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!.