A few things that may help you.

1) You will rarely, if ever, see "that" proceeded by a comma.
2) The proper construction for which when its not in a preposition (in which, of which, etc) is "This house, which is painted red, is owned by the Johnsons."

In terms of your questions both sentences 1 and 2 are wrong.

1)Temperature in US that, scientists estimate, is supposed to reach 30 degrees this year, is a complete bad luck.

You would need to say "Temperatures".
You would need to remove the comma after that
You would need to say something like "Future temperatures in the U.S., which scientists have estimated, are supposed to reach 30 degrees this year."

osirus0830 wrote:A few things that may help you.

1) You will rarely, if ever, see "that" proceeded by a comma.
2) The proper construction for which when its not in a preposition (in which, of which, etc) is "This house, which is painted red, is owned by the Johnsons."

In terms of your questions both sentences 1 and 2 are wrong.

1)Temperature in US that, scientists estimate, is supposed to reach 30 degrees this year, is a complete bad luck.

You would need to say "Temperatures".
You would need to remove the comma after that
You would need to say something like "Future temperatures in the U.S., which scientists have estimated, are supposed to reach 30 degrees this year."

Thanks man! you have been a great help to me!!