Scientific Forecaster Discussion
fxus62 krah 250112
Area forecast discussion
National Weather Service Raleigh NC
915 PM EDT Wed may 24 2017
a cold front will move east across North Carolina overnight into
early Thursday morning. A second cold front will move across the
region later Thursday. Fair and dry weather will return Thursday
night and Friday.
Near term /through tonight/...
as of 915 PM Wednesday...
A stacked low pressure system over the lower Ohio Valley will remain
nearly stationary overnight. A S/W rotating around the base of the
upper level low will usher a cold front west/southwest-
east/northeast across central NC overnight. The initial band of
convection that crossed the region late this afternoon/early evening
has aided to stabilize the atmosphere somewhat though bulk effective
shear is still strong enough to support organized convection in the
form of broken bands. Thus expect a 20-30 mile band of convection
along/ahead of the cold front overnight. Wind gusts 30-35 mph may
occur with the stronger showers and storms along with briefly heavy
Partial clearing may occur behind the sfc front toward morning,
especially across the SW half of the region. Though clouds will
quickly fill back in later Thursday morning ahead of the upper level
trough. Overnight temperatures should vary from the upper 50s/near
60 west to the mid 60s along and east of I-95.
Short term /Thursday through Thursday night/...
as of 115 PM Wednesday...
Another convectively active day expected on Thursday although
with a different character and a more limited coverage and
A vigorous closed mid and upper-level low and upper-level
trough axis over the eastern Ohio and Tennessee valleys on Thursday
morning will become negatively tilted and cross central NC
between 18 and 00 UTC. At the surface, an initial cold front
will extend north to south across the eastern Piedmont of NC at
12 UTC and then push east reaching the coast by 18 UTC as a
second cold front reaches the foothills of NC by 18 UTC and then
the coastal region toward 00 UTC.
The radar should be relatively quiet during the morning rush hour
although some scattered showers are possible along the initial
cold front that will be moving from the Piedmont into the
coastal plain. Ahead of the front, dew points will range in the
mid and even upper 60s supportive of convection. As the front
exits, dew points will fall into the mid and upper 50s.
Ahead of the approaching upper trough, large scale forcing for
ascent will increase during the day Thursday. This will be
supported by increasingly cold air aloft with 500 mb
temperatures of -18 to -20 deg c which combined with steep low
level lapse rates will lead to widespread cloudiness and the
development of scattered showers and thunderstorms very late in
the morning and into the afternoon. Cold air aloft resulting in
suppressed freezing levels and an evolving but still robust wind
field supporting shallow supercells, will support numerous
cells with small hail and a few storms producing larger hail. As
the previous shift noted, long and relatively straight forecast
hodographs would lead to some splitting supercells and the
potential for severe hail and damaging straight line winds.
Limiting factor for severe weather will be the amount of
instability while high cloud bases will reduce the tornado
threat, they will support enhanced downburst winds. The greatest
severe weather threat will be from 16 to 22z, especially across
the northern and western Piedmont. Convection will decrease
from west to east during the late afternoon as the secondary
cold front moves through and the air mass stabilizes. High
temperatures will range in the lower 70s northwest to upper 70s
Fair and dry conditions are expected Thursday night as heights rise
and a surface high pressure extends into the region from the south.
Low temperatures will range in the mid 50s to around 60. -Blaes
Long term /Friday through Wednesday/...
as of 125 PM Wednesday...
In the wake of the strong, negatively-tilted trough axis forecast to
exit the middle Atlantic coast late Thu and Thu night, height rises
aloft/ridging and associated subsidence, will sweep to the East
Coast and yield dry and mild-warm conditions on Fri, as underlying
surface high pressure settles across the southeastern U.S.
A warm front will develop ewd across the central Appalachians and
into the srn Middle Atlantic States (va/nc) this weekend, though
there remains uncertainty with respect to the precise location,
particularly given probable modulation by convection/outflow.
Nonetheless, there has been a consistent signal in model guidance in
recent days that would suggest a couple of episodes of convection
will likely develop along and north of the boundary Sat afternoon
through sun, before the front likely retreats nwd in advance of a
weakening cold front forecast to settle into NC on Mon.
Some storms may be severe, given relatively strong wind fields, and
steep lapse rates related to a large plume of eml that will likely
have overspread much of the sern U.S. And srn Middle Atlantic
States, around the top of a progressive sub-tropical ridge along the
The next trough aloft will amplify and progress slowly across the
central U.S. Early-mid next week, with associated surface lows
likely to lift across, and occlude over, the Great Lakes. Trailing
fronts will consequently weaken as they drift east of the
Appalachians and perhaps serve as a focus for primarily diurnal
convection. Continued warm, in the 80s.
Aviation /00z Thursday through Tuesday/...
as of 755 PM Wednesday...
24-hour taf period: showers and storms continue to move through
central NC as of the 00z issuance, lingering longest north and east
of krdu, which includes krwi and surrounding areas. Light rain
continues in and around kint, kgso, and krdu. A lot of uncertainty
still exists for the taf period with respect to cigs and visbys.
Largely expect visbys in the MVFR or VFR range with the showers, VFR
elsewhere. Cigs are generally VFR, although they may fluctuate
between VFR and IFR going from scattered to broken around 15 kft and
25 kft. Winds will be southerly, backing from south-southeast to SW through the
period. Winds may briefly decrease to around 5 kts or so, but
otherwise will be 8-12kts with gusts picking up Thursday morning.
Additional showers and thunderstorms are possible during the day
Thursday, accompanied by the usual reduced cigs and visbys.
Looking ahead: skies will clear out late Thursday night with a
return to VFR conditions Friday and Saturday. There will be a chance
for more convection late Saturday into early next week as a cold
front approaches, however VFR conditions should dominate through