This blog will be issued every Monday, Wednesday, and Saturday morning and updated if the weather is changing or critical or interesting weather events are occurring.
High pressure is along and over the west coast and will hold there through midweek. During this time Northern California will be mostly sunny with some afternoon cumulus clouds over the mountains. Temperatures will be 6 to 12 degrees above normal and winds will be light.
A "cutoff" or "closed" low pressure system is one that forms when a portion of a low pressure trough cuts off from the main jetstream and sinks southward. These are common during the spring and summer. Without the jetstream or strong steering winds these lows tend to move slowly and erratically. A cutoff low will be off the coast by late Wednesday and over the next several days will progress slowly eastward (see map below). The low does 2 main things...first it destabilizes the atmosphere as it approaches the coast and second the southerly flow ahead of it will sometimes pull sub tropical moisture northward.
Wednesday we will see the afternoon cumulus clouds begin to increase. Thursday and Friday there will afternoon and evening thunderstorms over the mountains with temperatures cooling to near normal. Saturday and Sunday the low will be close to or over the north state and there will be thunderstorms over the foothills and mountains with isolated activity in the valley.
The Old Forecaster