When the monsoon season really starts in Phoenix
The monsoon window opens on 15 June by the calendar and about four weeks later by the dew point. The 108-day season, its three phases, and why the two dates differ.
Short answer
The National Weather Service's monsoon season is a fixed calendar window, 15 June to 30 September, 108 days long. The storms do not keep that calendar: the normal dew point at Phoenix Sky Harbor does not reach 55 °F until 16 July, about four weeks after the window opens, and falls back below it on 8 September.
Two dates govern the Arizona monsoon, and they are a month apart. One is 15 June, when the season opens on the calendar. The other is around the middle of July, when the air over Phoenix becomes humid enough for the storms the season is named after. Everything confusing about the monsoon follows from the gap between them.
The National Weather Service fixes the season at 15 June to 30 September. Its Phoenix office states it plainly on its Monsoon Awareness Week page: “Our ‘monsoon season’ officially extends from June 15th through September 30th.” Sixteen days of June, then all of July, August and September: 108 days. The page gives the dates and not the day count; the 108 is the arithmetic on the dates, and this site checks it at build time so it cannot drift.
That window is a planning device, and it does its job. It is not a forecast of when the first storm arrives.
The calendar season and the moisture season
The National Weather Service window runs 108 days. The normal daily dew point at Sky Harbor does not reach 55 °F until 16 July and falls back below it on 8 September: 55 days inside a 108-day season.
Season window from NWS Phoenix; dew point dates derived from the NOAA NCEI 1991–2020 hourly normals for Sky Harbor, taking the first and last runs of three consecutive days whose mean daily dew point reaches the threshold. Both are normals (the average of thirty years) and a given season can be a fortnight out at either end.
15 June: the window opens
The season begins in the driest month of the Phoenix year. June’s normal rainfall is 0.02 inches, the lowest of any month, and the first fortnight of the window is the tail of that.
What that produces is a particular kind of storm. With little moisture in the air, thunderstorm bases sit high, and rain falling from them evaporates before it reaches the ground: the shaft of rain a pilot would call virga. The surface gets the wind and the dust and none of the water. It is why the first weeks of the season are the dust-storm weeks, and why the storms that do arrive in June are the ones most likely to start a fire rather than put one out.
16 July: the dew point arrives
The honest marker for when the monsoon has arrived is not rainfall but humidity, and the reason is that rainfall is a consequence of a storm that has already happened while the dew point is the condition the storm needs.
Averaged across thirty years, the daily mean dew point at Sky Harbor crosses 55 °F on 16 July, holds at or above it through the rest of July and all of August, and drops back below it on 8 September. That is 55 days of moisture out of a 108-day season: almost exactly half, sitting in the middle of it.
The 55 °F figure is the threshold this site measures against, and it is worth being careful about where it comes from.
The pre-2008 rule, and what cannot be checked
Before the season was given fixed dates, it opened on the first of three consecutive days whose average dew point reached a threshold: 55 °F at Phoenix, 54 °F at Tucson. That rule is quoted in monsoon explainers everywhere, and it is the reason this site uses 55 °F rather than an arbitrary line.
It cannot be sourced here. The NWS Phoenix Monsoon Awareness Week page no longer carries it, and no primary National Weather Service page stating the Phoenix 55 °F figure could be opened when this site went looking. The figure is marked as needing verification in this site’s own data, and the comparison on this page should be read with that in mind: the dates above are the arithmetic of this site on the NOAA dew point normals, against a historical threshold whose primary source could not be reopened. If the threshold is wrong, the dates move with it. What does not move is the shape: the moisture arrives weeks after the calendar says the season has begun.
How much the threshold moves the dates
The same NOAA dew point series gives a useful check on how sensitive those dates are, because running the same three-day rule at 54 °F instead of 55 °F moves the season by three days at each end. At 54 °F the moisture arrives on 13 July and leaves on 10 September; at 55 °F it is 16 July and 8 September. The length is nearly identical (60 days against 55) and the position barely shifts.
That is the reassuring part. The threshold is disputed and the wording of the historical rule cannot be reopened, but the underlying signal is not delicate: the dew point in Phoenix rises steeply through the first half of July and falls away through the first half of September, and any reasonable line drawn across it lands within a few days of the same place. What is genuinely uncertain is the label, not the shape of the curve.
What the dew point does not tell you
A dew point normal is a monthly average expressed as a daily figure, and it is not a forecast. Two seasons with the same normal can behave completely differently (one delivers its rain in six big storms, the other in twenty small ones) and the dew point says nothing about either.
What it is good for is the one question it answers well: whether the moisture the storms need is present yet. Before mid-July the answer is usually no, which is why the storms of the build-up are high-based and dry. After mid-July it is usually yes, which is why the rain finally reaches the ground.
The three acts of the season
Divided at its own wet points rather than at the calendar’s, the season reads as three phases.
| Phase | Days of the season | Dates | What it is |
|---|---|---|---|
| Build-up | 1–26 | 15 June – 10 July | Driest part of the season. The heat is at its worst and the storms are isolated and high-based, which is when the dust arrives without rain. |
| Wet core | 27–78 | 11 July – 31 August | The wettest stretch of the Phoenix year. July and August carry 0.91 and 0.93 inches against June’s 0.02. |
| Wind-down | 79–108 | 1–30 September | Storm frequency falls away, but the ground is saturated and the washes are primed, so flash-flood risk stays high. |
The boundary between the first two is set at the point where the dew point normal crosses the threshold, which puts it in mid-July rather than on 1 July. The boundary between the last two is the calendar’s: 1 September, with a month of window still to run and only a week of moisture left in it.
8 September: the dew point leaves
The moisture retreats before the season closes, and the gap is where September’s reputation comes from.
Storm frequency in September is lower than in July or August, but the ground under it is not dry. A season’s worth of rain has fallen and the washes and channels are primed, so a September storm can produce a flash flood from less rain than the same storm would have needed in June. The month is the driest of the three wet months and the one with the most water already on the ground.
The NWS Phoenix guidance for the season covers this directly, and it is worth reading as written rather than paraphrased, especially the instruction not to camp or park near a wash during the monsoon, which is about storms a person cannot see.
30 September: the window closes
The season ends on the calendar, whether or not it has rained itself out, and the day count resets to 108 for the following year.
What the fixed window buys is a shared vocabulary. It means a forecast, a warning and a news report all mean the same thing by “during the monsoon”, and it means a household can plan a season rather than react to one. The Monsoon Season Clock exists to say where today sits inside that window: which day of the 108 it is, which phase it falls in, and what the dew point is doing against the same 55 °F line drawn above.
The Rain Log is the other half of the same question, for anyone who wants to know what the season did at their address rather than at the airport. The gap between the two is often the whole story of a Phoenix summer: the official season and the one that actually happened on your street.
Once the season is under way, the next question is usually what the warning you have just heard means: that is the flash-flood warning guide, and the season’s rainfall in numbers is in the month-by-month normals.
- NOAA National Weather Service, Phoenix: Monsoon Awareness Week
The fixed 15 June to 30 September window, quoted from the page itself. The page gives the dates and not a day count.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, hourly summaries, Phoenix Sky Harbor (USW00023183)
The daily mean dew point normals behind the 16 July and 8 September dates and the 55-day count.
- NOAA National Centers for Environmental Information: U.S. Climate Normals 1991–2020, monthly summaries, Phoenix Sky Harbor (USW00023183)
The June, July and August rainfall normals used to place the wet core.
- NOAA National Weather Service, Phoenix: Monsoon safety
The Phoenix office's guidance for storms, washes and lightning, which is what the season is actually a season of.