What a difference a year and a shift in weather patterns makes: This time last year, drought was spread across the entire Pacific Northwest, but a spate of storms has wiped out much of it and could give cities like Seattle and Portland their wettest Octobers on record.

The shift in weather fortunes could be related to an emerging La Niña. How climate change may impact La Niña and its counterpart, El Niño, is still unclear, though the climate cycle could become more intense in the future.

Thanks to a series of storms, Seattle has had nearly 8 inches of rain in October, more than 5 inches above normal, making it the third wettest October on record. The wettest October there was in 2003, when 8.86 inches fell.

Portland’s airport has had more than 7 inches of rain this month, also giving it a shot at eclipsing its wettest October, which was in 1994 when 8.41 inches fell at the airport observation site ( Portland’s downtown records go back further, indicating 11.63 inches of rain fell there in 1882).

Just one year ago,  the Pacific Northwest was in the midst of severe drought. While moderate rains had slowly eaten away at this dryness earlier this year, as recently as two weeks ago, much of Washington State remained abnormally dry, with a moderate drought lingering along the Oregon Coast, according to the U.S. Drought Monitor. The recent rains have now wiped even that out, with only the interior of Oregon remaining in drought.


Left: Oct. 27, 2015. Right: Oct. 18, 2016. Dark reds and maroons indicated worse droughts.

The shift in weather patterns that has increased the frequency of storms in the Pacific Northwest is consistent with a developing La Niña, or colder than normal water in the Pacific Ocean near the equator. La Niña favors a jet stream pattern which more consistently sends storms into the Pacific Northwest, leaving the Southwest drier than normal. This also means the ongoing drought in California will probably get worse.

While the water along the equator in the Pacific is a little cooler than normal, it has not been cool enough for long enough to be officially classified as a La Niña. According the National Oceanic and Atmospheric Administration, for a La Niña to be officially declared, the temperature in the central Pacific should be at least 0.5°C (0.9°F) below normal for three months in a row.  

While NOAA’s Climate Prediction Center expects La Niña conditions to develop in the coming couple of months, there is very low confidence as to whether it will persist through the winter.  Many of the computer simulations that forecast ENSO (the larger cyclical climate pattern that includes La Niña and El Niño) suggest the Pacific Ocean temperatures will begin to be closer to neutral by the summer.

The effect of climate change on ENSO is still the subject of research, though some recent studies have suggested its impacts may be intensified. For example, areas that are normally wetter than normal during one phase of the ENSO could be even wetter, and drier areas could be even drier.

Satellite loop illustrating an atmospheric river

Satellite loop illustrating an atmospheric river. Taken from Tuesday, Oct. 25. White and blue shading from the Pacific Ocean to the Northwest Coast illustrates water vapor in the middle and upper atmosphere.


In the shorter term, the recent heavy rain with the parade of storms on the West Coast has been amplified by an atmospheric river. This is a narrow ribbon of high moisture content in the atmosphere that follows the flow of the jet stream, funneling moisture out of the tropics.

According to NOAA, these atmospheric rivers account for 30 to 50 percent of the annual precipitation to the West Coast.

Two more storms are following this flow and moving into the northern part of the West Coast before the end of the month, so Seattle and Portland both have a good chance to break their records for wettest October.