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August 2014
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Storm Recap

Posted: August 15th, 2014

KATX_20140813_1258.Z2.lrg

 

 

A wild, but not necessarily unprecedented storm provided Seattle with record rainfall early Wednesday morning. The event was associated with a low pressure system that slowly drifted through the Northwest over the course of four days. The cut-off low appeared to have tapped into the North American Monsoon, which normally affects the Desert Southwest at this time of the year. Additional atmospheric dynamics may have also been provided by the multiple tropical cyclones recently injected into the jet stream over the Pacific.

 

But yesterday’s event was reminiscent in some ways of the record rainfall from just three weeks prior, or perhaps even more-so, of the memorable lightning festival and deluge that occurred on September 6th of last year. At that time, a lot of energy was devoted to the hypothesis that drifting cut-off low pressure systems (and other slow-moving phenomena) would become more common due to Arctic Amplification and resultant jet stream “blocking” (Francis v Barnes, 2013). While that debate has cooled, the notion that the North American Monsoon could more frequently affect the Northwest (Retallack, 2005) will be re-examined by SPU’s Climate Resiliency Group in light of recent electric displays.

 

National Weather Service daily precipitation records for August 13th were broken by wide margins at Seatac and Sand Point (and, presumably, Boeing Field had equipment been functioning). The official airport rain gage recorded 0.85 inches, well over the previous record of 0.33 inches set in 1982, and good for the site’s eighth daily precipitation record of 2014. Had the event not been split by a calendar day, Seatac’s 1.33 inches in 24 hours would have been the 5th wettest day ever in August. The storm also nudged the month, which averages 0.88 inches, into the wetter-than-normal category. Six months in 2014 have been wetter than normal.

 

Across the City, individual rainfall cells approached from the ENE, over the top of the upper level low pressure system; however the mass of precipitation, or shield, associated with a zone of deformation, moved WNW. The most intense rainfall bands passed just north of the Ship Canal roughly between midnight and 3:00 A.M. Overall, despite coming from an atypical direction, the most rainfall fell toward SE Seattle, which more or less matches Seattle’s climatological rainfall pattern.

 

Rainfall intensity, as measured by SPU’s 17 rain gages, did not reach statistically significant levels (greater than a two-year return) at any standard duration, according to preliminary results. That said, an expanded data search did reveal 3-hour, 2-year returns at two sites, Magnolia and Sand Point, respectively.

 

Seattle RainWatch was, of course, more active than usual, providing SPU managers and operators with three one-hour accumulation alerts (greater than 0.30 inches), two 6-hour accumulation alerts (greater than 1.00 inches), but zero one-hour forecast alerts. Additional preliminary data indicate(s) that SPU received fewer than 10 storm-related customer calls, and limited drainage and wastewater system impacts.

 

Perhaps the best story to come out of the event was the performance of the forecast models. SPU’s weather team issued a single message on Monday at 11:50 A.M. At that time much focus—and caveated uncertainty—was devoted to output from a particular forecast model, University of Washington’s WRF-GFS. (Through its membership in the Northwest Regional Modeling Consortium SPU actively participates in on-going development of the “Wearf” model.)

 

Above is a RainWatch image depicting 6-hour rainfall totals across the City (between 3/4 of an inch to 1 inch). Those observations match quite amazingly well with “Wearf” 12 kilometer and 4 kilometer forecasts made 48-hours in advance. Weather modeling and forecasting has come a long way.

 

The meandering and monsoonal aspects of this week’s event will be looked at closely by SPU’s Climate Resiliency Group. Recent Augusts and Septembers have proven to be more challenging than expected, and if we’re witnessing a changing, wetter and more humid late summer climate, then we have to prepare. On the other hand if this weirdly wet weather only represents poorly understood natural variability, then learning from and responding to it is what climate adaptation and resilience is truly all about.

 

References:
Seattle RainWatch
Northwest Regional Modeling Consortium
SPU Climate Change program