A river of moisture arises from the Pacific Ocean and links up with a procession of enormous storms that bring heavy surf, flooding rains, and mountain snows to the U.S. West. It’s a weather narrative that one usually associates with a strong El Nino during winter time. But the powerful El Nino ended last year and it failed to bring the expected rains. Meanwhile, in early 2017, during a La Nina year in which typical trends would tend to point to drier conditions for the U.S. West, a procession of severe storms is now slamming into California.
El Nino Pattern During a La Nina Year
So what the heck happened? What could possibly cause such a crazy weather flip-flop in which record drought conditions extend through a time of El Nino but severe and extreme rains come with the onset of La Nina?
The answer appears to be that a record warm ocean combined with a strongly positive Pacific Decadal Oscillation to produce a powerful river of moisture aimed directly at California. And when the associated storms arrived it was with an extreme intensity — setting off numerous flash flood events.
(Water vapor models show an atmospheric river running out of the Western Pacific — crossing that vast ocean before engorging storms slamming into the U.S. West Coast on January 17 of 2017. This is a severe weather feature more typical of an El Nino year that is now occurring during a period of weak La Nina conditions. The difference being that rivers of moisture running into California typically issue over Hawaii. The present ‘Pineapple Express’ is coming all the way from the Philippines. Image source: Earth Nullschool.)
An almost continuous spate of heavy downpours since the first week of January has now unloaded enough moisture to fully slake severe drought conditions over Northern California and to considerably reduce the drought in the south. Overall, precipitation totals for the past 30 days have been as much as 2.5 times above the normal amount for California.
Another Batch of Heavy Rain on the Way
This week, NOAA expects another batch of powerful storms to come blasting out of the Pacific. Sections of Southern California are predicted to get hit with around 9-13 inches of rain over the next seven days while the north receives another 10 to 15 inches. These are notably severe rainfall totals for California. And NOAA model predictions have tended to range higher over the past 24 hours.
(NOAA 7-day precipitation forecast indicates a severe rainfall event for the U.S. West Coast with heaviest amounts hitting parts of Northern California. Image source: NOAA.)
According to Accuweather, the heavy rains are expected to spur flash flooding, increase the risk of mudslides and to possibly push some rivers over their banks. However, since many rivers are still at low levels following persistent drought during the last five years, over-topping is less of a risk than it otherwise would have been.
Storms tend to bring cooler weather to this region and the Western U.S. has cooled somewhat during 2017 compared to past years. However, the conditions in which these storms are firing are warmer than they have been in the past. As a result, mountain snowfall has occurred higher up on the slopes. Consistent with the warmer than normal storms, Accuweather predicts this week’s storm system will not produce big snowfall totals for the Cascades as snow levels are driven above 7,000 feet by the warmer than usual temperatures.
Very Warm, Moist Pacific; Positive Pacific Decadal Oscillation
There’s been very little weather and climate discussion as to why heavy rains are falling in California during a year when the odds stacked against such an event would tend to be higher due to La Nina. The elephant in the room at this time is a major excursion of global surface temperatures in the range of 1.2 C above normal during 2016. A notably severe climate change related insult to the Earth system. Such extreme atmospheric warmth will tend to hold more water vapor aloft in suspension. As a result, when the rains do fall, they will tend to be heavier and come more in the form of downpours and deluges than as moderate or lighter precipitation.
(This sea surface temperature anomaly map shows that despite La Nina, the Pacific Ocean, on balance, is much warmer than normal. These warmer than normal sea surfaces are pumping out a considerable amount of moisture — which is helping to feed the powerful storm systems running into the U.S. West Coast. Image source: Earth Nullschool.)
To this point, despite a La Nina blanketing the Pacific’s central Equatorial region in cooler than normal waters, most of the Northern Pacific is considerably warmer than normal. And all this extra warmth is helping to pump a lot of water vapor into the atmosphere above the ocean zone. A feature that is not typically consistent with La Nina, but one that is consistent with a considerably positive Pacific Decadal Oscillation acting in conjunction with overall global warming. Positive Pacific Decadal Oscillation (PDO) values are associated with above normal sea surface temperatures in the Eastern and South-Central Pacific. Positive PDO tends to produce longer and strong El Nino events. And it is also associated with strong storm tracks running from west to east along the 40 N latitude line.
Storm Track Runs All the Way to U.S. West Coast
To this point, it’s worth noting that PDO has been in a positive range for the past three years running. But it wasn’t until recently that a persistently strong storm track stretching all the way to the U.S. West Coast has developed. During past years, strong storms veered north into Alaska and Canada, deflected by powerful ridges over the U.S. West.
(The crazy, wavy jet stream with a strong storm track hitting California and a ridge riding up into Central Canada is rather changed from the Ridiculously Resilient Ridge blocking pattern that helped to spark severe droughts along the U.S. West Coast during 2013-2015. Now, severe flooding rains are the rule of the day. Under human-caused climate change, we can expect weather patterns to tend more toward extremes. For the U.S. West Coast extreme drought has been replaced by heavy rains. Image source: Climate Reanalyzer.)
Assisting the process of storms running toward the U.S. West Coast was the removal of a hot blob of water off coastal Washington and Oregon as a zone of somewhat cooler than normal waters formed. These cooler waters extended from just off Northern Japan to south of the Aleutians and on toward the U.S. West Coast. This zone is providing a dipole temperature anomaly between the cooler than normal surface waters in the north and the warmer than normal waters in the south. As a result, the Jet Stream has a nice slot along which to produce a powerful, flat storm track. These two features — a strong temperature dipole between the 40 and 50 degree latitude lines and a very warm Pacific producing copious amounts of moisture south of the 40 degree latitude line — are the key ingredients that appear to be fueling the powerful West Coast storms in a counter-La Nina fashion.
In contrast to the 2013 to 2015 period, high pressure ridging along the U.S. West Coast is not now strong enough to deflect the storms running across the Northern Pacific. In other words, it appears that the influence of the Ridiculously Resilient Ridge and hot Ocean blobs off Washington and Oregon during 2013 to 2015 is has now faded out. However, the new climate and weather trends driving this most recent influx of heavy rainfall to the U.S. West Coast are almost as odd and notable.
Hat tip to Colorado Bob