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CSTAR - High Shear Low CAPE

Our regional partner forecast offices have identified substantial challenges in prediction,
nowcasting, and warning for severe storms in environments with large vertical wind shear
but small instability (i.e. CAPE). Such high-shear/low-CAPE (hereafter, “HSLC”) severe
storms may take the form of squall lines/bow echoes or, alternatively, the form of “miniature”
(also known as “low topped”) supercells. These HSLC outbreaks are often characterized by
widespread severe wind reports, and less frequently by tornadoes. HSLC tornado days are
especially challenging because such tornadoes are difficult to identify on radar, and they often
occur during the cool season when many people assume that severe convective storms are highly
unlikely.

The mean tornado environment in the southeastern US (i.e. the corridor from Georgia through
Virginia) is characterized by CAPE values below 1000 J kg-1, an amount that is quite low in
comparison to the values that are routinely observed in the tornado environments of the Great
Plains. It is fairly well-known that vertical wind shear can dynamically compensate for limited
buoyancy in HSLC environments. However, in operational settings it is difficult to usefully
quantify the degree to which shear will offset low CAPE on a given day. The long range goal
of this effort is to improve forecasts and warnings for severe weather and to reduce false alarms
in HSLC convective environments. The specific objectives of the proposed activity are: 1) to
catalog and understand the spectrum of southeastern HSLC convective storms in terms of their
most common radar signatures and their severe weather production; and, 2) to identify the most
effective environmental parameters for operationally discriminating between high-end (i.e.
widespread severe winds or tornadoes) HSLC cases and null cases in the Southeast.