Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC-- Highlight Changed Discussion --
-- Discussion containing changed information from previous version are highlighted. --
FXUS62 KRAH 270736
Area Forecast Discussion
National Weather Service Raleigh NC
335 AM EDT TUE SEP 27 2016
-- Changed Discussion --A cold front will move into the Carolinas today and will become
quasi-stationary over the area through late in the work week,
bringing unsettled weather through the period.
-- End Changed Discussion --
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --As of 330 AM Tuesday
Deep mid/upper level cyclone over Southern Ontario and the Upper
Great Lakes will drop south into the Ohio Valley this evening and
overnight. Attendant cold front has made its way into eastward into
the Southern Appalachians this morning, but will move very little
over the next 2 to 3 days, becoming quasi-stationary across the
Western Carolinas through Wednesday night as the cyclone is forecast
to cut off from the westerlies and drift farther south towards the
TN Valley/Southern Appalachians. A pre-frontal band of convection
over the NC Piedmont that developed within a swath of strong low-
level moisture convergence in advance of the cold front coming into
the mountains has made little progress eastward,
diminishing/weakening along the eastern flanks as it attempts to
move into the Central Piedmont. Rainfall rates and amounts have
significantly decrease within this convective band, ending any
flooding concerns. Will see a chance of some lingering light rain
showers across the Triangle for the morning commute before further
waning as the showers move east into coastal plain.
Expect a midday to early afternoon lull before weak to moderate
destabilization, maximized across western NC along the zone of
strongest moisture convergence, is expected to lead to the re-
development of scattered to numerous showers and some storms by mid
afternoon and continuing into the evening and overnight hours as the
front settles in the lee of the mountains.
The threat for severe storms will be limited by weak mid-level lapse
rates and cloud cover, but cannot rule out an isolated severe storm
if moderate instability develops as proposed by the NAM. But still
think the primary threat will be localized heavy rain and isolated
flooding given high PWAT airmass of 1.6-1.8".
Highs in the mid to upper 70s north to lower/mid 80s south. Lows
tonight 65 to 70.
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.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/...
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
.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.