Snow in San Francisco? Could happen late Friday

By Jason Samenow

Unusually cold storm impacting West Coast

Temperatures are on the upswing across the eastern third of the United States, but a very stormy and cold pattern is taking hold over the West Coast. A winter storm warning is in effect for Seattle, where three to six inches of snow is expected through Thursday (including potential snowfall rates of one inch per hour north of the city into this afternoon). The same storm will dive southward, impacting much of California, Thursday into Saturday (from north to south). In the San Francisco Bay Area, the potential for some very rare snowflakes is on the table.

The San Francisco National Weather Service Forecast office writes:

ON FRIDAY INTO SATURDAY A RECORD COLD AIRMASS IS FORECAST TO DROP OVER THE REGION WITH SHOWERS AND POSSIBLE VERY LOW ELEVATION SNOW SHOWERS BETWEEN 500 FEET AND SEA LEVEL.

Measurable snow is very rare in San Francisco, and has only occurred in the area 11 times since 1856 according to meteorologist Jan Null of Golden Gate Weather Services, who has compiled a record. The last measurable snow to occur in San Francisco occurred in 1976 when one inch fell.

As for the snow potential Friday into Saturday, Null wrote in an email to colleagues: "The toughest part of the equation is whether the precipitation will phase with the coldest air during the coldest time of day. Any of a number of subtle changes to the airmass and its trajectory over the ocean could make dramatic differences to the eventual outcome."

In the North Bay Hills of San Francisco, at elevations above 1,000 feet, snow is a better bet than at sea level and a winter storm watch has already been issued for Thursday night and Friday:

ACCUMULATING SNOWFALL OF SEVERAL INCHES WILL BE LIKELY FOR ALL ELEVATIONS ABOVE 1500 FEET WITH A GOOD POSSIBILITY OF AT LEAST 1 INCH OF SNOW DOWN TO 1000 FEET.

As you head south toward Los Angeles, unseasonably cold temperatures are expected by Saturday with highs in the 50s and lows in the upper 30s Saturday night. Snow will be possible in its nearby hills. The National Weather Service notes model forecast "thicknesses" - a measure of how cold it is - are unusually low:

IF THESE VALUES VERIFY IT WILL BE THE LOWEST [COLDEST] THICKNESSES RECORDED IN AT LEAST THE LAST 10 TEN YEARS.

Be ready to bundle up, Hollywood.