Area Forecast Discussion
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FXUS62 KILM 211027
Area Forecast Discussion
National Weather Service Wilmington NC
627 AM EDT Tue Mar 21 2017
A cold front will move through the Carolinas late tonight,
accompanied by scattered showers and thunderstorms. Cold
Canadian high pressure will build southward from the Great Lakes
Wednesday night through Thursday night, perhaps bringing
freezing temperatures to some inland locations. The high will
move offshore Friday with a warming trend expected over the
weekend, along with an increasing chance of showers or
thunderstorms by Sunday.
.NEAR TERM /THROUGH TONIGHT/...
As of 630 AM Tuesday...An update has been made to the forecast
to account for the scattered convective showers erupting 30-40
miles inland from the South Carolina coast. These appear to be
situated along an axis of elevated instability rooted around
4000-5000 feet AGL and being supported by a southeastward-moving
upper level disturbance. Showers should be offshore in the next
90 minutes. Discussion from 300 AM follows...
Mid-level impulse moving overhead early this morning is
creating a few showers and widespread cloudiness across the
Carolinas. Do not expect any showers locally as precip advects
SE into a drier airmass, but the cloud cover and steady SW winds
of 5-10 mph will keep mins much warmer tonight than the
previous few nights, and many spots will remain around 50
degrees through daybreak.
Elongated area of high pressure along the Gulf Coast and across
Florida will retreat to the south today as a cold front sags slowly
down from the north. Much of today will become sunny, and with W/NW
/downslope/ flow through much of the column, and 850mb temps
climbing to 12-14C, highs will soar above climo for a change. Expect
max temps to reach the low 80s across the Pee Dee, to the upper 70s
closer to the coast. Along the beaches, a sea breeze will develop
albeit pinned, but highs here will be much cooler in the low 70s,
upper 60s possible across the Brunswick County coast.
Thereafter, concern turns towards potential severe weather late
tonight. The aforementioned cold front will drop slowly southward,
taking its time to move through the area as it becomes aligned
nearly boundary parallel to mid-level flow. As this front sags into
the CWA, a potent vorticity impulse rotating through the main
longwave trough will move overhead overnight, and the interaction of
this feature with the baroclinic zone below will spawn weak
cyclogenesis and convection. The surface wave will be weak, only
deepening once it gets offshore on Wednesday, but convection beneath
the shortwave will affect the area tonight. SPC has much of our CWA
in a MRGL risk for severe, and while surface based instability will
be lacking due to a nocturnal event, other parameters suggest at
least a chance for strong storms. Elevated CAPE of 600-1000 J/kg
thanks to steep lapse rates above 7C/km between 800mb and 500mb will
support strong updrafts capable of hail, while unidirectional bulk
shear values rise to 30-40 kts, supportive of multicell structures.
These multicell clusters will move quickly along the front from W to
E, and downshear corfidi vectors suggest motion of up to 40 kts,
with this forward momentum aiding the potential for strong wind
gusts already in place by 400-600 J/kg of DCape. If this was an aftn
event, would be supportive of a more significant severe risk, but
very stable air at the surface will preclude more than isolated
damaging winds, while decreasing updraft intensity and scattered
nature of storm coverage limit a more widespread hail risk. The best
chance for convection will likely be in the 8pm-2am timeframe, with
the highest risk for severe across the Pee Dee, but again, only
isolated strong storms are expected.
Once convection moves offshore, the weak surface low will drag the
cold front further south, finally pushing through the entire CWA by
Wednesday morning. Continued cloud cover and only a short duration
of CAA behind the boundary will prevent rapid cooling, so mins
Tuesday night will remain above normal, low 50s far north, to around
60 along the South Santee River.
.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 300 AM Tuesday...A potent upper level disturbance should
be exiting the Carolina coast around sunrise Wednesday. Surface
low pressure along a southward-moving cold front will accompany
the upper system eastward. While it was looking over the past
couple of days like there would be significant clouds remaining
behind the front within the frontal inversion for most of
Wednesday, latest models show a somewhat drier airmass aloft
which should allow clouds to fragment during the afternoon
hours. Despite cold advection the strong late-March sunshine
should be able to push temps well into the 60s if not near 70
degrees over the Pee Dee region, with slightly cooler readings
forecast along the coast and over SE North Carolina. It`s worth
noting there are still significant inter-model timing
differences with the onset of cold advection Wednesday.
Historically models have struggled with accurately depicting
temperatures during a cold frontal passage around sunrise, and
any meaningful delay in cold advection could lead to
temperatures far surpassing our latest forecast Wednesday.
Cold advection will definitely be ongoing by Wednesday evening.
850 mb temps plunging to near 0C could bring surface temps
close to freezing across portions of SE North Carolina. After
three freezes last week the last thing area berry farmers need
is another freeze...but it`s looking quite possible. Average
winds in the Sfc-1000 foot layer should remain 15-20 kt with
fairly steep low-level thermal profiles, suggesting advection
rather than radiation will dominate this possible freeze event.
Dewpoints plunging deep into the 20s will make frost too
unlikely for mention in the forecast.
Thursday: we are ignoring the NAM and its impact on the SREF for
the 00z cycle, instead going with the dry GFS, ECMWF, and
Canadian models. A region of 700 mb moisture will overspread the
region, particularly during the morning hours, however plenty
of sub-cloud dry air and lack of any meaningful isentropic lift
within the moist layer aloft should preclude precipitation.
There will be an impact on temperatures from the clouds however,
with highs probably holding in the lower to mid 50s for most
areas. Another chilly night is expected Thursday night as winds
die away and radiational cooling overcomes developing warm
advection to again drop lows into the 30s across much of the
area. Pender and Bladen counties in SE North Carolina again
stand the best chance of seeing freezing temperatures.
.LONG TERM /FRIDAY THROUGH MONDAY/...
As of 300 AM Tuesday...High pressure off the East Coast should
produce a warming trend through the period. Highs in the 70s and
lows in the 50s are expected through the period. The best chance
of showers or t-storms will probably be late Sunday as a
weakening upper system shears out through the Great Lakes but
drags a ribbon of moisture and modest lift through the
Carolinas. It`s getting close to that time of year where
daytime instability could become a large enough factor to pop
some showers or t-storms, and Sunday might be our first bout of
that for the year.
.AVIATION /12Z TUESDAY THROUGH SATURDAY/...
As of 12Z...Mid-level cloud ceilings and isolated showers this
morning are the result of a weak upper level disturbance passing
across the area. This feature should be offshore by 14Z, with
ceilings fragmenting into a scattered cloud layer for much of
the remainder of the daylight hours.
Action should begin tonight between 00-03Z as a rather potent
upper level disturbance approaches just ahead of a southward-
moving cold front. A significant cluster of showers and
thunderstorms should move quickly southeastward across the area
tonight. Gusty surface winds are expected in storms along with
the small potential of hail. The strongest activity should pass
offshore between 05-08Z, with lighter showers lingering through
12Z Wednesday. Surface winds will shift from southwest to
northerly with the cold front after local midnight.
Extended outlook...Lingering MVFR/IFR conditions from low clouds ceilings
are possible for a few hours early Wednesday morning. Otherwise
NEAR TERM /THROUGH TONIGHT/...
As of 300 AM Tuesday...Elongated high pressure across FL, and a cold
front approaching slowly from the north, will pinch the waters in a
tightening SW gradient today. Light W/SW winds this morning will
steadily increase through the day, becoming 15-20 kts this aftn and
evening. These winds will weaken a bit immediately along the front
as it sags south, but then shift abruptly to the N/NW and increase
once again with the frontal passage by daybreak on Wednesday. Seas
of 2-4 ft this morning will rise steadily as the winds increase,
becoming 3-5 ft this aftn and eve. Attm it appears conditions will
remain just below SCEC thresholds thanks to cold ocean temps
precluding stronger winds/seas in a WAA regime. Wave heights are
expected to fall back to 2-4 ft tonight, but a strong NE surge
behind the front will quickly raise seas to advisory thresholds just
after the near term.
A cluster of showers and thunderstorms is also expected to impact
the waters tonight ahead of the cold front, mostly after midnight
and into the first few hours of Wednesday. Within these storms,
briefly stronger wind gusts and torrential rainfall is expected
along with cloud to sea lightning and locally higher seas.
SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
As of 300 AM Tuesday...Low pressure just off the coast should
be driving a cold front southward across the South Carolina
coastal waters when the sun comes up Wednesday. The low is not
expected to strengthen appreciably, however the strong Canadian
high moving across the Great Lakes will more than make up for
the modest strength of the low. Strong northeast winds should
develop by Wednesday night and continue through most of
Thursday. A Small Craft Advisory will almost certainly be needed
for this period for 25+ knot winds and 6-8 foot seas across
open waters exposed to northeast winds.
By Thursday night the high should move off the Mid-Atlantic
coast, with winds veering a bit more easterly and diminishing in
LONG TERM /FRIDAY THROUGH SATURDAY/...
As of 300 AM Tuesday...Canadian high pressure off the East Coast
will weaken through the period, assuming a position only a bit
farther north than the typical summertime Bermuda High by
Saturday. Light easterly winds Friday should become more