Excessive Heat Warnings in effect across parts of California’s Central Valley region as well as the Desert Southwest; Extreme Heat Risk continues across south Texas… …Thunderstorms and heavy to Excessive Rainfall possible from Southern Plains to Northeast through Thursday… A very pronounced ridge will build across the Western U.S. over the next few days while upper-level lows influence the pattern across the lower 48 from their respective stations off the Baja and Nova Scotia Coasts. The upper ridge will support a heat wave which will expand across the region. High temperatures in the 90s and 100s will be 20-30 degrees above average for this time of year, while low temperatures in the upper 60s and 70s will offer little respite at night. Widespread high and low temperature records may be tied or broken through Friday. There are Excessive Heat Warnings in effect for California’s Central Valley as well as portions of the Desert Southwest including southeastern California, southern Nevada and western/southern Arizona. Mild night time temperatures will continue to plague southern Texas through the end of the week. These extremely hot conditions will affect anyone without effective cooling and/or adequate hydration. A pair of low pressure systems tracking across the eastern half of the country will trigger scattered to isolated showers and thunderstorms across much of the region today before shifting into just the East Coast on Thursday. Deep layer moisture paired with instability will support locally heavy rainfall over parts of the Ohio Valley, Central Appalachians and Mid-Atlantic where a Slight Risk (at least 15%) of Excessive Rainfall leading to Flash Flooding is in effect for today. The threat for Excessive Rainfall shifts into parts of the interior Northeast and New England on Thursday, where another Slight Risk of Excessive Rainfall is in effect. A pattern change will ensue across the Eastern U.S. on Thursday, in which a digging trough will usher in cooler air temperatures and continue through the weekend.