This paper was presented at the 20th Severe Local Storms Conference in Orlando Florida, September 2000.
13.8 DOCUMENTATION OF HIGH BASED THUNDERSTORMS DEVELOPING ON A BOUNDARY WHICH BECAME TORNADIC
This paper documents two events in which thunderstorms developed over extreme southeast Colorado and became tornadic. In the first event, synoptic conditions were favorable for rotating storms (moderate shear and moderate CAPE), while in the second case synoptic conditions were only favorable for strong convection (weak shear, high CAPE). What is common between these two tornadic events is: 1.) A well defined low level boundary was in place prior to tornadogenesis, 2.) The storms were high based, and 3.) Both storms produced multiple tornadoes, some tracking on the ground for many kilometers.
2. CASE I: HIGH BASED ROTATING STORM PRODUCING MULTIPLE TORNADOES: 29 JUNE 1999
The combination of an upper level short wave associated with a 50 ms-1 jet approaching the Centennial state from the northwest, a developing surface low over the southeast part of the state, and sufficient low level moisture set up a favorable pattern for rotating convection over extreme southeast Colorado. In addition to the favorable synoptic pattern, a well defined and broad boundary was located over the southern sections of southeast Colorado. Surface data at the time of convective initiation over extreme southeast Colorado indicated dewpoint depressions of 13-16 oC. Using the 12 UTC Dodge City sounding data and observed surface temperature/dew point values in the region of interest indicated CAPE values of ~1600 J/KG, and an LCL of 725 mb, or ~2.6 km above ground level.
The first reflectivity echo associated with the storm of interest developed along the boundary shown in figure 1 at 1842 UTC in Baca county, in far southeast Colorado. Radar reflectivity character of this cell prior to and up to the time of tornadogenesis indicated this storm was small both vertically and horizontally. In addition, 0.5 degree reflectivity values were modest, with values generally below 50 dBz prior to tornadogenesis. These radar characteristics are typical of low precipitation (LP) supercell storms (Doswell and Burgess, 1993). However, the cell did exhibit radar reflectivity signatures very early in its lifetime indicative of rotation, as a low level reflectivity gradient, at times evolving into a weak appendage feature, was noted in the reflectivity data. In addition, a manually identifiable, albeit weak, mesocyclone was observable in the 0.5 degree storm relative velocity display beginning at 1925 UTC. This weak rotation persisted until the time of the first tornado report (2000 UTC). After this time the mesocyclone intensified, reaching moderate to strong criteria (OSF 1995). This mesocyclone continued at moderate to strong criteria until it moved out of the operational range of the KPUX WSR-88D. This storm would produce a total of three tornadoes in Baca county between 2000 and 2040 UTC. One of the tornadoes was on the ground for over 10 kilometers.
A storm chaser in the region who observed the storm when
it was producing the third tornado reported the cell had visual "LP" supercell
characteristics. He observed the storm was very high based and no precipitation
was observable during the time of the tornado.
3. CASE II: HIGH BASED NON ROTATING STORM PRODUCING
MULTIPLE TORNADOES: 2 SEPTEMBER 1996
The second case was different from the case above, in that this tornadic cell developed in an environment which deep layer shear was weak, but instability was high. Upper air data on this date indicated the jet stream was well to the north of Colorado (over the northern tier of the western United States), with a dominant area of high pressure over the southwestern section of the country. Examination of the 12 UTC Dodge City sounding on this date indicated a potentially very unstable atmosphere was in place. Realizing a temperature in the mid 90s would support CAPE in excess of 4000 J/KG, with a cloud base located at 700 mb. As discussed above, deep shear (0-6 km) was negligible as maximum wind speed values within this layer were less than 8 ms-1 (a strong low level jet was in place in the lowest kilometer of the 12Z sounding, but this feature likely dissipated as the morning progressed).
Similar to case I, a low level boundary was evident in the KPUX radar data extending across southeast Colorado. Unfortunately, surface observation data and regional satellite data was not available for examination of this event. Surface RUC analysis indicated a large gradient of theta-e air over far southeast Colorado. In addition to the radar data, the development of convection was indicative that a boundary was in place, as convection developed linearly during the afternoon hours from southwest to northeast, developing first in northern New Mexico, across southeast Colorado, and then into Kansas.
Radar analysis indicated the cell which produced the tornadoes developed approximately 10 km south of the boundary, which was located just north of Springfield at 2123 UTC. This cell moved slowly north and intercepted the boundary shortly after 2200 UTC. According to STORM DATA (NOAA, 1999), all three tornadoes developed simultaneously at 2220 UTC in the vicinity of Springfield. Two tornadoes were short lived, while the third tornado was on the ground for over 10 kilometers. A WATADS analysis of KPUX radar data indicated no discernable rotation, or other supercellular characteristics, throughout the lifetime of this tornadic cell.
4. Discussion and Conclusion
Recent research has shown thunderstorms (both supercell and non-supercell) are more likely to produce tornadoes if the storm interacts with a pre-existing boundary. Regarding rotating convection, Markowski et. al. (1998) documented nearly 70 percent of tornadoes which developed in the VORTEX 1995 domain were associated with preexisting low level boundaries. In addition, other research over the years has implied boundaries were associated with tornadic supercell storms (see Markowski 1998 for a review). Boundaries have also been shown to be important for non-supercellular tornadoes. Brady and Szoke (1989) documented a tornado which developed in a weakly sheared environment. Using high resolution radar data, they showed how preexisting horizontal vorticity associated with a boundary developed into a tornado when this vorticity was stretched into the vertical under a rapidly developing cumulonimbus cloud.
It is believed the tornadogenesis mechanism associated with case II in this study is similar to what Brady and Szoke described in their research. Case I of this study however, is not as clear. The tornadoes which developed on this day developed from a rotating LP storm which had a very high base, estimated to be 2.6 km above ground level. This value is believed to be quite high for a tornadic storm. Rassmussen et. al. (1998) observed tornadic activity was more likely to occur if the lifted condensation level was below 1.2 km. In this case, the cloud base was more than twice as high.
It is possible a hybrid type of tornadogenesis mechanism occurred in case I of this study. Frequently, LP supercell storms are non-tornadic (Doswell and Burgess, 1993). In this case, multiple tornadoes occurred with an LP supercell storm. It is believed the boundary in which this storm initiated on played an important roll in tornadogenesis. It is possible low level horizontal vorticity associated with the boundary was stretched into the vertical by the high based rotating updraft, producing the tornadoes (similar to the non supercell tornadogenesis mechanism discussed in Brady and Szoke ). What is different from non supercell tornadogenesis is the updraft in which the vortex encountered was already rotating.
Forecasters should have high situation awareness when strong convection develops on, or interacts with a boundary. In case I, an LP supercell storm, frequently not associated with tornadoes, interacted with a boundary and became tornadic. In case II, convection which was in a very unstable, but weakly sheared environment interacted with a boundary and became tornadic. In both cases, multiple tornadoes occurred, with some of the tornadoes remaining on the ground for a significant distance.
The author thanks Al Pietrycha for reviewing this document
and Lyle Barker (SOO NWS GLD) and Paul Wolyn (SOO NWS
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Brady, R. H. and E. J. Szoke, 1989: A case study of nonmesocyclone tornado development in northeast Colorado: similarities to waterspout formation. Mon. Wea. Rev., 117, 843-856
Doswell, C. A., and D. W. Burgess, 1993: Tornadoes and tornadic storms: a review of conceptual models, in The tornado: Its structure, dynamics, prediction, and hazards. Edited by C. Church et. al., Geophysical Monograph 79, pp 161-172.
Markowski, P. M., E. N. Rasmussen,, and J. M. Straka, 1998: The occurrence of tornadoes in supercells interacting with boundaries during VORTEX-95. Wea. Forecasting., 13, 852-859.
NOAA, 1999: STORM DATA. National Climatic Data Center, Asheville, NC.
OSF, 1995: The WSR-88D operators guide to mesocyclone recognition and diagnosis. WSR-88D Operational Support Facility, Norman, OK. pp 111.
Rasmussen, E. N., and D. O. Blanchard, 1998: A baseline climatology
of sounding derived supercell and tornado forecast parameters.Wea. Forecasting.,13,