California: Storms Bring Need Rain North But Little South

A series of storms will be moving across the Northwest and northern California over the next few days. However, there will be huge differences between how much rain and snow falls in the north, to what falls in the south.

The initial storm is now west of southern Oregon.

Much of the energy, and therefore much of the rain, with this storm is going to mainly be from northern California on north Tuesday into Tuesday night. The second storm will move farther south Tuesday night and Wednesday, bringing more significant rain with it down into parts of central California, with snow over most of the Sierra. Snow levels in the Sierra Tuesday will be around 7,000 to 7,500 feet, but likely is to drop to 5,000 to 5,500 feet by Wednesday.

One of tools from the computer models we use to help predict how much rain/snow is expected comes from what is called QPF (quantitative precipitation forecast). This is a guide, but certainly not holy grail. This is especially true when dealing with the quite varied terrain we find in California. Mountains can both enhance precipitation on the windward side or hinder precipitation on the leeward side of mountains. At times the QPF the models have on the windward side of the mountains can be off by a factor of 4,5 or more. There are also times that our computer models for the same site greatly disagree amongst each other with how much precipitation will fall. This is the case today.

Below is a chart of what three different models have for precipitation totals over the same time period from Tuesday morning through Thursday afternoon.

There are huge differences, especially in the northern third of the state, between the models with a decrease in that difference as you get into south-central and then Southern California. Given all other parameters, it seems like the middle of the road from the Bay Area to Stockton on north is most likely which is closest to the NAM numbers except for the Sierra. In the remainder of central California, north of Pt. Conception, probably the European has the best numbers. By the time one gets south of Pt. Conception, the difference does not really matter as all that is expected is a little drizzle or very light rain being squeezed out of the deep marine layer.

In the Sierra expect one to two feet of snow above 7,000 feet from Yosemite on north with 4 to 8 inches down to 6,000 feet. South of Yosemite 6 to 12 inches is likely above 7,000 feet with 2 to 5 inches down to 6,000 feet.

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