January 01, 2015

High Pressure Means Low Sea Level

You would think you could trust sea level/tide predictions, and generally you can.  Such water level forecasts are based on the dependable impacts of the sun and moon on tides.  

But sometimes the tide tables are way off, and the last few days are a great example.  And the cause? The record breaking high sea level pressures!

Here is plot of predicted (blue) and observed (red) water levels at Seattle (Puget Sound, of course) from December 30th through New Year's Eve.  Predictions generally about a foot too high!  Enough so that folks can really notice.

Now if we plot the same water levels, starting December 10th, an interesting thing is apparent: just the opposite situation was occurring earlier in the month, particularly around Dec 10-12th.

So why the errors?   The astronomical information about the lunar and solar orbits and distances are well known.   Why the mistakes?

The answer is the anomalous sea level pressure we have experienced this month.   Higher than normal sea level pressure produces lower water levels and the opposite (lower pressures) cause sea level to rise.

The average sea level pressure in December  for Seattle Tacoma Airport is 30.15 inches of mercury or 1020.9 hPa.  Implicit in the sea level calculations done by NOAA is average pressure.  

But what happened this month?  Here is the answer at Seattle Tacoma Airport for the last 4 weeks, with the December average pressure shown by the black line.

During the past few days, particularly on Tuesday, pressure was way above normal (by roughly 20 mb or hPa), consistent with the water levels being pushed down.   But earlier in the month (before Dec 20th) , and particularly around the 10th through 12th pressures were equally below normal, resulting in higher than normal sea levels.   I talked about this effect in an earlier blog for a low-pressure period.  But this week we saw the substantial impacts of the other direction.

Finally, there is another memorable weather factoid:  December was the warmest on record at several Northwest stations.   Perhaps this is not shocking considering we had the warmest December day in Seattle history on December 10th, with many stations getting into the upper 60s!.


  1. I never thought about this before! Cool.

  2. Stupid question... is there any evidence that high atmospheric pressure causes increased volcanic or earthquake activity, either while the pressure is being applied to an area, or as it's released? I'm guessing the effects, while apparent on my poor sinuses, are too small to be noticeable in terms of volcanoes and tectonic plates, but instruments get better every year and I wondered if there was any correlation. The Earth seems more like an orange or grapefruit than a glass marble when you think about the impact of pressure changes.

    Happy new year! I hope the weather is interesting but not dangerous in 2015. :-)

  3. Does the high pressure have an effect on wood stoves? Since this high pressure event has been happening I'm having a heck of a time getting my stove to catch. Thanks!

  4. Cliff-
    My experience is that water level is affected by several factors; among them are warm temperatures causing snow melt, persistent wind direction and the barometric pressure you so aptly discuss.
    Snow melt causes a rush of water in the rivers which can raise the level of Puget Sound or the San Juans. I've noticed it especially in the South Sound from the Nisqually. A good Northerly can push surface water down sound, causing a rise in water levels, too.
    Usually the various factors cancel each other out and the NOAA tide tables are accurate. However, sometimes we get a solid North wind in the spring when the rivers are full. For most boaters, it doesn't cause any problems. But sometimes it can get a little exciting going into, say, Gig Harbor or Fisherman Bay on Lopez Island. Watch your depth sounder and keep in mind that the tidal predictions are mathematical calculations, not actual measurements.

  5. I think we have seen the impact of the very high pressure system in terms of low tides with the cancellation ofr our PT to Coupeville Ferry runs


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