The hottest heat wave to ever hit the Pacific Northwest is continuing to scorch the typically cool region with triple-digit temperatures.

       Seattle is forecast to hit its all-time high of 104 degrees Sunday -- one day after breaking the record for hottest temperature in June at 101 degrees. The previous all-time high in Seattle was 101 degrees.

       MORE: Pacific Northwest braces for historic heat wave, and experts say climate change could be to blame

       Seattle has only recorded triple digits twice in history, Karin Bumbaco, the assistant state climatologist for Washington state, told ABC News.

       In Portland, temperatures reached 108 degrees on Saturday, breaking the all-time record of 107 degrees. The city is set to break its record again on Sunday with a forecast high of 112 degrees.

       Nathan Howard/AP

       Paramedics Cody Miller, left, and Justin Jones respond to a heat exposure call during a heat wave, June 26, 2021, in Salem, Ore.

       The upcoming heat wave follows several recent record-breaking heat waves that have developed due to a blocking pattern in the upper air flow across the Northern Hemisphere. When the sun is at a higher angle, it weakens the jet stream, meaning the hot air mass gets even hotter.

       In the Midwest, heavy rain is expected to come down in parts of Texas and Oklahoma.

       A flash flood watch is in effect in parts of the region through Sunday night, but the ground is already saturated from clusters of storms in the Midwest on Saturday.

       MORE: Why water levels in megadrought-impacted Southwestern states have some experts concerned

       Since Thursday afternoon, the highest rainfall totals so far are in Chanute, Kansas, at 8.57 inches, Edmond, Oklahoma, at 7.14 inches and Bloomington, Illinois at 6.32 inches.

       The highest reported storm gust on Saturday came in at 88 mph in Lamesa, Texas, and golfball-sized hail was reported in Wickett, Texas.

       Showers and storms will continue to develop around the slow-moving frontal boundary that extends from western Texas and New Mexico into the Great Lakes.

       Rainfall rates could approach 2 to 3 inches per hour at times.

       MORE: 'Megadrought' in West directly linked to climate change, experts say

       Strong storms with gusty winds are also possible from Missouri to Michigan, where widespread flooding occurred near Detroit on Saturday due to the storms.