Here is GSP's reasoning:
.SHORT TERM /SATURDAY THROUGH MONDAY/...
As of 250 PM Thu: Still high confidence on the occurrence of a
winter storm developing late Saturday into early Sunday across
the region. Trends in the models unfortunately do not lend much
additional confidence to precip types or transitions. Cold-air
damming still is a key component of the event. The models suggest
the CAD will be a bit stronger at the onset of precip, which
looks most likely Saturday afternoon as a stationary front sets
up between the incoming continental sfc high and the developing
coastal low. The general track of the low is still expected to be
across the Coastal Plain to our south, so the timing of the event
has changed little.
One appreciable difference with this fcst package is lower
overall QPF. Unfortunately for the sake of forecasting p-type
distribution/transitions, models continue to show midlevel temps
flirting with 0C for much of the event. Temps are expected to
remain above freezing for most of the area Saturday afternoon,
though the trend has been colder in latest guidance. A deep but
relatively cool warm layer aloft implies a mix of precip types
is possible late Saturday; increasing precip should strengthen
the wedge through diabatic cooling and profiles will support all
snow over an increasing portion of WNC through Sunday morning. Sfc
temps Sunday are especially tricky given guidance split across
the freezing mark. Have biased the fcst in favor of a GFS-based
blend that did well in last month`s wintry wedge event. Still,
where temps are above freezing sleet is expected to be the main
p-type. The NAM is substantially warmer with temps aloft than
any of the global models we have at our disposal. For now the NAM
profiles are being discounted and not used in the fcst process,
not only due to the consensus in the other guidance, but to help
maintain better forecast continuity--i.e. not to shift too much of
the accumulation from the snow to ice category. But this is a trend
that we will have to monitor in subsequent fcst shifts. We continue
to expect 8-12" of snow over the climatologically coldest part of
the CWA during CAD, i.e., the east-facing Blue Ridge Escarpment
and area extending east along I-40. "Snow" totals have declined by
a few inches over the northern GA/SC zones and in the more western
NC mtns, as more of the QPF is now expected to fall as sleet. Ice
accums remain at least advisory criteria along the fringe of the
sleet area. Confidence is already high enough to warrant a Winter
Storm Watch for all of our NC zones; one is being hoisted with
this package. This is at least 12 hours earlier than we would
normally issue a watch. We thus have decided to leave out all the
GA/SC zones for now and perhaps try to better define the p-type
transition zone. The expected sleet/ice amounts in these zones
will be more sensitive to forecast QPF and therefore a downward
trend could keep some areas out of warning crtieria altogether,
thus not warranting a watch there. Stay tuned.
WAA continues, albeit more weakly, as the sfc low pushes out to sea
Sunday night. At that time we will begin a slow transition from the
CAD/Miller-A forcing to that supplied by a shortwave/deformation
zone transitioning over the area. PoPs decline very slowly thru
Monday as a result. With the WAA, whereas we might normally expect
a transition to all snow across the area, we will continue to see
mixed p-types toward the south and east, wherever sfc temps remain
near or below freezing. Moisture becomes more shallow during this
time, too, putting the possibility of additional snow/sleet in
question as ice nuclei become scarce.
Impacts from the combination of what is expected to be a very wet
snow, combined with icing in some areas, are not looking any less
significant than what the previous shift expected, even though
total amounts may have come down with this package. Very windy
conditions are expected Saturday night and most of Sunday, which
makes falling trees/powerlines likely.
