Fall has come arrived (<--"Arrived" sounds better here.) here in Korea
Fall's already here in Korea.
and (<--Try not to start sentences with conjunctions.) Plus, the weather center doesn't know why it's unwontedly unusually (<--Is this the word you were looking for?) chilly this year.
So Thus, if anyone's headed to Korea soon, make sure you packed enough warm clothes!
Climate change is why we have strange weather more often now =)
Fall has come here in Korea
Fall's already here in Korea.Actually it's colder than you might think, almost like mid-September. It is so [or very] unusual.
[Usually ellipsis marks come in sets of three with a space between each, for example . . . In this case, you don't really need ellipsis marks (they usually indicate that something has been left out or is implied, but an exclamation point (!) could be used to show emphasis and surprise if you like.]
The weather center doesn't know why it's unusually chilly this year.
So, if anyone's headed to Korea soon, make sure you packed enough warm clothes.
I've corrected some of the same things as Mr. Zhang did (he pointed out some excellent changes if you want to be more formal), but did not correct others. Some of your phrasing is perfectly acceptable in a casual conversation, though it might not be if you were writing a more formal essay or report. Since your topic is more informal "small talk," you can use more casual vocabulary and it actually sounds more natural to do so.
Make sure you check that you have included capitals where they are necessary and have the correct punctuation in your sentence. It will help to keep your ideas clear and accurate.
