Area Forecast Discussion
Issued by NWS Wilmington, NC-- Remove Highlighting --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KILM 240528
Area Forecast Discussion
National Weather Service Wilmington NC
128 AM EDT Sat Sep 24 2016
A cold frontal passage followed by modest and dry high pressure
will build southward across the Great Lakes and into the Mid-
Atlantic region this weekend. Moisture and rain chances will
return by Tuesday as the high moves offshore. The next cold front
will arrive from the west Wednesday, followed by autumn high
pressure accompanied by a dry and cool airmass.
.NEAR TERM /UNTIL 6 AM THIS MORNING/...
As OF 1000 pm Friday...Could see a lingering shower along the
immediate coast but for the most part, via latest 88D mosaic
trends, the pcpn over land areas looks done. Could see activity
fire up over the adjacent Atl waters overnight, but for the most
part this activity will be drifting/moving with a southerly
trajectory. It could swipe Cape Fear or Cape Romain, but for the
most part land areas, away from the immediate coast, should stay
pcpn-free. Still a dilemma of whether to go with a predominate
stratus or fog night across the ILM CWA. The progged soundings are
not clear cut for stratus nor ground based fog. But given dewpoint
depressions, although not huge, within the 1k foot agl, will lean
toward more fog with indications of areas to widespread. Not very
much, if any, tweaking applied to overnight mins. If any, it was
towards a degree lower due to the lack of a low stratus blanket
across the FA.
As of 700 PM Friday...The Flood Watch has been cancelled do to
lack of additional pcpn expected.
The evening will feature a diminishing and dissipating trend to
the showers and isolated thunderstorms as witnessed by the latest
mosaic 88D trends and the latest near term models, HRRR RAP and
HiResWrf. Will indicate a lowering of POPs over land areas this
evening, leaving widely scattered activity over the adjacent Atl
local waters. Trajectories in the lower levels tonight indicate
the pcpn that develops over the Atl Waters will not have that
onshore and inland movement like the past 5 days. May see a few
shra develop nearshore and clip possibly Cape Fear or Cape Romain.
For the pre-dawn Sat hrs, low level moisture will remain avbl as
drier air aloft works its way southward. Will either see low
level stratus or ground based fog. Not bought on which wx
phenomena will dominate. For now, will include variable amounts of
clouds and patchy to areas of fog during the pre-dawn Sat hrs up
to a few hrs after daybreak. No tweaks to the min temps needed at
.SHORT TERM /6 AM THIS MORNING THROUGH SUNDAY NIGHT/...
As of 300 PM Friday...As our low drifts farther south Saturday and
high pressure to the north begins to build in, the airmass should
dry appreciably from the top down. Precipitable water values in the
1.8 to 2.0 inch range today should fall to 1.5 inches Saturday
afternoon and to 1.25 inches Saturday night. Using 850 mb relative
humidity as a guide, I am limiting showers Saturday afternoon to
just the seabreeze convergence zone of Horry and Georgetown counties
in South Carolina, anticipating the airmass will become too dry
elsewhere to overcome the convective cap. With skies becoming mostly
sunny by late morning, highs should reach the upper 80s inland, 82-
86 closer to the coast.
High pressure north of the Great Lakes Saturday will build southward
down the Mid-Atlantic coast on Sunday. This should push a cold front
through the coastal Carolinas early Sunday. With a building ridge
aloft and meager moisture, this front will probably come through dry
with only an increase in low-level wind speeds expected. Winds
behind the front Sunday should become east-northeasterly, perhaps
picking up enough Atlantic moisture for scattered showers to develop
and advect onshore. Since this is heavily modified Canadian air that
will have spent a thousand miles or so over the warm western
Atlantic, low-level lapse rates should be steep which may help
support these shallow convective showers.
Given the significant post-frontal influence off the Atlantic, I am
not forecasting Saturday night and Sunday night low temperatures to
fall quite as low as recent MOS guidance would indicate,
particularly along the coast.
.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 300 PM Friday...High pressure will be to our north on
Monday but rather than a wedge-like NE flow wind will remain out
of the E. This will keep shower chances in the forecast as
Atlantic moisture is handily advected ashore. They may tend to
favor coastal areas though as mid level ridging keeps the upper
levels of the column quite dry. This onshore flow will turn more
southerly with the approach of a cold front on Tuesday, its
passage slated for Tuesday night. Wednesday and Thursday could
feature the strongest cool advection we`ve seen in a while and
bring some unseasonably cool weather with Thursday night possibly
slipping below 60 away from the coast. A wedge then sets up on
Friday continuing the cool weather.
.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
-- Changed Discussion --As of 06Z...IFR likely overnight, but moderate nocturnal boundary
layer winds may cause periods of VFR as well.
Fog has been spotty so far across the area, but with dewpoint
depressions approaching 0, and winds decoupling at the surface,
expect fog to become more widespread in the next few hours. Local
UPS Fog procedure has widespread fog away from the coast, and with
the wet ground in place, expect this to occur. The caveat to this
is VWP winds showing up at 20-25 kts presently, well above any
available guidance. The RAP soundings are closest to reality this
morning and are preferred, and these winds could lift the fog into
an IFR stratus deck, especially at the coast. Of course, if the
winds remain too strong stratus may be fractured leaving transient
VFR, and this will need to be monitored for possible AMDs. A few
showers developing offshore are not expected to affect the
terminals, and have shown IFR at all sites through daybreak
Saturday with very light north winds.
IFR will break an hour or two after sunrise, with VFR expected
thereafter. A much drier column forecast for Saturday will preclude
any showers, and only sct diurnal CU is forecast beneath winds
becoming easterly at less than 10 kts. More fog/stratus is possible
Saturday night, but any restrictions are expected beyond this valid
EXTENDED OUTLOOK...Chance for SHRA/TEMPO MVFR Sunday.
Chance for SHRA/TSRA Tue through Wed. Otherwise expect VFR.
-- End Changed Discussion --
NEAR TERM /THROUGH TONIGHT/...
As of 1015 PM Friday...Rather weak sfc pg and somewhat diffuse
pressure pattern to continue across the local waters due to the
weak sfc low now well south of the ILM CWA. Will continue with
the NE wind directions and speeds at 5 to 10 kt. The pcpn
coverage over the local waters has shrunk a good deal this evening
but could temporarily peak back up mainly over the offshore waters
south of Cape Fear during the pre-dawn Sat hours. With pcpn
movements slated to have mainly a southward trajectory, the
majority of the pcpn will likely remain over the offshore waters.
Significant seas will be totally a function of the 2.0 to 3.5 foot
e to ese ground swell at 11 to 12 second periods from Tropical
Cyclone Karl. The higher end of this range of swell will occur
north of Cape Fear, with 3 to 4 foot significant seas being
fcst...and with 2 to 3 foot south of the Cape.
SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 300 PM Friday...Surface high pressure north of the Great
Lakes on Saturday will build southward down the Mid-Atlantic
coast Sunday. This should push a cold front through the coastal
Carolinas early Sunday. Winds both ahead and behind the front will
be northeasterly, and since the front will come through dry the
only change heralding the front`s arrival will be an increase in
wind speed. Winds Saturday and Saturday night around 10 knots
should increase to around 15 knots for Sunday. These winds, in
combination with swells from Tropical Storm (or Hurricane) Karl
out near Bermuda, will build seas to 3-4 feet with some 5-footers
probably lurking not too far from 20 miles from shore near Cape
LONG TERM /MONDAY THROUGH WEDNESDAY/...
As of 300 PM Friday...Monday starts off with easterly winds as
high pressure is centered well to our north. This flow will then
veer as the day wears on as the high progresses eastward and a
cold front approaches from the northwest. This boundary comes
through Tuesday night and it will usher in an increasingly strong
NE surge of cooler air. Ahead of the front on Monday seas will be
quite manageable and any prefrontal increase in seas Tuesday
should be quite gradual. This may not hold true with and
following FROPA Tuesday night into Wednesday where headlines may