May 20, 2018
Summary: A weak trough of low pressure continues to maintain itself over the western United States. Today will be similar to yesterday with temperatures near seasonal averages with just a few high clouds at times. With that trough in place, an on shore flow continues. In fact, there is an 11.40 millibar difference in pressure between higher pressure over San Francisco to lower pressure over Las Vegas. Temperatures will cool even more Monday as a closed low formation develops right over central California with a pool of relatively cold air aloft. This, in turn, will increase the risk of afternoon and evening thunderstorms over the Sierra Nevada Monday with current models suggesting there could be some activity over the Kern County mountains and even the Coast Range.
As the low drifts southeastward to near Las Vegas Tuesday, the winds aloft will become east or northeast, possibly nudging isolated showers or thunderstorms over the valley floor with the greatest risk being along the Sierra Nevada foothills. I must emphasize most locations will likely be dry with perhaps a 30% chance of measurable rain Monday afternoon through Tuesday evening, especially during the mid afternoon through the evening hours.
By Wednesday the low will begin to move northeastward into the northern Rockies, allowing pressures to rise and temperatures to begin to warm. A new and rather strong low shows up on models off the northern California coast Thursday, but models are pretty consistent in showing this storm moving northeastward into the Pacific Northwest with all but northwestern California remaining dry. If models are correct in the medium turn, upper level high pressure will take over with 90 degree plus weather due next week.
Forecast: Other than occasional high clouds, it will be mostly clear through this evening. Becoming partly cloudy later tonight. Variable cloudiness at times Monday through Tuesday night with a small chance of isolated showers or thunderstorms, especially Monday afternoon through Tuesday evening. Clearing Tuesday night. Mostly clear Wednesday through Sunday.
Temperatures: Highs today will generally run in the low to mid 80s with mid to upper 50s tonight. Readings will dive into the mid to upper 70s Monday with upper 50s and lower 60s Monday night. Temperatures Tuesday will warm back into the mid 80s.
Winds: Winds will be mainly out of the northwest and less than 15 MPH today with periods of winds out of the northwest at 10 to 20 MPH late tonight through Monday night with stronger gusts along the west side, decreasing Tuesday. Expect the usual light winds to return Tuesday night and Wednesday.