Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Sun Aug 20 2023 Valid 00Z Mon Aug 21 2023 - 00Z Wed Aug 23 2023 ...There is a Slight to High Risk of excessive rainfall over parts of Southern California, the Great Basin, and the Northern Intermountain Region... ...There is a Slight Risk of severe thunderstorms over parts of Southeastern California... ...There are Air Quality Warnings over the parts of the Pacific Northwest and Northern/Central Rockies... ...Excessive Heat Warnings and Advisories from parts of the Plains, the Mississippi Valley, and Central/Western Gulf Coast... T.S. Hilary is forecast to move northward over Baja California Peninsula on Sunday afternoon and into Southern California by late afternoon Sunday. Hilary is then forecast to continue to move northward and dissipate over central Nevada on Monday. Hilary will also produce potentially historic rainfall amounts expected to cause flash, urban, and steep-sided gully flooding, including landslides, mudslides, and debris flows. Localized flooding impacts, some significant, are also expected across northern portions of the Intermountain West. Therefore, the WPC has issued a High Risk of excessive rainfall over parts of Southern/Central California into Monday morning. Severe, widespread flash flooding is expected. Areas that normally do not experience flash flooding will flood. Lives and property are in great danger through Monday. Hilary will also produce showers and severe thunderstorms over Southeastern California. Therefore, the SPC has issued a Slight Risk of severe thunderstorms over parts of Southeastern California through Monday morning. The hazards associated with these thunderstorms are lightning, severe thunderstorm wind gusts, a few tornadoes, and a minimal threat of hail. The showers and thunderstorms will extend northward into the Northern Intermountain Region, Northern Rockies, and Northern Plains through Monday afternoon. The threat of excessive rainfall continues on Monday. Therefore, the WPC has issued a Slight Risk of excessive rainfall over parts of California, Great Basin, Southwest, Pacific Northwest, Northern Intermountain Region, and Northern Rockies from Monday into Tuesday morning. The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, small streams, and burn scars the most vulnerable. In addition, the threat of severe thunderstorms decreases slightly on Monday. On Monday, the associated moisture plume moves eastward into the Southwest and Great Basin, producing strong to severe thunderstorms. Therefore, the SPC has issued a Marginal Risk of severe thunderstorms over parts of the Southwest, Great Basin, and Northern Rockies from Monday into Tuesday morning. The hazards associated with these thunderstorms are frequent lightning, severe thunderstorm wind gusts, hail, and a minimal threat of tornadoes. The threat of excessive rainfall continues on Tuesday as the showers and thunderstorms with heavy rain over parts of the Great Basin, Southwest, and Northern Intermountain Region. Therefore, the WPC has issued a Slight Risk of excessive rainfall with these thunderstorms over two regions, the first area over parts of the Northern Intermountain Region and the second over parts of the Great Basin on Tuesday. The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, small streams, and burn scars the most vulnerable. In addition, on Tuesday, the threat of severe thunderstorms ends over the West. Furthermore, wildfire smoke over parts of the Pacific Northwest and Northern/Central Rockies has prompted Air Quality Alerts over parts of the Pacific Northwest and Northern/Central Rockies. Meanwhile, a front over the Great Lakes will move into the Northeast and settle over the Mid-Atlantic parts by Tuesday. The system will produce scattered showers and thunderstorms over parts of the Northeast on Monday into Monday evening. In addition, upper-level energy and tropical moisture will move across the Central Gulf Coast Sunday evening and Monday. The energy will produce scattered showers and thunderstorms over parts of the region. Further, onshore flow off the Atlantic will continue to produce showers and thunderstorms over the southern tip of Florida through Tuesday. As the energy and tropical moisture moves over the Western Gulf Coast, the system will produce showers and thunderstorms with heavy rain over parts of south-central Texas. Therefore, the WPC has issued a Slight Risk of excessive rainfall with these thunderstorms over parts of south-central Texas on Tuesday. The associated heavy rain will create mainly localized areas of flash flooding, with urban areas, roads, and small streams the most vulnerable. Furthermore, an upper-level high over the Central Plains/Middle Mississippi Valley will create oppressive daytime heat, limited overnight cooling, which will also contribute to the effects of heat, prompting Excessive Heat Warnings and Heat Advisories over portions of the Central/Southern Plains, the Upper/Middle/Lower Mississippi Valley, Central/Western Gulf Coast. In addition, dangerous heat will continue to intensify and expand into the Plains states, with numerous daily high-temperature records expected, especially in the Central Plains and Texas. The heat wave will then expand early next week to include much of the Central and Southeast U.S. Some locations in the Midwest may see their hottest day of the year so far this weekend. Portions of the South Central U.S. and Gulf Coast may further extend their record number of days with a heat index reading of at least 110ĀF. Moreover, the prolonged nature of the heat wave, combined with very warm overnight temperatures, will limit the relief from the oppressive daytime heat and compound overall heat impacts. Further, excessive heat is anticipated to focus over the southeastern quadrant of the country next week. The greatest risk of excessive heat is for eastern Texas and parts of the Gulf Coast States. Ziegenfelder Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php