Sunnyvale and Los Altos, CA Tornadoes
May 4, 1998

Updated 1/25/03


This Web Site will be in constant evolution.  The opinions on the storm have not been subjected to peer review in the refereed literature and are the opinion of John Monteverdi alone. They represent only a current working hypothesis on this very interesting case and are in a state of constant evolution as new evidence comes in. 


Please contact John Monteverdi if you have any additional video footage which shows the tornado or the developing funnel.

Photo Shot By Greg Yannazzo and Published in the Chronicle 5/5/98

Estimate that the view is towards the northeast.  Video shows that this feature was rotating clockwise (anticyclonic). There is now mounting evidence that this indeed was the Sunnyvale F2 tornado. Analyses of radar and other videos taken by citizens suggest that the Sunnyvale tornado was anticyclonic and, possibly, multiple vortex. This image suggests a double funnel, with an additional nearly horizontal small funnel off to the left about half-way up the main lowering. The image suggests that another funnel cloud was present on a more distant lowered cloud base.


  • New sections on radar, synoptic and surface controls
  • New high quality graphics in all sections

New evidence and damage reports show that the second tornado (which hit Los Altos about 10 minutes after the Sunnyvale tornado) caused substantially more damage than previously thought.  In addition, the Los Altos tornado caused one serious and several minor injuries.  Post-analysis of damage suggests that the Los Altos tornado was a high-end F1 or an F2.

Forming Funnel.   Contraction and spinup.

Between 0425 PDT and 0432 PDT, 5/4/98, an outflow boundary encountered a developing thunderstorm in the Sunnyvale area.  Both analyses of radar and hodograph information suggest that this storm had a clockwise rotating updraft through a relatively deep portion of the storm.  As the outflow boundary (with a spin around the horizontal axis) encountered the storm, it probably was tilted upwards.  Lateral contraction and vertical stretching would "spin up" the rotation into strong motion, visible as a funnel cloud if cloud material was present.

The picture above left (taken by Karl Anderson) shows both the forming funnel and another anticyclonic vortex visible on its left (north) side. Picture is taken towards the south.  It probably was taken at the same time as the larger picture above. (Click on image to see larger version)

The picture above right (also taken by Mr. Anderson) shows the intensification of the funnel, and propogation downwards.(Click on image to see larger version)

Anticyclonic Rotating Funnel

Anticyclonic striations clearly visible.  This picture is roughly simultaneous with the tornado damage in Sunnyvale.

Looking Up The Throat of The Vortex

(Images Courtesy of D. Misaki)


These three remarkable images were shot from directly underneath the funnel shown above.  Foreshortening has made the funnel itself appear like a dimple.

Click on image to see full-sized version.

Rotation is characterized relative to the ground, as if one is looking down on the motion. Since the images are shot looking at the vortex from below, at first glance it appears as if the vortex were rotating counterclockwise.  However, since rotation is characterized as the motion around a vertical axis perpendicular to the ground, as if the observer were looking down, these photos provide striking corroboration of the clockwise (anticyclonic) nature of the rotation.

The full-sized version shows flecks which, in my opinion, are chunks of debris whirling up and around the vortex.

Click on image to see full-sized version.

The last image here has more of a three dimensional look, with shadowing highlights. This is probably because at this point the funnel had extended downward considerably, and probably was at the stage of the last Anderson picture shown above.

More debris is shown in this picture.

Click on image to see full-sized version.

 Animation of Misaki Images Illustrating
Anticyclonic (Clockwise) Rotation of Vortex

Click here to see an animated gif looking up at the vortex. Since the sense of rotation around a vertical axis is relative to the ground LOOKING DOWN, you will note the clockwise rotation.

Location Maps
Radar (SFSU)
Radar (NSSL SWAT Case Study)
Synoptic Controls
Surface Data Analysis
Conclusions and References

WSR-88D Imagery Received at San Francisco State University, Department of Geosciences, Weather Graphics and Simulation Laboratory 

Page Produced by John Monteverdi and I welcome your comments.


Take The California Tornado Quiz