Severe Weather Probability (SWP) Product
The Severe Weather Probability (SWP) product
provides an objective assessment of each storms
potential to produce severe weather. Unfortunately, its
sole input is VIL, and inaccuracies in VIL values will
affect the SWP output. Like VIL, SWP is a volumetric
product that uses vertically stacked grids to calculate
severe potential. Why then would SWP be any better
than VIL? SWP grid boxes are much larger than VIL
(24.2 x 24.2 nmi). Each SWP box contains 121 VIL
grids. Since SWP analyses more area, it builds a larger,
more reliable picture of each storm and is less likely to
miss or underestimate them.
SWP automatically extracts strong convective
cells from current reflectivity patterns and estimates
their probability to produce severe weather. Each
convective cell is assigned a number that represents the
probability that the cell will develop into a severe storm
within 30 minutes. By assigning percentage values,
SWP draws the users attention to these dangerous cells
and provides an objective tool for evaluating them.
SWP can be displayed alone, but is best used as an
overlay on other products, perhaps VIL or Base
Reflectivity (REF). Figure 2-41 is an example of the
Severe Weather Probability product.
SWP data is calculated from VIL and ultimately
base reflectivity. Therefore, SWPs are affected by all
of the same factors that affect VIL and REF.
Mesocyclone (MESO) Product
Mesocyclones are areas of strong cyclonic rotation
found in supercell thunderstorms. Such storms are
normally accompanied by severe weather, although
not all mesocyclones produce tornadoes.
The WSR-88D uses extensive computer
processing of velocity data to build an extremely
valuable product called MESO (mesocyclone). The
algorithm continuously searches for rotating wind
fields produced by areas of strong shear. Shear is a
speed and/or directional variation in the wind field
with height. The algorithm is designed to identify
three types of shear and categorize them accordingly;
2-D shear, 3-D shear, or mesocyclone.
2-D SHEAR.This is an area of horizontal
rotation that meets the algorithms shear and symmetry
tests. Symmetry determines the areas balance and
uniformity. 2-D shear lacks vertical consistency. It is
only found at one elevation angle and cannot be linked
at adjacent levels. For this reason, 2-D shear is said to
be "uncorrelated" shear.
3-D SHEAR.3-D shear has vertical consistency
and can be linked to other elevations. However, 3-D
shear fails the symmetry tests required for
mesocyclone classification. The shear area is not
balanced or uniform. 3-D shear is termed correlated
shear because of its vertical link.
MESOCYCLONE. This category identifies
shear regions that meet all algorithm requirements of
size, shape, symmetry, and vertical consistency
associated with a mesocyclone.
MESO, like any product, is just a tool. It should
NEVER be used as a stand-alone source of
information, and its findings should always be
confirmed with other products such as VEL. Although
velocity data is processed up to 124 nmi from the RDA,
the optimum effective range of this product is severely
restricted by beam broadening. The mesocyclone
detection algorithm does not establish time continuity;
this is left up to the operator. Figure 2-42 is an example
of the MESO product.
It indicates that a single
mesocyclone (large circle) has been identified just
northwest of the station.
Tornadic Vortex Signature (TVS) Product
Just as with the MESO product, the Tornadic
Vortex Signature (TVS) algorithm performs extensive
reanalysis of base velocity data to build each TVS
product. This product is designed to search out
tornadic signatures within mesocyclone bearing
The TVS product is a small area of abnormally high
shear commonly associated with tornadoes. Like
mesocyclones, TVSs are first detected at the storms
mid-section and grow, both up and downward, with
time. They reach cloud bases coincident with the
appearance of a funnel cloud (as viewed from below).
Studies suggest that TVSs are detectable 20 minutes
prior to tornado touchdown (on average). Most TVSs
detected by the WSR-88D are associated with
tornadoes. However, not all tornadoes produce a TVS.
Like the mesocyclone product, TVSs primary function
is to alert users of high rotation and shear. An area with
possible tornadic activity is indicated by an inverted