Area forecast discussion
National Weather Service Wilmington NC
323 PM EDT Tuesday may 21 2013
Synopsis...
scattered showers and thunderstorms this afternoon will dissipate
this evening. Shower and thunderstorm activity will decrease on
Wednesday with temperatures warming a little. Showers and
thunderstorms will increase on Thursday ahead of a cold front that
will cross the region early Friday. Cooler and drier weather will
develop for the weekend. Temperatures will return to normal early
next week as high pressure moves off the East Coast.
&&
Near term /until 6 am Wednesday morning/...
as of 3 PM Tuesday...convection now focused along what appears to
be differential heating boundary. Heating really the only forcing
in the absence of any ascent above the boundary layer but LCLs are
very low. Lapse rates so weak through the column that lightning
has been minimal despite some of the taller cells reaching the el
of about 40kft. Despite this implied very weak updraft speed the
column is nearly saturated and rainfall will still be heavy under
most of the showers but their short duration in any given location
will minimize flooding concerns...which are low but still non-
zero. Being borne of nothing more than heating the convection
should dwindle through the evening and a rain-free night is
expected. How much cloud cover lingers is a little uncertain and
is probably the main forecast challenge at this time with possible
minor ramifications for the low temperature forecast.
&&
Short term /6 am Wednesday morning through Thursday night/...
as of 3 PM Tuesday...deep south to southwest flow for much of the
period will continue to pump plenty of Gulf moisture into the
region. Bermuda high and middle level riding over the western Atlantic
briefly expand west Wednesday. Increase in middle level subsidence will
limit coverage of afternoon showers and thunderstorms. Diurnal
instability along with precipitable water values around 1.7 inches
and convergence along the sea breeze however suggest pop will not
be zero. Given the limiting factors slight chance to low chance
seems reasonable with inland SC favored. Worth noting that the 12z
NAM is not as aggressive with the building ridge and is slower to
exit the shortwave remnants. Discounting this scenario for now as
it is a significant change from previous solutions but should it
verify pop would need to be much higher.
Middle level ridge quickly weakens Thursday with 500 mb trough approaching from
the west. While the local environment remains relatively
unchanged...deep southerly flow pushes precipitable waters near 1.8 inches...the
loss of subsidence will result in a significant increase in storm
coverage. In addition another stretched shortwave will move into the
area from the Bahamas early Thursday. Positive vorticity advection ahead of it reaches the area
late Wednesday night or Thursday morning and should enhance convection. As has
been the case over the past few days the environment is not
supportive of severe weather. However deep warm layer and relatively
slow storm motion with potential for training storms will keep the
flood threat elevated...especially in areas that have already
received significant rainfall the last few days.
Convection will be winding down Thursday evening as dry air just ahead of
the 500 mb trough axis and surface cold front starts to move in. Cold
front is slow to cross the area...not reaching the coast until close
to the end of the period...but aloft westerly flow starts developing
before 00z Friday. Highs will be near to slightly above climatology...with
numbers tempered by clouds in some areas while lows continue to run
above climatology.
&&
Long term /Friday through Tuesday/...
as of 3 PM Tuesday...change in air mass expected on Friday as
cold front makes its way off shore and cool Canadian high pressure
builds in. Expect cooler and drier air to make its way into the
coastal Carolinas in deep west to northwest flow. Pcp water values
up above 1.5 inches over the days leading up to Friday will drop
out to less than an inch by Friday afternoon and be down to less than a
half inch by Sat morning as atmospheric column dries out.
Therefore do not expect any pcp through the weekend in a
relatively cooler and drier northerly flow.
As for temperatures...Friday should make it into the 80s in most places but
will feel cooler and drier by the end of the day. The 850 temperatures drop
from close to 15c previous days down to 8 to 10c through the
weekend. Saturday and Sunday temperatures will remain between 75 to 80 most
places with much lower dewpoints reaching down into the 40s inland
to around 50 along the coast as compared to 65 the previous days.
Will also see much greater diurnal swings due to the dry air mass in
place. Overnight lows will reach down to the lower end of the 50s
both Friday night and Sat night in cool advection and better
radiational cooling Sat night as high pressure shifts overhead.
By late Sunday into Monday the high pressure will shift off shore
giving way to a warmer moister return flow through early next week.
Therefore will see a warming trend. The GFS shows a weak cold front
dropping south into North Carolina late Monday into Tuesday. Therefore
will show clouds and chance of pcp back in the forecast holding to the
north on Monday and over local area by Tuesday.
&&
Aviation /19z Tuesday through Sunday/...
as of 18z...mainly VFR/MVFR conditions are being reported across the
County Warning Area this afternoon with south winds 8-12 kts. Convection has already
fired along the sea breeze boundary this morning...mainly near
kmyr/kcre. More showers/T-storms are possible today with continued
diurnal heating. Have included vcsh/thunderstorms in the vicinity at all terms through
00z...although the latest hrrr runs suggest precipitation this afternoon may be
more concentrated inland. Very brief periods of IFR are possible in
heavier downpours. The abundant low-level moisture in place could
allow MVFR ceilings/visibilities to develop overnight. Expect mostly cloudy
conditions again on Wednesday with south winds around 8-12 kts.
Extended outlook...slight chance of showers Wednesday. Showers/T-storms
possible Thursday through early Friday. Otherwise VFR.
&&
Marine...
near term /through tonight/...
as of 3 PM Tuesday...not much changing through the period from
the conditions that have been in place for quite some time now.
South to southwesterly winds will run in the 10 to 15 knots range on
average. Seas will be in the 3 to 4 feet range and be a little on
the choppy side with a period of 4-6 seconds. There is still some
swell energy out of the east-southeast at about 9 second but it is showing up as
a much lesser wave component in spectral plots.
Short term /Wednesday through Thursday night/...
as of 3 PM Tuesday...Bermuda high remains in control Wednesday before
retreating east Thursday ahead of a cold front expected to cross the
waters late Thursday night. Wednesday Bermuda high maintains southerly flow
with speeds of 10 to 15 knots range and seas 2 to 4 feet. Wind speeds
increase slightly then veer Thursday into Thursday night as the cold front
approaches from the west. Winds will generally be 10 to 15 knots with
periods of sustained 15 knots likely...especially in the afternoon.
Winds eventually veer to west late Thursday night as front passes. Seas
briefly rise to 3 to 5 feet Thursday then drop back to 2 to 4 feet as period
ends.
Long term /Friday through Saturday/...
as of 3 PM Tuesday...cold front moving farther on Friday as
cooler and drier Canadian high pressure builds in behind it
through the weekend. Winds will veer around from west around 10
kts Friday morning...to north to northeast at 15 to 20 kts Friday
night into Saturday in cool surge and tightened gradient. Winds
will lighten and continue to veer as high pressure shifts overhead
on Sunday.
Seas will be entirely wind-wave dominated between 3 and 5 feet through
Friday into Saturday. By late Saturday into Sunday...the seas will
subside and winds lighten as high shifts closer overhead. Expect
seas down to 3 feet or less by Sat night into Sunday.
&&
Ilm watches/warnings/advisories...
SC...none.
NC...none.
Marine...none.
&&
$$
Near term...mbb
short term...iii
long term...rgz
aviation...bjr
