Area Forecast Discussion
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FXUS62 KRAH 270026
Area Forecast Discussion
National Weather Service Raleigh NC
826 PM EDT MON SEP 26 2016
A cold front will approach from the west tonight and then move
very slowly southeast across central NC through Wednesday.
.NEAR TERM /THROUGH TONIGHT/...
As of 250 PM Monday
Slow erosion of stratus continue across the heart of the Piedmont,
with destablization in the Foothills and western Piedmont owing to
higher dewpoints/theta e across the periphery of the retreating
high off the New England coast. Scattered convection out ahead of
the synoptic front, which stretches from Memphis to Pittsburgh,
will drift east toward the western Piedmont through this evening,
though with time the deep layer flow will back to more
southwesterly and the entire system will slow. Thus, highest POPs
(50-60%) will be over the western Piedmont tonight, with 20% or
less east of US-1. Most Cams show the front hanging up over the
northwest Piedmont, with a couple rounds of showers overnight,
which could lead to some localized minor flooding, particularly
given an area of 2+ inch PW off the SC coast they should be tapped
by southeasterly flow this evening. However, there isn`t enough
support from guidance to raise flags about any significant
flooding threat. Lows 66-69.
.SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/...
As of 250 PM Monday...
The cold front will inch eastward into the Piedmont on Tuesday,
slowed further by system parallel flow induced by the upper low
that will close off and dig toward the Tenn Valley. PW will remain
relatively high at 1.75", though the source of higher PW off the
coast looks to be cut off by a surface wave that lights northeast
along the NC coast. Forcing is pretty much limited to moisture
convergence along the front, with better height falls later
Tuesday night ahead of the aforementioned upper low. Despite weak
lapse rates aloft, highs in the upper 70s/lower 80s should result
in weak instability and trigger scattered to numerous showers and
a few isolated, mainly in the Piedmont. Increasing mid-level wind
fields will increase the shear over the region Tuesday afternoon,
but weak instability (better Wednesday) will ultimately limit the
threat of severe storms. Based largely on WPC guidance, 0.5-1.0"
of rain is generally expected.
As mentioned above, forcing aloft is a little better late Tuesday,
albeit with less instability (though forecast soundings do show
some thin MUCAPE that may enhance parcel lift). The front isn`t
expected to make much progress eastward, so there may be a
limited flooding threat worth watching Tuesday evening, largely
dependent on how active the front is tonight and Tuesday. Lows
again in the upper 60s.
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
As of 345 AM Monday...
PoPs will be higher...chance category...in the east on Wednesday
due to proximity of the lingering frontal zone where moisture
advection will maintain precipitable waters in excess of 1.5
inches. Dry air wrapping into the area around the upper low will
be sufficient to produce a chance of thunder...especially I95
eastward. Highs will be similar to Tuesday...mid-upper 70s with
some lower 80s in the southeast.
Upper cutoff low over the Great Lakes throws a lot of uncertainty
into the forecast heading into the late week...but at this point
both GFS and ECWMF stall the southerly and easterly progress of the
system in the Tennessee Valley and lift it back north into the Great
Lakes over the weekend. Dry trajectory and maintenance of neutral
temperature advection around the low supports our ongoing dry and
seasonable forecast with highs mostly in the mid to upper 80s.
Morning lows will feel nippy...with mins falling into the mid to
.AVIATION /00Z TUESDAY THROUGH SATURDAY/...
As of 825 PM Monday...
A line of showers and thunderstorms well in advance of the
approaching cold front has moved through KINT/KGSO, with light rain
continuing behind it. This activity will continue overnight, moving
as far east as KRDU. IFR stratus will likely develop at KINT/KGSO
Tuesday morning before lifting Tuesday afternoon. Scattered showers
and storms will develop across the area Tuesday afternoon and may
bring periods of sub-VFR conditions.
Looking beyond the 24 TAF Period: The combination of the stalled
front and an upper low over the Tennessee Valley will continue the
unsettled weather Tuesday night and Wednesday. Conditions are
expected to improve to VFR from northwest to southeast by Wednesday
night as drier air filters into the area.