Potential for King Tide Flooding this Week?

 King Tides occur when the Earth, Sun, and Moon are aligned and can be particularly high during the winter months when the Earth is closest to the Sun. Also called spring tides, King Tides can result in localized coastal flooding.

Since water levels rise as atmospheric pressure falls, something called the reverse barometer effect, King Tides can be particularly problematic if a low-pressure system is overhead.   

So what is going to happen this week?

Let's start with the predicted tide levels based on astronomical conditions, with no consideration of atmospheric pressure.   NOAA's Tides and Currents website has all the information!

Let's consider Bush Point on the western side of Whidbey Island (see red marker on the map below).

The predicted tides are shown below.  The vertical green line shows yesterday (Monday) evening.  The diurnal range between high and low tides gets larger and larger until November 18th ( Monday), with a range from -3.26 ft to 10.65 ft above Mean Lower Low Water (MLLW), the average level of the lowest tide for each day computed over a 19-year period.

Very large range.  This level is probably not enough for significant flooding without the assistance of low pressure or wave action.

The influence of low pressure is called the reverse barometer effect (see figure).  For every  1 hPa drop in pressure, the water level tends to rise 1 cm (0.39 inches).  And high pressure can push water levels down.

So any low-pressure systems expected this week?

The latest WRF forecast for atmospheric pressure on northern Whidbey Island is shown below (time is on the x-axis in GMT/UTC).  Average sea level pressure is about 1013 hPa and there appears to be a low-pressure event (to around 1007 hPa) on the 17th.

Worth checking on further.  Below is the forecast sea level pressure for Sunday around 7 AM, when pressure will be the lowest (looks like about 1007 hPa over Whidbey).   About 8 hPa below normal pressure or about 8 cm rise in water level... so should add another 3 inches to the astronomical tides.

Not the end of the world, but there could be some minor localized flooding.   

The real threat may be on the coast, where the low pressure and high astronomical tides will add to wave action from storms over the northeast Pacific.   The NOAA ocean wave model shows modest wave heights on Sunday morning offshore (below, in meters).  Perhaps 2-3 meter wave heights in the morning, increasing during the afternoon.  2-3 meters is 6-10 ft.

Will have to watch this carefully regarding the potential for coastal flooding.