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September 2014
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Storm Recap

Posted: September 3rd, 2014

 

Radar image of PSCZ crossing NE Seattle at 6:26 PM, via UW.

Radar image of PSCZ crossing NE Seattle at 6:26 PM, via UW.

 

A strong Puget Sound Convergence Zone (PSCZ) passed through last night and provided parts of the City with rainfall intensities not seen in years. The event was associated with a fast moving, compact low pressure system that rapidly formed over the Gulf of Alaska on Monday. Typically quasi-stationary or slow, this PSCZ was fueled by vigorous mixing of cold air aloft and more closely resembled a Great Plains gust front.

 

While forecast models did predict the formation of a PSCZ at least 12 hours in advance, they did not accurately foresee rainfall amounts. Only 0.15 inches was expected over the course of the entire day. In the end, total rainfall across the City ranged from 0.09 to 1.14 inches. The National Weather Service reported in its 9:14 PM Area Forecast Discussion, “THE AIR MASS WAS A LITTLE MORE UNSTABLE THAN EXPECTED THIS AFTERNOON AND EVENING AS CONVECTION DEVELOPED IN A COUPLE OF RATHER INTENSE LINES OF SHOWERS.”

 

Preliminary data from SPU and other area rain gages indicate the most rainfall fell over parts of NE Seattle, especially across South Fork Thornton Creek, as well as over an area stretching from the Waterfront to South Lake Union. South Seattle only experienced light rainfall throughout the event.

 

The most intense rainfall was recorded by SPU’s Myrtle Edwards Park rain gage. It received 0.60 inches in the ten minute period ending at 6:56 PM. After six hours the waterfront site picked up 1.14 inches total. SPU’s Sand Point gage recorded the next most intense rainfall with 0.55 inches in the ten minutes leading up to 6:34 PM. Other neighborhoods that experienced statistically significant rainfall included Ballard (Locks), Crown Hill, Woodland Park, Green Lake, Maple Leaf, Ravenna, UW, and Capitol Hill.

 

Addendum:

 

Details are still under investigation, but if SPU rain gage data holds, Tuesday’s storm will go down as the most intense short-duration event on City record. (SPU’s citywide period of record is 37 years.) Below is a table with SPU’s most intense events citywide, by duration. Each event listed classifies as a 100-year storm, though for a number of reasons caution is advised when communicating as such. Finally, preliminary precipitation intensities (10-minute recurrence isohyetals) from Tuesday’s event have been sketched on this map.

 

Duration Date Amount SPU RG Location Storm Type
24hrs 2007-12-03 5.16″ 12 Magnolia AR
12hrs 2007-12-03 4.27″ 14 Admiral AR
6hrs 2007-12-03 2.38″ 12 Magnolia AR
1hr 2004-08-22 1.16″ 25 Central District FROPA
30min 2004-08-22 0.77″ 25 Central District FROPA
10min 2014-09-02 0.60″ 11 Waterfront PSCZ