Reading a Valley forecast: the products and the numbers

A forecast for Phoenix reads differently from a forecast almost anywhere else, and the difference is not the number at the top of the page. It is how much of the page is given over to warnings: a heat product, a dust product, a flood product, each with its own name, its own trigger and its own instruction.

This page takes those one at a time, in the order a household tends to meet them. Every figure quoted is either a climate normal from the 1991–2020 record at Phoenix Sky Harbor, station USW00023183, or a statement from the National Weather Service’s own Phoenix office. Where a number is this site’s own arithmetic rather than a published figure, the text says so.

Why does the same temperature feel like two different days?

Because the temperature is not the whole of the hazard. What the heat does to a person depends on the air that the sweat has to evaporate into, which is what the dew point measures, and on how used to the heat the person has become.

This site works that through in one place and labels it as arithmetic rather than observation. Its heat-index figure is computed from a normal temperature and a normal dew point using the standard National Weather Service regression, and a normal high and a normal dew point do not necessarily occur in the same hour. That caveat is printed wherever the figure appears, and it is worth carrying over to any heat index you meet anywhere: it is an estimate built from two inputs, not a measurement.

The acclimatisation point comes from the forecast office directly. Its own explanation for why there is no fixed temperature cutoff for a heat warning is that people adapt as the summer goes on: 105 °F in May is a different experience from 105 °F in July, in the same city, on paper the same number. The Heat-Safe Hours Planner takes a month into account for that reason.

What does a 20% chance of rain actually mean?

It is a chance at a point, not a share of the map. Monsoon rainfall is convective (storms that build over a few miles and drop everything they have in a short time) which is why the same afternoon can bring a running wash in one part of the Valley and nothing at all in another. Reading the number as “20 per cent of the area will get wet” or “it will rain for 20 per cent of the day” gets you the wrong answer twice.

The practical reading is about hours and place. A number that low in June means almost nothing is expected; the same number in mid-July is worth watching, because the season’s rain arrives in a handful of events, and the number in that pattern is the chance that one of them lands on you. The Monsoon Season Clock shows where today sits in the season.

What is HeatRisk, and how do I read the 0 to 4 scale?

HeatRisk is the National Weather Service’s heat scale, and it is deliberately a scale rather than a temperature. It comes in five categories, numbered from 0 to 4: green for little to no risk, yellow for minor risk affecting those extremely sensitive to heat, orange for moderate risk affecting people who are sensitive to it, red for major risk affecting anyone without cooling or hydration, and magenta for extreme risk from rare or long-duration heat with no overnight relief.

Two things about it are worth knowing. The first is that it is built from how far above normal the highs and lows run, how long the heat lasts, and how much overnight relief there is, so a merely hot week in a cool month can score higher than a hotter week in July. The second is that the service describes HeatRisk as supplementary to the official watches, warnings and advisories. It is a way to read the day; it is not the product that carries the instruction.

Why does the forecast say Extreme Heat Warning?

Because the product was renamed, and older guides still carry the old name. The Phoenix office’s warning product is now called an Extreme Heat Warning, with an Extreme Heat Watch ahead of it and a Heat Advisory as the lesser alert. A page written before the change will refer to the same thing as an Excessive Heat Warning, and the two are not different products.

The Heat Advisory has a wrinkle of its own that the office points out: much of the lower-elevation Southwest has chronic heat, so an ordinary hot day there is not automatically an advisory day. The advisory is issued when the heat is a hazard for sensitive groups but below warning level, not simply when it is hot.

What triggers a heat warning, if not a fixed temperature?

The office’s stated route is through HeatRisk. When major or extreme HeatRisk is forecast, an Extreme Heat Watch or an Extreme Heat Warning follows. The difference between the two is confidence rather than severity: the office attaches a moderate, roughly 50 per cent confidence to a watch, and a high, 80 per cent or better confidence to a warning.

Where both warnings and advisories are warranted, the office says the choice and the messaging come down to forecaster judgement and coordination with neighbouring offices. That is the honest explanation for a question people ask every summer: why one hot week brought a warning and a hotter one did not.

When does the monsoon start, and why does the humidity disagree?

The season has fixed dates and the humidity does not follow them, and both halves of that sentence are true.

The National Weather Service runs the monsoon from 15 June to 30 September (108 days) so that outlooks and warnings share one calendar. The weather arrives on its own schedule. The figure on this page makes the gap visible: this site’s count of the hourly dew point normals at Phoenix Sky Harbor does not put the daily mean dew point at 55 °F until 16 July, a month after the window opens, and drops it back below that on 8 September, three weeks before the window closes.

So the window is what the forecast uses, and the dew point is what the atmosphere is doing. The build-up across late June and early July brings high-based storms that deliver dust and lightning and very little rain; the wet core from mid-July to the end of August is when the season’s rainfall actually arrives. When the monsoon season really starts takes that disagreement apart on its own page.

What do the dust and flood products mean?

The two products are both about acting early, and both are unusual in how little time they allow.

A Blowing Dust Advisory and a Dust Storm Warning are the graduated version of one hazard. The guidance on the subject is written around the fact that a dust wall does not build: visibility collapses in seconds, so the instruction is to act before the air stops being see-through rather than after. That is why the standing advice is to get off the roadway the moment a wall is visible on the horizon, and why the safety instruction for a driver already caught in one is counter-intuitive: switch off every light, including the hazards, because a driver behind you in zero visibility steers at whatever they can see.

A Flash Flood Warning is the same shape of problem in the other direction. The hazard in a Valley wash is moving water, and the water does not have to look deep to be dangerous; agencies do not agree on exactly how shallow it can be and still take a vehicle, which the site says plainly on its flood pages rather than picking a figure. The Monsoon Prep Checklist Builder is where a household’s own storm preparation lives.

How far ahead is a Valley forecast worth reading?

The seven-day view is the horizon the office publishes, and the detail worth planning around sits in the first three or four days of it. Beyond that the useful information is about pattern rather than prediction: whether the ridge is holding, whether the moisture is coming back, which way the flow is turning.

Summer is where that limit bites hardest, because convective rain is local by nature. A forecast that says storms are likely across the Valley is a statement about the ingredients being present, not about your street getting wet, and the heat products carry the same grain: the office’s own HeatRisk display for Phoenix is, in its description, an average of six points around the metro area rather than a reading for a single one of them.

What does frost actually mean in a Valley winter?

It means the one night of the year that a Valley garden notices, and the useful thing to know about it is how rare it is rather than what the headline calls it.

The normals make the risk look smaller than it is. Sky Harbor averages 0.6 nights a year below freezing across the 1991–2020 period, which is to say that a normal year does not freeze at all, and the record back to 1933 shows individual winters that went well past that. The gap between those two facts is the whole reason the Frost & Freeze Date Explorer works from the record rather than from the average, and it is the reason a citrus tree in a Valley yard is a plant with a December plan.

A cold night here is a night for a decision you have already made: what gets covered, and with what. The forecast’s job is to give you the few hours of warning to carry it out.

The short version

Read the product before the number. An Extreme Heat Warning asks for something a HeatRisk level of 3 does not, a Dust Storm Warning asks for something a temperature forecast never will, and the monsoon’s fixed dates are a calendar for warnings rather than a promise about the weather. For the year those products fit into, The low desert year sets out the twelve months and what each one asks of a household.