Wind-Pushed Wildfires on Maui • Watts Up With That?

From the Cliff Mass Weather Blog

Cliff Mass

Large fires are now burning on Maui, Hawaii with severe damage to the historic town of Lahaina on Maui’s west side.

Downtown Lahaina burning.   Picture courtesy of Alan Dickar

Satellite imagery shows two major fires… one near Lahaina and the other in the high country east of Kihei (see images below, the orange and red dots show the fires).

As I will discuss below, these fires are the results of strong winds and bountiful grass, plus human ignition sources.  Wetter and cooler than normal conditions during the past six months played a role.

Wildfires and Hawaii

Believe it or not, Hawaii is one of the most fire-prone states in the U.S. (see map below of some historical fires)

With persistent trade winds out of the east or northeast, the eastern side of the Hawaiian Island is quite wet (with moist upslope flow), while the lee (or Kona) sides are dry (see climatological precipitation map of the Hawaiian islands below).

That is why many people vacation at Kona (Big Island), Lahaina (Maui), or  Poipu Beach (Kauai), where warm, dry weather dominates.

There is sufficient precipitation on the lee side of the islands for substantial grasslands to develop, with grasslands increasing during the past several decades as agriculture has declined on Hawaii (e.g., the sugar cane industry is essentially gone).  

To illustrate, here is an image taken by Google near Lahaina.  Lots of grass.

An inferno ready to happen.

You can also view the large grassy areas on Maui on this satellite image (grassy areas are brownish in color)

Such grasses are highly flammable, even a day after rain, with fire growth greatly encouraged by strong winds.

And Hawaii frequently gets strong trade winds.

Why did the Maui fires happen now?

Very strong trade winds developed yesterday, as a large pressure difference (gradient) formed between a strong subtropical high and hurricane Dora to the south.

To show how unusual this pattern is, the figure below shows the sea level pressure at 11 PM PDT last night, and the normalized anomalies from the typical sea level pressure (colors). 

MUCH higher pressure than normal was found just north of Hawaii (red and brown colors), while an area of lower than normal (blue colors) was found south of Hawaii (due to Hurricane Dora). 

The result of this anomalous pressure pattern was greatly enhanced winds that fanned the flames and I suspect helped start fires (by downing powerlines).   A familiar story to those of us in the Northwest.

How strong were the winds?     Below are the maximum gusts on Tuesday provided by a NOAA website–some as high as 62 mph.  Other reports were as high as 80 mph.

Model simulations/forecasts suggested strong wind acceleration on the lee slopes of the substantial terrain of Maui and downstream of gaps in  its terrain  (the University of Hawaii high-resolution surface sustained wind prediction for 6 PM PDT Tuesday is shown below)

The situation this year on Maui was made even more dangerous on Hawaii because the past half-year was WETTER and COOLER than normal, which enhanced grass growth.   I repeat wetter and cooler.    

I don’t have to tell you what some media will be ascribing the Hawaii wildfires to😆

To demonstrate wetter/cooler conditions, here is the departure of precipitation from normal for the past six months (in inches).   Western Maui was wetter than normal.  Much of the big island was crazy wet.  This excessive moisture was the result of several powerful Kona Storms this winter.

Temperatures the past six months?   As shown below, a bit below normal over western Maui.

So we started with a region prone to grassfires (the western side of Maui).   We had enhanced grass growth due to extra precipitation, and then the area was hit with unusually strong winds.

Many of the same ingredients came together to produce the recent large grass fires in eastern Washington.

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