Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
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FXUS62 KRAH 270643
Area Forecast Discussion
National Weather Service Raleigh NC
243 AM EDT MON SEP 26 2016
A cold front will stall across the Carolinas today through late in
the work week, bringing unsettled weather through the period.
.NEAR TERM /OVERNIGHT/...
As of 910 PM Monday
Have adjusted PoPs upward over the western-nw Piedmont as a band of
convection has arrived and has begun to slow its eastward
progression. This is raising concerns for a localized excessive
rainfall, mainly just south of Winston-Salem and Greensboro,
encompassing Lexington and Asheboro, and west of RDU, affecting the
Chapel Hill/Carrboro area. Some locations in the area described
above may receive 2-3 inches of rain in less than 3 hours before 2
Am Tuesday. This may cause a few creeks and streams to leave their
banks and may lead to an isolated road closure or two. Believe the
threat is too isolated for a flash flood watch but will continue to
The scenario is a classic set-up with a slowing band of convection
becoming perpendicular to a sly feed of moist air. Tonight, this
west-sw/e-ne band is encountering a sly south of 15-20kts in the 925-
850mb layer, advecting moist air with precip water values 1.6-1.8
inches, close to 150 percent of normal. Additional lift being
supplied aloft by 100kt jet crossing the eastern Great Lakes,
placing the nw Piedmont in the favored right entrance region.
Farther east, mainly for areas east of highway 1, overcast skies and
and a few spotty showers are expected though overall coverage and
intensity are expected to be spotty and light.
Overnight temps will be fairly uniform due to the overcast skies and
light sly flow. Min temps mid-upper 60s.
.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 /THURSDAY THROUGH TUESDAY/...
-- Changed Discussion --As of 245 AM Tuesday...
An upper level low over the northern Great Lakes will dig southward
through the OH valley today/Wed, cut-off over the TN Valley Thu/Fri
as an expansive upper level ridge builds over the eastern 2/3 of the
CONUS, then lift northward back to the Great Lakes Sat/Sun as the
ridge breaks down in response to potent shortwave energy digging
southeast from the Pacific NW to the Intermountain West/4-corners
region. What remains of the ridge will shift downstream of the OH/TN
valleys over the Mid-Atlantic/Carolinas early next week as
additional shortwave energy moving ashore the Pacific NW digs SE
through the Intermountain West, further amplifying a deep upper
level trough over the Rockies. With the above in mind, expect
slightly below normal temps Thu/Fri and lingering precipitation in
assoc/w the cut-off upper low possible on Thu. A dry mid-upper level
airmass wrapping around the southern periphery of the upper low will
likely preclude any potential for precipitation on Fri. Dry
conditions and near normal temperatures are expected to prevail for
the remainder of the long term period, Sat/Sun as the upper low
lifts north back to the Great Lakes and Mon/Tue as the
aforementioned ridge aloft shifts eastward from the OH/TN valleys
into the Mid-Atlantic/Carolinas. -Vincent
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.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
As of 156 AM Tuesday...
A pre-frontal band of showers and thunderstorms over the NC Piedmont
will move slowly eastward this morning and into the midday hours,
diminishing/weakening along the way. IFR to MVFR ceilings through
12z should remain confined to KINT, KGSO, and KRDU, along the axis
of stronger low-level moisture transport. These sub-VFR ceilings
should gradually lift to VFR between 15 to 18z, with additional
showers and storms re-developing across western NC and moving
eastward this afternoon and evening as the sfc cold front stalls
across western NC. Otherwise, at KFAY and KRWI, expect predominately
VFR ceilings with some isolated shower or two possible.
Looking beyond the 24 TAF Period: The combination of a quasi-
stationary front across the Carolinas and a cut-off upper level low
settling over the Tennessee Valley will keep unsettled weather in the
form of scattered to numerous showers and storms through Thursday.
Drier air from the west should gradually advect east into the area
late Thursday and into Friday, leading to improving conditions
with dry VFR conditions expected Friday and Saturday.