Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
803 PM EDT Tuesday Jul 22 2014
a stationary front along the coast will slowly move inland and
dissipate tonight. Bermuda high pressure will build into the area
Wednesday and Thursday. A cold front will approach from the
northwest late Thursday...stalling near the coast for the weekend.
Another cold front will approach from the northwest Monday or
Near term /until 6 am Wednesday morning/...
as of 715 PM Tuesday...latest kltx 88d indicates a diminishing
trend to the shower and thunderstorm activity across the ilm County Warning Area.
With the days heating overwith...convection will become more
difficult to sustain. A slow moving middle-level short wave trough or impulse
off the Carolina coasts...is prognosticated to push off to the northeast
overnight into Wednesday...away from the ilm County Warning Area. Enough
forcing/dynamics from this upper short wave trough will produce convection
over the adjacent atl waters that may drift onshore...mainly from
Little River Inlet north to Surf City during the pre-dawn
Wednesday hours. Latest sref model highlights this diminishing
trend to probability of precipitation over land but does keep precipitation occurring over the
adjacent atl waters. Low to middle 70s for tonights lows may need to
be bumped up higher across the coastal counties to the upper 70s
especially with latest surface dewpoints holding in the middle 70s.
Cloudiness more widespread and opaque in nature at the moment...
will slowly thin overnight and in turn could lead to some patchy
or areas of fog development. Will include the fog at the next
update if needed.
as of 330 PM Tuesday...its been a tough day diagnosing why more
widespread convection with heavy rain has not developed across the
area. An upper level low near the mouth of the Mississippi River and
an upper ridge in the eastern Bahamas are lifting a tropical
moisture plume north across the Carolinas. An old stationary front
at the surface is merging with the weak seabreeze boundary...
providing a little extra low-level convergence. Wind speeds through
the lowest 20kft of the atmosphere are very light which is limiting
moisture advection...although the airmass itself remains loaded with
moisture as shown by precipitable water values of 2.1 to 2.3 inches.
The only factor that really sticks out is that continued presence of
multiple large convective clusters over the Gulf Stream from SC
south to Florida. With so much sustained upward vertical motion
centered there...there has to be a corresponding downward vertical
motion located downstream over our heads. The Flood Watch has been
cancelled early as a result. Scattered showers and thunderstorms
will continue into the early evening...but coverage and rainfall
rates will be much lower than thought this morning.
The middle-level flow will veer a little more tonight as the Bahamas
ridge builds a little more westward across Florida. The mean flow
should remain oriented onshore for the Cape Fear region...and for
the Southport/Wilmington area I will indicate an increasing trend
in probability of precipitation after midnight. 12z models show nocturnal marine
convection passing very close to Cape Fear in the 06-12z time
Both GFS and NAM MOS showed a pronounced cool bias last night so I
have adjusted my forecast lows upward. Lows should range from around
73 inland to 75 at the coast. Like last night widespread low stratus
and some fog will probably develop by midnight.
Short term /6 am Wednesday morning through Thursday night/...
as of 330 PM Tuesday...upper low over the Gulf Coast will continue
retrograding on Wednesday...allowing a 500 mb ridge to briefly
build into the southeast. Middle level flow will become more westerly
during the afternoon/evening hours...finally bringing an end to
the continuous channel of deep moisture that has funneled into the
area over the past couple days. Most ongoing shower activity
Wednesday morning will be offshore...but will mention higher probability of precipitation
along the coast early. Precipitable water values will remain in the 2.0 inch
range during the day...so still expect isolated to scattered
convection in the afternoon along the sea breeze. But overall precipitation
coverage should be less widespread than we have seen over the past
couple days. 500 mb heights will gradually fall on Thursday ahead
of an approaching cold front. Rain chances increase again late
Thursday into early Friday as the front nears our forecast area.
By 12z Friday the boundary will be draped across the
Carolinas...perhaps even bisecting our forecast area as it stalls.
The surface pressure gradient will tighten between Bermuda high pressure
and a pre-frontal trough...with strengthening S-SW winds through the
period. 1000-500 mb thicknesses indicate a warming trend as the flow
acquires a more westerly component. Expect highs to climb into the
low 90s Wednesday and middle 90s Thursday across our inland zones...
with heat indices nearing or exceeding triple digits. Low temperatures both
nights will be in the middle 70s.
Long term /Friday through Tuesday/...
as of 330 am Tuesday...cold front may be hung up right along the
coast Friday morning but NAM wants to push it south and east of area
leaving high pressure in control. Not confident that the boundary
will get too far south as 500 mb trough lifts north leaving a very
broad middle to upper trough anchored over the Great Lakes
maintaining more of a SW flow aloft and not giving much of push to
the cold front. Therefore will keep front just to our northwest
on Friday. Deep SW flow will keep a warmer and more humid air mass
over the area leaving potential for localized convection through
Middle to upper trough will dig down from the Great Lakes come Monday
pushing another cold front into the Carolinas. This will produce
more widespread convection in increased upper level support later
on Monday. Once again upper level trough will remain to our west
leaving a flow running more parallel to front and not giving it
too strong of a push through the area. Therefore may not see too
much of an air mass change behind front.
Overall expect greatest potential for more widespread convection
early Friday and again Monday afternoon into early Tuesday...but expect more
typical diurnal type shower activity most days. Some drier air should
make it into inland areas on Friday and on Tuesday behind fronts...but
will most likely remain just to the west of forecast area. Temperatures will
basically be above climatology with overnight lows in the 70s and day
time highs into the 90s most days with lower temperatures expected. As
trough digs down Thursday into Friday and again Monday into Tuesday may see
below normal temperatures in lowered 500 mb heights.
Aviation /23z Tuesday through Sunday/...
as of 00z...convection is over for the most part...however some
nocturnal showers are possible near the coast through the overnight
hours. Still a lot of convective debris...however most of that will
be dissipating in a few hours...allowing moderate radiational
cooling to commence. Fog will return to the region...however low
level moisture is not as prevalent as last night so will back off a
bit on the intensity at this time. Could see pockets of IFR...as
well as stratus. Wednesday...an impulse will move up the coast...and
coastal terminals are expected to see convection start fairly early
in the day...becoming more intense by early afternoon with thunder
expected. Weak southwest flow tomorrow.
Extended outlook...chance of showers/thunderstorms will persist through the
extended period. Expect predominantly VFR conditions with tempo
periods of MVFR/IFR ceilings/visibilities in storms.
near term /through tonight/...
as of 745 PM Tuesday...latest surface pressure pattern remains
somewhat chaotic...hence the variable wind direction at times.
The surface pg to remain relaxed this evening. Towards daybreak
Wednesday...the surface pg is prognosticated to tighten as the ilm waters
becomes nestled between strong Bermuda high pressure centered well
off the southeast U.S. Coast...and a strengthening Piedmont trough across
the central Carolinas. Overall...winds prognosticated to veer to the south-southeast
through west-southwest directions. Speeds to increase to 10-15 knots.
Significant seas will be a product of a somewhat decayed 2 foot southeast
ground swell exhibiting 8 to 9 second periods...and building 2 to
3 foot wind driven waves at 5 to 6 second periods.
as of 330 PM Tuesday...an old stationary front that has been
stalled near the coast all day should largely dissipate
tonight...leaving slowly veering wind pattern in its wake. By this
evening winds everywhere should be turning southerly and will
increase to 10-12 knots nearshore and 15 knots offshore. Seas are
currently 2-3 feet mainly in a southeast 7 second swell. This will
be supplemented by a short period wind chop overnight...but with
little significant increase in sea heights.
Short term /Wednesday through Thursday night/...
as of 330 PM Tuesday...southwest winds around 15 kts will prevail
Wednesday afternoon through Thursday as the gradient is pinched between
Bermuda high pressure and a pre-frontal trough. Seas will be 3-4
feet...building to 4-5 feet on Thursday across amz250/252 where winds
may gust to 20 kts. The dominant wave group will be a 5-6 second
southerly wind chop. Winds and seas will relax a bit toward the
end of the period as a cold front dropping southward nears the
Long term /Friday through Saturday/...
as of 330 am Tuesday...cold front reaches into the Carolinas early
Friday but will most likely stall and dissipate over inland Carolinas
over the weekend. Expect to maintain a SW flow around Bermuda high
weakening as gradient relaxes through Friday into Saturday as
front dissipates over the Carolinas. Seas up to 3 to 5 feet early
Friday will fall back down to 2 to 4 feet through late Friday into