By AI Chat-Human Synthesis-13 September 2026

Two successive cold-air outbreaks, rain, wind and an unusually unsettled transition toward spring.

Ubatuba is experiencing an unusual end to winter. Although the most extreme temperatures — including readings around 5°C and below — have occurred farther south and inland, two successive atmospheric disturbances have brought their effects right to the São Paulo coast. For Ubatuba, the result has been cold-feeling days, persistent cloud, rain, strong humidity and periods of rough sea, rather than the extreme freezing temperatures seen elsewhere in Brazil.

The first cold wave

The first significant outbreak arrived around the 7 September Independence Day period.

A cold front moved northward through Brazil, bringing markedly colder conditions to São Paulo. Even Ubatuba and other Litoral Norte municipalities were affected by the combination of the cold air and persistent maritime cloud.

Reports at the time specifically warned of cold, overcast conditions and rough seas around Ubatuba, with waves potentially reaching about 2.5 metres and authorities advising caution with nautical activities.

That first event was therefore not an isolated "cold day." It was part of a substantial South American cold-air incursion.

Then came the second system

The more important development occurred from approximately 10–13 September.

A new frontal system became associated with an extratropical cyclone developing off the southern Brazilian coast.

That produced a very different kind of weather:

cold air + Atlantic moisture + frontal instability + strong winds + mountainous terrain.

The Litoral Norte, including Ubatuba, was placed under continuing rain alerts.

By 12–13 September, the system had produced serious weather across São Paulo state, including flooding, fallen trees, landslides and damaging winds. Some areas recorded wind gusts exceeding 90 km/h.

The important point for Ubatuba is that the Serra do Mar immediately behind the city greatly amplifies the effect of moist south/southeasterly airflows.

So Ubatuba can remain relatively mild in absolute temperature while nevertheless experiencing extremely unpleasant conditions:

18°C + rain + saturated air + wind + no sunshine

can feel considerably colder than a dry 12°C day.

Ubatuba's temperatures

This is where the distinction between Ubatuba and the rest of Brazil matters.

September is normally already becoming a transition month in Ubatuba.

Typical September conditions are approximately:

• Average high: around 25°C
• Average low: around 18°C
• Increasing rainfall as spring approaches
• Increasing daylight

AccuWeather's September 2026 climatological/forecast data puts Ubatuba's expected September overnight range roughly around 16–20°C, with daytime temperatures generally around 21–30°C.

So we should not describe the present Ubatuba episode as a 5°C cold wave.

That would be misleading.

The remarkable feature here is instead the persistence of cold, wet conditions for a coastal city that normally begins warming toward spring at this time of year.

Current Ubatuba forecasts are showing temperatures around the mid-teens to low 20s, with substantial cloud and rain continuing.

Why does Ubatuba feel so cold?

There are three important ingredients.

1. Cold air from the south

The successive frontal systems have transported colder air northward from southern South America.

The first outbreak produced exceptionally low temperatures in parts of southern and southeastern Brazil.

The second system followed before the atmosphere had completely settled.

That is why it feels as though winter has returned just when it should be disappearing.

2. The Atlantic Ocean

Ubatuba sits directly against the Atlantic.

When cold continental/subtropical air meets the relatively warmer and moisture-rich ocean, enormous amounts of moisture become available to the atmosphere.

The result can be:

rain + cloud + wind + low temperature

rather than the clear, dry conditions normally associated with a cold spell.

3. The Serra do Mar

This is particularly important for Ubatuba.

Moist air coming from the ocean encounters the Serra do Mar almost immediately.

The air is forced upward, producing additional cloud and rainfall.

That is one reason Ubatuba can have extraordinarily wet weather even when nearby parts of inland São Paulo are considerably drier.

The rain is actually one of the most remarkable aspects

September is normally a transition toward the wetter spring/summer regime in Ubatuba.

But the current atmospheric pattern has accelerated that feeling.

São Paulo city has already recorded 198.2 mm of rain during September through 12 September, making September 2026 the fourth-wettest September in 83 years of records there.

That number is not an Ubatuba rainfall measurement, so it should not be transferred to Ubatuba.

But it demonstrates the scale of the atmospheric disturbance affecting southeastern Brazil.

And the same frontal/cyclonic system is directly affecting the Litoral Norte.

What happens next in Ubatuba?

The interesting question is whether winter is really "coming back."

Probably not.

The astronomical beginning of Southern Hemisphere spring is 22 September.

The current pattern is better understood as:

late winter → cold outbreak → brief recovery → second cold/frontal outbreak → unsettled transition → eventual spring warming.

Current extended forecasts continue to show several days of cloud, showers and relatively cool temperatures in Ubatuba rather than an immediate return to hot beach weather. One current forecast shows temperatures remaining roughly in the 19–21°C range during several of the coming days, with frequent rain/showers.

So Ubatuba may remain distinctly cool and damp for a while yet.

The larger Brazilian picture

This is where your original observation was absolutely correct.

While Ubatuba has not experienced 5°C, other parts of Brazil have experienced genuinely remarkable cold.

The current situation illustrates the enormous climatic contrast across Brazil:

Southern Brazil
→ very cold air
→ temperatures near or below freezing in some locations

São Paulo interior/highlands
→ pronounced cold

São Paulo city
→ unusually cold and very wet

Serra do Mar
→ cool, saturated, rainy and windy

Ubatuba
→ comparatively mild in absolute temperature, but unusually cold-feeling, cloudy and wet

Northern Brazil
→ largely outside the direct influence of these polar incursions.

And there is an interesting contradiction: El Niño

One might expect El Niño to mean that Brazil should simply be warmer.

But that is not how atmospheric weather works.

El Niño changes large-scale probabilities and circulation patterns. It does not prevent individual cold fronts from travelling north.

Current meteorological analysis specifically notes that the continuing September cold is perfectly compatible with El Niño: the long-term climatic tendency and short-term weather events operate on different timescales.

So we can simultaneously have:

El Niño → warmer overall tendency

and

strong frontal system → several days of unusually cold weather.

There is no contradiction.

The Ubatuba conclusion

So I would describe what you and Dinora have been experiencing this way:

Ubatuba has not been hit by the extreme 5°C temperatures occurring elsewhere in Brazil. Instead, the town has been caught on the southern/eastern edge of two successive late-winter cold-air and frontal systems. Their combination with Atlantic moisture and the Serra do Mar has produced unusually cool, cloudy, wet and windy conditions for this late stage of winter.

And that makes the present episode particularly interesting.

Winter is almost over — yet the atmosphere has delivered two substantial reminders that the seasons do not change like a light switch.

The cold air can still surge north.

The Atlantic can still supply enormous moisture.

And the Serra do Mar can turn that combination into several days of cold rain and heavy cloud in Ubatuba, just as spring is approaching.