Unusually Low Pressure Caused Sea Level to Rise

Have you felt lightheaded during the past day?  

Perhaps you should have, because the atmospheric pressure has been unusually low.

Low enough to cause a significant rise in sea level.

Consider a plot of sea-level pressure for SeaTac Airport since July (below).  The pressure dropped to 990 hPa, the lowest observed during the period, by a LOT.

Our extremely low pressure was associated with a deep low-pressure center immediately to our west (see the analysis at 4 AM Friday).   At this time, the low center had a central pressure of 968 hPa, which is low for this latitude.

The average pressure for Seattle in November is 1017 hPa (the average around the nation is 1013 hPa), so our pressure on Friday was 27 hPa below normal.

Localized low pressure causes sea level to rise by approximately 1 cm for every decline of 1 hPa.  (See schematic below).

Thus, sea level over Seattle was elevated by 27 cm or 10.6 inches by the low pressure.  Almost a foot!

You want to see the low pressure in action on sea level?   

Below is a plot below of predicted (blue) versus observed (red) sea level in Seattle.   The difference between the two has increased this week as the low-pressure center approached, with sea level 2.2 feet above normal.  

So about 40% of the anomalous sea level rise was due to the low pressure.

But the effects of the low center on sea level do not end there.

The low center is associated with powerful westerly (from the west) and southwesterly winds (see forecast surface winds at 1 PM Friday below).   Winds near the coast were strongly from the south (southerly)

Such southerly winds push on the ocean surface, which flows TOWARDS the coast by the Coriolis force, resulting in a HIGHER water level.  See schematic below of this situation.  Want to impress your friends?  This is called Ekman Transport.

But the high water fun caused by the low center does not end there!

The offshore low with strong winds over a large area (long fetch) produces big waves that can bring the water levels up even more on the coast (see forecast wave heights for Friday morning below).

There are other, more minor contributors to short-term high water levels, such as a Kelvin wave associated with the El Nino moving up the coast (several inches).     

The good news is that water levels should drop as the low center weakens over the weekend.

The Large Effects of El Nino On Our Current Weather

Our current weather is being profoundly influenced by El Nino.   

And that influence is not about to end.

Let me demonstrate this to you.

The following is a map of the anomaly (difference from normal) of upper-level heights at 500 hPa pressure for the past week.  You can think of this as the difference from normal of pressure around 18000 ft.

A huge trough of low pressure is off our coast (purple color).  Higher pressure/heights over the western U.S. (yellow color)

Here is the forecast of the same upper-level heights/pressures for the next 5 days.  Very similar pattern.  Deep low off our coast aloft.

Now, let me show the pattern for very strong El Nino winters. 

Mamma Mia!  A strong offshore trough/low just like the last week!

This upper-level El Nino pattern is associated with a persistent low-pressure area off our coast (see below).

Which is VERY like the pressure pattern we have seen the last few days.

To illustrate, here is the sea level pressure forecast for tomorrow morning at 8 AM. Looks just like the strong El Niñoino pattern!  Impressive low off our coast.

Moisture tends to wrap around such "El Nino" lows, resulting in the heaviest precipitation on the coast and south of us (see plot of moisture transport below)

During El Nino years, southern Oregon and northern California tend to get a lot of precipitation, and the current forecasts show exactly that (see total precipitation forecast through Wednesday morning below).

Vancouver Island gets a lot of precipitation as well because of the strong southerly flow circulating around the low-pressure system, which is forced to rise by the island's terrain.

There is a lot of gloom and doom in some of the media about this pattern, but it is actually quite favorable. 

The northern CA reservoirs are currently below normal, as are the reservoirs of southwest BC.  They will get a welcome refill.

El Niño brings substantial precipitation to the Colorado River drainage, which is badly needed to fill the reservoirs such as Mead and Powell. 

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Announcement

I will hold a special online Zoom session at 10 AM on Saturday for Patreon supporters. I plan to talk about the seasonal forecast in much more depth and answer your questions.