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Slight Risk for Excessive Rainfall and Severe Thunderstorms Thursday

Additional rounds of thunderstorms are expected Thursday and Thursday night across Oklahoma and north Texas, with risks for both large hail and flash flooding. Strong winds may accompany any storms from east Texas northeast through the lower Great Lakes. Read More >

Overview -  December 14, 2024 Convection

 

A dynamic and energetic environment developed across the Bay Area and Central Coast on December 14, 2024,  thanks in part to: an elongated upper level trough off the California coast, a robust surface cold front, cold advection aloft resulting in an unstable atmosphere, and upper level jet stream support.  Given the larger scale set up, the Storm Prediction Center Day 1 Convective Outlook highlighted a good portion of the Bay Area in a Marginal Risk for severe thunderstorms (see below). On average, a Marginal Risk touching a portion of the Bay Area occurs roughly once a year. In other words, it's not super common.  The day began with a very potent line of pre-sunrise thunderstorms rolling through San Francisco, which prompted the issuance of a rare Tornado Warning for the Bay Area.  A storm survey was conducted by the NWS Bay Area and it was determined the damage observed was associated with straight-line winds.  This initial surge of storm activity was associated with the a strong cold front. The cold front ultimately pushed through the Bay Area and Central Coast late morning into early afternoon.  Behind the cold front the atmosphere remained unstable due to cold advection aloft.  The lingering instability resulted in another wave of thunderstorms into the afternoon.  Special Marine Warnings and Severe Thunderstorms were issued to convey potential threats (see right).  One particular thunderstorm stood out from the rest.  This low-topped supercell initially developed over the coastal waters.  As it moved inland near Davenport it began to intensify and peak intensification occurred near Scotts Valley resulting in a tornado.  A storm survey was conducted and based on damage indicators the strength of the tornado was determined to be an EF-1.  For more details on the storm survey and radar analysis see below. 

 

MMehle/LBain

Video from Scotts Valley showing the intense nature of the tornado.  Video from @Bushmantribe
Radar Image
Storm Prediction Center Day 1 Convective Outlook - While not unheard of, Marginal Outlooks , are less common for the Bay Area.

 

Scotts Valley Tornado
Picture Scotts Valley Tornado 12-14-2024

Radar Loop
Radar Loop - Special Marine Warning and Severe Thunderstorm Warning

Damage Survey Results
Preliminary Damage Survey Results 

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