How can weather forecasters predict that we will get another "heat wave" 1 week in advance?

Update: I was trying to determine if I should continue to water the newly planted fescue grass seed in my front lawn, and saw the weather forecasters predicted "more normal" temperatures for the next 5 days, but that by next weekend we will have , again, near 100 degree temps. how can they predict that far in... show more I was trying to determine if I should continue to water the newly planted fescue grass seed in my front lawn, and saw the weather forecasters predicted "more normal" temperatures for the next 5 days, but that by next weekend we will have , again, near 100 degree temps. how can they predict that far in advance?
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