Area Forecast Discussion
Issued by NWS Raleigh/Durham, NC

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FXUS62 KRAH 271421

Area Forecast Discussion
National Weather Service Raleigh NC
1020 AM EDT TUE SEP 27 2016

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.


As of 1020 AM Tuesday

Not making too many changes to the forecast this morning.  The cold
front appears to be just barely east of the spine of the
Appalachians, and will combine with modest differential heating,
from both the terrain and morning low clouds in the western
Piedmont/Foothills to be the main focus for scattered showers and a
few storms this afternoon. With highs expected to reach the
mid/upper 70s north to mid 80s south, RAP forecast soundings suggest
an axis of 1000-1500 j/kg MLCAPE through the southern/western
Piedmont during peak heating, and CAM ensembles show scattered
convection moving east into the Piedmont this afternoon. Mid-level
wind fields are still generally weak with the upper low still over
the upper Midwest, but 25-30kt of effective bulk shear in the
vicinity of the front could support a few strong updrafts, and these
would be more likely across the northern portions of the Piedmont.
An isolated flooding threat may exist with some training cells,
though convection is expected to remain mostly north of the areas in
the southern Piedmont that received the heaviest rain last evening.

Widely scattered convection may continue in the presence of the
front throughout the night. Lows tonight 65 to 70.


As of 358 AM Tuesday...

As the mid/upper level cyclone drifts south into the TN Valley,
models indicate a weak surface wave will develop along the quasi-
stationary frontal zone in place across Western NC. Enhanced low-
level moisture convergence coupled with strengthening vertical lift
from the associated height falls and mid/upper level speed max
spreading east into the area late in the day and into the overnight
hours may lead to the "wettest" period and potentially greatest
heavy rain/flooding threat. The strengthening mid-level flow also
results in improved shear parameters to 30 to 35kts, so cannot rule
out an isolated severe storm Wednesday afternoon.  Highs again
in the mid 70 north to lower 80s southeast. Lows in the 60s.


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


As of 745 AM Tuesday...

The lead 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 ceilings in the Triad
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.  Sub-VFR ceilings are expected again tonight,
especially in the Triad invof the stalled front.

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.





LONG TERM...Vincent
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