The Upcoming Northern Californa Summer Deluge

A highly unusual summer deluge will soon hit northern California.  June is typically quite dry over the Golden State (it isn't called golden for nothing!).  A climatological (1980-2010) precipitation map (see below) tells the story... from the Bay area south and west the monthly totals are .1 to .4 inches and even the northern Sierra Mts. only typically receive about an inch.

But take a look what the models are now consistently forecasting for the 48 h ending
5 AM on Wednesday:   1-5 inches over the northern Sierra (the pink and black colors) and .5 to 1 inches for much of the remainder of northern CA.   We won't escape the rain in the Pacific Northwest, but California will be hit far harder than we.

The origin of this unusual rain is an extraordinary summer atmospheric river, one that is stronger and extends farther west than most of us can remember seeing in decades during this time of the year.  (an atmospheric river is a long, narrow current of high moisture values originating in the tropics or subtropics).  The UW WRF model illustrates the forecast atmospheric river with a plot of the total water vapor in a column for Monday evening.  The dark blues are very high values.  Really quite unusual for this time of the year.

Even today (Saturday) we can see the atmospheric river setting up, as viewed by weather satellites that can sense the amount of water vapor in a vertical column (see image).  You can observe an initial system (green colors of moderate water vapor amounts) that is approaching the NW (yes, we will get some rain tomorrow), but look at the LONG stream of moisture coming off of southeast Asia (highest values are red)!  That atmospheric river is heading for northern California.

The wet pattern on Monday and Tuesday is associated with a highly perturbed weather pattern, with a deep low forming over the eastern Pacific (see graphic for 5 AM Tuesday of the upper level flow pattern).  The flow (which is parallel to the solid lines) will be strong and from the west/southwest...and headed into northern California.

Good news.  The cool, wet pattern should be a memory by next weekend--the models are suggesting a huge high pressure ridge developing over the western U.S. during the second half of the week.