Doppler Radar data received and processed at San Francisco State Univesity's Weather Graphics and Simulation Lab

See NSSL's analysis of the Level II Archive for this event (authored by Greg Stumpf)

See technical discussion of radar plots, with real imagery from KMUX for the event and a stunning animation

KMUX Reflectivity  

2225 UTC BREF 1/2 KMUX

A=Milpitas storm; Y=storm on outflow boundary. Developing Sunnyvale Storm just to west of storm at "Y."  Outflow boundary also spreading eastward.

2245 UTC BREF 1/2 KMUX

Storm "Y" and outflow boundary merges with Sunnyvale storm. Landspout F2 (marked by "X") occurs about this time.  Storm "Z on other outflow flank. "

Base Reflectivity, 1/2o tilt, KMUX

2300 UTC 4/4/98 - 0030 UTC 4/5/98

Same cell associated with Los Altos F1 and possible tornado in Palo Alto mudflats.

Note that this cell produced tornado in Sunnyvale when outflow from Milpitas storm encountered it and when another cell along the outflow boundary merged with it.

Note that another cell forms on east flank of outflow, in eastern portions of San Jose.

Tornadic storm moves with outflow boundary and becomes portion of a small Mesoscale Convective Complex that extends towards Livermore.

KMUX Storm Relative Velocity Plots 

Storm Relative Velocity, 1/2o tilt, KMUX

2300 UTC 4/4/98 - 0030 UTC 4/5/98

Outflow from Milpitas storm is green area.  Eventually weak couplet forms on Sunnyvale storm (green on east, magenta on west) as anticyclonic rotation builds in the air ingested from the environment through the midlevel of the storm.

The hodograph (shown in previous section) shows the wind shear vector backing with height in the lowest layers. This favors clockwise rotation in the updraft.This anticyclonic rotation is known as a "mesoanticyclone"  (as opposed to a cyclonically-rotating updraft--"mesocyclone").

The wind shear profile also favors leftwards motion of the storm relative to other storms in the area. Interesting to see the Sunnyvale storm move northwestward on radar while other storms were moving northward or northeastward.

Negative numbers inbound storm relative speeds(kt), positive
numbers outbound storm relative speeds (kt)

The evidence is now mounting that the impressive feature shown on the opening page was indeed the Sunnyvale tornado.

At the same time that the environmental shear produced the anticyclonic rotation through a deep layer (shows up higher in the storm, and is visible in video tapes of the funnel cloud shown on first page), the outflow boundary from the neighboring storms intercepted this storm from the northeast. The rotation in the storm initially was not sufficient to stimulate the cascade of processes which result in a supercell tornado.

This outflow boundary was oriented north-south, with warmer air on the west, and just reached the Sunnyvale storm from the northeast at around 2030 UTC. Outflow boundaries have rotation around a horizontal axis with warm air rising and cold air sinking . When this is ingested into the updraft of the Sunnyvale storm, it is stretched and contracted, and, just as a ballet dancer spins faster, this produced an anticyclonic rotation that built from the ground upwards into the base of the storm. There is evidence that this rotation actually was such that the Sunnyvale tornado was briefly multiple vortex. (Two or more subvortices rotating counterclockwise around a parent weaker vortex. This can also be seen on video--the main swirling debris is clockwise-rotating, while brief suction vortices can be seen on one side).

Thus the storm was rotating clockwise (anticyclonic) (also known as a supercell), and produced a clockwise (anticyclonic)-rotating landspout.

KMUX Storm Top Plot

Storm Tops at 2345 UTC 5/4/98

(Click for animation)

Storm Tops

2300 UTC 4/4/98 - 0000 UTC 4/5/98

(Click on Image For Animation)

Maximum top on this storm was around 34000 feet. (Gold area on 2345 UTC Image)

Storm Tops (1000's feet)