Splitting Thunderstorms Along CA Coast--2/28/96

"...a particular concern is that such storms (shallow supercells or mini-supercells or small supercells) are often forecast poorly because of the unusually low CAPE. Our results help to clarify how such environments may still produce shallow supercellular updrafts that could lead to tornadogenesis or other severe weather manifestations..."

from:

McCaul, E. W. Jr. and M.L. Weisman, 1996: Simulations of shallow supercell storms in landfalling hurricane environments. MON.WEA.REV., Vol 124, 408-429.

OAK 12 UTC Sounding + Modification

 

 1200 UTC Sounding

This sounding is typical of those associated with thunderstorms in California. In this case, the morning surface lifted parcel is indicated in green. In gold is shown the temperature of a surface lifted parcel, given the conditions at SFSU at 17 UTC, the approximate time of the split.

As is also typical of California thunderstorm soundings, note that most of the instability is contained in the lower troposphere, with the 500 mb Lifted Index (LI) a poor indicator of the instability. For California cases, the 700 mb LI is more representative. Of course, Convective Available Potential Energy (CAPE) is the best indicator of buoyancy; the LI is often used thoughtlessly.

Note also the directional (veering) and speed shear suggested by the wind barbs on the right margin of the image. This hodograph showed favorable shear for development of supercells.


For this sounding and hodograph, the following apply (these are preliminary calculations: (a) CAPE=712 J/kg; (b) BRN=5; (c) Positive Shear=3.5 X 10 exp-3.


Fort Ord Profiler

 







 Fort Ord Profiler, 22 UTC 2/27 to 22 UTC 2/28

The wind veer with height is well-illustrated by the hourly plots. The observations for the latest times are given on the left side of the diagram. Note that the mid-tropospheric flow was lighter at Fort Ord than at Oakland, as indicated by the wind barbs in the previous figure.





KMUX Base Reflectivity, 1/2 Degree Tilt

Radar displays from San Francisco State University's Weather Graphics and Simulation Lab



 

 


1700 UTC

The left moving portion of the initial cell shows up on the 1/2 degree tilt reflectivity data (approximately 8000 feet Above Ground Level) as a >50dBZ (red) (approx. VIP 5) core. The right moving portion can be seen as a much weaker echo on the southeast flank of the strong cell. Both cells move in such a fashion that a line intersecting their centers is almost parallel to a line of latitude (actually, slightly canted from northwest to southeast). To maintain this geometry, the right mover had to have moved more quickly.





 

 
1706 UTC

The left moving portion of the initial cell has moved northward and weakened. The right moving weaker echo on the southeast flank has moved northeastward and has crossed the coastline just south of Pacifica.

 

 

1712 UTC

The left moving portion of the initial cell has moved northward and continues to weaken. The right moving weaker echo on the southeast flank has moved northeastward and is now on the coastline.

 

 
1718 UTC

Both cells have moved to the latitude of Pacifica, with the left mover moving due northward and the right mover slightly east of north. The left mover continues to weaken but the right mover will intensify to nearly VIP 4 (46 dBZ).

 

 

1724 UTC

Two new small cells are developing on the left flank of the western-most storm.


 

 1730 UTC

The cells have moved to the latitude of southern Daly City. The left mover continues to weaken but the right mover has gotten larger and intensified to about 40dBZ. The smaller developments on the left flank of the westernmost storm have weakened.


 

 1736 UTC

The echoes on the left flank of the system have continued to weaken while the right mover continues as a VIP 3-4 echo. After this time, the right mover weakens considerably, while the western most echo intensifies to between 40 and 50 dBZ once again.

 

 
1742 UTC

Both cells continue their trajectories, the right mover now only a VIP 2 to VIP 3 echo (30 to 41 dBZ) while the left mover is now VIP 4 once again. Note that the motion of the western most storm is parallel to the coast--it never did pass across San Francisco State University. The outflow boundary between the two cells does progress northward across all of San Francisco.

 

 


1748 UTC

The western echo is VIP 4 and moving due northward. The eastern echo is now over San Francisco Bay and moving towards Alameda and Oakland.



 

 

1754 UTC

The western echo is VIP 4 and is now parallel to Golden Gate Park. The eastern echo is now over Alameda and Oakland. The ultimate fate of the western most echo is to merge with the train of echoes responsible for the 1.5 to 2" rainfall totals in south-central Marin County.


 

 
1800 UTC

This case illustrates the fact that forecasters in California MUST be familiar with all the facets of severe weather, including splitting of supercells. It must be kept in mind that splitting is essentially a supercellular process. Also evident here was echo training.