GSP afternoon disco:
.SHORT TERM /FRIDAY NIGHT THROUGH SUNDAY NIGHT/...
As of 215 PM EST Thursday: The latest trends suggest more of a warm
nose developing in model profiles for the weekend wintry weather
event. This will toggle our forecast a bit further toward the
freezing versus frozen end of the ptype spectrum, but still with a
very mixed mag of ptypes across mainly western NC.
We expect this event to unfold from the influence of a 500 mb trough
deepening slightly over the southern plains on Friday night. The
resulting upper trough is expected to remain strongly positively
tilted, with the main shortwave and associated DPVA moving toward
the southern Appalachians Saturday through Saturday night. A coupled
upper jetlet structure will set up east of the trough to provide the
best deep-layer forcing Saturday night. Isentropic lift could start
up from the south a bit earlier on Saturday as southerly flow sets
up over a developing surface-based cold layer strengthening from
1033 mb high pressure over Pennsylvania. The Friday night through
Saturday forecast remains difficult because profiles will be far
from saturated, but the cool and dry air could wet bulb down to
support some sleet and snow at onset across a good chunk of the
western Carolinas and northeast Georgia, especially if moisture flux
from the eastern Gulf of Mexico arrives at lower levels a bit sooner
than expected. However, the deepest moisture and forcing should
begin to arrive Saturday evening and cross the region overnight
through Sunday. Although the upglide diminishes a bit as winds turn
more westerly through the day on Sunday, steeper lapse rates under
the lower heights aloft could increase precipitation rates under
redeveloping showers.
Regarding the thermal setup, the 1033 mb surface high pressure will
provide a very favorable cold air damming setup by daybreak on
Saturday. Surface wet bulb temperatures will become increasingly
critical Saturday night as the wedge becomes entrenched. The cooler
raw model guidance will be followed in this regard. A NAM/GFS blend
on profiles will bring a more robust warm nose across the region to
trend precipitation from frozen to freezing Saturday night from
south to north, with the best chance of advisory level, or better,
snow/ice accumulations occurring along and north of I-40. Since the
heaviest of the wintry precipitation will occur in period 5, and we
are we still bouncing around on QPF and ptypes given the likely
transitions, we will hold off on any Watch issuance for at least
another model cycle and keep the Outlook information going.
Mid-level dry slotting will likely wrap in from the southwest late
Sunday through Sunday night. In addition, the briefly warmer temps
and profiles by Sunday afternoon will start cooling again from the
north through Sunday night as the surface low pressure system gets
wrapped up offshore and begins to pull away. This could lead to a
transition back to snow showers from north to south across western
NC for late weekend. Additional vorticity will remain very channeled
south of the elongated trough axis through Sunday night, so
precipitation rates should be a bit lighter.