Scientific Forecaster Discussion
fxus62 kilm 142335
Area forecast discussion
National Weather Service Wilmington NC
635 PM EST Thu Nov 14 2019
a very slow moving area of low pressure will bring increasing
rain and wind over the weekend most focused in coastal areas
while the cloud cover keeps temperatures below normal. A weak
upper disturbance will bring a small chance for rain on Tuesday.
Sunshine and seasonable temperatures finally return Wednesday.
Near term /through Friday night/...
an up surge in precipitable water, strong coastal convergence, and
deep-layered Omega, will lead to widespread rain tonight and Friday.
By nightfall of Friday, a coastal low will be positioned just off
Cape Fear, with intensifying north winds late Friday as retrograde
and deepening of the low run in tandem, but still offshore. Rising
wind-speed trends to continue beyond this period.
Centerline of quantitative precipitation forecast ensembles through 00z Saturday/7pm Friday, shows
1.5-1.75 inches across the area, which likely translates to isolated
spots around 3 inches, and a few under 1.25 inches, through tonight
and the day Friday.
Rain only, no thunder expected this period as stability will have
extensive depth west of the low center. Dewpoints will rise very
gradually through the period, in the 40s to near 50 at the coast,
with temperatures nearly flat-lined, very shallow rises Friday. 2-
meter relative humidity shows 95% to saturation all of Friday.
All this, but no warnings or advisories over land for the cold rain,
the exception, coastal flood advisory for the lower Cape Fear
River and beaches will be approached, and may be needed Friday.
Through the Friday day however, any flooding will remain minor in
Short term/Saturday through Saturday night/...
complex/occluded area of low pressure off the coast on Saturday will
be slow to move on account of how deeply stacked the system is in
the vertical. There will be a considerable gradient in wind and rain
between the coast and inland, with the highest prospects for both
focused in the Cape Fear region. Observed wind and wind gusts are
going to hinge upon two as of yet somewhat uncertain factors: the
actual track of the low (which will be dictated in part by the
complex frontal structure) and low level atmospheric stability.
Strong low level wind fields should have difficulty mixing down to
the surface by a very stable inversion that could be based as low as
1.5 kft, not reversing its course until 8kft. Some Wind Advisory-
worthy winds will certainly be found in this layer (especially on
the wrf), but unless the rain is heavy enough for downward momentum
Transfer they should remain aloft.
Long term/Sunday through Thursday/...
rain may linger in coastal NC on Sunday depending on how quickly the
still occluded low decides to lift out to the NE. Guidance is
trending a bit slower so would not be surprised to see a trend
towards wetter and more breezy. Low level thermal advection still
cool so highs well below climo. Trough axis remain locked in the
eastern U.S. Monday and Tuesday with disturbances rotating through
to enhance cloud cover some and keep temps below climo in the cool
advection regime though some weakening and thus moderation slated
for Tuesday. Final piece of energy aloft should be through Tuesday
night allowing a long due return of sunshine and seasonable
temperatures for Wednesday and Thursday.
Aviation /00z Friday through Tuesday/...
expect widespread IFR ceilings to develop over the area tonight and
continue through the valid taf period. Light north winds tonight
should increase to about 10 knots with higher gusts after daybreak
Extended outlook...mainly MVFR and IFR into Fri evening and again
Sat into sun. This a result of 2 coastal low pressure systems.
Becoming VFR Sun night thru Mon.
gale watch in effect, mainly targeting Friday night and Saturday
beyond the near term period, but advisory winds and seas will lead
up to this during the day Friday. Marine visibilities will begin to
deteriorate late tonight, after midnight, as rain-rates gains more
intensity. Seas will be dangerous Friday and worsen into the
weekend. Seas will feature NE waves 4-6 feet every 7-8 seconds much
of this period, with winds and seas increasing beyond the end of the
day Friday. Mariners of small craft should not venture offshore
Friday as the treachery will only become more imposing and intense.
A strongly cutoff and occluding low will be off the SC coast on
Saturday. There are still some nagging differences in the way models
portray the complex frontal structure of the low, which in turn will
have ramifications regarding the shape of the isobars and the
resulting wind fields. Even so-a Gale Warning for 35+kt winds seems
like a certainty, while some guidance is hinting at storm force/48kt
gusts. While certainly possible it seems quite likely that guidance
may not be adequately respecting the impressively deep and strong
inversion that will be in place preventing such winds being realized
down to the surface save for perhaps in very occasional gusts. As
such will hold gale watch as-is. Blended probabilistic 48kt gusts
show an area of 75% along the outer reaches (5 nm or so) of amz 250
and 252-the targeted area for these occasional gusts. Should the low
end up tracking just a bit closer to the coast then future shifts
may need to consider a storm watch or warning. Similarly NC waters
will host the most inhospitable seas as steep waves grow to 15ft out
near 20 nm. Wave shadowing will lead to lesser seas off Brunswick
and Horry, with some 10+ ft seas developing well offshore the gtown
The low should be pulling away to the NE on Sunday, though some
guidance like the ec and Canadian are slower and imply gales into
Sunday. With such a strong mid level trough approaching to act as a
"kicker" this seems unlikely, though possible. If not gales then Small Craft Advisory
will certainly be needed through Sunday if not into Sunday night.
Upper level trough will persist through the remainder of the period
into early next week keeping winds offshore, generally northwesterly.
strong northeasterly winds associated with slow moving, occluded
low pressure offshore will start piling up water along
correctly oriented beaches beginning on Saturday. East facing
beaches may see advisory level coastal flooding Saturday and the
southeast facing beaches of SC may experience the same on Sunday
(while they continue along the east facing). The Cape Fear River
should also have advisory level flooding both days. Advisory
level flooding is only expected with the higher of the two high
tides, falling roughly mid morning.
Marine...gale watch from Friday evening through Sunday afternoon for