Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Wed Jul 31 2024 Valid 12Z Wed Jul 31 2024 - 12Z Fri Aug 02 2024 ...Dangerous heatwave expands from the Plains/Mississippi Valley into the Southeast as another heatwave develops over portions of the West... ...Severe weather and flash flooding expected for the Northern/Central Plains and Midwest Wednesday into Thursday... ...Another threat for flash flooding Wednesday in northern New England... ...Monsoonal shower and thunderstorm chances continue for the Southwest... Widespread, dangerous heat will be the big story for much of the country this week as one heatwave expands from the Plains/Mississippi Valley into the Southeast and another heatwave begins over portions of the West. A strong upper-level high will remain in place over the southern-tier of the U.S., with forecast high temperatures Wednesday and Thursday once again into the low to mid-100s over much of the Central Plains and southern High Plains and mid- to upper 90s for the Middle and Lower Mississippi Valley. High temperatures will also begin to climb into the mid- to upper 90s over much of the Southeast and southern Mid-Atlantic. High humidity values, especially for eastern portions of the Plains into the Mississippi Valley and Southeast, will bring heat indices soaring into the 105-110 degree range for many, with some locations reaching as high as 115, and many heat-related warnings and advisories are in place. The combination of hot temperatures to the west and high heat indices to the east, as well as very warm morning lows only dropping into the mid- to upper 70s, will be dangerous to anyone without access to adequate air conditioning. An approaching cold front will bring temperatures and humidity down a bit from the Middle Missouri Valley into the Central Plains Thursday. Unfortunately, an upper-level ridge will also begin to build northward over the West as well. This will quickly cause high temperatures across much of the northern tier of the West to rise from close to average on Wednesday to 10-20 degrees above average Thursday. Forecast highs will be into the 90s for the Pacific Northwest, northern Rockies, and northern High Plains, with 100s over much of the northern Great Basin. Similar to areas further east, many heat-related watches and advisories have been issued as this heat will be dangerous to those without adequate air conditioning. Smoke from area wildfires will also continue to plague parts of the region, resulting in hazy conditions and poor air quality. An upper-level wave/surface frontal system along the northern side of the upper-level high over the Northern/Central Plains will continue eastward into the Midwest Thursday, bringing a threat for severe weather and flash flooding the next couple of days. Plentiful moisture at the surface, strong to extreme instability, and accompanying strong flow aloft will help lead to some more intense thunderstorms. First, on Wednesday, the Storm Prediction Center has outlined an Enhanced Risk of severe weather (level 3/5) over the Middle Missouri Valley, with a broader Slight Risk west into portions of the Plains and southeast into Middle Mississippi/Ohio Valleys and southern Appalachians. Some initial isolated thunderstorms over the Plains may produce very large hail and some significant wind gusts in the late afternoon. Then, into the evening hours, storms are expected to expand in coverage and develop into an organized convective system over the Middle Missouri Valley, with an accompanying threat for more widespread significant wind gusts. Additional storms are expected along a warm front southeast into the Middle Mississippi/Ohio Valleys and southern Appalachians, potentially organizing into another convective system here as well, posing a threat for some damaging wind gusts. The expanding coverage of storms into the evening hours, as well as the plentiful moisture and stronger storms capable of producing intense downpours, will bring the chance of some heavier rainfall totals. A Slight Risk of Excessive Rainfall (level 2/4) covers the same region for the threat of some scattered instances of flash flooding. A similar pattern will be in place Thursday as the system shifts east into the Midwest, with the threat focusing on portions of the Great Lakes and Ohio Valley in vicinity of a lifting warm front and ahead of the approaching cold front. Scattered thunderstorms are forecast to increase in coverage during the evening, organizing into yet another convective system, likely moving southeastward from northeastern Illinois into central Indiana and northern Kentucky/southwestern Ohio. Another Slight Risk of severe weather is in place mainly for the threat of damaging winds, with a Slight Risk of Excessive Rainfall for some additional instances of flash flooding. Showers and thunderstorms are expected ahead of another upper-level wave/cold front approaching the Northeast on Wednesday. High moisture lingering over the region may lead to some locally heavy downpours, with a heightened threat of flash flooding over portions of northern New England given the very wet antecedent conditions following historic amounts of rainfall on Tuesday. Another Slight Risk of Excessive Rainfall as well as Flood Watches are in place for the region. The front will weaken/push eastward on Thursday with some lingering shower chances, most likely over Maine. High temperatures will also rise into the upper 80s to low 90s following the departure of the system, with humidity producing some muggy conditions. Showers and storms are also expected further south over portions of the Southeast/Florida on Wednesday with some moderate to locally heavy rainfall possible, with some lower storm chances lingering into Thursday. Daily Monsoonal shower and thunderstorm chances will remain over portions of the Southwest with some heavy downpours possible and an isolated threat for flash flooding. A passing upper-level shortwave will lead to greater coverage of thunderstorms over south-central Arizona on Wednesday, with a Slight Risk of Excessive Rainfall in place for a locally higher threat for some flash flooding. Forecast high temperatures more broadly in the region will remain closer to average the next couple of days compared to northern areas of the West, with highs in the 60s and 70s for coastal California, 80s and 90s for the Four Corners region and higher terrain of the Southwest, the 90s to low 100s for interior California into the central Great Basin, and mid- to upper 100s for the Desert Southwest. Putnam Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php