Tropical Storm Lala and Hawaii: The Weekend Timeline, the Rainfall Numbers, and What It Means for a Trip You Already Booked

The National Hurricane Center started issuing advisories on Wednesday for a system it is calling Potential Tropical Cyclone One-C, sitting roughly 900 miles east-southeast of the Big Island. It is expected to become a tropical storm late Wednesday night or early Thursday, and when it does, it takes the next name on the Central Pacific list. That name is Lala.

If you live in Hawaii, you have already heard this. If you have a flight to Kona on Friday, you probably found out about twenty minutes ago and are now reading forecast discussions written for meteorologists.

So here is the plain version: what the forecast actually says, why the rainfall number matters more than the category, and what the decision points are between now and Sunday. I have sat through a good number of these in Asia, and the single most useful thing I can tell you up front is that the part people fixate on is almost never the part that ruins the weekend.

What the forecast actually says

Take the numbers one at a time, because they get garbled fast once they go through headlines.

The system is expected to reach tropical storm strength before it gets anywhere near land, and to hold roughly that intensity as it approaches. Forecasters have said it could gain just enough to touch Category 1 hurricane strength at its peak, which is a real possibility rather than the main expectation. Either way you are looking at a tropical storm or a low-end hurricane, not one of the monsters that gets a documentary made about it.

The timing runs like this. Rain arrives at the Big Island first, most likely Friday. The system then tracks across the island chain from east to west through Saturday and Sunday, and moves off to the west of the islands by Monday. That east-to-west progression matters for planning, because it means the islands do not all get hit at the same time and do not all clear at the same time. The Big Island and Maui are currently flagged as the areas of greatest concern.

The rainfall forecast is 5 to 10 inches across much of the island chain, with maximum totals reaching 20 inches in some areas through Sunday. Alongside that, forecasters are warning about life-threatening surf and rip currents from the swells the system is throwing ahead of itself, and coastal flooding concentrated on east- and south-facing shores. Wind is expected to be enough to bring down trees and cut power, with outages potentially lasting from hours to days.

A watch was expected to be required for parts of the islands by early Thursday. By the time you read this it may already be up.

One piece of terminology is worth clearing up here, because it confuses people every single time it appears. Potential Tropical Cyclone is not a weakened kind of storm or a category below tropical depression. It is an administrative designation that lets forecasters issue watches and warnings for a system that has not organised into a named storm yet but is expected to threaten land within a couple of days. Before this designation existed, there were cases where a disturbance was clearly going to hit somewhere in thirty-six hours and the forecast office had no mechanism to post a warning for it, because the rules required a named system. So the category was created to close that gap.

What it tells you in practice is that the threat is considered near-term and land-directed. A system out in open ocean with nobody in its path does not get this label no matter how promising it looks. One-C got it because the islands are in the cone and the clock is short. The number and the letter are just bookkeeping: it is the first such system of the season in the Central Pacific basin, which is what the C stands for.

The rain is the story here, not the category

Twenty inches of rain is the number to hold onto, and it is worth explaining why a merely moderate storm can produce it.

Hawaii is not a flat coastline. It is a set of very steep volcanic islands sitting in the middle of an ocean, and when moist tropical air is pushed against terrain that rises thousands of feet in a few miles, that air has nowhere to go but up. Rising air cools, cooling air drops its moisture, and the result is that the same storm which would deliver four inches to a flat coast delivers three or four times that to the windward side of a mountain. This is called orographic lift, and it is the reason Hawaii holds rainfall records that have nothing to do with hurricane strength.

Then the water has to leave. On steep terrain it leaves fast, through stream channels that are dry most of the year and are not dry during this. Flash flooding in Hawaii is not a slow rise you watch from a window over the course of an afternoon. It is a channel going from nothing to dangerous in a very short span, and it happens in valleys where people are hiking, camping and parked. The specific warning language being used is life-threatening flooding and mudslides, especially in areas of steep terrain, and that phrasing is deliberate.

This is the mismatch that catches people out. A tropical storm sounds survivable, and in terms of wind it usually is. But intensity categories are a wind scale, and they say nothing at all about how much water a storm is carrying or how the ground underneath it will respond. A slow-moving tropical storm over mountains can be considerably more dangerous than a fast-moving Category 2 over flat ground, and if you only track the category, you will feel reassured at exactly the wrong moment.

The surf is a separate problem with the same character. Swells arrive ahead of the storm, often under a blue sky, and every year they injure people who looked at the weather, saw sunshine, and went to the beach. If Lala behaves like most systems of this kind, dangerous surf on east- and south-facing shores will start before the rain does and outlast it on the back end.

What a Watch actually means, and where the real decision point is

The word watch gets treated as a synonym for warning, and the difference is the most practically useful piece of terminology in the whole system.

A watch means conditions are possible within about 48 hours. A warning means they are expected within about 36 hours. Those two numbers are the entire message. A watch is telling you to get ready and to stop making plans that would be hard to reverse. A warning is telling you the window for getting ready has closed and you should be where you intend to ride it out.

For Hawaii this week, that maps onto a fairly clear schedule. A watch going up Thursday for the Big Island and Maui would mean conditions are possible from roughly Saturday. If that converts to a warning on Friday, the expected arrival is Saturday. If the track shifts north or south by even sixty or seventy miles in the meantime, the practical outcome for any one island changes a lot, which is why the advice is to check the forecast again rather than to lock in a decision on Thursday morning based on Wednesday's map.

For anyone holding a booking, the useful sequence is roughly this. Airlines issue weather waivers when a named storm threatens a destination, and those waivers usually let you change dates without a change fee inside a defined window. They tend to appear once a watch is posted, sometimes a little before. If you rebook before the waiver exists, you may well pay for a change that would have been free a few hours later. If your flight has not been cancelled and no waiver has been issued, the most common mistake is paying money to solve a problem that has not happened yet.

And if you are already on the islands, the decision is not about flights at all. It is about whether you are in a valley, on a low-lying coastal road, or somewhere that loses power and stays lost for two days. Those are answerable questions on Thursday. They are not answerable on Saturday afternoon.

What to actually do between now and the weekend

None of the above is a reason to panic, and most of it is a reason to be slightly more organised than usual.

Check the Central Pacific Hurricane Center directly rather than headlines. Advisories update on a fixed schedule and they say what is known and what is not. Any article, this one included, is a snapshot of a moving thing.

Assume power goes out even if the wind forecast looks modest. Trees come down in saturated soil at wind speeds well below what they survive when the ground is dry, and Hawaii's outage risk in this event is a soil problem as much as a wind problem. Charge everything Thursday night.

Stay out of stream valleys and off the beach, including on days that look fine. Both of those kill people in storms that produced no wind damage at all.

Do not cancel anything you do not have to cancel yet. The storm is still two days from the islands and the track has room to move.

If you are watching the forecast for a category number, watch the rainfall totals instead. That is the number that will decide how this weekend actually goes.

More on typhoons and storms

• Hurricanes Almost Never Hit Hawaii. Three Have in Recorded History, and That Number Explains More Than It Looks
• What a Hurricane Actually Sounds Like, and Why the Noise Is the Part Nobody Warns You About
• What the Eye of a Hurricane Is Actually Like: I Stood In One, and the Silence Was the Strange Part