Microbursts and the wind that comes first

A monsoon storm does not build the way a temperate storm does. It arrives. The first thing you notice is not the cloud, which may be ten miles away, and not the rain, which may never reach you at all: it is a wall of wind and dust that shows up several minutes before anything falls out of the sky. By the time the rain gets there, the damage has usually already been done.

That leading wind has a name and a set of published numbers, and knowing them explains most of what the safety guidance tells you to do.

What a downburst is

The mechanism is ordinary once you have it. A thunderstorm’s updraught holds up a great deal of water. As that water evaporates and the air around it cools, the cooled air becomes denser than what is under it and falls: a downdraught. When it hits the ground it has nowhere to go but sideways, and it spreads outward in every direction at once, at speed.

The spreading air is the outflow, and its leading edge is the gust front. The Phoenix forecast office’s figures for it are short and worth remembering as a pair: gusts commonly exceed 40 mph, and they can top 100 mph. For scale, 40 mph is enough to take a patio umbrella out of an unweighted base, and 100 mph is the bottom of the range at which a well-built house starts losing roof cover.

The term microburst describes the small, short-lived, most intense version of the same thing. The forecast office’s page groups the phenomenon under downbursts and gives the figures once for all of them, which is why the numbers in this article are quoted that way.

Why the wind arrives first

Three things happen in the order the guidance is written around.

The outflow travels outward from the storm while the storm itself drifts; the gust front can be five or ten miles in front of the rain shaft that produced it. So the wind reaches you minutes before the water does. Then in the build-up months, June and the first half of July, when the dew point at Phoenix is still in the thirties and forties (about 36 °F on 15 June and 46 °F on 1 July, averaged from the hourly normals) and the rain evaporates long before it lands, the storm can deliver a full-strength outflow with almost nothing falling out of it. That is a dry microburst, and it is why a dust storm can arrive out of a sky that looks like any other afternoon.

Third, the dust itself is diagnostic. Dust only lifts when there is strong wind near the ground, so a haboob is evidence that an outflow has already reached the surface somewhere upwind of you. The Dust-Storm Decision Flow is written for exactly that moment, and the reason its first step is to act before visibility falls is that the horizon is the last good information you get.

The two products that carry wind

The forecast office describes two warnings in terms of wind, and they are worth keeping apart.

Neither is a forecast of a specific street. A dust storm is forecast as a possibility over an area rather than as a time and a place, and treating the issuance of a warning as the moment to start thinking about it is already late.

What the gusts do

The figures above map onto damage in a fairly direct way, and the forecast office says so in its own guidance. Mobile homes are unsafe in a downburst, and even an anchored one can be damaged above 80 mph. That is not a statement about build quality; it is a statement about surface area and mass. A single-wide has a great deal of wall and very little weight, and 80 mph on that ratio is enough to move it.

For a site-built house the exposure is different. The roof is the largest flat surface on the building, the outflow arrives at an angle that gets under its edges, and the debris the wind is carrying (gravel, tiles, palm fronds, a neighbour’s patio cover) does the rest. That is why the city’s pre-season advice about trees is about the wind rather than the rain, and why the roofing article in this series treats tree work as roof work.

If you are driving when it hits

The sequence the campaign gives is the whole of the advice, and its order is the point. Do not drive into it. Get off the pavement as far as the ground allows, and make that decision while you can still see well enough to make it safely: Maricopa County’s instruction is not to wait for low visibility, because every option closes as it arrives. Never stop on the travelled portion of the roadway, which the Phoenix office states flatly. Then, once you are stopped, turn off every light you have: headlights, tail lights and the emergency flashers.

The lights are the counter-intuitive one, and it is the step people argue with. The reason is that a driver coming up behind you in zero visibility steers toward the only thing they can see, and your tail lights are it. Being visible is not the same as being safe. Set the parking brake and take your foot off the pedal, because brake lights are lights too, and stay belted in the vehicle until it passes. The campaign’s own description of the timing is that visibility can fall to near zero in a matter of seconds, so the road is not where you want to be when it does.

If you are outside or at home

Outdoors on foot is the worst of the three places, and the guidance is thinner because the good options run out fast. Get to a building rather than to a vehicle or a shade structure. If there is no building, the city’s high-wind advice is to lie flat in a ditch or a low-lying area with your hands protecting your head, and to keep away from anything that can come down: trees, poles, signs and the loose material the wind has already picked up.

At home, the same wind that carries the dust is a wind event in its own right, so the preparation is the high-wind preparation: get everything loose off the patio and into the garage, because the utility names umbrellas, kiddie pools and trampolines as things that end up in power lines, and a storm on one street becomes an outage on another. Then go inside, shut the house up (that part is about dust rather than wind, since airborne soil gets into everything) and take an interior room without windows. In a mobile home the guidance is to leave it rather than ride it out.

Afterwards, expect the grit to have reached the roof drains and the drywells. Clearing them is the same job as the pre-season one, done in the wrong order, and the Monsoon Prep Checklist Builder puts it where it belongs on the list.

What the wind does not tell you

Two limits are worth stating plainly.

A downburst is short. The same outflow that produces a 70 mph gust is usually spent within a few minutes, and the flat calm that follows it is not the all-clear: it is the gap before the rain. And the wind figures here are the forecast office’s published guidance for what a downburst can do, not a measurement of any particular storm. Nobody is going to tell you your street is about to get 40 mph or 100. The forecasting problem is that these things are small, and the practical answer is to watch the horizon rather than the phone.