December was exceptionally active for tornadoes, even marking the historic issuance of San Francisco's first-ever tornado warning.
What's happening?
2024 was a prolific year for tornadoes.
There were over 1,800 preliminary reports of tornadoes, more than 30% above average for the year. A Memorial Day weekend storm outbreak with tornadoes killed at least 22 people. Over a five-day period during the week of Christmas, there were dozens of preliminary reports of tornadoes in the South — and about 30 confirmed tornadoes. This was a significant outbreak of twisters during the first month of the meteorological winter.
December also saw San Francisco's first-ever tornado warning. On the morning of December 14, a strong thunderstorm swept into the northwest part of the city. A tornado was reported in Scotts Valley, about 67 miles to the south.
"Based on video, photos, firsthand accounts, and radar signatures a tornado occurred [roughly] 1:40 PM," the Bay Area National Weather Service (NWS) posted on X, formerly known as Twitter, later that day. "Social media reports of cars overturned and tree damage in and across Scotts Valley. Also video evidence of a tornado," its report read.
A survey crew from the Bay Area NWS office later determined the damage from the storm within San Francisco was consistent with straight-line winds, not tornadic winds. The crew's preliminary peak wind speed estimate from the storm as it moved across the Richmond District and Golden Gate Park areas was around 80 mph. The NWS storm survey crews also documented several large trees that were either uprooted or snapped at their base. The storm also damaged a roof on at least one structure and downed several power lines.
Why is a tornado warning in San Francisco important?
A Climate Central study found that the threat of supercells, the primary type of thunderstorms that produce tornadoes, is increasing dramatically in parts of the U.S., including regions that don't typically see this type of dangerous storm. The study also concluded that conditions conducive to supercells and tornadoes are expanding their reach into what were once less active winter months.
Instability in the atmosphere that leads to warm, rising air is one of the essential elements needed for the formation of thunderstorms. One of the measures of instability that meteorologists use is "convective available potential energy," or CAPE, which is projected to increase on our overheating planet, as Climate Central detailed. More CAPE could mean more intense storms that produce even more storm-related damage.
What's being done about increasing instability in the atmosphere as our world warms?
While there are uncertainties involved, cooling our planet could reduce the atmospheric volatility that could help fuel tornadoes. This will require a shift away from a reliance on dirty energy sources to power our planet and a move toward cleaner, renewable energy options.
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We can reduce the amount of heat-trapping gases in our atmosphere by installing solar panels, replacing an old HVAC system with a heat pump, and using an induction stove.
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