A cold upper-level trough will drop from north to south Wednesday through Friday in the West. This is not going to be a big precipitation producer, but it will be followed by a noticeable change to colder weather. The colder air will press through the Northwest Wednesday then drops down through California and the central Great Basin Thursday and into the Southwest on Friday.
There will be some rain showers west of the Cascades Wednesday that could mix with snow in the afternoon and night down to 1,000 to 1,500 feet. In the Cascades, periods of snow are likely to bring 4 to 8 inches, and east of the Cascades a few rain and snow showers turning to all snow showers are likely. Thursday, behind the upper-level trough, temperatures will be slow to rise to even near 40 in the afternoon along the I-5 corridor from northwestern Oregon on north and stay in the 20s and 30s east of the Cascades.
As the same storm moves through California Wednesday night and Thursday, it will be accompanied by a few showers in northern to central areas with maybe a shower or two southwest. Snow levels will fall to only 1,000 to 1,500 feet in the north to 1,500 feet to 2,000 feet central by later in the day. Luckily there will not be a lot of precipitation, just scattered snow showers. In southern California, gusty winds will announce the cooling in the deserts, and breezy conditions elsewhere will accompany the change to much cooler temperatures. Highs in the Los Angeles Basin will be mostly in the middle and upper 50 Thursday and upper 50s to near 60 in San Diego. Here, snow levels will drop to 2,000 to 2,500 feet.
Even colder air will follow on Friday into Saturday for some of the coldest weather so far this season. Temperatures in the Central California will be mostly in the 40s Friday with a very cold night Friday night. Many parts of the Central Valley will drop below freezing along with inland portions of the coastal plain. The coldest parts of the Central Valley will be down into the middle 20s with some lower 20s possible. Southern California will have a brisk, chilly day Friday with highs in the lower to middle 50s then a widespread frost Saturday morning with sub-freezing temperatures in parts of the valleys. The colder Upper Desert locations drop into the teens.
The colder air will arrive Friday in Arizona with snow showers in the mountains and plateau region and maybe a shower in parts of the lower deserts. Here again, it’s the cold, not the amount of snow or rain, that is important. Highs in Flagstaff Friday and Saturday will be in the 20s with even a place like Phoenix not getting out of the low to middle 50s.
In the inter-mountain West, snow showers are likely Thursday with 4-10 inches of snow possible in portions of the Wasatch as the cold front moves through. Temperatures in the Salt Lake Valley are going to be in the 40s Wednesday, upper 30s Thursday but only middle 20s for Friday and Saturday.
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