A temperatura está alta desde ontem.

Because estar is usually used for a state or condition, especially one that can change.

• A temperatura está alta = the temperature is high right now / has been high
• ser would sound more like a permanent characteristic, which does not fit well here

Since temperature changes over time, estar is the natural choice.

In Portuguese, it is very common to use the definite article with nouns in situations where English often does not.

So A temperatura means the temperature.

• A temperatura está alta = the temperature is high

Leaving out the article here would usually sound unnatural in standard Portuguese.

Because temperatura is a feminine noun in Portuguese, and adjectives usually agree with the noun in gender and number.

• a temperatura → feminine singular
• so the adjective must also be feminine singular: alta

Compare:

• o preço está alto = the price is high
• a temperatura está alta = the temperature is high

For temperature readings or general temperature level, alto / alta is very common and natural.

• A temperatura está alta = the temperature is high

You may hear quente, but it does not mean exactly the same thing in all contexts.

• Está quente usually means it is hot
• A temperatura está alta focuses more directly on the temperature level itself

So if you are talking specifically about temperature, alta is a very good choice.

Desde ontem means since yesterday.

It shows that the situation started in the past and continues up to now.

So the sentence means that the temperature became high yesterday and is still high now.

Portuguese often uses the simple present with desde to express something that started in the past and continues into the present.

So:

• A temperatura está alta desde ontem

can correspond to English:

• The temperature has been high since yesterday

This is very normal in Portuguese. You do not need a special present perfect form like English does.

Usually, that would not be the best choice here.

• está alta desde ontem = it has been high since yesterday and still is
• ficou alta usually emphasizes the change into that state: it became high

So ficou focuses more on the moment of change, while está ... desde ontem focuses on the ongoing condition from yesterday until now.

Yes, it can move, although the most neutral order is:

• A temperatura está alta desde ontem

You may also hear:

• Desde ontem, a temperatura está alta

Both are correct. Putting desde ontem first can give it a little more emphasis.

Está has a written accent because the stress falls on the final syllable: es-TÁ.

The accent mark helps show the correct stress and distinguishes it from other forms.

In Brazilian Portuguese, it is pronounced approximately like:

• es-TAH

The final a is stressed.

By itself, A temperatura está alta is most naturally understood as the temperature in some context being high, such as:

• the weather
• a room
• a machine
• the body temperature of a person

If you want to be more specific, Portuguese often adds context:

• A temperatura do quarto está alta = the room temperature is high
• A temperatura do corpo está alta = body temperature is high
• Ele está com febre = he has a fever

So yes, it can refer to different kinds of temperature, depending on context.

Yes. Desde can be used with many starting points in time.

For example:

• A temperatura está alta desde manhã = since this morning
• A temperatura está alta desde domingo = since Sunday
• A temperatura está alta desde a semana passada = since last week

The pattern is:

• estar + adjective + desde + starting point in time

Yes, it is completely natural.

It sounds like standard, everyday Brazilian Portuguese and is grammatically correct. It is a good model for talking about an ongoing condition that began in the past:

• A temperatura está alta desde ontem
• O preço está alto desde janeiro
• Ela está doente desde segunda-feira