmass change as upper winds shift to northwest...bringing
cooler and drier conditions for a good chunk of the Holiday
weekend. Moderate to high confidence continues for the Thursday-Friday
timeframe with periodic precipitation and scattered convection...then
cold front should sweep through Friday night into early Sat. Moderate
confidence beyond Friday as may see some timing issues with the
passage of the upper trough sometime Sat. Few showers are
possible...but timing is difficult. Expect high temperatures this weekend
to run up to 5 degrees below seasonal norms. Does look to be dry
into early next week with close to seasonable temperatures.
Details...
Wednesday night...broad upper ridging starts to break down...
while stationary front remains across central and northern New
England. May see weak wave moving on this front...which could
bring scattered showers/thunderstorms across far interior areas of S New Hampshire/west Massachusetts/north
central CT. Kept chance probability of precipitation going there. With SW flow in
place...expect warm and humid conditions with lows mainly in the
Lower-Middle 60s...except the upper 50s to around 60 along the
immediate coast...Cape Cod and the islands.
Thursday...500 mb trough will begin to approach with its associated
surface cold front. Rather amplified upper system...so cold front
will be slow to move across.
Will remain well in the warm sector ahead of the front on
Thursday...so expect rounds of showers and scattered thunderstorms. Very humid
airmass in place as well...with precipitable water values around 1.7 inches. Will
likely see some heavy downpours with some showers...along with hail
and gusty winds. May not quite reach severe thresholds as clouds
will be more prevalent. Expect daytime highs only to reach the middle
and upper 70s...though a few spots may reach 80 if the sun breaks
through. Will also see gusty SW winds...up to 20-25 knots during the
afternoon/evening.
Timing of cold front is still in question...though appears it
should slowly cross the region Thursday night...may reach the coast by
12z Friday. Have carried likely to Cat probability of precipitation as cold front passes.
Looks like best instability may pass east overnight...but could still
see some elevated convection so kept scattered thunder in the forecast.
Temperatures do start dropping back to the Lower-Middle 50s late across far
interior areas.
Friday...cold front should clear the East Coast during the morning
but some showers may linger into middle morning across east Massachusetts.
Otherwise...expect conditions to improve though it will be
noticeably cooler and less humid.
Saturday through Tuesday...blocking upper pattern continues
across Labrador into Greenland through this timeframe...which will
keep high pressure building out of central Canada down the eastern
Seaboard. Long range models and ensembles are in pretty good
agreement with this portion of the forecast. Expect dry and cool
conditions through the Holiday weekend into early next week. On
Saturday...expect daytime highs only in the 60s...running 4 to 8
degrees below seasonal norms...though will modify a bit through
the remainder of this timeframe.
Due to severe weather watch this afternoon...kept previous forecast
going from Sunday Onward...but looks in good shape with latest model
guidance.
&&
Aviation /03z Wednesday through Sunday/...
forecaster confidence levels...
Low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.
Overview...moderate confidence in timing and areal coverage of
stratus and fog through Wednesday morning.
Overnight...expecting scattered convection to weaken through 22/06z.
Storms likely still strong enough to steer clear of their path
though. Will see widespread IFR-LIFR ceilings/visibilities mainly across
central and eastern areas with MVFR-IFR across the CT valley.
Wednesday...expect MVFR-LIFR conditions to gradually improving to
VFR 14-16z as SW winds develop...but IFR may persist most of the
day at ack.
Kbos terminal...high confidence in trends...lower confidence in
timing. IFR-LIFR ceilings/visibilities will redevelop...possibly as soon as
late afternoon.
Kbdl terminal...high confidence that conditions lower to IFR
tonight. Exact timing uncertain.
Outlook...Wednesday night through Sunday...
Wednesday night...low to moderate confidence. Expect variable
conditions depending upon shower and fog development. Better shot
for IFR visibilities in fog along S coast on S-SW winds and mild temperatures
mainly after midnight. Scattered showers/thunderstorms across most of the region
though best shot across central and western areas...with MVFR-IFR
cigs/vsbys. May see local heavy downpours. Low confidence on
timing and placement.
Thursday...low to moderate confidence. Very unsettled pattern
ahead of approaching cold front. Variable conditions between VFR
and MVFR/IFR in rain showers/tsra/patchy fog. Cold front crosses the
region Thursday night...so will see improving conditions across western
areas after midnight Thursday night...then into central sections after
06z-07z. May see SW wind gusting up to 20-25 knots during the day as
well. Winds shift to west-northwest across central and western areas after
midnight.
Friday...moderate to high confidence. May see leftover pockets of
MVFR ceilings/visibilities possible across east Massachusetts through 14z-15z Friday.
Otherwise...VFR.
Saturday-Sunday...high confidence. VFR.
&&
Marine...
forecaster confidence levels...
Low...less than 30 percent.
Moderate...30 to 60 percent.
High...greater than 60 percent.
High confidence through Wednesday.
Winds expected to remain below Small Craft Advisory criteria
through Wednesday. Increasing SW flow will result in seas building over
the S coastal waters. Leaned more heavily on the Swan guidance for
seas forecast as the wnawave guidance appears to be too high. Seas
could build to 5 feet over the S coastal waters on Wednesday and a Small
Craft Advisory may eventually be needed. Areas of dense fog will
affect the coastal waters tonight into Wednesday morning.
Outlook...Wednesday night through Sunday...
Wednesday night...high confidence. Expect SW winds gusting to 25
knots and seas up to 6-8 feet...highest on the southern waters. Visibilities
lowered in patchy fog especially across the southern waters.
Thursday...moderate to high confidence. Approaching cold front
will keep small craft criteria winds and seas in place. Expect low
visibilities in heavy showers...patchy fog. Scattered thunder as well. Winds
may shift to west-northwest across the western waters after midnight.
Friday...moderate to high confidence. Cold front will be slow to
clear the waters...so expect SW winds in place for a portion of
the day before shifting to west-northwest across the eastern waters.
Saturday-Sunday...moderate confidence. Seas will remain above
small craft for most of the weekend especially on the outer
waters. Northwest gusts may approach 25 knots.
&&
Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...none.
&&
$$
Synopsis...kjc/evt
near term...Belk/kjc/evt
short term...kjc
long term...evt
aviation...Belk/kjc/evt
marine...kjc/evt
