On June 23, the United States National Weather Service warned of an approaching heat wave on the Pacific Northwest,[12] whose origins could be traced to torrential rains in China.[13] There, the warm, moist air rose and was eventually sucked up by the jet stream, which transported it east over cooler waters. When that air current encountered an upper-level high-pressure zone, also called a ridge, it started to significantly deform on June 25,[14] forcing to accommodate the high-pressure area south of the jet stream's meander.[15] At the same time, the Southwestern states are enduring an intense drought,[16] which have earlier allowed higher-than-average temperatures over the Southwestern United States, leading to a similar hot wave earlier in June.[17][18] Its remnants then moved north to the Pacific Northwest.[19][20]Six days later, Environment Canada issued a heat warning for Alberta, Saskatchewan, British Columbia, Manitoba, Yukon, and Northwest Territories.[21]