An unseasonably strong storm will invade California Thursday night and Friday, and it will bring wild weather with it. There are not many times in a year that one can say that severe weather is likely with a storm. This looks to be one of those few times. I am impressed.
A very cold trough is centered off the coast of Oregon and northern California. The leading edge of that storm is already affecting parts of northern and central California.
Showers can occur anytime Thursday night in the northern two-thirds of the state, and a couple of showers will develop in Southern California later at night. In the increasingly unstable air mass in the northern half of the state through early tonight, scattered thunderstorms are likely. Where thunderstorms do develop they could become strong and even locally severe with heavy downpours and hail and maybe an isolated funnel cloud or weak tornado.
However, the wildest weather looks to occur Friday into Friday evening in central and southern California. The upper-level trough will move south then east, become what we call negatively tilted and move inland during the daytime and evening hours. The negative tilt will produce good lift to generate widespread showers and thunderstorms as well as adding some twist to any thunderstorm that develops. It becomes very cold aloft making for strong lapse rates which means increased instability. All the ingredients will be in place for thunderstorms to turn severe with any severe thunderstorm producing very heavy downpours, strong and gusty winds, large hail and even a waterspout or tornado. The greatest lift and threat of severe weather is aimed at the southern half or so of the state. The upper-level disturbance itself will move right over the Los Angeles Basin during the afternoon and early evening hours. Rainfall amounts will be highly variable because of the shower nature but probably average 1/2 to 1 inch but locally higher amounts possible. Rain could fall at fast enough rates below the heaviest showers and storms to produce flash-flooding problems.
Snow levels will become quite low, dropping to 3,000 feet in the Sierra and to 3,500 feet or so by later in the day in the southern California mountains. Accumulations of 1-2 feet of snow are likely in the central and southern Sierra above 6,000 feet with 5 to 9 inches down to 4,500 feet. In the southern California mountains, snow levels will start Friday around 5,000 to 5,500 feet then drop to that 3,500-foot level later in the day. Snow accumulations above 5,500 to 6,000 feet will be 8 to 16 inches with locally higher amounts possible. Snow will fall at all passes by late in the day and evening and could cause travel problems tomorrow night for a time.
If you are headed to the big Coachella Music Festival be warned that the weather is not going to be very good Friday midday through Friday night. A few showers and thunderstorms are likely even in the desert as this storm moves through. It becomes much cooler and very windy as well. Winds by Friday afternoon and evening are likely to be in the 20- to 35-mph range with gusts near 50 mph in spots. Saturday will be brisk and unseasonably cool with the only really nice day being Sunday.
Stormy weather will move into Arizona late Friday night and Saturday with snow likely in the Arizona mountains of 4-8 inches with higher amounts possible.
As of the end of June there had been no named storms in the Eastern Pacific basin.
This is some serious and dangerous heat. Outdoor activity is just not at all recommended during the daytime.
A strong ridge of high pressure in the West brings the highest heat of the season so far to a large area.
Combine the cold with the wind and some precipitation and there is a real danger of hypothermia.
Any shower and thunderstorm can contain heavy downpours, heavy enough to cause temporary, low-lying ponding.
According to all long-range models, the warmest area in North America compared to average will be over the Northwest.