La Nina Slams Pacific Northwest
After a six week break, La Nina is back with a vengeance across the Pacific Northwest and she is bringing plenty of fury with her! Several cities and counties in both Washington and Oregon have declared states of emergency. In just the past four days alone, Oregon and Washington have been pounded with record setting snowfalls of 15-20 inches in valley locations, an ice-storm with power outages affecting more than 100,000 customers at a time, mudslides, 110mph coastal winds and now record setting rainfall and flooding. Oregon and Washington are currently split in half by an arctic frontal boundary. Most of Washington state remains below freezing with many locations reporting freezing rain or snow for the past several days. Oregon also received a large dump of snow earlier this week, but is now on the south (warmer) side of the frontal boundary. Temperatures rose from near freezing to above 50 degrees in a very short period of time over western Oregon this week. At the same time, a steady stream of precipitation known as a pineapple express reached deep into the tropical Pacific and is aiming directly at Oregon. Rapid rises in freezing levels over Oregon have lead to very fast snow melt. Combine that 5-10″ of rainfall in just 72 hours and you have the perfect set up for flooding. This is a classic La Nina signal. Seems like the only thing we are missing this winter is a region wide windstorm, which the area is overdue for.
A similar set up to this week’s event occurred in the first week of February 1996 and lead to the largest flood in many locations around Oregon and Washington since December 1964. This time it is occurring on a smaller scale all around, including less snowfall, rainfall and cooler temperatures. However, the same ingredients are at play. In Oregon’s Willamette Valley from Salem south to Corvallis, residents are seeking higher ground as flood waters rose rapidly on Thursday. The Cascade mountains are also getting hit hard. After picking up plenty of fresh snow, many resorts are now facing increased avalanche hazard. Winds gusted to 119mph on Mt Hood, just above Timberline Lodge at the 7,000ft level this week. Seattle is still waiting to thaw out from the deep freeze and ongoing ice storm. Anything is possible in a La Nina winter and is usually marked by swings from one extreme (cold) to another extreme (pineapple express / warm) in a very short period of time. This meteorological phenomenon occurs when both the sub-tropical and polar jet streams collide over the Pacific Northwest at the same time, just as they did in February 1996.
In October, meteorologists at the 19th annual Oregon AMS Winter Weather Forecast Conference in Portland warned residents that the majority of winter’s wrath would likely arrive in January and February. Those forecasts are available for review on the Oregon Chapter of the American Meteorological Society’s web site under the “meetings” page at: http://www.ametsoc.org/chapters/oregon/. I am impressed with the skill of our forecasters at this year’s conference. They issued detailed forecasts months in advance. So far they have hit the nail on the head with the active January pattern we are now experiencing. Hats off to them. If their forecasts continue to verify, we should expect to see a lot more active and potentially record setting weather to come this winter.
The Pacific Northwest is in the beginning stages of what is known as the cold phase of the PDO (Pacific Decadal Oscillation) which is marked by 25-30 year spans of cooler and wetter than normal weather in Pacific Northwest. The last warm phase of the PDO lasted from about 1976-2007. The switch to the cold phase of the PDO started off with a bang when record setting Christmas snow fell across the Pacific Northwest in December 2008. In the winter of 2008/09, Portland received the most seasonal snowfall (24″) since 1968/69 (34″). Since the change to the cold phase of the PDO, Portland has since experienced snow and/or arctic air every winter. That’s four winters in a row (2008-2012) and is the first such feat since the winters of 1953-1957, which occurred during the last cold phase of the PDO from 1948-1975. If this week’s weather isn’t enough for most folks, longer range computer models suggest the Pacific Northwest could be in for a “rinse and repeat” near the end of the month. After all, it’s a la nina winter and nothing would surprise me.
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