Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Raleigh NC
548 am EST Thursday Dec 25 2014
Synopsis...a cold front will sweep eastward through the region this
morning. High pressure will build into the Carolinas from the
southwest tonight and persist through Friday before shifting
offshore Friday night into Saturday.
Near term /today tonight/...
as of 545 am Thursday...
Overview: a positively tilted /increasingly sheared/ upper level
trough centered along the Appalachians at 07z will progress eastward
through the Carolinas this afternoon and offshore this evening. An
associated cold front (crossing the I-77 corridor at 07z) will
progress eastward through central NC this morning. In the wake of
the front...surface high pressure will build into the Carolinas from the
southwest late this afternoon into tonight in associated/west 500 mb height
rises over the southeastern US.
530 am update: the latest observational data indicates prior
forecast reasoning is in error. The synoptic cold front (approaching
Highway 1 at 10z) has accelerated the residual wedge airmass eastward
through the coastal plain...wiping out the remaining warm sector. As
a result...the thermodynamic environment in central NC is no longer
supportive of additional shower development along the synoptic cold
front...and precipitation chances have been removed. -Vincent
Prior reasoning through 12z: scattered shower activity in advance of
the approaching cold front (still located west of the Yadkin at 07z)
near the Highway 1 corridor is developing along a baroclinic zone
marking the interface between the residual wedge airmass over the
western Piedmont and the pre-frontal warm sector over the coastal
plain...apparently in associated/west shallow low-level convergence and DPVA
attendant small amplitude waves in SW flow aloft in the presence of
weak warm sector instability. Expect the aforementioned activity to
evolve into a more distinct north-northeast-south-southwest oriented line of showers along
the Highway 1 corridor between 08-10z as low-level convergence increases
and stronger DPVA arrives from the west as the synoptic cold front
overtakes (or merges into) the residual cad wedge. Regardless...
shower activity will exit the coastal plain with the front ~12z.
Today and tonight: expect clearing by afternoon as synoptic
subsidence /height rises/ commence in the wake of the sheared upper
level trough and surface cold frontal passage...with highs ranging from
lower 50s northwest to upper 50s southeast. With clear skies...calm winds and a
dry airmass in place once again...expect excellent radiational
cooling with lows in the upper 20s (typical rural and low-lying
aras) to lower 30s elsewhere. -Vincent
Short term /Friday and Friday night/...
as of 345 am Thursday...
Clear skies and dry conditions will prevail as heights continue to
rise in associated/west surface high pressure centered over the Carolinas and an
amplifying upper level ridge over the southeastern Continental U.S.. highs
similar albeit slightly warmer than today...in the middle/upper 50s to
lower 60s. Lows Friday night similar to tonight... in the upper 20s to
lower 30s. -Vincent
Long term /Friday through Wednesday/...
as of 230 am Thursday...
Saturday and Saturday night: high pressure centered over coastal NC
at 12z Saturday will shift offshore as a cold front crossing the
Midwest moves toward the Ohio Valley. Look for clouds to gradually
increase and thicken through the day as SW flow ahead of the front
increases. Otherwise we should see fair-weather with temperatures still 10+
degree above normal...highs around 60 and lows in the Lower-Middle 40s.
Sunday and Sunday night: the chance for rain will return as the
aforementioned cold front drops south and stalls across the
Carolinas...while waves of energy and moisture in the SW flow aloft
move across the region. Daytime highs will be a few degree cooler given
the clouds and rain...but little change for night-time lows.
Early next week...Monday through tuesday: unsettled and potentially
damp weather will continue as the aforementioned boundary drifts to
our south and moist SW flow aloft overruns the cool NE low level
flow north of the boundary. The airmass will gradually cool during
this time as the surface boundary drifts south...allowing the cooler
airmass to settle into the region from the north.
Midweek and beyond: a brief break in the damp weather may come on
Wednesday as high pressure and slightly drier air settles over the
region. This may be short-lived though...as energy ejecting
northeastward from the trough over the SW states...and moisture
lifting north from the Gulf Coast region...approach the Carolinas
for the late-week period.
Aviation /12z Thursday through Sunday/...
as of 545 am Thursday...
24-hour taf period: VFR conditions will rule through the taf period as
a drier airmass advects into the region from the west in the wake of
a recent cold frontal passage. Winds will become westerly/west-southwest at 10-
15 knots with occasional gusts up to 20 knots this morning...eventually
decreasing to ~10 knots during the afternoon...then becoming calm after
sunset as high pressure builds overhead.
Looking ahead: high pressure will linger directly over central NC on
Friday...progressing offshore (though still extending westward into
the area) Friday night and Saturday. As a result...VFR conditions
/mostly clear skies/ and light winds will prevail through Sat.
MVFR/IFR ceilings and a chance for rain can be expected Saturday
night through Sunday night as an approaching cold front temporarily
stalls over the southeastern Continental U.S.. -Vincent