Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wakefield Virginia
953 PM EDT Friday Jul 31 2015
high pressure builds into the area from the west tonight into
Saturday. A weak frontal boundary slides offshore late Saturday
into Saturday night...before high pressure returns to start next
Near term /until 6 am Saturday morning/...
more comfortable conditions this evening compared to 24 hours
ago...as dewpoints have settled into the 60s and precipitable waters have
dropped below 1 inch. This thanks to a cold front that has
stalled south of the region this evening and high pressure that
has centered over the Midwest. Aloft...GOES water vapor imagery
and radiosonde observation plots depict an upper level trough over the Great Lakes
region into the Ohio Valley. Height falls have produced little more
than some middle level clouds over the eastern Virginia Piedmont...as
bufr soundings depict modest middle level moisture under a strong
subsidence inversion. Regardless...still anticipate a clear to
mostly clear sky. Near normal lows expected tonight...generally in
the middle 60s inland to low 70s near the coast.
Short term /6 am Saturday morning through Monday/...
generally dry conds in the short term period. Despite the upper-
level trough over the eastern Continental U.S....moisture will be lacking and surface
hi pressure will be in the vicinity. A weak surface boundary/trough of low pressure
slides offshore late Sat into Sat night as the surface hi pressure center
builds in from the west. Expect temperatures to maximum out in the lower 90s...with
lows Sat night again in the middle 60s to lower 70s.
Similar temperatures into sun with the surface hi still over the area. Did
include a 20% chance of a thunderstorm but only over NE NC with some
seabreeze effects possible.
Hotter conds then for Monday as S/SW flow develops. Expect highs in
the middle 90s with heat indices peaking at or above 100 degrees. Probability of precipitation again
no higher than 20% for an afternoon/eveng thunderstorm.
Long term /Monday night through Friday/...
pattern begins with with a middle/upper level ridge centered over the
SW/southern Continental U.S. And an upper level low near georgian/Hudson Bay Canada.
Latest GFS/European model (ecmwf) are into improved agreement compared to
earlier runs with this overall setup through the middle of next
week. This translates into hot weather Tuesday/Wednesday with 800 mb temperatures rising
to 19-21 c across the area. Will forecast highs in the middle 90s over
much of the area...a little cooler (highs upper 80s to lower 90s)
closer to the coast due to some potential for afternoon winds shifting
more to the se/onshore. Also a little cooler over the far north as
surface front will not be that far north of the region and could lead to
more afternoon clouds and somewhat higher chances for precipitation. Dew points are
forecast by models to mix out into the upper 50s/low 60s west of I-
95 Tue/Wed...generally stayed a little higher than this...lower to middle 60s
west to lower 70s east. For now this yields heat indices peaking
around 100 f or a tad higher but remaining below 105 f advsy
thresholds. As for probability of precipitation Tue/Wed...models show very little in the way
of deep forcing/moisture but given fairly low 500 mb heights and hot low
levels will maintain at least a 20% pop both days in the
afternoon/evening timeframe (up to 30% Wednesday across the north).
By Thu/Fri...upper ridging amplifies across the western Continental U.S. Gradually
carving out a deeper trough over the eastern Continental U.S.. this should be
enough to eventually drive the cold front south into the middle
Atlantic/Carolinas Thursday into Friday (though hard to say if the front clears
the area on fri). Raised probability of precipitation to 40% north and 30% S Thursday afternoon/Thursday
night...with a 20-30% pop for Friday. Highs trend down into the
middle/upper 80s north to the lower 90s S on Thursday...and into the 80s all areas
Aviation /01z Saturday through Wednesday/...
a weak inverted trough over eastern portions of the Carolinas will
veer winds to southeast/S during the evening. A weak cold front moves
across the region Saturday morning. High pressure builds in from the
west during the remainder of the weekend.
With a mainly clear sky and relatively low dew points...will
maintain a VFR forecast. Will need to monitor due to some onshore
component to the winds. Some high clouds are forecast Saturday with
light west/SW winds.
Outlook...dry weather prevails through the weekend. A slight chance of thunderstorms
is in the forecast Sunday night through Tuesday...with best chances
confined to southeastern portions. Thunderstorm chances increase by
winds have shifted to the north/NE over the southern areas and northwest
farther north. Waves in the Bay average 1-2 feet and coastal seas average
around 3 feet/3-4 feet NC waters.
Winds turn back to the south-southeast this evening and veer to the south-southwest later
tonight. Fairly light flow/good boating conds this weekend with weak
pressure gradient conducive to afternoon seabreezes...light west winds in
the morning...turning to the east/southeast at around 10 knots in the late
aftn/evening. Waves average 1-2 feet in the Bay with seas 2-4 feet. Pressure
gradient increases Monday/Monday night and expect to see southerly flow
into the 15-20 knots range (possibly to marginal Small Craft Advisory conds) with waves
2-3 feet and seas 3-5 feet.