Fog In the Wrong Season

This morning, fog was widespread over the lowlands of western Washington, with many of the river valleys in the murk (see visible image below around 7 AM)

Several local weather cams showed the fog.  Olympia had almost no visibility.

And it wasn't much better on the Chehalis River Bridge.

All this foggy action is a bit unusual in late April, which is close to the time of minimum frequency of dense fog for most locations in our region. (see below)

Why is dense fog unusual in April and May?

Because days are much longer and the sun is getting quite strong.  Thus, the surface is getting warmer. But the air is still relatively cold aloft.  

That leads to a rapid decrease in temperature with height, which results in instability and vertical mixing.

Vertical mixing is the enemy of fog, which generally forms when cold, dense air is near the surface, and warmer air is aloft.

Exactly, the situation that was occurring this morning (see the temperature above SeaTac Airport this morning below).

So why the fog this morning?  

A strong high-pressure area aloft developed to our northwest (highs of the 500 hPa pressure surface are shown below...think of pressure around 18,000 ft).  Red indicates the high pressure.

Such an offshore high-pressure aloft causes sinking, which warms the middle atmosphere.  Furthermore, high pressure causes clear skies aloft, which allows the lower layers to cool by emitting infrared radiation.

Warming above and cooling below is perfect for fog formation and the development of a low-level inversion.

California and Oregon Are Getting Our Rain: But That is OK

 There is always a yin and a yang in the weather.

Because of the structure and limited extent of weather systems, one area's precipitation bounty inevitably means less precipitation than normal for an adjacent region.

This is often true on the U.S. West Coast.  When California is wet, we tend to be dry and vice versa.

In April, such a reversal of fortune is occurring, with northern California and Oregon receiving above-normal amounts, and less than normal precipitation over Washington State.

Let's start with the current water year (October 1 to now) precipitation totals (the percentage of normal is shown).

Western Washington and the Cascades are in good shape, which is why our reservoirs are full.  Southern California has been wet.  But Northern California and much of Oregon have been drier than normal.

Climatologically, precipitation tends to shift north in the spring as the moisture-transporting jet stream moves northward, leading to a drying of California, while maintaining substantial precipitation in Washington State and British Columbia.

You can see this effect in the climatological precipitation maps for the region (below).

For January, substantial precipitation (dark blue color) extends from Washington State to Northern California and down the Sierra Nevada. Even Southern California gets a piece of the wet action.

But April is different, with far more precipitation falling over the Northwest than California, with profound drying over the southern portion of the state (see below).  In May, the California lack of precipitation is even more profound

But this year, something different is happening. 

The total precipitation forecast for the next 15 days (below) is for heavier-than-normal precipitation in California, with southern Oregon getting a substantial wet bounty as well.    Washington State will get some welcome precipitation as well, but the real action will be south of us.

The folks in LA and San Diego will be startled by all the rain, but the thorough wetting of California will greatly benefit the huge agriculture industry of the Golden State.  

What is going on?  

This is not climate change, but rather the development of anomalous upper-level lows that are heading into California.  

Below are predicted upper-level (500 hPa pressure, about 18,000 ft) maps, with blue showing troughs of low pressure (which are associated with precipitation).

11 AM Monday....a deep trough over northern California.

5 PM Saturday... another CA trough

Monday, May 4?   Another low-pressure area is heading to California.

Finally, although Washington State has a dry period ahead, we received substantial precipitation last night from a wet system that dropped very heavy precipitation exactly where we needed it:   on the eastern side of the Cascades and over the mountain barrier (see 24 hr below).   

Perfect for supporting agricultural needs over the Yakima Valley region.  

The weather gods are taking care of both California and our region.😊