Cliff Mass: The Extended Drought In Hawaii Did Not Contribute To The Lahaina Tragedy

Cliff Mass has published an important and quite fascinating post on the tragic Lahaina fire.

https://cliffmass.blogspot.com/2023/08/the-real-cause-of-maui-wildfire-disaster.html

The take home message is that the prevalent highly unusual synoptic weather conditions were the principal, if not the sole cause of the tragic wildfire - including ignition via the downing of the powerlines by exceptional winds. Prior drought conditions had little to do with it.

The Smoking Gun

Last week, I asked David Ovens, a highly skilful atmospheric modeler in my group at the UW, to run a forecast of the Maui event using the WRF model at very high resolution (1.3 km grid spacing).

I was stunned by the simulation, which revealed the real cause of the disaster:  a high-amplitude mountains wave with very strong downslope flow on the West Maui Mountains.  

Below is a  forecast vertical cross-section of the situation around 2100 UTC August 8th (11 AM Hawaii time).   Winds are shown by color shading and arrows.  The solid lines are potential temperature.

An extraordinarily high amplitude wave had formed, with air descending the western side of  the Maui Mountains, accelerating as it plummeted towards Lahaina.

The descending flow was not only strong, but very, very dry, as shown by the vertical cross-section of relative humidity (see below).  Relative humidities below 20% descended down the mountains, with the driest air well under 10%.

The combination of strong winds and dry air hugely promotes fire, including rapidly drying surface fuels such as grass. 

It did not matter whether the grass or light vegetation were wet or dry the days or weeks before:  this extraordinary atmospheric animal would ensure they were dry enough to burn.   Prior dry conditions during the weeks before were immaterial.

How extraordinary was this ‘atmospheric animal’? 5 sigma extraordinary. That makes it a Black Swan I think.

The wind anomaly from normal of the winds at this level is shown in color.   Look carefully and you will see gray color over Maui.... five standard deviations from normal....which means VERY unusual.  You will also note the clear separation of the strong winds of Dora from what hit Maui.

If Cliff is correct, what this means is that any attribution (or not) to climate change of the Lahaina wildfire is going to have to disregard prevailing drought conditions, which Cliff maintains were irrelevant, and instead concentrate solely on the cause of the exceptional synoptic setup which generated these fierce downslope winds on the west side of Maui - plus shoehorn in a role for the very modest long term rise in regional temperature since 1900 of course. So, will Friederike Otto and others at World Weather Attribution pick up the gauntlet, or maybe the redoubtable extreme weather attributors at the Met Office will?