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November 2015

All Treat, No Trick

Posted: November 2nd, 2015



Seattle RainWatch image showing two-day rainfall and a clearly defined rain shadow.


We keep calling atmospheric rivers doubled-edged swords for their simultaneous abilities to replenish water supplies and flood cities. Sometimes, when mountain reservoirs are already full, they’re completely unwanted. But seldom are they considered win-wins. This weekend’s storm, though, was more or less all treat, and no trick.


It’ll take a few days to properly measure just how beneficial the storm was to SPU’s water supply, but a lot of rain fell. Because it occurred during a drought some of the old hydrological rules, for example, a foot of reservoir elevation for every inch of rain may not apply. But between the time the first tropical wave passed early Friday morning and cool post-frontal showers tapered of on Sunday more than 8 inches of rain fell over parts of SPU’s Tolt and Cedar watersheds. About a foot of rain fell over the course of the week!


As for the urban drainage system that SPU also manages, the storm was good as well. Sure, we’d prefer that it not rain at all when autumn leaves hit the ground. But we expect the two to go hand in hand, so we look for other ways to measure success.


It should be noted that Olympic rain shadows frequently protect the city from flooding while the mountains get hammered. Westerly flow aloft does that well, and it certainly did this weekend. Over the same 72-hour period in which SPU’s watersheds received all that rain, the city only picked up between 1.4 and 2.2 inches. The least fell at SPU’s U-District rain gage, and the most fell at our Georgetown rain gage. That pattern closely follows that which was described earlier in October when SPU featured its rain gage network for Take Winter by Storm.


During the main atmospheric river event, rainfall never reached significantly intense levels city-wide. As a result, SPU received only 57 calls from customers during the three-day period, the vast majority of which were related to leaf-clogged storm drains.


Perhaps the best aspect to the storm, however, was the forecast. Documented in the previous blog entry, the GFS model hinted at the event nearly two weeks ago, and more or less held firm last Monday through Friday. The European model gets plenty of praise for the way it handles hurricanes, but the American model can do well in other areas. U-S-A! The GFS’s ghoulish consistency enabled SPU to prepare days in advance and avoid those leafy hobgoblins of little storm drains.



After partial clearing today and tomorrow, a little bit of light rain is expected to return to the region on Wednesday and Thursday. Then sometime during the weekend a low pressure system and cold front should deliver some wind and rain. At the moment, less than an inch is expected Thursday through Sunday in Seattle, and 1.5 to 2.5 inches over SPU’s watersheds.



NWS Area Forecast Discussion
NWS Forecast Table Interface—Seattle
NWS Forecast Table Interface—Tolt Reservoir
NWS Forecast Table Interface—Chester Morse Lake
SPU Water Supply Outlook