Lightning safety in Arizona

Arizona gets its thunderstorms in one stretch of the year, and lightning is the hazard inside that stretch that reaches furthest and gives the least warning. The National Weather Service’s Phoenix forecast office fixes the season at 15 June to 30 September, and every rule below comes from that office or from the service’s national lightning pages. There is nothing here that is this site’s own advice, and no figures that were estimated to fill a gap.

The two rules, stated plainly

There are two instructions, and between them they cover almost everything a person needs.

The first is the slogan the National Weather Service has attached to the hazard for years: when thunder roars, go indoors. The service’s national lightning page adds the reason in one line: if you hear thunder, you are likely within striking distance of the storm. Phoenix’s forecast office puts the same idea in Arizona’s terms: if you hear thunder, you are close enough to a storm to be struck by lightning.

The second is the 30-30 rule, which the service has published in two halves. The first thirty is the count. When you see a flash, count the seconds until the thunder arrives; a gap of 30 seconds or less puts the strike within roughly six miles, and it is time to be inside. The arithmetic behind it is the flash-to-bang method that NOAA’s own lightning pages state, at about five seconds for every mile. The second thirty is the wait. Stay in shelter for 30 minutes after the last thunder before going back out, and start the clock again if the thunder returns.

One thing worth saying about the rule, because it is sometimes taught badly: the modern National Weather Service wording no longer asks you to do the counting. Its published instruction is to treat thunder itself as the signal rather than to wait and see how far away the storm is. Read together, the counting half of the 30-30 rule describes the margin you have if you have already heard something; the slogan tells you not to wait for it.

What people get wrong

The dangerous beliefs about lightning are not exotic. They are the ordinary ones, and most of them are addressed directly by the two agencies that publish on it.

That a car is safe because of its tyres. The tyres have nothing to do with it. The National Weather Service lists substantial buildings and hard-topped vehicles as the safe options, and lists convertibles, open vehicles, rain shelters and small sheds as unsafe. A hard-topped car is a metal shell, and the shell is what does the work.

That if it is not raining, it is not a threat. NWS Phoenix says lightning can strike up to 60 miles from the nearest rainfall. That figure is the reason the guidance is written around thunder rather than around rain, and it is also the reason a dry, high-based storm in the first weeks of the season is not the harmless thing it looks like.

That the storm has passed when the rain stops. Rain stops before the storm does. Thirty minutes after the last thunder is the published rule, from both the Phoenix office and the national page.

That crouching or lying flat is a safe option. Getting low is better than standing tall, and it is not safe. The Phoenix guidance for someone caught outdoors is to find a low spot away from trees, fences and poles: a way of reducing the exposure you cannot avoid, not a shelter.

That the indoors is automatically safe. The Phoenix office’s list is specific: stay off anything that uses wires or plumbing while the storm is overhead, unplug appliances that are not needed, and keep phone use to emergencies.

That the pool is a small risk. Running water and plumbing are conduits, and the Phoenix guidance names swimming pools in the first line of what to get away from. A pool deck during a monsoon storm is one of the exposed places the rule exists for.

Where to be when it starts

The order is short and it does not change.

A substantial building is the first choice. A hard-topped vehicle is the second, and it is a genuine second rather than a consolation: it is on the service’s list of safe places. Outdoors with nothing to hand, the instruction is to get low and away from trees, fences and poles. On the water, or on a pool deck, get out of the water and indoors.

If someone is struck, the Phoenix office’s instruction is to call 911 immediately. Lightning-strike casualties are a medical emergency, and the county-level guidance on what to do while help is coming belongs with the people who publish it rather than paraphrased here.

When in the day the risk sits

Lightning in Arizona is an afternoon and evening hazard rather than a night one, and the Phoenix office’s planning advice reflects it. Its guidance is to arrange to be inside between the middle of the afternoon and the middle of the evening, and to be more careful again at higher elevations, where storms build earlier and the terrain gives you fewer ways out.

That matters most for the activities the Valley does outdoors in the season. A trail hike that starts at dawn and finishes by ten is a different proposition from the same hike at two in the afternoon, and the Mountain parks and preserves north and east of the city are closer to the storms than the flat ground in between. The same applies to a pool, a golf course or a job site: the window in which a storm can arrive is known, and it is a short one to plan around.

Two limits on this page are worth stating plainly. The first is that the rules above are the published ones and nothing here extends them: where the National Weather Service or a county agency gives a specific instruction, that instruction is the one to follow. The second is that lightning safety is not a topic this site gives medical advice on. If someone has been struck, the guidance is to call 911, and the treatment that follows belongs to a clinician.

Where this fits in the year

Lightning is a monsoon hazard, which means it belongs with everything else the season brings. The season opens on 15 June and its first weeks are the driest part of it: the stretch when storms are high-based and produce more wind and dust than rain, and when a strike can land a long way from any water falling out of the sky. The wetter core arrives in July and August, when the dew point chart above turns green.

The other two-thirds of a monsoon safety plan live in the same season. The Monsoon Prep Checklist is the household work that has to be done before the first storm rather than in the middle of one, and the Dust-Storm Decision Flow covers the hazard that arrives on the same afternoons as the lightning. The wind that comes ahead of both is its own subject (see microbursts and the wind that comes first) and the wall of dust is another, at what a haboob is and what to do.

None of it is complicated. It is a season with three or four things you do without thinking about them, and thunder is the trigger for one of them.