Cliff Mass Weather Blog: A Tropical Storm Is Approaching Hawaii: Should Residents Be Worried?

More than a year ago, Hurricane Dora passed south of Hawaii and its powerful winds blew down the western slopes of the West Maui Mountains, sparking wildfires that destroyed the town of Lahaina and killed about 100 people.

As I write this, another tropical system, Tropical Storm Hone, is approaching the Hawaiian Islands and will pass south of the Big Island within the next 24 hours (see satellite image below at approximately 6 p.m. PDT). The star indicates the center of the storm and the arrow points to Lahaina on Maui.

Over the past year, I’ve been working with UW research scientist David Ovens to understand the meteorological situation surrounding the 2023 Lahaina disaster… if you’re interested, you can read about it here. This.

So I’m very concerned about the tropical storm approaching today and have been carefully checking the forecast model’s diagnostics.

My overall assessment is that while this event poses little threat to Maui, it will cause strong winds in some locations as well as heavy rainfall.

What was the main difference from last year that made this event less threatening?

First, the storm is significantly weaker and closer than last year’s Hurricane Dora. Being closer means more rain will reach the islands, which helps to combat the risk of wildfires.

Below are the rainfall totals as of 11 p.m. PDT tomorrow. You can see a band of rain directly related to the storm south of the islands. The increased eastward flow of moisture associated with the storm’s passage will produce significant rainfall in Hawaii (up to 15 inches) and parts of Maui.

A key factor in last year’s storm was very high pressure north of Maui, which resulted in strong winds blowing up the Maui Mountains. These winds interacted with the mountains to create very strong dry downslope winds, which directly contributed to the fires.

Today, the high pressure to the north is much weaker than last year, resulting in a weaker north-south pressure gradient and therefore weaker winds as it approaches the West Maui Mountains (see pressure map (solid line) and pressure difference relative to normal (color) below). The blue circle area approaching Hawaii is the tropical storm.

But there’s something more…something more subtle…but very important. The downslope wind response is influenced not only by the wind speed and direction of the air stream approaching the barrier but also by the stability of the air, where stability depends on how temperature and humidity change with altitude.

The strong high pressure a year ago made the stability change near the crest ideal for strong downhill winds around Lahaina (there was a stable layer just above the crest). Today’s situation is much LESS favorable.

The strongest winds associated with the strengthening northeast trade winds tend to be near Lahaina, downstream of the West Maui Mountains, and on the northwest side of the Big Island, downstream of the Kohala Mountains (see red arrows)

Let’s take a look at the latest winds around Lahaina and northeast Maiu… and there are MORE wind observations today than there were a year ago. Today’s maximum winds (shown below) were just north of Lahaina at about 25 mph.

Winds are picking up on the Big Island and peak winds have gotten really strong on the Kohala coast… reaching 93 mph! (see below). That’s awesome.

The risk of these winds is mitigated by the high relative humidity and the fact that light rain is coming over the crest. Rainfall over the past 12 hours (shown below) has been heavy near the crest of the Kohala Range (up to about 5 inches), with very light rain reaching the gusty area on the leeward (southwest) side.

At this time, Hawaiian Electric is not shutting off power to prevent wildfires (see below). For Maui, I agree, but northwest Hawaii is a different story.

There is a lot of dry vegetation there. I hope they make the right decision.