Rain still on the way

The major change in the large-scale weather pattern is fortunately still on track (and actually is already underway).  Today, a cold upper-level trough moved into the Pacific Northwest, bringing rain and even snow down to low-elevation cities like Seattle and Portland.  This system also cooled our temperatures dramatically as onshore winds increased.  This system has also dislodged a stubborn ridge of high pressure that has been present much of this winter along the West Coast.  This will allow a parade of storm systems to impact northern California starting mid-week.

Monday, Tuesday, and much of Wednesday will still be dry in the Bay Area.  Despite mostly sunny skies though, the air mass aloft is much cooler, and cannot support the temperatures in the 60s to near 70 we have been enjoying lately.  The first in a series of cold fronts will impact our area late Wednesday, with rain developing in the North Bay and spreading south into San Francisco after dark.  Then, a couple more cold fronts will move in, with the strongest one expected late Friday into early Saturday.  This strongest front will bring rain even to southern California.

What is uncertain though is where the heaviest rain will be and how much will fall.  The models have been fairly consistent in producing the bulk of the heavy rain north of San Francisco, from Sonoma County northward into Oregon.  The image shown is model estimated cumulative rainfall from now until just after midnight on Saturday.  The model depicts a tight precipitation gradient from north to south across the Bay Area, with 2 to 5 inches across the North Bay, 1 to 2 inches around San Francisco, and perhaps only an inch or less across the South Bay and southward.  This creates a very difficult forecast - 5 inches of rain has considerably different ramifications than 1 inch, so nailing down exactly where the rainfall bulls-eye will be key.

The weather pattern looks to remain active into early next week before quieting down again.  A few inches of rain (and heavy snow in the mountains) will be much welcome.  But we must also keep in mind that such rainfall amounts are not unheard of for this time of year, and will not erase the huge deficits from November, December, and now the first half of January.