2014-03-26 MISC: CA

A tornado warning has been issued by NWS Sacramento office for a storm SW of Orland. The storm is showing some weak rotation on radar and is almost stationary. The closest station is Chico airport - 54/46 ESE at 17.

I elected to stay in Yreka and work - I thought that instability was going to be an issue this afternoon... Bummer.

TORNADO WARNING
NATIONAL WEATHER SERVICE SACRAMENTO CA
316 PM PDT WED MAR 26 2014

THE NATIONAL WEATHER SERVICE IN SACRAMENTO HAS ISSUED A

* TORNADO WARNING FOR...
NORTH CENTRAL GLENN COUNTY IN NORTHERN CALIFORNIA...

* UNTIL 345 PM PDT

* AT 308 PM PDT...NATIONAL WEATHER SERVICE DOPPLER RADAR INDICATED A
SEVERE THUNDERSTORM WITH EMBEDDED ROTATION CAPABLE OF PRODUCING A
TORNADO OVER CENTRAL GLENN COUNTY...OR 7 MILES NORTHEAST OF ELK
CREEK...MOVING VERY SLOW EAST NORTHEAST AT LESS THAN 10 MPH. IT
SHOULD REMAIN WEST OF INTERSTATE 5 FOR FOR THE TIME BEING.

* THE TORNADO WILL BE NEAR...
RURAL NORTHERN GLENN COUNTY AT 330 PM PDT
 
I found this interesting in that SPC did not mention any chance of severe of any kind yesterday, even as cells in California kept garnering tornado warnings. I'm not at all familiar with California climate, did the setup have to do with with the elevated terrain in the region?
 
Well... I'm sure the SPC guys are aware of the ingredients for severe storms in CA's Central Valley. There was mention in one SPC discussion of the shear profile - but the thought was that buoyancy was going to be inadequate. That's what kept me home yesterday - as I agreed that there wasn't going to be enough afternoon clearing to get things going.

I'm a wildlife biologist and not a meteorologist... so bear with me on this. California does have a tornado alley and density wise - rivals the Plains in the number of tornadoes that occur annually. What makes CA severe weather setup unique (compared to the Plains) is that they usually develop in a post frontal, cold core thunderstorm environment characterized by weak thermodynamic instability and high values of low level positive and bulk shear. I doubt that surface based CAPE got much higher than 500 J/Kg on yesterday's EF-1 Willows tornado - but bulk shear was around 50 knots. The echo top of that storm when the tornado was on the ground was 26,000 feet. The elevation of the Central Valley is under 1000 feet, however the Sierra Nevada Range provide the orographic relief to back winds during these events. Chico and Oroville were both ESE at 15-20 knots yesterday afternoon.

Dr. John Monteverdi (from San Francisco State University) has published several papers including a couple co-authored by Chuck Doswell on the subject. I believe you can find a few on the internet. Most of us see John on the Plains every year while on his chasecation.
 
What I found interesting is that the tornadic supercell had a persistent, classic look to it on radar for nearly two hours; yet SPC's real-time supercell index software (on their hourly mesoscale analysis page) never indicated any supercell potential (at least during the event).

As Bob mentioned, the terrain is important in the Central Valley to get the locally backed winds. The wind profile in the lowest one kilometer must have been very strong, and very impressive. The instability was the big question mark, and somehow a couple of storms found themselves in places with buoyant air.

I would be interested in knowing if any of the chasers in the Sacramento Valley yesterday saw any lightning or heard any thunder. It is not uncommon for California tornadoes to be spawned by storm cells that are not "thunderstorms"! (There was one CG indicated with the Willow storm about an hour prior to the tornado, but little or none thereafter.)

Bill Reid
Westlake Village, CA
 
Here are a cell phone photo of the tornado plus a few GR3 screen grabs I saved while the Glenn County tornado was in progress. As Bill notes above - the storm looked like a classic supercell on radar. I did speak with a chaser who witnessed the tornado - he heard some thunder while the tornado was on the ground... though he saw no lightning.
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As Bob mentioned, the terrain is important in the Central Valley to get the locally backed winds. The wind profile in the lowest one kilometer must have been very strong, and very impressive.

Several of the models that morning had 1km Bulk Shear ~20kts, and 6km ~40kts...far beyond what we need here to generate some rotation. Coastal mountains help to develop a bit of a lee trough aiding the wind direction at the SFC.

Pulled the CANSAC model from that time:

SLP_WINDS10m_24_D3.gif


You can see what it does to surface winds in the forecast sounding for Corning (not too far away):
CLICK FOR CANSAC SOUNDING

And a note on the SPC...we almost never make it into the probabilities, and warnings here from the local office can be interesting...not all storms producing funnels get a warning, frustrates us to no end. If I remember correctly, we were not in SPC notes when we had our "outbreak" in October of 2012 (7 warnings, 5 tornadoes). We sometimes get included in the green "see text" area but the text never mentions us.
 
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