The origins of the Los Angeles wildfires—what does Watt think about them?

From Cliff Mass Weather Blog

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My team and scientists at the University at Albany are studying the meteorology of the Los Angeles wildfires earlier this month and have important early results. At last week's American Meteorological Society meeting, I attended a number of wildfire meteorology sessions and spoke with several colleagues who are actively studying such events. The Los Angeles fires are a widely discussed issue.

We knew exactly what was happening: an extreme/unusual Santa Ana event associated with severe downslope winds. In this blog I'll show you some early simulations and explain why this happens.

I will also describe the reasons Climate change does not play a significant role.

As I will explain below, this was not just a Santa Ana event, but an unusual event with extreme winds descending to lower elevations. Some stations at lower elevations, such as Burbank, experienced some of the strongest wind gusts ever recorded. Model simulations demonstrate what is known as “ High-amplitude mountain waves cause downslope storm events.

Santa Ana 101

Hurricane Santa Ana is associated with strong northeasterly winds (north to east) over Southern California. These strong winds are accompanied by very low relative humidity, which is very conducive to wildfires in southern California.

Typical large-scale weather patterns associated with St. Anas include a large area of ​​high pressure over the Great Basin, with the strongest events also having a low pressure center to the southwest (see below). This pattern causes strong northeasterly winds to approach the mountains of southern California.

The observed sea level pressure pattern (below) at 4 pm on January 7 shows many of these factors, with the low pressure to the south being Very strong (Warm colors indicate higher than normal pressure, cool colors indicate lower than normal pressure).

The emergence of this pattern is extreme and perhaps unprecedented. Northeasterly winds approach the region in the lower atmosphere. To illustrate this, the color shading in the image below shows the difference between the wind speed at 925 hPa (about 2500 feet above sea level) at 10pm on Tuesday (January 7) and the normal wind speed. Light gray indicates very unusual winds (more than six standard deviations from normal). Wind direction and speed at certain points are also shown.

timeHe was no ordinary Santa Ana.

High resolution simulation

To understand what was going on, my research team member David Ovens, a research scientist at the University of Wisconsin-Madison, performed a very high-resolution prediction/simulation of the event using the WRF model, with a grid spacing of 1.3 kilometers (which is very high-resolution).

Let me show you the surface gust forecast for Wednesday, January 8th at 12 noon (32 hours after the forecast).

Extreme winds (gusts over 70 knots, 81 mph) are expected over and downwind of the San Gabriel Mountains. A huge threat. Winds will be lighter but still strong (gusts 50-60 mph) in the Palisades area to the west and south.

In order to understand what is happening, it is useful to draw a vertical cross-section of the fire area to show the 3D atmospheric structure. Below is a cross section of the Eaton Fire (Tuesday at 8 p.m.); a section through the San Gabriel Mountains. The shading shows the sustained wind speed in knots, the x-axis is horizontal distance and the y-axis is pressure altitude (700 is about 10,000 feet)

Wow. The air accelerated very strongly as it descended over the southern slopes of San Gabriel, with the winds being strongest near Altadena, where the fires were most intense. This is a very powerful downslope storm with a highly amplified mountain wave pattern.

At 10 a.m. Wednesday, another cross section, this time through the Palisades Fire. January 8, as shown below. Leeward (southern) winds are very strong in the interior mountains and the southern Santa Monica Mountains (left side of the image).

These, as well as early simulations from the UW WRF model and NOAA/NWS models (e.g., HRRR, High Resolution Rapid Refresh), consistently predict extreme downslope winds days in advance

It was clear that there was an unusually large amount of dry “fuel” available to burn. Fuel loads are particularly high after two unusually wet winters (2022-2023, 2023-2024). A dry early winter ensured there was plenty of fuel to burn.

This is clearly an unusual and extreme wildfire danger situation. This is why I issued a strong warning on my blog the day before. Why does the National Weather Service do this too. Los Angeles officials should have known there was a serious threat.

All that is needed is an ignition source. For the deadly and massive Eaton Fire near Pasadena, a faulty transmission tower appears to be the source of the fire.

For Fence Hell, it's either the smoldering embers from the New Year's Eve fireworks blaze or the failure of some aging electrical wires found north of Skull Rock. Perhaps arson is another possibility. Several fire experts I spoke with at the AMS Wildfire Conference believe power lines are the most likely cause, but there's no clear assessment yet.

What yes What is clear is that even after the fire, Los Angeles did not turn off power to the area and had only limited firefighting resources in place before the fire broke out. not good.

Bottom line: A very cleverly predicted Santa Ana event hit Los Angeles earlier this month with record-breaking winds. Two wet winters resulted in unusually high levels of dry fuel. The fire was started by human ignition.

effects of global warming

Many media outlets and climate activists believe that the Los Angeles fires are the result of global warming, or a serious intensification of man-made warming.

These claims are contrary to the best science, This shows the minimal impact of anthropogenic warming.

This narrative diminishes the importance of key social failures, from not turning off the power and fire-hardening homes to not having adequate firefighting capabilities or proper warning and evacuation protocols.

Main causes of global warming not an important factor include:

1. There is no evidence that global warming will increase Santa Ana winds. In fact, there are strong physical reasons (supported by scientific literature) for global warming weakening Santa Ana winds.

2. The available fuel was unusually plentiful because the first two years were much wetter than normal. There is no evidence that this is caused by global warming. There is no long-term trend in wetter winters, which would be obvious if this were a climate issue.

3. The first few months were drier than normal. Global warming projections did not predict this, and there is no historical trend for drier early winters if climate is a cause (see chart below, Los Angeles precipitation from October 1 to January 6).

3. There are some outrageous claims that the Los Angeles fires were caused by “weather whipping“Because of climate change. This 'whiplash' theory is not supported by observations such as the one shown above.

I am particularly troubled by the media promoting untrue weather/climate theories. Case in point, there is an unsubstantiated theory that the Lahaina fires are related to the hurricane's southward movement. This turned out to be wrong.

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