Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
1020 am EDT Tuesday Apr 21 2015
a cold front will move off the coast around daybreak. Dry and
seasonable high pressure will build across the area today and
Wednesday. Another cold front will cross the coast early Thursday
followed by more seasonable weather Friday. A storm system may
affect the area Saturday. Cooler high pressure will follow into
early next week.
Near term /through tonight/...
As of 1030 am Tuesday...
12z area radiosonde observations show very dry column especially above weak inversion
at around 7kft. Other than a stray flat cumulus at that level expect a
day of full sunshine, seasonable temps, and lower humidity than days
past as drier air advects in from the northwest behind the cold
front. No changes to the forecast.
For tonight high pressure along the Gulf Coast should move east
to coastal Georgia. Since we're north of the ridge this means the
gradient will support a generally westerly wind. In the upper levels
channeled vorticity should remain north of the Carolinas with
insufficient moisture expected for clouds. This means radiational
cooling should proceed uninhibited...and lows should fall into the
48-52 range...except some middle to upper 50s at the beaches.
Short term /Wednesday through Wednesday night/...
as of 300 am Tuesday...large closed low spinning across southeast Canada
will drive nearly flat/zonal flow across the Carolinas through
middle- week. This will create fairly seasonable and quiet
weather...but a weak cold front is prognosticated to drop across the area
very early Thursday. High pressure will be offshore Wednesday with
return flow ahead of the cold front driving temperatures towards 80
beneath a lot of sunshine. Cloud cover will gradually increase Wednesday
night into Thursday as the cold front approaches from the
northwest...driven by a piece of vorticity energy rotating around the large
Canadian vortex. Guidance still suggesting only a small chance for
showers with this feature as forcing is weak and the column
remains dry. Will keep inherited schc pop for Thursday...but not
expecting widespread or significant rainfall Thursday.
However...clouds and any showers will keep temperatures Thursday a bit
cooler than Wednesday...with widespread middle 70s expected. Drying will
occur Thursday night...allowing temperatures to fall well into the
50s...a solid 5+ degrees cooler than Wednesday night lows.
Long term /Thursday through Monday/...
as of 300 am Tuesday...biggest concern for the extended is a
significant low pressure system prognosticated to move across the
southeast and tap Gulf moisture during Saturday. Still lots of
uncertainty due to lack of model agreement...and while the GFS has
been consistent bringing moisture in Friday night with soaking
rains on Saturday...the European model (ecmwf) is slower and more suppressed with
the baroclinic gradient leading to less quantitative precipitation forecast and a better Friday
night/Saturday. It is getting late in the year for such an
equatorward push of the baroclinic gradient...so the European model (ecmwf) may be
too suppressed...but will make very little change to inherited and
maintain a split of the guidance which agrees with current wpc
forecasts. Thereafter...cooler weather will prevail as drying
occurs Sunday...and these below normal temperatures will persist through
the end of the period even beneath full late-April insolation.
Aviation /12z Tuesday through Saturday/...
as of 12z...VFR this taf valid period. Low clouds 3-6k associated
with a cold front will move quickly offshore early this morning
leaving clear skies and northwest winds around 10 knots for the remainder of
the morning hours. This afternoon heating will allow for a few low
clouds around 5k to develop...and some high clouds will move in
from the west as winds back to a west direction ... at the coastal
terms. This evening winds become light and variable with scattered skies
through the overnight hours.
Extended outlook...possible showers Wednesday night and Thursday. Showers
/isolated thunderstorms/tempo MVFR Friday night and Sat. Otherwise
Marine... near term /through tonight/...
As of 1030 am Tuesday...cold front making good progress now having
cleared most if not all of the area. As a result the gradient is
easing and the scec will be lowered as previously forecast. Other
than headline drop no changes to the forecast.
Short term /Wednesday through Thursday night/...
as of 300 am Tuesday...high pressure offshore will maintain SW
return flow Wednesday...but speeds increase through the day thanks
to a pinching gradient ahead of a cold front. Wind speeds will
rise to 15-20 kts Wednesday night before shifting to the northwest early
Thursday behind the front. These northwest winds will persist and be
light most of Thursday...but veer more to the north and increase
late to 10-15 kts. Wave heights will be highest immediately ahead
of the frontal passage...rising to 3-4 feet with a SW wind chop and
southeast ground swell comprising the spectrum. North/northwest winds Thursday will
push highest seas away from shore...leaving 1-3 feet waves in the
Long term /Friday through Saturday/...
as of 300 am Tuesday...a cold front will waver back and forth
across the waters Friday...leaving highly variable wind
directions...from north early...to SW late...and then to north/NE
Saturday morning. Fortunately the diffuse gradient allowing this
to occur will keep speeds around 10 kts except very early Friday
when they may reach 15 kts. These north/NE winds will persist Saturday
ahead of a low pressure system approaching from the SW...but the
gradient remains lax so speeds will hover around 10 kts. With the
light gradient in place and no significant swell...seas will be
mostly 2-3 feet through the period...although a bit of an
amplification is possible late Saturday.