Heavy surf and high tides undermined a dozen beach-front homes in Malibu yesterday as southern Californians were warned to prepare for yet another -- and probably stronger -- Pacific storm expected to sweep ashore today. By midday the current storm had dumped 4.8 inches of rain on the Civic Center, bringing the season total to 26.33, more than 2 1/2 times normal.

As this storm proceeded eastward, a sheriff's deputy drowned and hundreds of Phoenix homes were threatened.Up to 8 inches of rain fell on parts of normally semi-arid Arizona.

Meanwhile, a blustery snowstorm swept across the Great Plains and the Midwest, the second in two weeks in Kansas, Missouri and Nebraska, and at least the sixth mighty storm to pound the nation in the winter of 1978. Kansas City was socked by 8 inches of snow by midmorning, with 6 inches more forecast. Schools shut down there and in Omaha as bitterly cold air frosted the area. The mercury plunged to 23 below zero at Butte, Mont., rising to 15 below at Elkins, W. Va.

The worst may be yet to come, according to the National Weather Service, which said some regions face a spring of potentially disastrous flooding from the melting of the unusually heavy snow that fell this winter.