Severe Weather Potential
A band of showers with a few embedded thunderstorms crossed central North
Carolina during the early morning hours on Friday, January 14th. The
thunderstorms had a history of producing severe weather (largely damaging
winds) as they moved across the Mississippi and Ohio Valleys.
In advance of these severe weather producing storms, the pre-storm environment
characterizing much of central North Carolina indicated the potential for
additional severe weather. A deep southerly flow into North Carolina was originating
from the Gulf of Mexico, transporting considerable moisture into the
area. Indeed, the precipitable water values were around 1.33 inches, well
above normal for the time of year.
The 00Z January 14, Greensboro (GSO) sounding depicted a strong southerly flow.
In particular, note the strong speed shear (rate of change in wind speed
with height) in the low level winds. The winds quickly increased from 15 knots
at ground level, averaging around 50 knots through a deep 10,000 foot layer.
There is also some turning of the wind (helicity) seen as southerly surface winds
quickly became southwesterly. The presence of strong speed shear near the
surface and through a deep layer indicated a high potential for wind damage from
convective storms. Since there was also some turning (spin) to the low level
winds, the potential for tornadoes also needed to be closely followed.
The 00Z January 14 GSO sounding
revealed additional clues regarding the potential
for severe storms. The change in temperature with height (lapse rate) is
relatively small from near the surface through a deep layer.. The relatively small
lapse rate limited the air mass's instability and thunderstorm intensity;
however some passing showers did precede the stronger main convective line and might
have resulted in a little more instability via cooling of the
temperatures in the previously unsaturated layers and hence a stronger lapse rate
(change in temperature with height).
The above pre-storm characteristics, especially the very strong winds just off the
surface, prompted the National Weather Service radar operators to closely
monitor those convective storms featuring any type of bowing or shearing pattern as
indicated by radar signatures. Such signatures are indicative of strong
winds. Most of the severe storm warnings were either based on bowing segments or
convective elements where gate-to-gate shear (large changes in wind
speed and direction over a short distance) appeared more significant than in other
parts of the line of convection. Only a few thunderstorms exhibited rotation
indicative of mesocyclones. A tornado may develop from a mesocyclone's rotation
and extend below cloud to the ground. Fortunately, the mesocyclones found in this
severe event were short-lived and relatively shallow with depths ranging from between
7,000 to 12,000 feet. The storms appeared to intensity as they approached the northeast Piedmont
counties where there was a little more turning (helicity) of the low
level wind while the air mass remained just marginally unstable (lifted indices ranging
from 2 to -1).
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