Blocking Ridge, Dry Pacific Northwest, and Southern California Rain

 Not a boring time for meteorologists.  New Orleans Airport is closed with record-breaking snow and cold, while the Pacific Northwest is strangely dry and southern California will get its first decent rain of the season.

The Big Easy

All because of a highly amplified upper-level wave pattern.

The upper level (500-hPa) height map (think of this as pressure around 18,000 ft) today shows a ridge of high pressure aloft over the eastern Pacific while a trough of low pressure is found over the eastern U.S. (red indicates high pressure and blue indicates low pressure).  

Ridges/high pressure are associated with sinking air and dry conditions on their eastern sides.  Like over us.   Troughs are associated with rising air and precipitation.  Between them, cold air moves southward.

By Thursday the eastern Pacific ridge becomes crazy strong.  

Keep your sunglasses handy, as it will be sunny and dry.   What a nice break in the middle of winter for us.   But cold and wet over the eastern US.

By Saturday, something interesting...and important... will occur.   A trough will develop south of the West Coast ridge, resulting in a pattern called a Rex Block.  Very stable pattern.

Such a southern California trough will produce upward motion and precipitation.   Southern California will get wet.  Significantly wet.    

Below are the forecast precipitation totals through next Monday evening.  As much as 1.5 inches in the mountains near LA.  Significant rain over the entire coastal zone.

Such rain will moisten the light fuels and greatly reduce the fire threat until the next Santa Ana.  Very good news.

 

Finally, something else occurred today that I thought was interesting.....a very energetic Chinook wind on the western side of the Rockies around Great Falls, Montana.

Two days ago,  shallow cold air was entrenched over the lower elevations of Montana, with lows of -5 to -20F. (see below)

But with high-pressure building over the east Pacific and lower pressure inland, strong winds from the west descended into eastern Montana.  As the air descended the eastern slopes of the Rockies, warming by compression occurred.   This descending air also displaced the frigid dense air that was in place.

Today's low temperatures were MUCH warmer around Great Falls and neighboring locations near the eastern slopes of the Rockies, with highs getting into the 20s!

Check out the temperature and relative humidity observations at Great Falls during the past day (below).

Yesterday morning at 7 AM, the temperature dropped to around -9F.  By noon it was 12F and by noon today is was around 36F.   All associated with air sinking on the eastern slopes of the Rockies.  

   

The UW WRF model simulated this transition well. The forecast surface temperatures for 4 AM Monday were very cold over most of Montana on Monday (white is very, very cold air).  The winds are shown by the wind arrows.  Weak at low levels.

But by 10 PM PST Monday, the white was gone as westerly winds descended the Rockies.  Blue colors are relatively warm.

Chinook winds are also Foehn winds and are sometimes referred to as "snow easters."

The Origin of The Los Angeles Wildfires

My group and scientists from the University of Albany are now studying the meteorology of the LA wildfires earlier this month and have significant early results.  At the American Meteorological Society meeting last week, I attended many wildfire meteorology sessions and talked to several colleagues who have actively studied such events. The LA fires were a topic of considerable discussion.

We have a good idea of what happened:  an extreme/unusual Santa Ana event associated with severe downslope winds.   In this blog, I will show you some early simulations and explain why this event occurred.  

I will also describe why climate change did not play a significant role.

As I will explain below, this is not only a Santa Ana event, but an unusual one, with extreme winds descending to lower elevations.  Some lower-elevation stations, such as Burbank, experienced their strongest wind gusts on record.  Model simulations suggest the development of what is known as a high-amplitude mountain wave leading to a downslope windstorm event.

Santa Ana 101

Santa Ana's are associated with strong northeasterly (from the north to east) winds over southern California.  These strong winds are accompanied by very low relative humidity and are very favorable for southern CA wildfires.   

The typical large-scale weather pattern associated with Santa Anas includes a large high-pressure area over the Great Basin, with the strongest events also having a low center to the southwest (see below). Such a pattern results in strong northeasterly winds approaching the mountains of southern CA.

The observed sea level pressure pattern at 4 PM January 7 (below) showed many of these elements, with the low to the south being particularly strong (warm colors indicate above-normal pressure, cool colors show below-normal pressures).

This pattern produced extreme, perhaps unprecedented, northeasterly winds approaching the region in the lower atmosphere.  To illustrate,  the color shading in the map below shows wind differences from normal at 925 hPa (about 2500 ft above sea level) at 10 PM Tuesday (January 7) evening.  The light gray indicates very unusual winds (more than six standard deviations from normal).  The wind direction and speed at some points are also shown.

This was not your normal Santa Ana.

High-Resolution Simulations

To understand what happened,  UW Research Scientist David Ovens, a member of my research group, ran a very high-resolution forecast/simulation of the event using the WRF model, with a grid spacing of 1.3 km (this is very high resolution).

Let me show you a surface wind gust forecast (32 hours into the prediction) for 12 AM on Wednesday, January 8.   The areas of the Palisade and Eaton fires (the two biggest ones) are shown by red outlines.   

Extreme winds (with gusts exceeding 70 knots, 81 mph) were forecast over and downwind of the San Gabriel Mountains.  HUGE threat.    Lesser, but still strong winds (50-60 mph gusts) are noted in the Palisades area extending to the west and south.   

To understand what is going on, it is useful to plot vertical cross-sections through the fire areas to show the 3-D atmospheric structure.   Below is a cross-section (at 8 PM Tuesday) through the Eaton fire; a section that crosses the San Gabriel Mountains.  The shading shows sustained winds in knots, the x-axis is the horizontal distance and the y-axis is height in pressure (700 is about 10,000 ft)

Wow.    Very strong acceleration of the air as it descends the southern slopes of the San Gabriels, with the strongest winds near Altadena, where the fire went crazy.  This was a very powerful downslope windstorm with a highly amplified mountain wave pattern.

Another cross section, this time going through the Palisades fire at 10 AM on Wed. January 8, is shown below.   Very strong winds to the lee (south) of both the interior mountains and Santa Monica mountains to the south (left side of the figure).

These and earlier simulations by the UW WRF model and NOAA/NWS models (such as the HRRR, High Resolution Rapid Refresh) consistently forecast the extreme downslope winds DAYS in advance.

It was also clear that there were unusually large amounts of dried "fuels" ready to burn.  The fuel load was particularly high after TWO unusually wet winters (2022-2023, 2023-2024).  A dry start of winter ensured massive amounts of fuels were ready to burn. 

This was clearly a situation of extraordinary extreme wildfire danger.  That is why I blogged a strong warning the day before.  Why the National Weather Service did the same.  LA officials should have know a severe threat was in place.

All that was needed was an ignition source.  For the deadly and large Eaton Fire near Pasadena, it appears that a problematic electrical transmission tower was the origin of the fire.

For the Palisades inferno, it is either some smoldering embers from a New Year's Eve fireworks blaze or the failure of some aged powerlines found north of Skull Rock.   Perhaps, arson is another possibility.  Several fire experts I talked to at the AMS wildfire meeting thought the powerlines were the most probable cause, but a definitive evaluation is not yet available.

What is clear is that LA did not shut off the power to the region even AFTER the fire was initiated and that only limited fire-fighting resources were in place before the fires began.  Not good.

Bottom Line:   A very skillfully predicted Santa Ana event with record-breaking winds hit LA earlier this month. Two wet winters resulted in unusually high levels of dried fuels. Human ignitions initiated the fires.

Influence of Global Warming

There are several media outlets and climate activists that are suggesting that LA fires were the result of global warming or significantly enhanced by human-caused warming.  

These claims are contrary to the best science, which indicates minimal impacts of human-caused warming.  

Such claims undermine the importance of key societal failures, from not shutting off power and fire-hardening homes to not having sufficient fire-fighting capabilities or adequate warnings and evacuation protocols.

Key reasons why global warming is not an important factor include:

1.  There is no evidence that global warming increases Santa Ana winds.  In fact, there are strong physical reasons (with supporting scientific literature) that global warming WEAKENS Santa Ana winds.

2.  The available fuels were unusually abundant because the two previous years have been much wetter than normal.  There is no evidence this is caused by global warming.  And there is no long-term trend for wetter winters, which would be evident if this was a climate issue.

3.  The immediately prior months were drier than normal.  This is not predicted by global warming projections and there is no historical trend to drier early winters which would occur if climate was a cause (see plot below of Los Angeles precipitation for Oct. 1-January 6 ).

3.  There are some outrageous claims that the LA fires are the result of increasing "weather whiplash" due to climate change.  This "whiplash" theory is unsupported by observations, such as the plot shown above.

I am particularly troubled by the media pushing weather/climate theories that are untrue.  A prime example is the unsupported claim that the Lahaina fire was associated with the passage of a hurricane to the south.   This was proven to be false.