California is about to see a weather whiplash from rain to extreme heat

Rain showers. Then, just three days later, 100-degree heat.

In parts of Northern California, that sequence is possible next week as weather whiplash takes hold of the West Coast, with increasing odds of a widespread heat wave by mid-June.

On Monday, a broad low-pressure system is slated to approach the West Coast. This system will have a deep moisture tap and could wring out as much as an inch of rain along the Oregon-California border. Showers are possible as far south as San Francisco on Tuesday, but probably won't add up to more than a tenth of an inch of rain.

A strong high-pressure system is expected to follow Wednesday as the area of low pressure pushes into the Great Plains. That's when a rapid warmup may begin.

The area of high pressure will first build over the Pacific Ocean on Wednesday and move eastward, either parking over California or the Pacific Northwest by the weekend. Where its center settles will influence which areas experience the most extreme heat.

Most of the time, California heat waves stem from high-pressure systems over the Four Corners in the Southwest. But occasionally, more often in spring and fall, high-pressure systems form over the ocean and then move over California - and those are more prone to coastal heat.

That means San Francisco could heat up rapidly. Triple-digit heat is possible in Northern California as early as Thursday. Downtown San Francisco could reach the 80s, with 90s possible in parts of Oakland. By Friday, the Bay Area could hit 100 degrees for the first time this year.

Coastal heat probably won't last long - with a cooldown favored by Saturday, June 13 - but inland areas may continue to bake. Long-range weather models predict up to a 40% chance of temperatures at least 110 degrees in the San Joaquin Valley and northern Sacramento Valley the week of June 14.

Southern California will probably get warm with this pattern, too, but the most anomalous temperatures are likely to come in Northern California, Washington and Oregon. There's an increasing likelihood of 90-degree weather in Portland, Ore., and Seattle. Meanwhile, Los Angeles isn't currently forecast to get that hot.

The exact evolution of the heat wave is still uncertain, but global climate patterns - including tropical storm remnants and an emerging El Niño - provide some confidence to weather model predictions.

A ribbon of winds in the upper atmosphere that extends roughly from south of Hawaii to Florida - known as the subtropical jet stream - is stronger during El Niño. Its influence usually takes an edge off Desert Southwest heat while raising the odds of above-normal temperatures in the Pacific Northwest, said Daniel Swain, a climate scientist at the UC Agricultural and Natural Resources extension.

In the weather service's summer heat forecast, the Pacific Northwest is the bull's-eye, as it is the place where probabilities of above-normal temperatures are the greatest. Next week's heat wave, if it forms as forecast, may be a preview of this expected summer pattern.

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