Area forecast discussion
National Weather Service Wilmington NC
942 am EDT Thursday may 23 2013
Synopsis...
showers and thunderstorms will precede a cold front today. The
front will move through early Friday and usher cooler air into
the Carolinas over the weekend. Next week will bring a gradual
warmup.
&&
Near term /through tonight/...
as of 9:30 am Thursday...plume of moisture and associated
precipitation now offshore although some light rain or drizzle may
scrape the coast north of Cape Fear this morning. Have adjusted
forecast accordingly. No other changes anticipated...previous
discussion from earlier this morning follows:
Vapor analysis shows one short-wave moving off the Georgia/SC coast
early this morning...tripping off nocturnal ocean convection from
Cape Fear south through east. Areas of low-level moisture
convergence offshore was additionally helpful in blossoming showers
with embedded thunderstorms over the waters. The Bermuda high circulation
over our locality is marked by deep southerly flow that will guide
the ocean convection onshore this morning...generally east of a kmyr
to kcpc to keyf line. This activity will begin to wane in the middle
morning as buoyancy over the water decreases with diurnal warming.
The convective focus will shift from water to land by midday and
low-level wind convergence will spread inland along a resultant
sea breeze boundary helping to produce lift. Short-wave energy
will approach from the west in the afternoon to provide upper
support. The inland advancing marine layer will shift the highest
pop values inland away from the coast through the afternoon.
Forecast soundings indicate 1100-1700 j/kg of cape at 18z inland
with precipitable water values around 1.6 inches. Projected storm motion 210/10
knots. Light environmental winds and bulk shear parameters will limit
strength of convection...but certainly some strong wet down-bursts
can be expected.
Loss of heating will take a toll on convection this evening and
drying in the middle levels of the atmosphere after 6z tonight should
begin to curtail shower activity. Middle-level drying will out-pace
the surface front. The front will bring a wind-shift to the coast
around 12z Friday.
&&
Short term /Friday through Friday night/...
as of 3 am Thursday...strong upper trough especially for late may
swings across on Friday. The cold front and deep moisture will be
offshore before the start of the period but if the trough axis lags
enough to prevent downsloping/drying then there could be enough SW
flow and positive vorticity advection for some instability sprinkles. Forecast soundings show
enough middle level dry air to preclude measurable precipitation at this time
so the forecast will remain dry. Otherwise Friday looks increasingly
breezy with plummeting dewpoints and perhaps even strong enough late
day cold air advection to make for a rather flat diurnal temperature curve. Cold air advection
sufficient to bring a downright cool Friday night. Wind stays up to
preclude rad cooling but lows could still dip down close to records.
Northerly flow and cold advection continues into Saturday keeping
temperatures a solid category or more below climatology despite full
sunshine. As the trough deepens and cuts off off the New England
coast by Saturday night the cold advection will be maintained into
the period. With the solar modification fighting the process the
whole time Sat night lows may end up very near those of Friday
night.
&&
Long term /Saturday through Wednesday/...
as of 3 am Thursday...large vortex more typical of cooler season
weather to only very slowly lift out of New England states Monday
through Wednesday. Local result will be an abating of the surface
high pressure that until then will have been pushing in from
points north. The speed at which this happens is still up for
debate but the trend will be one of gradually increasing warmth
and moisture. At some point moisture may be sufficient to support
isolated or scattered convection or there could even be an actual surface
warm front that develops in the return flow...which could add to
coverage a little bit. Either way both scenarios seem to offer
little in the way of rain chances area-wide. By the end of the
period the surface high will be off the coast and an upper level
ridge building so temperatures should end up near or above climatology.
&&
Aviation /14z Thursday through Monday/...
as of 12z...most of the convection is offshore...associated with a
weak short wave rotating around a shallow longwave trough. Ilm will
see a shower or two...with the other terminals remaining rain free
until this afternoon...as heating and sea breeze resultant will
combine to fire off some low topped stuff. Ceilings this morning
will be IFR inland...and mainly MVFR along the coast...becoming VFR
at all terminals by late morning. Light southwest flow will continue
throughout the day...a bit more south southeast along the coast this
afternoon.
Extended outlook...showers and thunderstorms possible Thursday
through early Friday. Otherwise VFR.
&&
Marine...
near term/through tonight/...
as of 9:30 am Thursday...no changes to the forecast with the
latest update. Previous discussion follows:
South winds will increase today into this evening ahead of an
approaching cold front. The winds will remain below advisory
levels...but gusts to 20 knots can be expected this afternoon and
tonight. South-southwest winds will shift to west-northwest after daybreak on Friday.
Seas may reach 5 feet outer portion later today and tonight...and
3-4 feet most location. Showers and thunderstorms will impact the waters
this morning and moving from south to north but decreasing by late
morning. Seas will be composed of a mix...south-southeast waves of 3 feet
every 7 second and south-southwest waves of 1-2 feet every 4-5 seconds. Winds and
seas will be locally high in and near thunderstorms.
Short term /Friday through Saturday night/...
as of 3 am Thursday...the period begins with a cold front moving off
to the east of the waters. The sharp veer associated with frontal passage will
give way to a more gradual veering of wind direction as high
pressure/cool air spills southward across the eastern U.S. Friday
night will also bring a category of speed increase in the cool
surge. Too early to tell whether or not any headlines will be needed
as model guidance wind speeds seems to have decreased slightly
resulting in slightly lower seas in Swan output. Even so expect
steep and choppy waves from all of the changing wind direction. High
pressure continues to build in from the north on Saturday keeping the
winds northerly but there should be a relaxation by Saturday night
as ridge center draws closer.
Long term /Sunday through Monday/...
as of 3 am Thursday...high that builds in behind Friday cold
front weakens as low pressure deepens and occludes New England
coast. As the low becomes the dominant wind maker locally a northwest to
west flow will be established but wind speeds may stay capped at 10
feet. Wind waves fairly diminutive with a little backswell
developing from the storm. As this system pulls away to the NE and
fills the gradient locally will start to be most defined by weak
high developing over the west Atlantic and a light and more
typical southerly component gets re-established.
&&
Ilm watches/warnings/advisories...
SC...none.
NC...none.
Marine...none.
&&
$$
Near term...mjc/rek
