Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 221437
Area Forecast Discussion
National Weather Service Raleigh NC
1036 AM EDT THU SEP 22 2016
Deep low pressure will hold over the eastern Carolinas through
Friday. This low will weaken Friday night through Saturday, as a
backdoor cold front approaches from the north.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 1035 AM Thursday...
With little change in the overall pattern, featuring a surface low
and associated inverted sfc trough off the Carolina coast and a
closed but weakening mid/upper level over the heart of the
Carolinas, the damp and unsettled weather will continue for
one last day.
DCVA in tandem with improving upper divergence, and with ample low
level moisture convergence, will likely lead to redevelopment of
scattered to numerous showers mainly along and east of Highway 1
this afternoon. While isolated pockets of heavier rain are possible
today within the above-normal PW environment, amounts should be
notably less overall given the weaker dynamic forcing for ascent and
slowly filling mid level low over the eastern Carolinas.
Under a veil of overcast low clouds, temperatures again will be held
in check, but not to the degree they were yesterday as rain/drizzle
this morning is not as widespread. Highs in the mid 70s to around
Tonight: With closed low devolving into an open wave trough,
decreasing upper level support and shallower saturation
through the column will support less in the way of rain/drizzle
overnight. Lows 65-69.
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.SHORT TERM /Friday through Friday night/...
As of 228 AM Thursday...
The mid level trough continues to tilt and ease toward the SE
Fri/Fri night, due in large part to a potent vortex moving through
the Intermountain West and the subsequent downstream height rises
through the middle CONUS and mid-lower Miss Valley. The surface low
as well continues to fill and shift enough toward the ESE to allow
the drying aloft to work down into the boundary layer, resulting in
diminishing rain chances and increasing sunshine. Remaining high PW
over 1.6" and the last lingering traces of the departing trough over
the SE half of the forecast area necessitate leaving a chance of
showers and perhaps an isolated storm in this area, while NW
sections will be drying out through the column. Low level
thicknesses are likely to recover sufficiently (around 10 m above
normal) with greater sunshine (mainly NW half) to push Fri highs
into the lower-mid 80s. The deeper dry air moves into the area from
the NW in full force Fri night, and will taper pops down and out for
Fri night with clearing skies. Lows again in the mid-upper 60s. -GIH
.LONG TERM /Saturday through Wednesday/...
As of 300 AM Thursday...
Extremely low confidence forecast in place as run to run consistency
in the models has been non-existent, particularly on the back end of
the long term forecast. As far as the weekend forecast, a backdoor
cold front is progged to move through late Saturday or early Sunday
and bring temperatures down from the mid to upper 80s on Saturday to
the upper 70s to low 80s on Sunday. Models and forecast soundings
indicate a mostly dry frontal passage at this time.
As the forecast moves into the early part of next week, model
agreement breaks down considerably with the GFS presenting a blocked
flow scenario and the ECMWF not buying it at all as the high to the
north behind the cold front is very transitory, migrating from New
England, out into the western Atlantic. The ECMWF solution allows a
low pressure system centered over the upper plains to move eastward
into eastern Canada and drag a cold front with it. This puts the
cold front over the Carolinas by Wednesday, whereas with the GFS
blocked solution, the same front is not allowed to progress into the
Carolinas until Friday. The upshot of all this is that through
midweek, the difference in model guidance should not affect the
precipitation forecast all that much as most solutions are fairly
dry with only slight chances in the forecast, mainly with moist
return flow around the high in the western Piedmont. Temperatures
may be another story, being much harder to forecast due to the
placement of the high with the GFS solution well north, and thus
cooler than the further south ECMWF scenario. For now will go with
temperatures starting out cool on Wednesday in the mid to upper 70s
and moderating into the low to mid 80s by midweek.
.AVIATION /12Z THURSDAY THROUGH MONDAY/...
As of 655 AM Thursday...
Adverse aviation conditions are likely through at least the morning
hours areawide, with IFR conditions expected at sites along/east of
highway 1 including RDU/RWI/FAY, and MVFR conditions at western
sites including INT/GSO. Areas of light rain will be departing
RWI/FAY but will move into RDU/GSO and perhaps INT over the next few
hours, prior to daybreak. Cigs will lift to MVFR by 16z-18z, and
will likely remain mostly MVFR through most of the afternoon
areawide, except at FAY where cigs should rise to VFR after 18z.
Periodic light rain is expected to persist at RDU/RWI/FAY late
morning through afternoon with occasional MVFR to IFR vsbys.
Cigs/vsbys will once again drop to IFR toward the end of the TAF
valid period (after 05z). Winds will remain from the northeast today
at 6-12 kts with gusts to around 15 mph possible.
Looking beyond 12z early Fri morning: IFR/LIFR conditions are
expected to hold through daybreak in the west (INT/GSO/RDU) and
through mid to late morning in the east (FAY/RWI). As surface low
pressure along the Carolina coast weakens and shifts eastward, with
lessening influence on central NC, conditions will trend to VFR
areawide by 22z with any lingering showers in the east ending. VFR
conditions will then dominate through Monday, although early morning
fog is possible early Sun morning, and MVFR cigs are possible Sun
night through Mon morning as a front drops into the area from the