Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
624 am EDT Wednesday Jul 29 2015
high pressure will slowly weaken through middle week. A Piedmont
trough will form this afternoon and linger on Thursday as weak
low pressure remains off the southeast coast. A cold front will
move into the area Friday and stall during the weekend before
Near term /through tonight/...
as of 538 am Wednesday...distant radar returns over the sea
reveal a weak low level circulation with bands of showers spinning
off and emanating from a center position around 33.08n and 75.88w
or about 140nm east-southeast of Cape Fear. Steering winds surrounding and
above this cyclonic swirl are weak...and this feature is apt
remain quasi-stationary...keeping convection mainly beyond our 20
nm waters. No significant changes to the forecast are needed with
the daybreak update.
Vapor animations show a dry intrusion aloft pushing southward
across the middle-Atlantic into the Carolinas early this morning.
Time-height analysis of relative humidity and wind trajectories show this dry air
embedded between 500 mb-300 mb in a 25-35 knots north wind flow. This
advection will cap local chances of precipitation for most areas
today. The exception area resides mainly west of I-95 where the
strongest low-level heating will occur...and closest in proximity
to dying disturbances aloft approaching from the north-northwest but even this
is uncertain. Maximums today 91-94 inland...around 90 coastal
interior and upper 80s beaches and icw corridor. Diurnal cooling
will kill what few showers crop up later this afternoon in the
early evening with a dry layer aloft covering the area. Minimum
temperatures at daybreak Thursday low to middle 70s inland and upper
Short term /Thursday through Friday night/...
as of 3 am Wednesday...precipitation chances will be on the increase
during the period as 500 mb ridge breaks down and 500 mb trough starts to
set up. Ill defined surface pattern on Thursday will allow sea breeze
and Piedmont trough convection but lack of significant surface or
middle level triggers will keep coverage limited. Precipitation chances
start to increase Thursday night as weakening cold front moves in from
the northwest. Front arrives as instability is waning which should
keep coverage for Thursday night in slight chance range at most despite
the presence of low level convergence and deep moisture.
Front remains in the area Friday...serving as a Focal Point for
convection. Diurnal instability will have plenty of deep moisture to
work with...precipitable waters exceed 2 inches later in the day.
Weakening subsidence and instability along with what appears to be
some slight positive vorticity advection Friday afternoon increases precipitation chances Friday into Friday
night. Will continue with inherited likely pop but wonder if middle
level ridging might hold on a bit longer than guidance is
suggesting. Should that be the case lingering subsidence could keep
convection in check...resulting in slightly lower pop than what
is currently forecast for the second half of the period.
Long term /Saturday through Tuesday/...
as of 3 am Wednesday...stalled front and deep moisture lingers over
the area Sat keeping weather unsettled through the weeks end. At
this point uncertainty in the forecast really starts to increase.
Both GFS and European model (ecmwf) show a 500 mb trough digging down the East Coast. Middle
level trough pushes the deep moisture off the coast for the start of
next week with the region drying out. Surface pattern remains ill
defined and whats left of the surface boundary will linger in the
area. However the lack of deep moisture keeps precipitation chances at or
below the typical summertime afternoon 30 pop to start the week.
Previously guidance had been showing a stronger shortwave dropping
across the area in the deep northwest flow Tuesday. While this feature
is still discernible...just barely...it is in a weaker state. As
these shortwaves are frequently convectively induced/influenced will
not go above climatology pop for the end of the period given the
heightened uncertainty. Temperatures will be near to slightly above
climatology...generally near climatology by day and slightly above by night.
Aviation /12z Wednesday through Sunday/...
as of 12z...a coastal low and trough will keep convection offshore
today. Very little moisture to work with today...although low level
moisture may produce a brief MVFR ceiling this morning. Onshore flow
will keep things fairly pleasant today...with slightly below normal
temperatures along the coast. Light winds this evening with light
fog possible toward daybreak.
Extended outlook...predominantly VFR through the period with a
slight chance afternoon shra/tsra. Best chance rain showers/thunderstorms and rain Friday and
near term /through tonight/...
as of 538 am Wednesday...east wind 10-15 knots to prevail today and
tonight...accompanied by 3-3.5 foot seas. Occasional gusts to 20
knots ought to be anticipated...especially in the afternoon
nearshore. Seas primarily a mix of southeast waves 2-3 feet every 6 seconds
and moderate easterly chop. Showers and thunderstorms are expected to
remain well beyond the 0-20 nm waters today and no restrictions to
visibility are expected.
Short term /Thursday through Friday night/...
as of 3 am Wednesday...ill-defined gradient and weak surface wave
passing east of the waters Thursday will induce light north to
northeast flow into Thursday night. Wave moves off to the northeast Thursday
night into Friday as weak surface front drops in from the northwest.
Light and variable winds Thursday night gain a southwest component Friday
and Friday night but lack of any real gradient keeps speeds around 10
knots through the end of the period. Seas 2 to 3 feet through the
Long term /Saturday through Sunday/...
as of 3 am Wednesday...southwest flow will continue over the
waters through the weekend. The remains of a stalled front will
linger near the coast as Bermuda high tries to build back west.
Gradient will remain on the weak side with speeds no more than 10
to 15 knots at any point during the period. Daily sea breeze will
result in enhanced onshore flow near the coast both days. Seas
will run 2 to 3 feet on Sat and 2 to 4 feet sun.