One of the Greatest West Coast Atmospheric Rivers on Record

The more I look at the strong atmospheric river that will strike California tomorrow, the more amazing it becomes.   In some, but not all, respects it is one of the more extreme atmospheric rivers on record.

Today's water vapor satellite imagery shows a huge plume of moisture stretching from the tropics towards California (see below)

And the values of vertically integrated water vapor (total water vapor in a vertical column of air) forecast for tomorrow (Friday) at 2 PM are immense, with the blue colors indicating values of 40 mm (1.6 inches) or more.  That means if one condensed out all the moisture in a vertical column, 1.6 inches of rain would result.  That is a very large amount.

This intense plume of water vapor will slowly move down the California coast, reaching southern CA on Saturday morning (below is the forecast map for 8 AM Saturday).

The latest forecasts promise major rain (and considering the warmth of this atmospheric river, that is what it will be, even over high terrain).  For the 72-h ending 5 AM Sunday, the total amounts reach 5-10 inches over the coastal mountains and Sierra Nevada of central and northern CA--and we also do "well" in the precipitation arena.

But for me, the most extraordinary part of this event is how unusual it will be, so let me demonstrate that to you.  Here is a plot of the forecast integrated water amount (also known as Total Precipitable Water) for San Francisco from the National Weather Service ensemble system (many forecasts using the GFS model).  The amounts get to 1.65 inches tomorrow and Saturday--and this is a confident forecast (all the ensemble members are going for it).  The black line is the ensemble average...a very good forecast to use.

We can get a feel for how unusual this is by looking at the climatology of these precipitable water values using the nearby radiosonde (balloon-launched weather observations) at Oakland, across the bay from San Francisco (see below).  The averages for each day of the year are shown in black and the daily records are indicated by the upper red lines that gyrate up and down.  The forecast value for tomorrow is shown by the black dashed line.

This is amazing: the forecast value tomorrow is higher than ANY DAY for the entire record at Oakland (about 70 years) from the period October 1 through July 1.  So tomorrow will not only beat the daily record, but the record for ANY day during the wet season of year in California.

There have been some higher values during the summer, but that reflects more moisture in the warmer summer atmosphere...moisture that doesn't result in heavy precipitation, mainly because the incoming flow is weak that time of the year.  To prove this to you, here are the extreme precipitation events at Yosemite National Park for each day of the year.  Extreme precipitation is wimpy during the dry summer.

 So to repeat...this is probably the strongest atmospheric river, in terms of water vapor content, to have ever hit central California when it really counted (during the non-summer portion of the year).    Just extraordinary.

But as strong as this atmospheric river is in terms of moisture content, it will not be the wettest on record (in terms of accumulated precipitation).  Why?  Because the incoming flow has been much stronger in some past events.   And what really counts in terms of producing precipitation on the California mountains is both moisture content AND the rate at which air is forced to rise on the terrain.  Stronger incoming winds promote more rising, upward motion, and thus precipitation.

What about the strong oceanic cyclone approaching the Northwest on Saturday?

  It is still coming.

Here is the latest sea level pressure forecast for 2 PM Saturday.  A 980 hPa low off our coast with a very large pressure gradient south of it.  Very strong winds offshore and along the WA and Oregon coastlines.

The low will slowly drift northwest of Seattle and the latest UW wind gust forecasts over Seattle for 5 PM Saturday shows 50-60 mph gusts over portions of Seattle, with 40 mph+ gusts over much of the city.  That will produce some power outages, with some mitigation by the fact that we are at the end of the winter, with a number of storms already clearing the weak stuff.  And the trees are not leafed out yet.

Anyway, the entire West Coast will be in for very active weather the next 72 hour.
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Announcement:  The Northwest Weather Workshop is on April 27-28

The NW Weather Workshop is the big annual meeting for those interested in Northwest meteorology.  This year we will have a major session on the meteorology of NW wildfires and others on other aspects of our regional weather.  The gathering takes place at the NOAA facility in Seattle.  To view the agenda and to register, go to the meeting website.  The workshop is open to everyone, but registration is required.