Does it rain in Phoenix in winter?

Everyone in the Valley knows the monsoon. It is loud, it is on the news, and it is genuinely the most violent rain we get. So the assumption follows naturally: if it rains here at all, it rains in July and August, and the rest of the year a roof has nothing to do.

The 30-year record says otherwise, and the gap is not small. Here are the NOAA/NCEI 1991–2020 precipitation normals for three Valley stations.

Two things in that table are worth sitting with. The first is that the winter column beats the monsoon column at every station. The second is that the further north and east you go, the more pronounced it gets — Scottsdale Municipal takes 51% of its annual rain between December and March.

What is different about this winter

The Climate Prediction Center had an El Niño Advisory in effect as of its ENSO Diagnostic Discussion issued 8 October 2026. The Niño-3.4 index stood at +2.1 °C, the discussion described El Niño as continuing to strengthen, and it put the odds of a strong-to-very-strong event persisting through January–March 2027 at greater than 83%. The chance of a historically strong event — a three-month relative ONI of +2.5 °C or more — was 83% for October–December and 70% for November–January.

El Niño winters tilt the Southwest toward wetter conditions. That is a shift in odds, not a forecast. Michael Crimmins, the University of Arizona climatologist, made the point this September that El Niño “shifts the odds” without guaranteeing anything: 2016 brought a strong El Niño, the storm track stayed north, and much of Arizona finished the winter drier than normal.

So the honest version is this. A historically strong El Niño is under way. The usual wet window here is January through March. Nobody, including us, knows what will actually fall on your roof.

Winter rain damages differently, not less

A monsoon cell is a capacity problem. Water arrives faster than a gutter can carry it, it comes over the front lip, and you find out in twenty minutes whether the system was sized right.

Winter systems are a duration problem, and they break three other things:

1. Wet stays wet

A monsoon storm dumps and then the sun comes out and everything is dry within the hour. A winter system can keep a fascia board damp for a day or more at a stretch. Water that would have evaporated out of a seam in July has time to sit in it in February, and that is the condition that rots wood.

2. The second storm is the one that bites

Valley soil is effectively a hard surface when it is dry. The Flood Control District of Maricopa County’s report on the January 2010 storms described exactly this: the first system produced relatively little runoff because the ground was dry and absorbed it, and the second system — arriving on near-saturated soil — ran off and flooded. Winter storms come in runs with dry breaks between them, so this is the normal pattern rather than the exception.

What that means practically: a downspout discharge point that behaved perfectly well in the first storm can start pooling or cutting a channel in the second one, because the ground it was draining onto has stopped accepting water. This is the single most common winter call we get, and it is almost never a gutter problem. It is a where the water goes problem.

3. It comes after the dustiest part of the year

By December a Valley gutter has spent the autumn collecting dust, palo verde litter, seed pods and roof granules, and nothing has washed through it since September. The first serious winter storm runs into a trough that is already partly blocked, which is why overflow in December often has nothing to do with sizing.

What to check before the first one

None of this needs a contractor. Fifteen minutes from the ground will tell you most of it.

• Look along the bottom edge of the gutter for a sag between hangers. A run that holds a puddle after a storm is a run that is holding weight it should not be.
• Check the corners. Mitered corners are where sectional systems leak first. A rust streak or a stain on the fascia under a corner is water that has already been getting out.
• Find where every downspout ends. Stand at each one and follow it. If it ends within a few feet of the foundation, on bare dirt, or at the top of a slope running toward the house, that is the thing to fix before the rain and not after.
• Look at the fascia itself. Paint bubbling, a dark line, or wood that gives when you press it means water has been getting behind the gutter rather than into it.
• If you can see into the trough safely from a window or a slope, look for a packed layer of silt. Dry dust turns to a crust that water runs over rather than through.

If something on that list looks wrong, it is worth saying plainly that none of it is urgent in the way an ad would tell you it is. It is worth handling before January, not tonight.

Where this matters most

The winter skew is not even across the Valley. The two gauges that put February ahead of August are both north and east of central Phoenix, which lines up with where we see the most winter drainage calls. If you are in one of these, your roofline is doing more work between December and March than the monsoon reputation suggests:

• Scottsdale — the Scottsdale Municipal gauge takes 51% of its annual rain December through March, the highest winter share of the three
• Paradise Valley — flat and parapet roofs, where a slow soaking finds a scupper problem a short storm would not
• Cave Creek — wash-adjacent lots and caliche, which sheds rather than absorbs once it is wet
• Peoria and Sun City — nearest the Deer Valley gauge, 8.90 inches a year with February the wettest month
• Mesa, Gilbert and Chandler — east Valley, same winter-leaning pattern

Where the water goes

Two of our own jobs where the discharge point, not the gutter, was the thing that needed solving.

Sources

• NOAA National Centers for Environmental Information, 1991–2020 U.S. Climate Normals — monthly precipitation normals for Phoenix Sky Harbor (USW00023183), Phoenix Deer Valley (USW00003184) and Scottsdale Municipal (USW00003192).
• NOAA Climate Prediction Center, ENSO Diagnostic Discussion, issued 8 October 2026.
• National Weather Service Phoenix — monsoon season definition (15 June – 30 September) and Phoenix climate normals.
• Flood Control District of Maricopa County — January 2010 storm report, for the soil-saturation sequence described above.
• Michael Crimmins, University of Arizona, quoted by KJZZ, 9 September 2026, on El Niño shifting the odds without guaranteeing an outcome.

Update log

This page tracks a live seasonal outlook, so it gets revisited rather than left to age.

• 8 October 2026 — published, against the CPC discussion issued the same day.

Next scheduled review: the CPC ENSO Diagnostic Discussion of 12 November 2026.