During the month of March, the "Storm Door" opened for much of California as several storms brought hazardous weather to the region. A plethora of weather was observed from floods to mudslides and waterspouts to tornadoes.

First let's define a waterspout and a tornado:

• Waterspout: In general, a tornado occurring over water. Specifically, it normally refers to a small, relatively weak rotating column of air over water beneath a Cb or towering cumulus cloud. Waterspouts are most common over tropical or subtropical waters. The exact definition of waterspout is debatable. In most cases the term is reserved for small vortices over water that are not associated with storm-scale rotation (i.e., they are the water-based equivalent of landspouts). But there is sufficient justification for calling virtually any rotating column of air a waterspout if it is in contact with a water surface.
• Tornado: A violently rotating column of air, usually pendant to a cumulonimbus, with circulation reaching the ground. It nearly always starts as a funnel cloud and may be accompanied by a loud roaring noise. On a local scale, it is the most destructive of all atmospheric phenomena.

Even though tornadoes and waterspouts are somewhat rare in the San Francisco Bay Area they do occur. Over the past 20 years 23 tornadoes have been reported in the Monterey WFO County Warning Area (CWA). The vast majority of these tornadoes were ranked EF0 to EF1 on the Enhanced Fujita Scale. However, on May 15, 1998, a F2 (original Fujita Scale) occurred in Sunnyvale, CA causing and estimated 3.8 million dollars in damage. Generally speaking, unlike 'Tornado Alley', most tornadoes in the Monterey WFO CWA are associated with low topped/mini supercells (<~15k feet) or along narrow cold frontal rainbands. Given the shallow nature of these storms, it should be noted that in some cases tornadic storms cells can be hard to detect on WSR-88D KMUX, which is at an elevation of 3,469 feet MSL. That is why the general public and trained storm spotters are invaluable in relaying severe weather reports to the National Weather Service.

A vigorous cold front moved through the San Francisco Bay Area on, March 18, 2011 bringing a round of severe weather with damaging winds, waterspouts, heavy rainfall and even a reported tornado. The frontal boundary first impacted the North Bay during the early morning with heavy rain and gusty winds reported. As the boundary moved through Santa Rosa a brief spin up occurred, causing damage to a metal building and bending a gate (see pics below). This 'spin up' was later classified as an EF1 tornado with estimated winds 86-110 mph . Farther south, the frontal boundary began to intensify as it moved toward the San Mateo Coast. In fact, the line of convective storms formed a LEWP(line echo wave pattern) . LEWPs are known for producing damaging winds and even tornadoes. Ahead of the LEWP a few discrete cells developed with weak rotation observed on radar. One person from the general public even filmed a discrete cell with a waterspout off the coast of Ocean Beach.

Photos By: Santa Rosa Fire Dept.	Photos By: Santa Rosa Fire Dept.
Waterspout off Ocean Beach - By : Rick Gutierrez	Radar Image of LEWP

About two hours later the convective line began to show embedded storm cells with rotation. The storm cell rotation was strong enough that the National Weather Service in Monterey issued a Tornado Warning at 11:02 AM PDT. The Tornado Warning was for portions of Western San Mateo County.

Green = Flow toward radar Red = Flow away from radar

Tornado Warning Issued

Radar Animation as storm cell moved toward the coast

Despite a storm cell with decent rotation and a Tornado Warning, no tornado was reported. However, there were a few reports of funnel clouds from the general public.