UW research finds rainfall over Olympic Mountains more complex than originally believed

Mobile "Doppler Radar On Wheels" has to be towed from flood waters after 14" of rain fell in Lake Quinault during a storm on Nov. 13, 2015 (Photo: Joe Zagrodnik, University of Washington)

The widespread, lush rain forests that cover the southwestern region of the Olympic Mountains already provide an easy clue that it rains quite a bit there -- the Olympic Rain Forest is home to the wettest weather in the Continental United States, averaging about 200 inches a year -- but recent studies surprised some local research meteorologists just how the area gets all that rain.

The data comes from the months-long OLYMPEX study that was conducted in the fall and winter of 2015-16 in the Olympic Mountains. The purpose was to get better ground data during extreme rainfall situations to help calibrate NASA's new Global Precipitation Measurement Satellite. Researchers with the University of Washington collected rainfall data from a number of sources, including ground radar stations, rain gauges, weather balloons, snowpack measurements, and devices called disdrometers that measure raindrop size and fall speed to identify several different types of rainfall during various atmospheric conditions.

They picked a good time to be there -- that fall and winter featured a variety of storms, including a few "Pineapple Express" (or the newer term, "Atmospheric Rivers") that bring in warm, tropical moisture to Western Washington, resulting in heavy rains and high snow levels.

One study, conducted by UW PhD Candidate Joe Zagrodnik with coauthors Dr. Lynn McMurdie and Dr. Robert Houze and published in the March 2018 issue of the Journal of the Atmospheric Sciences, aimed to learn how those atmospheric-river type storms produce extreme rainfall amounts upon encountering a coastal mountain range such as the Olympics.

MORE | Read the full study

"Not surprisingly, the most common rainfall type turned out to be quite boring," Zagrodnik said. "Cool, stable air and weak winds near the ground surface prevented moisture from rising up over the mountains--meaning that most of the rain originated from snowflakes formed high in the cloud. The raindrops were generally medium-sized and modest in concentration--basically the typical scenario where it rains all day but doesn't amount to much actual accumulation."

In other words, the rainy days you leave the umbrella at home.

But one particular storm on Nov. 12-13 in 2015 was more of the atmospheric river type, and let's just say the umbrella probably wouldn't have helped. Zagrodnik says the storm produced incredible rainfall over the lower elevation mountain slopes.

"Over 14 inches of rain fell near Lake Quinault, resulting in a rapid rise of the lake that forced researchers to move one of their radars to higher ground," he said. The storm would go on to create strong winds and flooding in the I-5 corridor and Cascades too

It was followed four days later by another big storm.

"Another foot of rain on November 17, 2015 caused landslides in Olympic National Park that blocked the Enchanted Valley trail for much of the following year," he said. "Rain rates of a half inch per hour lasted an entire day with an inch per hour for up to six hours in a row." That storm too, would pack a windy, rainy punch for Seattle.

But how did the Olympics get a foot-plus of rain in both storms? In general, as moisture reaches the Olympics, the vertical terrain creates lift which squeezes out moisture -- and with predominate moist southwesterly flow in the fall and winter, that process plays out quite often and is how they tally some 200 inches of rain a year.

But the secret to how the mountains get such extreme rainfall totals was unveiled by measuring the size of the raindrops.

"Rather than the big rain drops that they expected to find, the researchers instead measured tremendous concentrations of small rain drops less than 1.5 mm in diameter," Zagrodnik said. "Small drops form from a 'warm rain' process--meaning the rain drops are produced entirely from liquid cloud drops in the lower part of the atmosphere where the temperature is above freezing . By forming close to the ground, the small drops require less time and distance to reach the surface."

That meant since the rain was originating from lower levels inside clouds, the heaviest rainfall was occurring on the lower slopes of the Olympics, not near the higher elevations within Olympic National Park. Then again, some storms were a mix of both!

"Overall the new research shows that rainfall on the Olympic Peninsula is more complicated than was previously believed," Zagrodnik said. "The findings are helping improve rainfall estimates from NASA's Global Precipitation Measurement satellite and improve forecast models so that these heavy rain events can be better predicted."

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