Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
732 PM EDT Monday Apr 27 2015
dry and cool high pressure will remain over the area through
Tuesday. Low pressure will track up the southeast coast Wednesday
and Thursday and may bring significant rainfall to portions of the
area. Dry high pressure will follow Friday and into the weekend.
Near term /through tonight/...
as of 645 PM Monday...the current forecast remains on target with
1 exception...and that is tonights sky conditions. Latest visible
and 11-3.9 infrared imagery trends indicate the cirrus/cs is a bit more
widespread and opaque as well as further north than previous
thought. Using these trends and the latest 18z various model
runs...have become more pessimistic with mainly the evening cloud
situation. The SC/cumulus will dissipate after the days insolation
ceases but the mainly opaque and thin cirrus/cs will persist well into
the night. Will use the phraseology of partly cloudy becoming
mostly clear for all locations.
as of 300 PM Monday... what few clouds have developed inland will
dissipate towards sunset as heating abates and mixing becomes more
shallow and less vigorous. This leaves a mostly clear sky for much
of the period and light winds overnight. Not concerned about fog
however as continued dry air advection compliments of high well to
our north to eventually lower dewpoints much lower than expected
low temperatures. Some debris cirrus may impinge upon southern zones late
tonight as weakening mesoscale convective system crosses northeast Gomex and another upper
jet tries to become established behind the one currently exiting
to our east.
Short term /Tuesday through Wednesday night/...
as of 300 PM Monday...broad high pressure ridging into the area
Tuesday will slowly get displaced as a shortwave and associated
surface low begin to approach from the Gulf Coast. Tuesday will
likely end up a nice day with ample sunshine through most of the
day before middle-level clouds begin to overspread the region from
SW to NE. This cloud cover is associated with increasing
isentropic lift...and once clouds develop they will slowly lower
through Tuesday night. Guidance has slowed precipitation onset...likely
due to the dryness of the antecedent airmass...and it appears that
the NC counties will stay dry until Wednesday morning...but light
rain showers will begin to develop across SC late Tuesday night.
Wednesday shaping up to be a pretty wet day as isentropic lift
increases...especially along the 290 and 295 k surfaces...causing a
full saturated column and periods of rainfall. Additionally...the
surface low will move out of the Gulf Coast and off the eastern sea
board while lifting NE through Wednesday night. Good diffluence aloft will
help this low strengthen as it moves up the coast...and will likely
track along the immediate coast as it gets pulled north by a deep
middle-level closed low dropping into the Ohio Valley. Periods of MDT to
potentially heavy rain will develop late in the period...accompanied
by increasing winds within the tightening gradient.
Temperatures Tuesday will rise to around 70 as cold advection keeps highs
well below seasonable norms. Highs Wednesday will be similar...as
clouds and rain limit warming even as the air mass modifies. Lows
Wednesday night will be held above normal middle/upper 50s...which will
be several degrees warmer than Tuesday nights mins.
Long term /Thursday through Monday/...
as of 300 PM Monday...potent and deep storm system will be on the
move slowly across the area Thursday and Friday. The surface low
will be off Cape Hatteras initially and accelerate well to the
northeast by early Friday. The middle level trough will rotate
through Thursday night. We have a northeast to southwest gradient
of probability of precipitation from high chance to chance Thursday as the heavy and
organized activity should have ended. Same gradient for overnight
Thursday with lower probability of precipitation as the middle level trough swings across.
Weather still looks nice and quiet beyond this as the middle level
pattern transitions to zonal. At the surface high pressure builds
in from the Mississippi Valley eventually moving offshore by the
end of the period.
Aviation /00z Tuesday through Saturday/...
as of 00z...VFR expected this taf valid period. Light northerly
winds through the overnight hours should keep the lower atmosphere
well mixed which will inhibit fog formation. The exception may be
klbt where temperature/dew point spreads may become close enough
when winds are lightest in the pre-dawn hours to briefly allow br
to develop. But since moisture is limited confidence is low and
will not include in tafs.
During the day Tuesday there may be enough moisture to allow a
few low clouds around 5k to develop. Broken high clouds will be the
rule though. Winds will be light north-NE and may become light and
variable at kflo/klbt by 18z if the center of high pressure moves
directly over those terminals. During the afternoon winds will
become light east-southeast at the coastal terminals.
Extended outlook...a low pressure system will move towards the
terminals early Wednesday with rain and possible MVFR/IFR conditions
developing by middle morning. Showers/tempo MVFR Thursday and Friday as
a upper low passes. VFR Saturday.
near term /through tonight/...
as of 645 PM Monday...main wind and seas forecast still aok. With
the approach and immediate passage of a decent middle level short wave trough
overnight...the surface pressure pattern and a semi-tightened gradient
will yield northwest-north-northwest winds increasing to 15 to 20 knots for all waters.
The gusts up to 25 knots remains possible but should be far and in
between...and reserved for the warmer SST just beyond the 20 nm
mark from the coast. The mainly offshore component will limit
significant seas to 2 to 4 feet...with 5 footers at and just beyond
20 nm out. Average periods will run 4.5 to 5.5 seconds.
as of 300 PM Monday... high pressure centered over the Great
Lakes through the period. Initially this is creating a minor
gradient. However as the night progresses this anticyclone will
extend a weak ridge axis into the Carolinas and the gradient may
increase to the tune of a category Worth of winds. Latest run of
locally run Swan keeps seas capped at 4 feet. While a stray gust of
25kt cannot be ruled out sustained wind and seas still appear to
remain below scec thresholds.
Short term /Tuesday through Wednesday night/...
as of 300 PM Monday...quiet to begin the period but a low pressure
moving up the coast will create deteriorating marine conditions on
Wednesday. Broad high pressure Tuesday will create a weak pressure
gradient...and north winds around 10 kts most of the day. Late
Tuesday night into Wednesday...low pressure will develop and move
off Georgia coast before lifting NE along the coast through Wednesday.
This pinches the gradient and winds will increase steadily to 20-25
kts while veering to the east ahead of the system...and then turning
to the northwest behind the low late. Seas on Tuesday will generally be 1-3
feet...but then increase late and into Wednesday as the winds ramp up
near the low pressure system. Seas will rise to 4-6 feet on the
strongest east winds...and an Small Craft Advisory may be required during
Wednesday...before seas fall once again behind the low.
Long term /Thursday through Saturday/...
as of 300 PM Monday...surface low pressure will reside just off
the coast of Cape Hatteras at the beginning of the period. Winds
from the northwest on the order of 10-15 knots will continue
through Friday morning. Speeds tack on a couple of knots at this
time due to some cold air advection and these speeds will prevail
through most of the day Friday. Speeds drop off a bit by Saturday
morning. Significant seas will be 3-5 feet Thursday dropping off
somewhat Friday with the higher values confined to the outermost
waters with the stronger offshore flow.