The upper level pattern will transition to a zonal flow regime this weekend with the jet stream aligning from west to east.
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This pattern will produce focus most of the active weather over the western U.S., largely in the form of an atmospheric river which is expected to bring increasing intensity of precipitation to central California starting on Saturday.
While light rain/mountain snow was ongoing Friday afternoon from central California to the Pacific Northwest and northern Rockies, low level winds will strengthen on Saturday from the San Francisco Bay region to locations just northward, with a plume of anomalous moisture extending inland into the Great Basin.
The plume of moisture will vary in latitude over the next couple of days but remain focused over portions of central to northern California, sandwiched between low pressure along the coast of British Columbia and high pressure to the west of southern California.
Snow levels will be relatively high within the core of the atmospheric river which will keep heavy snow confined to elevations above 7000 feet in the Sierra Nevada.
The remainder of the precipitation will fall as rain with the Coastal Ranges, just north of San Francisco Bay into the Sierra Nevada, likely picking up the highest rainfall totals over the weekend.
Through Sunday, 2 to 4 inches of rain is expected for the Coastal Ranges with 3 to 7 inches for the Sierra Nevada (locally higher).
Rain is likely to continue through Monday as well.
While rainfall is needed for many of these areas, it may be too much at once, resulting in flooding concerns and potential for debris flows within areas of sensitive terrain and area burn scars.
Heavy snow with 1 to 2 feet is expected for the higher elevations of the northern Cascades into central Idaho, far northern Utah and northwestern Wyoming.
Below average temperatures will filter into the Pacific Northwest this weekend, perhaps allowing for snow into the cities of Seattle and Portland, though any accumulations are expected to be minor (if any).
Elsewhere across the lower 48, temperatures will be above average from the southern Great Basin into the Great Plains, Midwest, Gulf Coast and Southeast.
The core of the warmth will extend from the southern High Plains into the Midwest where high temperatures are expected to climb to 15 to 25 degrees above average for Sunday.
After the departure of a cold front in the East tonight, drier weather is expected for most locations east of the Rockies the weekend.
The exception will be from the Great Lakes to the Northeast where a quick moving disturbance will bring light snow to the region along with colder than average temperatures, especially across northern New England where high temperatures are forecast to be 10-20 degrees below average on Saturday, with some modest warming on Sunday. ■