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March 2016

The Wettest Wet Season

Posted: March 31st, 2016


Over the course of a “normal” year, 37.49 inches of rain falls in Seattle (Sea-Tac). Since the first of this January, we’ve received 18.49 inches. That’s an impressive 150 percent of where we’re supposed to be on this date, the final day of March.


But if you go back to the start of the water year, which when discussing precipitation one always should, our total is even more impressive. Since October 1st, 43.33 inches has accumulated. That’s a new record through March, and obviously much more than we’re supposed to get annually. (And if all we get is a half-an-inch next month, we’ll break the October-through-April record, too.)


To break the annual record we’ll need nearly 12 inches over the next 6 months, which will be difficult to do. Normally, about 10 inches falls between April and September, so our streak of wetter than normal months (currently at 6) would have to continue in earnest.


The reason is was so wet was primarily due to a strong El Niño. I know, I know, the majority of seasonal forecasts consistently called for generally warm and dry conditions around here this winter. But the truth is that there have only been a few strong El Niños to which this could have been compared and other under-predicted global processes (e.g. the Atlantic Multi-decadal Oscillation) appear to influenced the atmosphere.


Fact is the jet stream, which normally focuses on the Northwest in late November as it shifts south, and then periodically visits later on its way north again, persistently hung around. And it was strong, developing hurricane-type storms globally all winter long. Indications are that the Hadley Cell, which in a sense represents the extent of the tropics, expanded this year. The now-fading El Niño was stronger in a somewhat unexpected place, which seems to have invigorated and lifted the jet stream across the Pacific toward us—and away from California, at least compared to expectations there.



Heavy rain is often a double-edged sword for SPU, in that it benefits water supply while flooding the urban environment. This season we lucked out.


Back in October, recall that SPU was still managing a water shortage, caused by the combination of last winter’s dreadful snowpack and last summer’s extreme heat. We wanted to keep Seattle safe, but we needed storms badly.


Curtailments were avoided when the first atmospheric river (AR) of the season began to to fill reservoirs over Halloween. A few weeks later, the water shortage was completely over as another big AR amazingly nudged us toward flood mode. By December it was clear that something strange was up when one long AR struck. The hits kept coming in January when a “stalled front with entrained tropical moisture” passed through.


While it was all rain over the lowlands, snow accumulated nicely in the mountains. That long December storm was truly the main snowpack-building event of the season, but in general it was a close-to-normal year, which tends to peak on April 1st. SPU’s “third reservoir” is thankfully full for summer consumption.


As if there weren’t enough oddities and pleasant surprises this season, there were very few high-intensity, short-duration precipitation events across Seattle. Indeed, those three aforementioned ARs were not without impacts such as temporary street flooding, but typically such storms are accompanied by embedded downpours, which cause the biggest problems for SPU’s drainage infrastructure. The only briefly intense event of note occurred during the final few hours of the 96-hour event in December, when 0.25 inches fell in 5 minutes across West and Central Seattle.


This wet season was wild, but compared to last winter’s record warmth and last summer’s record heat it was just not that bad. Let’s take a break from records, though, am I right?