Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
324 PM EDT Thursday Jul 24 2014
a cold front will approach from the northwest today...bringing a
risk for strong thunderstorms this evening. This front will stall
across the area Friday before dissipating on Saturday. High pressure
will bring hot and humid conditions Sunday. A strong cold front will
move into the area late Monday...bringing cooler and drier
conditions for the middle of next week.
Near term /through tonight/...
as of 3 PM Thursday...so far this afternoon the concentrated area of
thunderstorms with heavy rain has remained just to our north from
near Goldsboro and Mount Olive eastward to the southern Outer Banks.
For US in southeast North Carolina and NE South Carolina this has left only
a smattering of thunderstorms...most concentrated in the northern
Pee Dee and sandhills region. This activity is spreading eastward at
nearly 25 miles per hour. The synoptic cold front is tough to find across
eastern NC due to thunderstorm outflow...but based on surface
pressures likely lies across the NC/Virginia border then southward across
the NC mountains.
One factor that appears to have limited thunderstorm coverage so far
today is a pocket of dry middle-level air discernible in water vapor
satellite imagery across eastern South Carolina and extreme southern
NC. Overlaying 12z model 700-400 mb dewpoints on satellite imagery
reveals the models see this feature too...and the NAM and GFS both
scoot it offshore before 00z. Moistening middle-levels plus the
favorable alignment of an upper level jet streak extending from the
Middle-Atlantic States to the New England coast should sustain at least
scattered thunderstorms through the evening and overnight. I have
trimmed back probability of precipitation from earlier projections based on latest model
data and observed trends...but forecast probability of precipitation tonight still range
from 40-70 percent...highest across southeast North Carolina and lowest
near the Santee river in South Carolina. With the 500 mb trough not
moving into the area until very late tonight I cannot safely remove
probability of precipitation from the forecast even after midnight.
500 mb wind as high as 40 knots is helping produce 0-6 km bulk
shear as large as 25-30 knots. While this would be a low value any
other time of the year it seldom occurs with dewpoints in the low to
middle 70s and surface-based cape near 3000 j/kg. Rather than single
cells...storms should organize into multicell clusters with gusty
winds possible due to dry air entrainment aloft. With large
potential instability through the -10c to -30c layer aloft frequent
lightning is also possible.
Low temperatures should reach the lower 70s for most areas...except
middle 70s near the coast.
Short term /Friday through Saturday night/...
as of 3 PM Thursday...a weak cold front will be stalled over the
eastern Carolinas at the start of the period. The boundary will
linger over our County Warning Area during the day as it slowly lifts northward and
eventually dissipates. Drier air will begin filtering into the area
Friday...but the magnitude and timing of this drying is where the
GFS/NAM solutions diverge. The 12z NAM is much drier above 700 mb
than the GFS as seen in forecast soundings. Will trend probability of precipitation downward
a bit for Friday but keep at least slight chance in the forecast...
with highest probability of precipitation along the coast...as the front wavers over the
area. Saturday will be warmer with dry west-northwest flow aloft as an upper
ridge over the central U.S. Expands eastward. Will mention slight
chance probability of precipitation as good surface heating could produce isolated sea breeze
thunderstorms in the afternoon...but overall coverage will be very limited
if anything develops. The NAM picks up on an upper disturbance near
the end of the period spilling over the top of the ridge...but no
signs of it in the GFS or European model (ecmwf)...so will keep silent probability of precipitation through
Expect seasonable highs on Friday...generally around 90 degrees
everywhere away from the beaches. On Saturday highs will climb into
the low to middle 90s...warmest across the Pee Dee region of South
Carolina. Though dewpoints will be a bit lower on Saturday...could
still see heat indices touch triple digits across our inland zones.
Low temperatures will only drop into the low to middle 70s.
Long term /Sunday through Thursday/...
as of 3 PM Thursday...middle level trough will dominate much of the
period keeping temperatures across the region below climatology. The
exception will be on sun with middle level ridge to the west
extending east. 850 temperatures will flirt with 20-22c which should
produce highs in the middle to upper 90s. Despite the hot temperatures do not
think there will be much convection around. Westerly middle level flow
and subsidence on the ridges periphery will keep middle levels dry.
Cannot rule out isolated convection...especially later in the day
associated with the Piedmont trough but see no reason to go above
inherited silent pop.
Cold front moves into the area late Monday...pushed by impressive middle
level trough. Does not look like Monday will be as hot as sun given
increased middle level moisture and more widespread convection later in
the day...just ahead of the front. Should the front be delayed a bit
Monday could end up as hot as sun given 850 temperatures hanging in the low
20c range for much of the day. Later Monday into Tuesday will be the most
active part of the period convectively speaking and will maintain
inherited high chance pop.
Middle level trough axis will remain west of the area through the
period with the front stalling not too far off the coast. Initially
middle levels are dry but increasing moisture aloft will result in a
slow increase in pop through the middle of next week. Temperatures will run
a little below climatology late in the period.
Aviation /18z Thursday through Tuesday/...
as of 18z...possible MVFR/IFR in any heavy downpours this afternoon
and evening as chances for rain showers/thunderstorms and rain increasing throughout the
day...along with patchy fog possible before daybreak on Friday.
Otherwise expect VFR.
VFR prevails this afternoon with west-southwest winds at or below 15 kts and
gusts already near 20 kts along the coast. A cumulus field has begun to
develop nicely as seen from latest satellite imagery ahead of an
approaching cold front...and expect cloud cover to be scattered/broken
throughout the rest of the day as chances for rain showers/thunderstorms and rain increases.
Though thunderstorms will taper off this evening...anticipate
showers to persist into the early overnight hours. Brief periods of
MVFR/IFR are possible in any rain showers/thunderstorms and rain especially with heavier
downpours and lower ceilings. Late overnight into the early morning
hours...MVFR/IFR is possible due to patchy fog...especially inland.
Into the morning hours...light winds and conditions will improve to
VFR with a chance of rain showers/thunderstorms and rain again on Friday.
Extended outlook...showers/tstms/tempo MVFR possible through
Friday...as well as Sunday and Monday.
near term /through tonight/...
as of 3 PM Thursday...a reasonably tight pressure gradient along the
northwestern periphery of the Bermuda high has pushed recent wind
gusts to 23 knots at Wrightsville Beach...22 knots at Oak Island...
and 15-18 knots at Myrtle Beach. These stronger winds should
continue through early evening (and through most of the night well
offshore) unless thunderstorm outflow disrupts this warm southwest
flow. An exercise caution headline remains on the forecast for the
NC waters for these winds.
A cold front across the NC/Virginia border will drop southward but should
remain inland even late tonight. This front will help enhance
thunderstorm chances this evening...with the best potential for
storms across the North Carolina waters through midnight. Seas
consist of a mixture of 9-second southeast swell plus a rough 4-5
second chop due to the breezy southwest winds. Combined seas are
averaging around 4 feet...with seas expected to begin subsiding
overnight as wind speeds die down.
Short term /Friday through Saturday night/...
as of 3 PM Thursday...light west winds at or below 10 knots early Friday will
become S-SW around 10 knots in the afternoon as the dissipating front
lifts back northward. Southwest winds strengthen to around 15 knots
during Saturday as the gradient tightens around high pressure well
offshore. Seas will remain around 3 feet through the period...building
to 3-4 feet across amz250/252 after 00z Sunday with an increasing
southerly wind chop.
Long term /Sunday through Tuesday/...
as of 3 PM Thursday...modest SW flow on sun will increase into Monday
as gradient tightens ahead of advancing cold front. Speeds later sun
may approach scec thresholds as Piedmont trough strengthens. Small Craft Advisory
headlines seem certain for at least Monday and possibly Sun night as
prolonged strong southwest flow builds seas to 4 to 6 feet. Front
moves across the waters Monday night with northerly flow developing.
Gradient relaxes behind the front and speeds will drop under 10 knots
by midday Tuesday. Front ends up stalled just east of the waters with
the proximity to the frontal zone keeping gradient weak and winds
light and variable. Seas will drop to 3 to 4 feet behind the front and
below 3 feet by the end of the period.