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Thread inspired by yesterday's truly spectacular and widely discussed Great Plains-style supercell in California's Sacramento Valley. #CAwx #Redding #ShastaCounty (1/n)
Thunderstorms in general are much less common along the West Coast of North America than across nearly all other parts of the continent. Some parts of coastal California see one (or even fewer!) thunderstorms in a typical year. #CAwx (2/n) 
Cumulonimbus clouds associated w/t-storms require strong upward vertical motion in order to form. Sometimes, this can be generated by boundary, like a cold front; other times, mere heating of humid lower atm can be sufficient for air to spontaneously rise, cool, &condense.(3/n)
Pacific Ocean temperatures along the West Coast of the U.S. are very cool relative to ocean temps at similar latitudes on East Coast due to intrinsic ocean circulation patterns (and resultant coastal "upwelling" of cold waters from the deep ocean). #CAwx (4/n)
In addition, proximity to subtropical high pressure systems (the descending branch of the Hadley cell) tends to promote *downward* (rather than upward) vertical motion (subsidence) in the atmosphere over California, especially during warm season. #CAwx (5/n)
The nearby cold ocean means there is rarely enough atmospheric moisture for air parcels to spontaneously convect, & downward motion from persistent subtropical high pressure acts to "keep a lid" on whatever clouds do manage to form. Hence: thunderstorms are unusual! #CAwx (6/n)
But there are a few exceptions. In spring, combination of strong sunshine, relatively abundant low-level moisture from winter rains, & "triggers" (e.g., low pressure systems) can occasionally destabilize atmosphere just enough to produce storms, mainly over Central Valley. (7/n)
When this happens, these CA storms are typically of lesser intensity than those elsewhere on the continent. Why? A subset of most powerful thunderstorms are called "supercells," the necessary preconditions for which are even rarer in the Golden State. (8/n)
Supercell thunderstorms require not only deep atmospheric instability, but also strong vertical wind shear (changing speed/direction of wind w/height). Strong shear allows updraft in supercells to rotate & tilt,allowing storm to become organized & partially self-sustaining. (9/n)
During that event, the #CarrFire generated an enormous smoke plume which essentially created a fire-induced supercell thunderstorm ("pyrocumulonimbus"). This plume began to rotate, and generated what was effectively a 145-mph tornado of flaming gas. spc.noaa.gov/publications/n… (12/n)
Obviously, these two events are not directly related. But it is interesting to note that both Carr Fire pyrocumulonimbus & last night's supercell formed in similar locations, albeit under quite different circumstances. @nplareau @NickyNaus @climate_guy #CAwx #CAfire (13/end)
