Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Remove Highlighting --
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FXUS62 KRAH 281935
Area Forecast Discussion
National Weather Service Raleigh NC
335 PM EDT WED SEP 28 2016
A potent upper level low over the Great Lakes will drift southward
through the Ohio valley tonight, stall over the Cumberland
Plateau in eastern Tennessee and Kentucky on Thursday and Friday,
then retrograde north through the Ohio valley and lower Great
Lakes this weekend.
.NEAR TERM /THROUGH TONIGHT/...
As of 330 PM Wednesday...
...Severe storms and flash flooding possible this evening...
Convective initiation is underway in two main areas: 1) along a
differential heating boundary over the northwest Piedmont, just
downstream of the synoptic front, and 2) an axis of weak mass
convergence near I-95. MLCAPE has risen to around 2000 j/kg
amidst good diabatic heating, and with mid level flow increasing
to around 40kt ahead of the upper low currently dropping toward
the Tenn Valley, effective bulk shear has increased to 40-45kt
north of roughly Highway 64. These bands may not move very much
given system parallel deep layer flow, which will likely lead to
training of cells and a flash flood threat. Not issuing a Flash
Flood Watch after coordination with neighboring offices, but the
threat of flash flood should not be ignored this evening and
tonight. Supercells will be possible within the bands, more so
toward the VA border where shear is better, and a tornado threat
may unfold this evening as low-level winds increase/back and
curved hodographs elongate in response to the approaching upper
Another band of convection (maybe not as intense) should develop
tonight as stronger height falls finally arrive and deep forcing
is better. There has been relatively good agreement among the
large scale models and the ensemble of CAMs of this in the
vicinity of US-1 in the 10pm to 3am time frame. Expect lows will
be in the upper 60s to around 70 given no movement of the front.
.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
As of 120 PM Wednesday...
Additional showers and storms with training moderate to heavy
rain remains likely over Central NC Thu, with a gradual shift to
the ENE on Fri.
The powerful mid level low will continue to sit and spin over KY on
Thu before drifting/wobbling slightly northward to IN on Fri, with
lobes of DPVA sweeping through western and central NC, along/atop
the very slow-moving north-south oriented cold/occluded surface
frontal zone. PW values remain elevated (over 1.5") over all but the
far west Thu, shifting to east of I-95 Fri, beneath nearly
continuous bands of pronounced upper divergence. The WRF-ARW/NMM
both start the day Thu with a band of convection bisecting the
forecast area, drifting slowly eastward through the far eastern
Piedmont and Coastal Plain through Fri, filling in with better
coverage with time, with a cell motion that presents a threat for
training and resultant localized flooding. Will retain, with minor
tweaks, the overall pattern of the earlier forecast, with lower
chance pops west and high pops in the central/eastern CWA Thu,
trending lower WSW to ENE through Fri but remaining high in the far
eastern/NE forecast area Fri, tapering off further Fri night.
Temps both days will be held down by clouds/precip in the east and
by lowering post-front thicknesses in the western CWA. Highs in the
upper 70s to lower 80s Thu and in the mid 70s to lower 80s Fri,
still slightly above seasonal normals. Lows 60-68 Thu night and 54-
65 Fri night. -GIH
.LONG TERM /Saturday through Wednesday/...
-- Changed Discussion --As of 320 PM Wednesday...
A few showers are likely to linger Sat into Sun in the eastern CWA,
along and east of the frontal zone, but otherwise this period
appears mostly dry. The large mid-upper low centered over IN early
Sat will drift across lower MI and slowly fill through Sun before
being drawn across the St. Lawrence Valley and Northeast states by a
shortwave trough rotating around a northern Atlantic polar vortex.
This, in conjunction with a large longwave trough shifting onto the
West Coast and over the western CONUS, will gradually build ridging
over the eastern CONUS heading into early next week. The surface
front over the East will continue to ease eastward and wash out with
a weakening low level wind field fostering frontolysis. A few mainly
afternoon showers are possible along this diffuse frontal zone over
the far eastern counties of NC early next week, but the PW will be
quite low elsewhere with a lack of instability, and will keep the
forecast mostly dry. Temps should hold a bit above normal through
the period, following thickness trends, with highs mostly in the
upper 70s and lower 80s, and lows in the upper 50s to mid 60s.
Uncertainty grows during the middle of next week, with a lot of
questions regarding the fate of what is now Tropical Storm Matthew,
located north of Venezuela this afternoon. However, at this time, it
appears that it will still be far from NC through Wed. Check with
the National Hurricane Center for the latest on Matthew. -GIH
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.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
As of 150 PM Wednesday...
Scattered of morning stratus has yielded VFR most everywhere, with
MVFR ceilings holding on in at INT and GSO. Daytime heating and an
approaching deep upper low will trigger storms that should become
more numerous with time. These will be scattered this afternoon and
early evening,but may become focused in a line tonight. Some of the
storms may be strong with very heavy rain, and thus LIFR conditions
are possible. Some IFR ceilings should develop once the main area
of showers moves east, primarily around GSO/INT/RDU, tough additional
showers will still be possible throughout the night.
Typical daytime improvement in ceilings is expected Thursday
morning, though with the the frontal zone still over the region and
the upper low settling over the Tenn Valley, another round of strong
storms is expected Thursday afternoon.
Outlook: Drier air is finally expected to filter into the region on
Friday, leading to improved aviation conditions through the weekend.