Last night and this morning featured unusual weather across northern California, as widespread thunderstorms developed over the Sierra, parts of the Central Valley, and the North and East Bay regions. Thunderstorms are relatively common over the Sierra during summer, but are highly unusual for the lower elevations, especially during the summer months. Thunderstorms typically require a combination of moisture, instability, and lift, and we rarely have these ingredients in combination over northern California. However, a weak upper-level low pressure system over the region helped create an environment favorable for thunderstorms. The cool air aloft associated with the upper level low helped increase instability - temperature decreasing rapidly with height. Southeasterly winds at the mid-levels of the atmosphere helped draw moisture into the region from the south. And, the upper low itself acted as a trigger, or lift, to generate thunderstorm activity.
The thunderstorms we experienced were highly unusual in that in some locations, a deep marine layer was present at the surface. The marine layer is very stable and not conducive for thunderstorm activity. However, the thunderstorms last night developed above the marine layer - these are referred to elevated thunderstorms, and are relatively common in the central and eastern U.S. during springtime. Thunderstorms pull in warm, moist air as inflow, which rises, cools, and condenses as clouds and rain. In an elevated thunderstorm, this inflow originates well above the
surface, whereas with most thunderstorms, the inflow originates near the
surface.
While the atmosphere was more humid than typical for summer in northern California, it was still relatively dry, and thus the storms that developed produced little rain. Most locations received a tenth of an inch or less. The storms did produce frequent lightning though, and according to CalFire, the lightning sparked numerous small fires across the region. The image on the right from the Sacramento NWS office shows that nearly 7000 lightning strikes occurred across northern California and western Nevada with these storms. The upper level low responsible for this active weather is already moving east, which will return us to our typical June pattern with a persistent marine layer near the coast and warm, sunny conditions inland.
Last night and this morning featured unusual weather across northern California, as widespread thunderstorms developed over the Sierra, parts of the Central Valley, and the North and East Bay regions. Thunderstorms are relatively common over the Sierra during summer, but are highly unusual for the lower elevations, especially during the summer months. Thunderstorms typically require a combination of moisture, instability, and lift, and we rarely have these ingredients in combination over northern California. However, a weak upper-level low pressure system over the region helped create an environment favorable for thunderstorms. The cool air aloft associated with the upper level low helped increase instability - temperature decreasing rapidly with height. Southeasterly winds at the mid-levels of the atmosphere helped draw moisture into the region from the south. And, the upper low itself acted as a trigger, or lift, to generate thunderstorm activity.
While the atmosphere was more humid than typical for summer in northern California, it was still relatively dry, and thus the storms that developed produced little rain. Most locations received a tenth of an inch or less. The storms did produce frequent lightning though, and according to CalFire, the lightning sparked numerous small fires across the region. The image on the right from the Sacramento NWS office shows that nearly 7000 lightning strikes occurred across northern California and western Nevada with these storms. The upper level low responsible for this active weather is already moving east, which will return us to our typical June pattern with a persistent marine layer near the coast and warm, sunny conditions inland.
