by Ken McKinley
During the second week of August 2023 catastrophic wildfires have occurred in the Hawaiian Islands. The fires have been most severe over western portions of the island of Maui, and the town of Lahaina has suffered near total destruction in some areas. Many news reports have cited Hurricane Dora as the primary cause of the development and rapid spread of the Hawaiian wildfires, but this is not the case.
Lahaina has been a significant yachting center for cruisers and racers in recent years and has a rich maritime history. Unfortunately there has been significant loss of life associated with the Hawaiian fires, and the final death toll is not yet known and is likely to increase.
Hurricane Dora, meanwhile, has been tracking generally westward through the tropical Pacific since it first formed as a tropical depression on the last day of July well off the coast of southern Mexico. Dora became a tropical storm during the pre-dawn hours on August 1st, and reached hurricane strength in the evening that same day. Rapid intensification occurred during the ensuing 24 hours, and Dora reached category 3 strength, becoming a major hurricane during the afternoon of August 2nd. Dora has remained a major hurricane even reaching category 4 status. Through this period the center of the hurricane has continued to track generally west or west-southwest, and its center passed due south of the big island of Hawaii and of Maui during the early morning of August 8th. The center of the hurricane remained well south of these islands with a closest point of approach to South Point of the island of Hawaii of more than 400 miles.
Figure 1 shows the history of the wind field of Dora from its formation through the evening of August 9th. It can be clearly seen that the track of Dora and its wind field have remained well south of the Hawaiian Islands.
It is necessary to look at the entire weather pattern over the central and eastern Pacific to determine which weather features have played a role in the fires. Figure 2 is a surface pressure chart valid at 1200 UTC 9 August 2023, right around the time of some of the worst fire conditions. The most prominent feature in the region is a rather strong subtropical high which is centered well to the north-northwest of the Hawaiian Islands near 38° N/146° W. The circulation around this high is rather large as shown by the isobars, and extends over the Hawaiian Islands. In fact, the isobars over Hawaii are fairly closely spaced indicating strong winds. Wind barbs on the chart indicate wind speeds of up to 25 knots from the northeast around Hawaii, and a gale warning is also indicated on the chart, meaning that sustained winds in the range of 34 to 47 knots are anticipated, driven by the circulation around the large high.
When considering winds around the Hawaiian Islands, it is necessary to remember that these islands are quite mountainous, and the extension of the terrain well up into the atmosphere impacts the wind field. In particular, air will funnel and accelerate through the areas between the islands, and also through gaps in the mountains. This results in enhanced wind speeds in these areas. This high has also led to a fairly long period of dry weather in Hawaii, and the combination of the dry weather and the strong winds produced conditions that were ripe for wildfires. Once the fires began the resulting enhanced thermal instability contributed to even stronger winds. In this situation in this area, it is not at all surprising to hear reports of hurricane force wind gusts in the fire areas.
The presence of Dora to the south of Hawaii has likely contributed to the pressure gradient across the islands, but not in a major way, and in fact if Dora was weaker, or was not present at all, weather conditions would likely still have been conducive for the formation and expansion of wildfires due to the presence of the large and strong high north of Hawaii.
One final comment, and this has to do with El Nino. A strong El Nino is in place across the equatorial Pacific at this time. This means that sea surface temperatures are warmer than normal in the tropical latitudes of the eastern and central Pacific. These warmer temperatures have likely been a significant factor for the longevity of Dora, particularly considering that the system has remained a major hurricane for so long, and as it has moved so far to the west. In a year without such a well developed El Nino, a hurricane like Dora would likely have weakened significantly well before reaching the longitude of the Hawaiian Islands.
Ken McKinley is an ON contributing editor and is a weather router and owner of Locus Weather based in Camden, Maine.
