Low Precipitation (LP)

The LP supercell storm was identified in the 1970s (Davies-Jones et al. 1976; Burgess and Davies-Jones 1979; Bluestein and Parks 1983). LP supercells are generally dominated by updraft with little precipitation reaching the ground. These storms are visualized by exposed updrafts and translucent to essentially transparent precipitation cores. They have small, less reflective and benign-looking radar echoes; produce little precipitation at the ground; lack a rain-cooled downdraft; and the updraft lies behind the radar echo at low levels. The relative lack of precipitation leads to poor downdraft development and thus these storms could be said to be outflow deficient. LP supercell updrafts often show significantly midlevel mesocyclones. However, low-level mesocyclones are rare owing to the lack of a well defined RFD. There is rarely a hook echo detected unless the radar is very near the storm, and most of the precipitation is carried well downstream of the updraft by the storm-relative upper-level winds. Maximum reflectivities in LP storms can be weak (< 50 dBZ), however, the reflectivity maximum likely consists of a few large hailstones.

Low Precipitation supercells require significant instability and shear; however, other conditions help to reduce precipitation efficiency. Relatively shallow moisture and abundant dry air aloft reduce available moisture and add to entrainment. However, LP storms can also exist where boundary layer moisture is high. Additionally, very high storm-relative anvil-layer winds (>30 ms-1) transport rising hydrometeors well away from the updraft before they descend from the anvil Rasmussen and Straka 1998). Hydrometeors may have little chance of recycling back into the updraft, especially if the mid-levels are dry. LP storms are severe and produce large hail and, although rare, can produce weak to moderate tornadoes while appearing innocuous on radar. Significant tornadoes appear to be rare but they have occurred (Burgess and Davies-Jones 1979). LP storms that move into a region of deeper, low-level moisture may transform gradually into CL or even HP supercell storms.

 

Radar chronology of an LP storm and a list of severe weather events associated
with that storm. Reflectivity contour values are 20 and 40 dBZ, respectively.
Hatching represents ground clutter (from Davies-Jones et al. 1976).
 
 
Top view of an LP supercell
 

References

Bluestein, H. B., and C. R. Parks, 1983: A synoptic and photographic climatology of low precipitation, severe thunderstorms in the southern plains. Mon. Wea. Rev., 111, 2034-2046. 

Burgess, D. W., and R. P. Davies-Jones, 1979: Unusual tornadic storms in eastern Oklahoma on 5 December 1975. Mon. Wea. Rev., 107, 451-457.

Davies-Jones, R. P., D. W. Burgess, and L. R. Lemon, 1976: An atypical tornado-producing cumulonimbus. Weather, 31, 336-347.

Rasmussen, E. N., and J. M. Straka, 1998: Variations in supercell morphology. Part I: Observations of the role of upper-level storm-relative flow. Mon. Wea. Rev., 126, 2406-2421.