Brentwood funnel cloud caught on video!

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By robmayeda

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.

By robmayeda

El Nino Strikes Back To Finish January

Our fairly dry start to January is about to take a fairly abrupt turn into some very rainy and occasionally windy weather.

A strengthening subtropical jet should begin to undercut and break through our fairly consistent ridge of high pressure allowing a series of tropics-enriched systems to slam into California during the next 10-14 days.

For those wondering what happened to El Nino where California was concerned – this is starting to look much more like it – as this upper level wind pattern matches what you’d expect in an El Nino year:  Central/Southern California get hit the hardest and the southern US sees much higher than average rainfall in the coming weeks.

The current forecast calls for some rain and wind on Tuesday with another weaker system late Friday into Saturday with another stronger system dropping in Sunday into Monday (perhaps packing the biggest punch of the three).

A very energetic long period swell (18 seconds) will reach the Bay Area coastline on Tuesday with wave heights possibly reaching 25 feet for some high potential for more beach erosion and localized coastal flooding around high tide times.  Wave heights should begin decreasing later on Thursday.

Rainfall estimation and wave model video below:


By robmayeda

Dry start to 2010

2010 looks to begin very dry by Bay Area standards with a blocking pattern that may hold up until the middle part of the month.

“So where’s El Nino?” many might ask.

Well, this is very El Nino-esque.  As San Francisco Weather Examiner Jan Null (CCM) points out quite eloquently in this article, the Bay Area is prone to dry spells in Winter regardless of a El Nino, La Nina or “average” Winter season.

What has our rain chances blocked up for now is what looks to be a tremendous ridge of high pressure building upward into Western Canada and down most of the West Coast. At the same time a fairly persistent upper level feature sometimes referred to as a “Hudson Bay” low will anchor itself across Eastern Canada.  This creates a highly amplified upper level pattern that is slow to break down in most instances.

Now here’s where “El Nino” should start looking more like what most folks see as being an El Nino Winter (heavier rains and milder temps for California).  As this large West Coast high builds deeper into Western Canada, the Pacific jet should begin to break through the lower half of the high pressure ridge around mid-month.  Given its lower latitude placement, the jet is likely to be infused with subtropical energy that should yield some pretty juicy rainfall potential that appears to be aiming in mainly along the Central Coast southward.

While this pattern may be a good reservoir filler for parts of the state, its warmer trajectory means there could be a potential for snowmelt if strong warm air advection rains boost snow levels to near 6-7,000 feet at times.  So this ridge-busting break though back to rain may be a case of one step forward and two steps back for the state’s on-going drought situation.

The Climate Prediction Center’s projections for California and most of the southern tier of the US is holding the El Nino-centric line for rainfall expectations:  milder + wetter than average especially for Southern California now through March.

For More Information: Climate Prediction Center


By robmayeda