Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington NC
356 am EDT Thursday may 7 2015
low pressure south of Cape Fear may become a subtropical cyclone
over the next couple of days. The system is expected to approach
the coast of South Carolina or southern North Carolina Friday and
Saturday with increasing chances for heavy rain...gusty winds...
and dangerous rip currents at the beaches. This system will only
gradually lift north away from the area early next week. A cold
front will approach next Wednesday.
Near term /through tonight/...
as of 330 am Thursday...extensive rain shield approaching and now
skirting the coast is having trouble holding together over land as
it has to overcome a very dry middle layer. The temperature-dewpoint spread
at the 850 mb level is around 45 degrees just inland of the
coastal counties this morning according to model soundings...so
although the beaches are finally seeing some patchy light rain
with ceilings around 10 kft...progress further westward will continue
to be slow and uneven. Radar shows a narrow band of heavier rain
about 20 nm offshore. Unless that band gets here intact we can
expect any accumulations to be only on the very light side this
NHC now has the non-tropical low pressure system around 220 miles
south-southeast of the SC/NC border and gives a 70 percent chance of cyclone
formation within 48 hours. Satellite loops show better organization
and a more distinct circulation center...which is being analyzed at
around 1009 mb. Guidance in relatively good agreement for the near term as
far as movement GOES...with the system drifting northwest towards coastal
NC/SC. All models show deepening with the NAM being the most
aggressive. Have leaned on a blend of guidance to account for both
the uncertainty of the situation and agreement between guidance.
Expect winds to pick up from the NE today as the system moves
northwest...but the relatively moderate associated gradient will keep winds
in the 15 knots range this afternoon with gusts of 20 kts or so. Precipitation
will be the tricky element of the forecast. Given recent trends and
projected movement of the system have upped probability of precipitation to categorical for
today for the coastal counties...trending downward as one moves
inland. Retaining possibility of thunder overnight as system
approaches the coast.
Short term /Friday through Saturday night/...
as of 330 am Thursday...the challenge...unsurprisingly...through
the short term remains determining what kind of impact a
strengthening low pressure will have on the area. Recon from NHC
was cancelled yesterday afternoon due to lack of organization...and
current WV/infrared imagery is less than impressive...although
improving. However...guidance is starting to come to some general
consensus on a slower and overall weaker system meandering just
off the coast through the period. Low pressure will lift north to
some where just off the SC coast before the period...and then
stall...loop...or retrograde back towards the SW as its motion
gets halted by ridge to the north. Very slow movement is expected
regardless of the actual track...and it is nearly guaranteed that
this will be impacting the area all Friday and Saturday. A slowly
looping system just offshore will maintain rain chances...some of
which may be heavy at times...the chance for some gusty winds
(focused almost exclusively along the coast however)...as well as
high wave action/minor beach erosion/strong rip currents.
The biggest impact locally is likely to end up being
rainfall...especially from Myrtle Beach northward across the Cape
Fear coast. Guidance suggesting that 250mb diffluence will be almost
exclusively on the north and east side of the system (more evident it will
never truly become tropical even as FSU phase diagrams suggest
symmetric warm core development) which suggests convection will be
confined to the North/East side of the system. With the low stalling just
southeast of the area...copious moisture will advect from an anomalously
high precipitable water airmass...so some very heavy rain will at least lurk
offshore...and may advect inland. A tight and significant gradient
will exist...and convective elements are possible as moist-adiabatic
lapse rates exist deep into the column. Highest pop will remain
Friday especially along the coast...but a good chance for rainfall will
continue right trough Saturday night.
Temperatures will struggle even in the tropical air mass due to widespread
clouds and rain. Highs will remain below climatology...middle 70s Friday and
upper 70s Saturday...but tempered diurnal ranges will keep mins in the
upper 60s or close to 70 both nights.
Long term /Sunday through Wednesday/...
as of 330 am Thursday...first half of the extended will continue
to feature a slow moving low pressure system somewhere in the
vicinity. This low pressure will meander just offshore or possibly
just inland of the coast Sunday and Monday as steering flow
remains generally weak. While a weakening trend is expected during
the long term...the highest precipitable water air may actually move across the
County Warning Area Sunday...so periods of heavy rainfall could continue at least
into Monday. Will not focus on any specifics at this time
range...and just show continued unsettled weather with chance
pop...highest across the Cape Fear region...with periods of MDT to
possibly heavy rainfall. The low will finally kick out to the NE on
Tuesday as an upper trough digs into the Great Lakes region. The
unsettled conditions will persist into Wednesday however as a cold
front digs across the Carolinas at the end of the period. There is
good news however...as this front should bring in a period of much
drier conditions beyond this forecast period.
Aviation /08z Thursday through Monday/...
as of 06z...confidence is still rather low...but improving as the
models are starting to get a bit more in sync. Low pressure will
slowly approach the region toward morning...with showers and MVFR
conditions expected an hour or two after daybreak along the coast.
Again...the uncertainty is there as the hrrr model is slower than
this. The gradient will tighten also around daybreak...with gusts
over 20 kts a good bet after 14z. The inland terminals will take a
while to get any precipitation...perhaps by middle afternoon. The column of
deep moisture will remain over the region through the end of the
forecast period...with intermittent IFR possible in the heavier
Extended outlook...rain/MVFR. Tempo IFR in heavier showers highest
confidence coastal terminals. Becoming VFR/showers Sunday. VFR
isolated showers Monday.
near term /through tonight/...
as of 330 am Thursday...Small Craft Advisory will continue
through the near term as a low pressure system 220 miles south-southeast of
the NC/SC border drifts northwest. Expect winds and seas to be highest
over our SC waters...with 20 to 25 kts and 5 to 8 feet respectively
for today. Conditions will be a bit better for our NC waters...but
will still be hazardous with seas in the 5 to 7 feet range.
Short term /Friday through Saturday night/...
as of 330 am Thursday...strengthening low pressure will meander
very near the coastal waters through the entire period...creating
periods of gusty winds and large waves. While uncertainty remains
in the exact intensity and placement of this system as it
evolves...regardless of the eventual solution conditions will be
difficult for mariners. With the current forecast path expected to
keep the low center east of the SC...winds will be generally
easterly across amz250 and amz252...and northerly over amz254 and
amz256. These should remain fairly consistent through the
period...although some fluctuation can be expected...especially as
the low wobbles in the vicinity. Speeds regardless of direction
will be 15-25 kts...with some gusts over 30 kts
possible...primarily over the NC waters.
Wave heights will be highest across NC waters thanks to the onshore
flow where 5-9 feet seas are expected Friday. 4-7 fters are
anticipated across the SC waters. Seas will gradually start to come
down the latter half of the period but this will obviously depend on
exactly how the system evolves beyond Friday. At this time show wave heights
lowering to 3-5 feet Saturday and the current Small Craft Advisory ending at 8am
Saturday across the southern waters. NC seas will likely remain
elevated beyond the 8am Small Craft Advisory end-time but will extend as needed with
Long term /Sunday through Monday/...
as of 330 am Thursday...gusty winds to start Sunday...especially
across the NC waters...as the persistent low pressure system moves
slowly near the area. At this time the best forecast takes the low inland
across Horry/Brunswick counties...and thus winds the first half of
Sunday will still be gusty around 20 kts from the east across
amz250/252...but much weaker and northerly over the SC waters. This
system will finally begin to lift NE through Sunday and into
Monday...so winds will veer to the SW and become 10-15 kts across
all the coastal waters. Seas initially may still be above Small Craft Advisory
thresholds across the NC waters but should fall through the
day...and 3-4 feet seas are forecast for most of the extended.
Marine...Small Craft Advisory until 8 am EDT Saturday for