FXUS62 KRAH 222339
Area Forecast Discussion
National Weather Service Raleigh NC
739 PM EDT Sat Sep 22 2018
A backdoor cold front will sag slowly southward through the area
late tonight through Sunday. Cool high pressure will build in behind
the front and will linger across central North Carolina through
early next week.
.NEAR TERM /THROUGH TONIGHT/...
As of 325 PM Saturday...
The extensive precip and cloud shield is still to our NW and W, NW
of the surface frontal zone, with a few showers over the NC
mountains and a few prefrontal storms from central TN into AL. A
subtle wind shift boundary has pushed SSW through NE sections of the
CWA, although the synoptic front (and the leading edge of lower
dewpoints) still stretches W-E through central VA. Mesoanalyses
depict around 2000 J/kg of MUCAPE across our north, and while mid
level lapse rates are somewhat poor, the low levels show sufficient
instability within 25-30 kts of effective shear to support scattered
to numerous storms, mainly across the northern third. This
convection, and its outflow, is expected to produce a low level cool
stable pool which will draw the synoptic front southward into the
far N sections tonight. As the weak upper divergence skirting by to
our NW (in the right entrance region of a jet streak pushing into
New England) departs tonight, this convection should be largely
diurnally driven, given the lack of substantial dynamic support for
ascent, and expect any storms to fade out late tonight, leaving a
southward-pushing outflow that should propagate southward to the S
and E CWA, while the actual synoptic front pushes further into the
N/W Piedmont overnight. The flow around the offshore-moving 850 mb
anticyclone will bring about enough light moist isentropic upglide
for widespread stratus along and NW of the front. Expect lows in the
mid-upper 60s except low 60s near the VA border. -GIH
.SHORT TERM /SUNDAY AND SUNDAY NIGHT/...
As of 350 PM Saturday...
A cold air damming event will begin Sun. The parent surface high to
our north will settle over NH/VT as it helps push the frontal zone
further south into the CWA Sun and Sun night. Overrunning flow will
abate somewhat during the morning, but considerable low level
moisture will persist over the Piedmont. Despite the lack of
instability or DPVA, minor waves of upper divergence will pass by W
and N sections of NC during the day, and this combined with renewed
moist isentropic upglide late will prompt the need for a chance of
light rain or drizzle across the N and W CWA, within the CAD wedge.
As is usually the case during such events, there will be a tight
gradient of temps and clouds, and as a result the forecast
confidence in both elements close to the frontal zone is low. Expect
highs to range from the upper 60s NW to the low-mid 80s SE. Lows
should be mostly in the low to mid 60s with skies becoming cloudy
areawide overnight as the front settles along S and E edges of the
forecast area. -GIH
.LONG TERM /MONDAY THROUGH SATURDAY/...
As of 345 PM Saturday...
Monday morning a surface cold front will be stalled across our
central zones with a CAD event firmly in place. During the day
Monday more clouds are forecast with highs only around 70 degrees
towards the Triad and highs in the mid 80s near the coastal plain.
One noticeable difference in forecast soundings for Monday is the
presence of dry air around 750 MB with the overrunning signal not
near as bullish as it was yesterday. To be more precise, the upglide
signal is there (as seen on 300 K and 295 K surfaces), but the
moisture is now lacking (as mentioned above). Condensation pressure
deficits also reflect this change as noted above in the GFS forecast
Tuesday morning, the mid level ridge will anchor off the coast of
North Carolina with a low level disturbance pulling northeast around
the ridge. This disturbance will help to finally break the CAD
Tuesday afternoon. Late Tuesday into Wednesday the upper level
trough axis will be centered over the central plains with North
Carolina in between the trough and ridge axis. Multiple rounds of PV
will move overhead across VA during the day with PWATs forecast to
be around 1.80" (90th percentile for this time of year is 1.60").
Due to the more than sufficient lift and moisture, the chance of
precipitation may have to be raised in subsequent forecast packages.
Late Thursday an upper level disturbance over Ontario will rotate
through the main upper level trough axis with a surface cold front
pushing into the western zones. The chance of showers and
thunderstorms looks pretty good with this front as a trailing jet
streak will provide lift in the form of a RRQ. The upper level
disturbance will then pull northeast with a secondary shortwave
diving southeast into Manitoba. This shortwave will provide a
reinforcing cold front late Friday into Saturday morning. It should
be noted though that confidence is low on frontal timing in the
extended as an upper level low is forecast to break off over the
Pacific as the longwave trough axis begins to modulate in amplitude.
Model trends have been for a less amplitude longwave trough and a
stronger ridge across the southern United States. This trend would
favor a warmer forecast with cold fronts potentially not making it
as far south.
.AVIATION /00Z SUNDAY THROUGH FRIDAY/...
As of 738 PM Saturday...
24 hour TAF period: Scattered showers/storms have developed along a
southward-moving backdoor cold front. As the effective front dips
south of the border and convection outflow progresses even farther
south, IFR cigs will develop and spread southward through tonight at
INT/GSO/RDU/RWI, with a trend to MVFR cigs after 05z at FAY. Cigs
will very slowly rise from IFR to MVFR at INT/GSO/RDU during the
late morning and early afternoon Sunday, with mostly VFR cigs at
RWI/FAY. Scattered showers/storms are possible near INT/GSO/RDU/RWI
23z-06z, followed by dissipation overnight. Redevelopment of a few
showers/storms is possible at all sites Sunday aft/eve. Surface
winds will be mostly from the E or NE, gusting periodically at
INT/GSO/RDU Sunday morning. MVFR cigs are expected at all sites
except FAY (VFR) through 21z. -KC/GIH
Looking ahead: Conditions areawide should drop to IFR Sun evening
and persist through daybreak Mon as the backdoor front settles to
our S, with patchy drizzle. Slow improvement to VFR is expected at
all but INT/GSO Mon, where no better than MVFR is expected through
the day. A return to IFR is expected Mon night areawide. Tue/Wed
will see sub-VFR conditions each night/morning with improvement to
VFR each afternoon, along with scattered showers and storms. -GIH
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