Mesoscale Data
Analyzed surface pressure and wind barbs from SPC at 20 UTC on Tuesday, May 20, 2008
A surface low was located off the Virginia coast with a trailing cold front
extending southwest across northern North Carolina.
Analyzed surface temperatures (red/purple), dew points (brown/green), and wind barbs from SPC at 20 UTC on Tuesday, May 20, 2008
A cold front stretched from Southeast Virginia southwestward across northwestern North Carolina.
Temperatures ahead of the cold front were in the lower to mid 80s with dew points in the lower 60s.
Analyzed low level lapse rates in the 0-3 km layer (blue, green, and orange) from SPC at 20 UTC on Tuesday, May 20, 2008
A lapse rate is the rate of temperature change with height and the image below
is for the layer from the surface to around 10,000 feet. Note the surface based, low level
lapse rates shown below range in the 6.5 to 7.0 deg C/km across much of central North
Carolina. Values less than 6 degrees C/km represent "stable" conditions, while values near 9 degrees
C/km are considered "absolutely unstable." The lower portion of the atmosphere on May 20
was obviously very unstable.
Analyzed mid level lapse rates in the 700-500 mb layer (blue, green, and orange) from SPC at 20 UTC on Tuesday, May 20, 2008
A lapse rate is the rate of temperature change with height and the image below
is for the layer from around 10,000 to 18,000 feet. Note the mid level
lapse rates shown below range around 6.5 deg C/km across much of central North
Carolina which was only slightly unstable. The greatest mid level instability,
associated with the elevated mixed layer was present across far southern North
Carolina, South Carolina, and Georgia.
Analyzed surface based convective available potential energy (SBCAPE) (red) and surface based
convective inhibition (blue lines - shaded) from SPC at 20 UTC on Tuesday, May 20, 2008
SBCAPE values ranged between 1500 and around 2000 J/kg ahead of the cold front in central North Carolina
with no significant convective inhibition (CIN).
0-3 Km Storm Relative Helicity (blue) and storm motion (brown) from SPC at 20 UTC on Tuesday, May 20, 2008
The 0-3 Km SRH values ranged between 100 and 200 units across central North Carolina with the
greatest values in southern North Carolina.
CAPE in the layer from -10 C to -30 C, 0-6-km shear vector, and the freezing level height from SPC at 20 UTC on Tuesday, May 20, 2008
Large CAPE in the layer from -10 C to -30 C favors rapid hail growth. 0-6-km shear in excess of 30-40 knots
supports supercells with persistent updrafts that contribute to large hail production.
Finally, lower freezing level heights suggest a greater probability of hail reaching the surface prior to melting,
though melting impacts small hail much more than very large hailstones.
Analyzed Lifting Condensation Level (red, blue, and green) from SPC at 20 UTC on Tuesday, May 20, 2008
The LCL height is the height at which a parcel becomes saturated when lifted dry adiabatically.
The importance of LCL height is thought to relate to sub-cloud evaporation and the potential
for outflow dominance. Low LCL heights imply less evaporational cooling from precipitation
and less potential for a strong outflow that would likely inhibit low-level mesocyclone development.
Thunderstorms that produce tornadoes generally have a lower LCL height with LCL heights
less than 1,000 meters typically favorable for significant tornado development. The LCL values during this
event were around 1,500 meters.
NWS Composite Reflectivity Imagery from 0130 UTC on Thursday, May 20, 2008 (930 PM EST).
The composite reflectivity imagery is from the approximate time in which the analysis imagery above is valid.
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