November 29, 2023

High Pressure and the Northwest's White Affliction

 The visible satellite image this morning was stunning.

Low-elevation clouds covered nearly all of Washington, except for the highest elevations (see image)

Some folks were in fog while others looked at a low-stratus cloud deck.

And this cloudy murk was cold, with much of the state below freezing (see minimum temps this morning).   Freezing fog was widespread.

Cold, cloudy, foggy and icy.  Yuk.

What can you blame for this unpleasant situation?   

Persistent high pressure.  A strong ridge of high pressure aloft.

The upper-level map (500 hPa pressure, about 18,000 ft) on Sunday evening shows the situation (see below).

The difference from normal of 500 hPa heights (or pressure) for the last week was way above normal  (see below, red indicates strong ridging...which is the same as high pressure aloft).

Translation:  the high-pressure aloft was very strong and persistent.

In the summer, such a pattern would give us a heat wave.

But in winter we get a cold wave!  But why?

High pressure is associated with sinking air aloft, which kills middle and upper-level clouds.

Without clouds, the Earth can effectively emit infrared radiation into space.  There are no clouds to get in the way!  Nights are very long and the sun is wimpy, so solar heating is weak.

The ground cools more effectively than the atmosphere aloft and this helps create an inversion, with temperature WARMING with height.   The inversion is also strengthened by the high-pressure aloft, whose sinking motion warms the air aloft (air warms as it sinks due to compression by the higher pressure at lower levels).

The inversion is a very stable feature and prevents the mixing of air at lower elevations.   It protects the low-level cold air from mixing out.

Want to see the inversion?  Here is a plot of temperature (red) and dew point on the Washington coast at Quillayute on Tuesday morning.  Temperature is increasing with height at low levels.  An inversion!

Each day the air got colder near the surface.  Cold air can hold less water vapor than warm air.  Water vapor starts to condense out into low clouds and fog.

And it is even worse than that.  The top of the clouds reflect solar radiation back to space, enhancing cooling.  Clouds are good emitters of infrared radiation.  More cooling.   Low clouds are cooling machines!

So if you want to blame the cold, cloudy weather on something, blame the high pressure!

We escape this run-away cooling in the summer because of the far stronger solar radiation, which is able to heat the surface and destroy the low-level inversion.

November 27, 2023

Are King Tides a Threat During the Next Two Months?

A number of media stories have been talking about King Tides and flooding during the past few days, and I have gotten several emails from worried readers.

So let's talk about the reason for King Tides and evaluate the threat.

The highest tides of the year...the King Tides... typically occur this time of the year for two reasons.

First, the astronomical setup is ideal, because the Earth is closest to the sun around the New Year.

Second, this is the stormest time of the year, with Pacific cyclones associated with low pressure, and low pressure causes the water level to rise.

Monthly high tides, also known as spring tides, occur twice each month when the sun, moon, and earth are aligned.

And this alignment is enhanced during the winter because the Earth is closer to the sun during our cold period.  Believe it or not, the Earth is nearest to the sun during our winter!  (see below).  Thus, the gravitational attraction of the sun is larger during winter, which produces a greater maximum daily tide:  the KING TIDE.

Finally,  regional low pressure increases the water level.  

How much?  When sea level pressure falls by 1 hPa (or 1 mb), the water level rises by about 1 cm.   So a deep storm that causes pressure to fall by 25 hPa can result in a water rise of around 25 cm or roughly 10 inches.

Now let's look ahead.

Take a look at the predicted astronomical tide levels for November, December, and January below (the green line is today).  For November, tides will max out during the next few days around 12.5 ft above Mean Lower Low Water (MLLW), the average level of the lowest tide for each day computed over a 19-year period.

December will have a number of days (mainly during the middle and end of the month) that will get to the same level.    Mini-King tides.

Mid-January will bring max tides that are a bit higher...around 13 that will be the time of greatest vulnerability.

There will be little problem of high tides and flooding without a strong low-pressure system moving we need to watch the forecasts for the next few months.   Right now pressure is well above normal...and will remain that way for several days.

Beyond, 10 days the forecasts have little we are going to have to wait and see.

Want to know about the forecast pressure for the next week?  Check out the ensemble of many forecasts below for Seattle's sea level pressure.  Forecast uncertainty gets very large after December 2 (the forecasts are not in agreement).

The latest model run has a decent storm on December 3 (see sea level pressure forecast below) with a deep low-pressure center heading into Washington State.  Fortunately, the astronomical tides will be low then.


High Pressure and the Northwest's White Affliction

 The visible satellite image this morning was stunning. Low-elevation clouds covered nearly all of Washington, except for the highest elevat...