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December 2014

Perihelion-King Tides

Posted: December 22nd, 2014


Update: the “storm surge” at 6:18 AM this morning (12/23) was a whopping 0.21 feet, according to preliminary measurements. SLP was higher than predicted (1025mb) and winds were also weaker (SE, ~3kts). SLP should dip to around 1012mb tomorrow, but gradually climb above 1030mb over the weekend, according to the latest UW WRF-GFS forecast. Minor coastal flooding is possible tomorrow; otherwise it looks like we’ll avoid problems this round.



December 2012 King Tide coinciding with a 980mb windstorm. Image: Nick Adams, West Seattle Blog.


It’s that time of year again. The first of this winter’s three King Tide events begins this week. Predicted water levels will be at least a foot higher than average daily high tides (MHHW) tomorrow through Monday. Fortunately, tidal flooding is not expected due to favorable weather conditions.



This week’s King Tide dates, times and water levels.


Each winter, the sun and moon’s gravity conspire to create extreme sea levels, both high and low, around the world. The events are often referred to as King Tides, in honor of Canute the Great who apparently had a special relationship with the sea in the 12th century. While some of us might prefer that we promote scientific literacy and call them Perihelion Tides, those us of who are working on sea-level rise adaptation appreciate the attention.


Long-range tide prediction, thanks to our understanding of astronomy, is highly accurate. There are, however, a number of environmental factors that can alter water levels on a daily or seasonal basis. They include water temperature, air pressure, and wind.


Seattle is protected from the open ocean and true hurricane-style storm surges by the Olympic Peninsula. Therefore the primary driver over relatively narrow Puget Sound is the Inverted Barometer Effect in which either relatively low atmospheric pressure coaxes the sea upward, or relatively high pressure exerts downward force on the water. The mathematics of the effect are well understood and forecasts for sea-level pressure can be reliably applied to tide predictions. Wind speed and direction may be slightly less reliable to forecast, but we usually have a good idea about tidal anomalies days in advance.


Tidal flooding usually begins when water levels reach two feet above average daily high tides. Given the present weather forecast, we’ll likely only get close to that mark on Friday morning. That said, water levels have been running high this season, possibly due to warm air and water temperatures; minor impacts may also occur tomorrow morning. The National Weather Service has issued a Coastal Flood Advisory. Particularly exposed parts of Seattle include Alki Point and the Duwamish Waterway—south-facing residents of Alki as well as Beach Drive SW property owners should pay close attention.


NOAA Tides and Currents—Seattle
Washington State Department of Ecology King Tide Information
SPU Sea-Level Rise Information



One more quick pineapple to go.

One more quick pineapple to go.


A brief reprise of this past weekend’s pineapply weather will pass through tomorrow. By Thursday the weather will turn much cooler and drier, and might stay that way for more than a few days. It had looked like we’d get an arctic blast this week, and while the forecast has obviously moderated, models are still having problems. See this morning’s insightful NWS Area Forecast Discussion for more details.


The latest National Weather Service forecast for Seattle shows 1.00″ tomorrow and 0.40″ Wednesday, followed by mostly dry conditions Thursday through Sunday. The latest UW WRF-GFS has slightly less for tomorrow, but about 0.70″ of it falls the 6-hour period ending at 10 PM. That’s respectably intense, but outside of isolated street ponding, few drainage impacts are expected.


Over SPU’s mountain reservoirs, this afternoon’s NWS forecast shows between 2-2.5″ tomorrow through Wednesday. Snow levels will climb to almost 7000′ tomorrow, but will then plummet on Wednesday when at least a few inches should fall at reservoir level.


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