Ken Clark

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Southern California and Arizona Snow/Big Changes Coming

April 29, 2010; 1:17 PM ET

It was hard to convince thousands of drivers this morning trying to negotiate travel through the Grapevine that it is really spring in Southern California. You know the time of year that shorts and t-shirts return as the daily outfit, with nice, warm sunshine. Nothing could have been more different today.

Snow fell on the Grapevine this morning, the main route between Southern and northern California along the I-5. It snowed hard enough to not only accumulate on grassy surfaces, but also on the pavement. With temperatures below freezing by several degrees, driving became very slick. The icy roads caused more than 20 accidents, and the CHP was forced to close both the north- and southbound lanes for several hours. This caused a huge backup on both sides of the pass. Travel has since returned to normal Thursday afternoon, but cold winds continue to blow everywhere with gusts over 40 mph.

The cold winds were just not in Southern California, but all across California and the rest of the Southwest. It also snowed today in Flagstaff, Arizona with a mid-afternoon temperature of 32, some 30 degrees below normal. It only reached the low 70s in Phoenix, almost 20 degrees colder than yesterday. The Las Vegas Strip was a cold place to walk today with the afternoon temperature near 60, but the wind making it feel more like 50.

The unseasonably cold 500 mb trough now in the West causing this unusual weather will be replaced by the end of the weekend and early next week by a ridge of high pressure. Here is a look at the current 500 mb pattern.

And here is what the 500 mb pattern looks like by Monday morning.

This change will bring about a big warm-up across the Southwest this weekend into early next week. By Monday, it will be in the 80s in most of interior and southwest California, while the deserts head back into the middle and upper 90s. Similar warming, but probably a day later, takes place in Arizona with the southern deserts by Tuesday in the middle 90s and the plateau region in the 70s and 80s.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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Ken Clark
Ken Clark's Western U.S. weather blog tackles daily weather events with commentary from one of the most experienced and trusted Western U.S. weather experts.