Scientific Forecaster Discussion
fxus62 kilm 201001
Area forecast discussion
National Weather Service Wilmington NC
601 am EDT Thu Jul 20 2017
showers and thunderstorms will be isolated in the days ahead,
as an upper ridge builds from the west. The ridge will bring hot
and humid conditions through the weekend. Rain chances will
increase early next week, as a surface trough deepens inland.
Near term /through tonight/...
as of 321 am Thursday...upper level low clearly evident on WV
imagery this morning spinning off the SC coast will continue to
drift slowly southward while 500mb ridge begins to expand from the
MS valley. At the surface, broad high pressure will remain but with a
very diffuse gradient in place. This leaves the upper low as the
primary influence on today's weather.
An upper low overhead in July typically means steep lapse rates and
convection. While the former is true, with stronger than moist-
adiabatic lapse rates seen on forecast profiles between 700 and
500mb, the latter will be tough to come by today. Anomalously strong
easterly flow aloft (-2 to -3 sigma) will advect drier air into the
column with pwats falling to around 1.6 inches this aftn.
Additionally, coupled 250mb jet structure atop the 500mb upper low
will actually drive confluence overhead, likely forcing some sinking
motion noted by weak negative Omega. Despite these features working
against convection today, MLCAPE climbing over 1500 j/kg could still
support an isolated tstm, most likely along the sea breeze, and have
carried schc pop for the sea breeze boundary only. Do not expect
activity on the Piedmont trough today.
This cape is driven by what will become the hottest day so far this
week (although this will most certainly be eclipsed Fri and sat)
combined with still high dewpoints. Some drier mixing is forecast
this aftn so heat index values are expected to remain in the upper
90s to around 100 as temps climb into the upper 80s NE zones, to as
warm as 95 in the far SW Pee Dee counties. Developing southerly flow
this evening and tonight will keep mins elevated despite mostly
clear sky conditions, and forecast lows are only around 75 most
Short term /Friday through Saturday night/...
as of 321 am Thursday...primary feature of note this period is
broad upper ridging exerting eastward into the area, as a hot
dome of air aloft expands into the Carolinas. This may squelch
rainfall across the area altogether and have kept mentionable
pop values below the 15% threshold both Friday and Saturday.
Enough h8-h7 moisture remains trapped below the warm air and
moderate diurnal cumulus is a certainty each afternoon. The heat
indices will be on the uptick and a heat advisory may be needed for
portions of NE SC and southeast NC on Saturday. Apparent temps will range
from 98-103 Friday and 100-106 on Saturday.
Long term /Sunday through Wednesday/...
as of 3 PM Wednesday...the first half of the extended period,
basically the weekend will feature mostly dry conditions with
temperatures somewhat above normal. These conditions will be
provided by a weak mid level pattern and Bermuda high pressure
at the surface. The mid level capping is not quite as prominent
on soundings as it has been in recent days, but with mid level
moisture lacking convection will still drive a hard bargain. The
second half of the extended, early next week shows more
favorable conditions for convection as cooling aloft and an
increase in column moisture to well over two inches warrants.
Temperatures fall slightly from the weekend's warmer readings.
Aviation /12z Thursday through Monday/...
as of 10z...diffuse pressure gradient in place across the area
has left light to calm winds at the terminals. A hot and dry
day forecast after sunrise with sct diurnal VFR cumulus and maybe an
isolated tstm. The best chance for this will be near The Myrtles
this aftn, but coverage is expected to be too limited for a taf
mention. Winds will remain light, but become S/SW at 5-10 kts
all terminals, highest at the coast within the sea breeze. Most
cumulus will erode with loss of heating at the end of the valid
period leaving mostly clear sky conditions once again and VFR.
After 6z 3-4sm br possible.
Extended outlook...VFR. Scattered showers and thunderstorms
near term /through tonight/...
as of 321 am Thursday...a land breeze through early morning
will gradually give way to more typical return synoptic flow
this aftn through tonight. The pressure gradient today is very
weak, so whether the winds are coming from the west this
morning, or the S/SW this aftn and later, speeds will be 10 kts
or less. The exception to this will be brief increases within
the sea breeze circulation. These light winds allow the
2-3ft/7sec southeast swell to be the primary driver to significant seas
today, creating 2-3 ft seas will little fluctuation.
Short term /Friday through Saturday night/...
as of 321 am Thursday...the intensifying heat inland will bring
a robust sea breeze each afternoon Friday and Saturday, and
gusts to 20 knots like certain at this time. Otherwise expect
sustained winds from the SW 10-15 knots through the period. Seas of
around 3 feet should be expected, in a mix of SW waves between
1-2 feet every 3-4 seconds, and southeast waves 2 feet every 9 seconds.
The dry air aloft will keep thunderstorms away from the area this
period but a few showers near the Gulf Stream may pop up.
Long term /Sunday through Monday/...
as of 300 PM Wednesday...Bermuda high pressure will maintain a
summertime flow across the waters. Mariners can expect a
southwest flow of 10- 15 knots with some late morning/afternoon
acceleration with the sea breeze near shore. Significant seas,
without the assist from any meaningful swell component will fall
into a range of 2-4 and mostly 2-3 feet.