Good synopsis from the NWS Raleigh AFD
.DISCUSSION...
As of 122 PM Saturday...
KEY MESSAGE 1...
A strong mid-level system will move across the region tonight
through early evening Sun, where a band of rain changing to
rain/snow and then snow on the back end will favor a slushy
accumulation of snow over northern and northeast sections of central
NC.
A rather vigorous upper-level shortwave, presently on satellite over
northern TX, will round the base of a larger longwave trough set to
move into much of the eastern US Sun. As the shortwave rounds the
base of the trough over the Carolinas Sun aftn/eve, it will become
negatively tilted before exiting off the Mid-Atlantic coast Sun
night into Mon. An impressive 150-180 kt jet streak will be
positioned from SW to NE from the southern Appalachians into the Mid-
Atlantic, putting central NC in the favored right-entrance region
for enhanced lift at mid/upper levels, primarily from overnight
tonight into early evening Sun.
At the surface, we will see a cold front slowly sag south and east
from the west on Sun, crossing over the mountains and oozing into
eastern sections of the state Sun/Sun night, as an area of low
pressure develops off the coast and tracks off the NE US Sun night.
Precipitation amounts have increased since the last forecast
issuance. This is driven by strong isentropic ascent and some
frontogenesis driven by the jet/shortwave and attendant PW-values
120-130 percent of normal. QPF amounts could range from a a tenth to
three tenths of an inch in the Triad, to one half to three quarters
of along/east of US-1 to the Coastal Plain. Rain should overspread
the region as early as late this evening, but should really get
going after midnight into early Sun morning, when the forcing really
blossoms with the jet. A changeover from rain to rain/snow is
favored for northern areas Sun morning to early Sun afternoon,
becoming briefly all snow in the mid to late afternoon across the
north before exiting early Sun evening.
The main challenge with this system continues to be how fast the
cold air can arrive before the precipitation shield exits early Sun
evening. The other question is whether mesoscale bands within the
precip shield can favor diabatic cooling for a quicker changeover to
snow. If one takes a look at the mesoscale and global models from a
forecast sounding standpoint, there is actual decent agreement on
thermal profiles. Thicknesses would support a narrow 1-3 hour window
when precipitation would changeover to all snow, which is roughly 9a-
12pm in the NW, 1-4pm in the Triangle and points N and NE, and 3p-
6pm in the central/northern Coastal Plain. Little if any snow is
expected in the Sandhills and southern Coastal Plain. Overall,
expected snow amounts have not changed much, with a slushy trace to
half inch or less of snow on mainly elevated/grassy surfaces.
Where could this forecast go off the rails and reach the high
end/worse case amounts of 1-1.5 inches? This scenario could happen
if we see some isolated mesoscale precip bands that enhance diabatic
cooling of the column. There could also be a narrow 1-3 hr window
when mid-level lift/fgen could coincide with saturation in the
dendritic growth zone to favor a narrow zone of heavier snow rates
and support this higher worse case scenario. If this were to occur,
it would roughly be from just east of Burlington to Roxboro to NE of
Raleigh to Henderson to Roanoke Rapids, where HREF/LREF snow
probabilities for this worse case are highest. Confidence in this
scenario remains low but HREF probabilities for 1-1.5 inches do show
30-40 percent values in this region, a little higher than the global
LREF of 15-percent. For now, will continue to message the most
favored outcome noted earlier.
Precipitation wraps up early Sun evening and temperatures fall into
the 20s overnight into Mon, which may result in some refreezing and
icy patches on roadways Sun night to Mon morning.