Some of the information in this article was originally published on June 22.The weather has already left its mark on this World Cup. Thunderstorms have delayed play and forced supporters from their seats. With the knockout stage underway, extreme heat is expected to create a different challenge.Some host cities are forecast to experience their hottest conditions of the tournament this week, bringing the temperatures into sharper focus as the stakes rise.FIFA’s mandatory hydration breaks have proved controversial, particularly when they have interrupted games in comfortable conditions. In the days to come, those pauses may become an essential safeguard.The Athletic explains what to expect this week, the rules around extreme heat, and how it can affect players, spectators and games.How Extreme Heat Could Decide The World CupJJ BullWhat extreme weather is forecast this week?Aaron Mentkowski, chief meteorologist at WKBW-TV Buffalo, and The Athletic’s weather expert during this World Cup:Expect a sizzle this upcoming week, bringing extreme-heat concerns for players and spectators alike. Get ready for dangerously high temperatures near 100F (38C), with a heat index (a combination of temperature and humidity measuring how hot it really feels) of 110F (43C).A change in the weather pattern is expected across the eastern United States as we head toward the end of June and the beginning of July. The pattern will bring above-normal temperatures for Toronto, New York City, Boston, Kansas City and Philadelphia. The heatwave could continue beyond the first week of July.Atlanta, Dallas, and Houston will also experience soaring temperatures, but those matches take place indoors. Miami is always hot and humid in the summer, so there’s not much of a forecast change for South Florida.The West Coast will stay seasonable. San Francisco, Seattle, Los Angeles, and Vancouver (Canada) will be much cooler than the other sites during this stretch. The weather in Mexico City is forecast to remain reasonable.This week’s matches:Monday