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August 2015

Surprise Thunder

Posted: August 13th, 2015



A small, unexpected disturbance rippled through the region yesterday and reacquainted parts of Seattle, albeit very briefly, with an old friend: rain. While colorful and noisy, the event caused only isolated street flooding and provided very little, if any, drought relief.


Thunderstorms first appeared on the radar near Tacoma at around 10:00 AM. Ninety minutes later a cell passed over Sea-Tac airport on its way to southeast Seattle. Sea-Tac is the where the National Weather Service keeps official records and the short-duration event provided it with 0.30 inches. That’s 34% of normal rainfall for the entire month of August and more rain seen there in a single day since April.


SPU’s Rainier Beach rain gage recorded 0.21 inches in just five minutes, which was good for a 10-year statistical recurrence. KOMO’s Scott Sistek featured Seattle RainWatch while reporting the event.


While lightning strikes caused a few power outages, there were only minor impacts to SPU’s drainage and wastewater system. SPU received and responded to 7 calls from customers, 5 of which were related to “ponding,” or minor street flooding.



The drifting low pressure system responsible for directing yesterday’s quasi-monsoonal weather toward us will get picked up by the jet stream tomorrow. On its way out the hemispheric door another round of showers and thunderstorms will occur starting as soon as tonight.


Previous model runs had suggested that both the city and mountains could see significant rainfall tomorrow, upwards of an inch. The latest guidance supports the notion of spottier activity, peaking tomorrow afternoon and remaining under 0.50 inches total. Beyond tomorrow, additional rain is unlikely save perhaps for some barely-measurable on-shore drizzle early next week.


Temperatures will also be much cooler tomorrow as on-shore flow fills in behind the departing system. In fact, tomorrow’s forecast high of 68 degrees would represent the first day sub-70-degree day since June 12. That’s notable because at 11:00 AM this morning, Seattle reached 70 degrees for a record 62nd consecutive day.


Temperatures are expected to rebound to near normal next week. Extended models suggest slightly warmer temperatures beyond next week, but they still do not show any signs of additional heat waves.


NWS Area Forecast Discussion
SPU Water Supply Outlook