Water Levels Approaching or Reaching Record Water Levels in the Northwest – Rise Thanks to That?

From the Cliff High Volume Weather Blog

By Cliff Mass

The King Tides represents an unusually high water level, which occurs when the shapes of the sun and moon are just right.

But this year, the tides are predicted to have risen sharply in Seattle, thanks to the atmosphere.

At about 9:15 a.m. Friday, the water level in Seattle rose 14.47 feet above sea level and possibly higher 20 minutes ago (there was an outage at the site from 8:42 to 912:AM)

The previous record high tide in Seattle was 14.51 ft in December 2012. Chances are we beat it.

The result was coastal flooding throughout the region.

Near Alka Point that morning (courtesy of West Seattle Blog).

Here are the predictions (blue) and actual water levels (red) in Seattle from the NOAA Tide and Currents website as of January 3. Friday’s prediction (about 12.5 ft) is WAY too low ( about 2 feet). (red arrow indicates Friday)

Something has greatly increased the high tide!

But what?

The secret is a powerful low-pressure system and the front is working EXACTLY at the right time (see sea level forecast at 8am on Friday, in perfect sync with the forced natural spring tides). Literature.

Sea level pressure (solid line)

And a pressure trace at SeaTac Airport records the drop in pressure (see below). Pressure drops all the way down to 990 hPa from ~1025 hPa…. It was a big and quick change.

Why is low pressure important? Because the low pressure causes the surface of the water to rise in what is known as the inverse barometric effect (see below)

Courtesy Naval Graduate School.
But as in a late-night trailer: WAIT more!
The passage of the trench causes winds to shift southwest (from southwest) over Puget Sound, pushing the water into Eliot Bay, Seattle’s harbor (see below). Sea level is even higher!

You might ask how the weather-related increase in high tides compares with the steady increase observed over the past century.
Good question. Below is a chart of sea levels over the past 120 years, a very steady rise with no signs of recent acceleration. Sea level rises about 2.06 mm a year or 8 inches per century. So a weather-related increase of about 24 inches has reduced the steady rise in sea levels over the past century, some which may be related to global warming.