By Alan Smith, Meteorologist Posted 4 months ago August 10, 2023
Strong Winds Fuel Devastating Maui Wildfires
This article has been edited from its original version due to new scientific analyses regarding the meteorology that contributed to this event.
The island of Maui in Hawaii has been dealing with a horrific wildfire situation this week. Strong winds combined with dry air and preceding drought conditions contributed to a rapidly spreading wildfire that devastated the town of Lahaina on the southwest side of the island. Fires have also impacted central portions of the island.
Here is a look at fire hot spots on OpenSnow's Active Wildfires Map when fires were quickly spreading during the morning hours on Wednesday, August 9th.
As of Thursday morning, winds have settled down to "moderate" levels and the rate of fire spread has slowed. Although no rain is in the forecast, a further decrease in winds is expected from Friday on.
Let's take a look at the meteorology and climatology factors that led to this outbreak of wildfires...
The Climate of Hawaii and Maui
Wildfires might not be the first thing you think of when you think of Hawaii. While it's true that many areas on the islands experience a wet tropical climate, there are significant variations and microclimates due to the mountainous terrain of the islands.
The predominant wind direction in Hawaii is out of the northeast. This is associated with trade winds that move in more of an east-to-west direction closer to the equator, as opposed to the more typical west-to-east flow experienced in the mid-latitudes of the Lower 48 States.
Due to mountain orographics, the northeast coast of Maui and northeast-facing slopes on the windward side receive the heaviest rainfall, while the southwest-facing slopes on the leeward side receive the least amount of rainfall.
Significant terrain changes on this small island (the highest point on the island is over 10,000 feet in altitude!) result in microvariations in rainfall as well.
The town of Hana on the northeast coast of the island receives 80 inches of rain per year on average and other areas just inland average well over 100 inches of rain per year.
The town of Lahaina, on the other hand, only averages 14 inches of rain per year. So this area already experiences a dry climate to begin with and grassland vegetation is common as a result. There are also pockets of drier microclimates in interior sections of the island.
Moisture and Drought Conditions Leading Up To The Event
During most of 2022, Hawaii experienced below-average rainfall with widespread drought conditions taking hold throughout the islands.
A wetter period did take hold during early to mid-winter (approximately November 2022 to February 2023) with above-average rainfall for most areas of Hawaii. The wet period on Maui was a bit shorter, lasting through January.
Abundant mid-winter rain resulted in improving drought conditions heading into the spring, while also providing moisture needed for grassy vegetation to thrive. It's possible this could have played a role in fire danger down the road.
Learn More → How Spring Weather Affects Wildfire Season
By the time spring arrived, most of Hawaii was out of a drought. However, a shift to below-average rainfall also began during March and rainfall has been below average every month since then. As a result, the once thriving grassy vegetation quickly dried out as summer progressed.
While summer is the dry season for Maui, the dryness has been even more pronounced than normal this year.
As of early August, central and southwest Maui were under a moderate to severe drought.
The National Interagency Fire Center also forecasted "Above Average" significant wildland fire potential for the month of August for all of Hawaii.
High Wind Event Contributes to Rapid Wildfire Spread
A major wind event took hold across Hawaii and on Maui on Tuesday night and Wednesday. Wind gusts of 40-70 mph were recorded across the islands over an extended period of time.
It's unknown exactly how the fires started, but once they did, strong winds resulted in rapid fire spread with little time for people to evacuate.
A strong area of high pressure in the Central Pacific Ocean moved from east to west, passing north of Hawaii on Tuesday (August 8) and Wednesday (August 9). A tight pressure gradient (the difference in air pressure over a horizontal distance) to the south of the high pressure center developed, resulting in a pocket of strong winds in the lower atmosphere.
This narrow region of strong winds in the lower atmosphere is known as a low level jet, and it tracked from east to west (the typical direction of trade winds in the subtropics), passing just north of Hawaii. This feature, along with the center of high pressure to the north, can be seen on the left side of the image below:
Previously, Hurricane Dora passing south of Hawaii was thought to be a major contributor to the high wind event, due to the pressure gradient between high pressure to the north of Hawaii and low pressure associated with Dora to the south of the island.
However, recent analyses by National Hurricane Center Specialist Phillipe Papin and University of Wisconsin-Milwaukee Atmospheric Scientist Clark Evans indicate that the high wind event was a result of the low level jet passing over the islands and that Hurricane Dora likely contributed very little to the event.
While Dora was a strong hurricane, it was also very compact and passed about 600 miles south of Maui. In other words, too far away to have much of an impact.
As the low level jet reached Hawaii, surface winds were blowing from northeast to southwest, which had a downsloping effect on the southern and eastern side of Maui.
Winds that move downslope (from high to low elevations) often gain acceleration and pick up speed if a stable layer exists in the mid-levels of the atmosphere, which appears to have been the case here.
Also, when air is forced to descend, the sinking motion has a warming and drying effect due to the compression of air molecules, which can further exacerbate the fire situation. This is the same process we see with Chinook winds and Santa Ana winds, which have both been associated with rapid-spread wildfires in the Western U.S.
The airmass passing over Hawaii on Tuesday and Wednesday was already very dry to begin with as the amount of water vapor in the atmosphere was at least 1 standard deviation below average.
If you look closely at Maui (the second island from the bottom) in the image below, it appears that downsloping winds also had a localized drying effect with water vapor levels of 2 to 3 standard deviations below average.
To summarize, the following climatological and meteorological factors likely played a significant role in escalating the fire situation in Maui:
- Pre-existing drought conditions and dry fuels.
- Low relative humidity associated with a dry airmass over Hawaii.
- Localized downslope winds, which led to a further decrease in relative humidity on the southern and western sides of Maui.
- An extended period of strong winds due to a low level jet passing over Hawaii, along with localized downslope winds.
High wind situations such as this when vegetation fuels are already dry and "primed" represent the most dangerous situations when it comes to wildfires, since fire spread can occur so quickly and with little time to react. This was a similar situation to devastating fires in recent years in Colorado and California that were driven by high winds.
The OpenSnow team sends its deepest condolences and well-wishes to all who have been affected by the wildfires in Maui.