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Weekly Rainfall Review

Posted: November 6th, 2016
2016-11-05-500

An “ejecting trough axis” racing overhead Saturday.

 

Last Monday’s (Halloween) forecast summary:

• A few showers with wind today and tomorrow.
• Somewhat steadier rain is likely late Wednesday into Thursday.
• More substantial rain is possible by the weekend.
• Extended forecasts suggest continued wet conditions.

 

The 7-day precipitation forecast called for 2.5- to 4.0-inches across the Tolt and Cedar watersheds, with perhaps 1.0- to 1.5-inches of uncertainty toward day-7. Maximum forecast intensities, or “bulls-eyes,” were for 0.71 inches during the 12-hour period ending at 11am on Monday, and 1.00 inches during the 24-hour period ending at 11pm Saturday.

 

With falls rains having obviously returned, eyes are starting to look toward freezing levels. The forecast had a few hours below 5000 feet; otherwise between 7000 and 9000 feet, which simply meant no early snowpack development—not that we were expecting it yet.

 

In the City, the 7-day forecast had 1.5- to 2.0-inches, with a “bulls-eye” of 0.61 inches during the 18-hour period ending at 5pm on Saturday. There was also a minor threat of convergence zone activity across North Seattle around trick-or-treat time; otherwise high-intensity, short-duration events were not expected.

 

7-day Precipitation Observations:

Watersheds: 2.5- to 4.3-inches.
City: 1.9- to 3.2-inches.

 

At the start week, day-7 uncertainty was emphasized. Notably, the GFS-WRF model was showing heavier rainfall over the weekend. As has been the case for much of this season, we indeed ended up with even more than the wettest case scenario as a strong jet raced overhead (see image above). As the National Weather Service put it in their Area Forecast Discussion that day:

A shortwave trough axis ejecting eastward out of the larger scale eastern Pacific trough will cross western Washington late this evening, shoving the widespread rain out of the area overnight.

The event did not cause significant impacts, but certainly required careful management of the Cedar River, especially with respect to turbidity.

 

Down in the City, the “ejecting trough axis” caused 5 combined sewer systems to overflow almost 3 million gallons into Lakes Washington & Union, and Puget Sound.

 

Finally—mercifully—the wettest October on record came to a close.

 

JRH