SPC
...Day5/Saturday - Mid South and the Tennessee Valley to the
Southern Appalachians...
Severe weather will likely be ongoing at the beginning of the period
from the early morning thunderstorms across Arkansas and vicinity.
There may be a slight lull in storm coverage/intensity during the
diurnal minimum Saturday morning, as the lead wave continues
northeastward and the primary trough approaches from the west.
However, by midday, significant height falls will start to
overspread the warm sector and widespread storm development is
anticipated. The location of overnight convection will have a
significant impact on the zones within the broader risk area where a
higher severe weather threat will exist. However, all guidance shows
a broad warm sector with a broad 50 knot low-level jet, which would
foster an environment favorable for all severe weather types. The
greatest uncertainties are related to the northward extent of the
risk and the surface low track. Given the widespread convection
expected north of the warm front, a convectively reinforced warm
front could set up farther south than the surface low track may
suggest. Also, global guidance still varies with the intensity and
location of the surface-low as it tracks northeast. The track and
intensity of this surface-low will have some impact on the storm
evolution and intensity within the warm sector, especially as it
relates to storm mode and the tornado threat. Maintained 15 percent
probabilities from the previous Day 6 forecast with an expansion on
the southern and eastern extent to account for the threat continuing
late Saturday night.
...Day6/Sunday - Portions of the Southeast into the Carolinas...
A severe-weather threat will likely persist into the Southeast and
Carolinas after 12Z on Sunday, but the magnitude of the threat
varies greatly. Most guidance shows a remnant squall line moving
through the region. With extensive cloud cover, slightly lower
moisture quality, and decreasing ascent, a significant severe threat
would not necessarily be most likely. However, there is still
considerable uncertainty during the Day 6 timeframe, and several of
the GEFS members, including the operational GFS, have a deepening
surface cyclone somewhere east of the Appalachians on Sunday. This
would lead to a more significant severe-weather threat than ECMWF
guidance would suggest and will need to be monitored as a
possibility.
..Bentley.. 12/28/2021