A series of strong storms will begin impacting the Pacific Northwest over the next couple of days, each bringing the opportunity for incredibly heavy precipitation through this weekend. Lower elevations could see up to a foot of rain, while mountain peaks could see nearly ten feet of snow.
Several favorable factors will help a run-of-the-mill weather event turn into a prolific precipitation producer (alliteration unintended), with an unsettled jet stream producing numerous low pressure systems in the northeastern Pacific over the next couple of days. Each of these lows will sweep through and affect areas from southern British Columbia through northern California
The above animation shows sea level pressure and precipitable water in the atmosphere over the next seven days. Precipitable water is the amount of rain that would fall if all of the water vapor in a column of the atmosphere were to condense and precipitate to the ground. High levels of precipitable water (shown by progressively warmer colors) indicate high levels of moisture in the atmosphere, and higher chances of heavy precipitation.
The stream of moisture flowing northeast out of the tropics is a classic "atmospheric river" event known as the Pineapple Express, so called because the moisture comes from the vicinity of Hawaii. Heavy rainfall events along the West Coast influenced by the Pineapple Express events are common during this time of the year, and they're responsible for some of the most extreme rainfall events recorded in this region of the country.
The predicted rainfall and snowfall totals across the region are impressive to say the least, and could result in flash flooding, landslides, and possibly even avalanches in the mountains where the heaviest snowfall is anticipated. The above graphic shows the Weather Prediction Center's forecast precipitation amounts over the next seven days, beginning this morning.
The big question on everyone's mind is how this will impact the devastating drought in the region.
According to last week's Drought Monitor, many of the areas expecting a major deluge are under some form of drought. Parts of northwestern California and western Oregon stand to receive the most rain relative to their droughts, but it could prove too much of a good thing. Too much heavy rain at once hurts more than helps; the water can't effectively absorb into the dry ground, causing it to run off and cause flooding.
A new drought monitor is released every Thursday.
For reference, here are some selected year-to-date rainfall figures as of yesterday:
- Rainfall at Sea-Tac Airport is 10.49" above normal so far this year
- Olympia, Washington is 5.38" above normal.
- Portland, Oregon is 2.99" above normal.
- Klamath Falls, Oregon is 1.59" below normal.
- Mount Shasta, California is 12.59" below normal.
- Eureka, California is 4.72" below normal.
- Redding, California 4.87" below normal.