The rain gauges at Portland International Airport were busy yesterday, recording 1.30 inches of precipitation over the course of the day, tying the record for December 11th which was set in 1969. The amount of rainfall measured yesterday is more than eight times the average precipitation for this time of year. It was a bit warmer than usual, too, which increases the dangers associated with a rain event like this one. The greatest threat is caused when rain at high elevations melts snow rapidly and leads to flooding, avalanches, and landslides.
We call events like this a Pineapple Express due to their origins in the Pacific tropics near Hawaii. The map to the right shows the rather direct path that the current storm system took from the tropics to the Pacific Northwest.
As I discussed in this post, we expect more and heavier rain events during La Niña events, so this comes as no surprise to forecasters. Put into the context of our current weather, that graphic showing the path of the jet stream during La Niña should make more sense.
Hydrologists track the effects of storms like this using monitoring sites called SNOTEL stations. These sites report vital information like snow depth, precipitation, temperature and what’s called snow water equivalent (SWE) from remote locations across the west.
The SNOTEL site shown to the left is on Mount Hood near Clear Lake. Over the last 24 hours the Clear Lake site measured a decrease in snow depth of seven inches, with a reduction in SWE of 0.6 inches. That means that while the visible snow is now seven inches shallower than it was mid-day yesterday, the water run-off that the melting snow produced is equivalent to just over half an inch of rain. At the same time, 1.2 inches of actual rain fell, so the total water run-off from this event would be the same as having 1.8 inches of rainfall (1.2 in. of actual rain + 0.6 in. of snow melt water).
Recent weather advisories have warned that flooding is possible or even likely for rivers like the Puyallup, Skokomish, and Snoqualmie rivers in Washington and the Nehalem, Siletz, Tualatin, and Johnson Creek in Oregon. If you live near or on one of these rivers, please monitor your local forecasts for alerts should flooding develop. In fact, as I write this, the Snoqualmie River is already above flood stage.