Bay Area severe weather – brief tornado warning (!)

Our El Nino-infused stormy weather of the last few days reached its peak on Wednesday with more localized flooding and wind damage, and also yielded a brief tornado warning for Santa Cruz and southeastern Santa Clara county.

The National Weather Service in Monterey’s radar had shown signs of lower level rotation within a strengthening thunderstorm.  A trained weather spotter near Morgan Hill, one of the towns under the tornado warning area reported one inch diameter hail on the ground.

The warning lasted for nearly a half hour before being allowed to expire before 3pm.

So what is going on with California’s weather, this event one day after tornado warnings were issued for parts of Los Angeles County to the south?

A trip through the following slideshow shows why.  An incredibly robust jet stream is firing in across Central and Southern California with some spreading of the wind barbs noted at the 300mb level.  Wind speeds are strong and turn fairly dramatically with height down to the surface with decent available moisture and some sunny breaks to destabilize the atmosphere even more for a brief time midday on Wednesday around the Bay Area.

Our severe storms tend to be the larger hail producers (Bay Area standards that’s anything above pea-sized hail).  Once thunderstorms enter environments with strong winds moving at different directions in the atmosphere (for instance westerly high up, southwesterly mid-level and southerly/southeasterly at ground level) we can see these storms beginning to rotate and on the rare occassion drop a waterspout or tornado.

Given the current upper level wind setup, it would appear that Thursday will offer at least another chance for possibly severe thunderstorms especially for Central and Southern California.  Those opportunities for severe weather may continue into Friday for Southern California before a weak ridge of high pressure nudges in for Saturday.

As our energetic Pacific jet goes from being nearly zonal into California and heads further north again, our opportunities for severe, tornado/waterspout inducing weather should also move out of the region.  But for now, let’s check out one of the few patterns that can bring tornadic weather to the Bay Area and the rest of California.

The NWS issued tornado warning for near Morgan Hill/Santa Clara county and parts of Santa Cruz county.

Jet stream level chart just before midday: Note incredible wind speeds just offshore and turning of the jet winds near the Bay Area/Central Coast.

Good “spin” to the atmosphere as well, dynamic lift.

Lower level moisture field along and behind the cold front also favorable for strong thunderstorms.

More turning of the wind down to the 850 mb level. A narrow notch between San Jose down to San Luis Obispo appears to be in a region of directional shear between 850/300mb.

“Mammatus” clouds passing by my house in San Ramon, on the northern flank of this line of thundershowers. The southern cell down towards Morgan Hill was more isolated and showed rotation and yielded 1″ hail.

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By robmayeda

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