US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 429 PM EDT Thu Jul 29 2021 Valid Sunday August 01 2021 - Thursday August 05 2021 Hazards: - Heavy rain across portions of the Northern Rockies, northern Great Basin, Central Rockies, Central and Southern High Plains, Southern Plains, Mid-South, and the interior Northeast, Sun, Aug 1. - Heavy rain across portions of the Southeast and southern Mid-Atlantic, Sun-Thu, Aug 1-Aug 5. - Heavy rain across portions of the the Southern Appalachians, Southeast, Tennessee Valley, Lower Mississippi Valley, and the Southern Plains, Mon-Tue, Aug 2-Aug 3. - Heavy rain across portions of the Southeast, Mon-Thu, Aug 2-Aug 5. - Excessive heat across portions of the Southern Plains and Lower Mississippi Valley, Sun, Aug 1. - Heavy rain across portions of western mainland Alaska, Sun, Aug 1. Detailed Summary: Multiple threats for heavy rain, specifically across the South, highlight the hazards outlook for the medium-range forecast period (Sunday, August 1 - Thursday, August 5). The period will feature an upper-level ridge over the central/western U.S. and a trough over the Midwest and Northeast that will begin to dig into the central/southern U.S, with a closed low expected to form. A low pressure center will be located over the eastern Great Lakes region, with a warm front extending into New York and a cold front extending west across the Midwest, Plains, and along the Rockies. Additionally, a quasi-stationary boundary will be draped across the Carolinas west through the Southern Plains. As the low pressure center over the eastern Great Lakes moves eastward, increased moisture advecting northward with the warm front into the interior Northeast will lead to the threat of showers and thunderstorms producing 1-2 inches of heavy rain on Sunday (August 1). There is a significant signal for widespread heavy rain across the South as showers and thunderstorms form along and ahead of the quasi-stationary front and the cold front moving southward, with additional reinforcement from the closed upper-level low. Daily rainfall totals between 1-2 inches and locally higher appear likely. The quasi-stationary front in place across the eastern Carolinas along with the cold front moving in from the north will trigger showers and thunderstorms beginning Sunday. The quasi-stationary front will begin to move to the southeast towards the Florida Panhandle as the cold front continues to push further south on Monday (August 2), expanding the coverage of showers and thunderstorms deeper into the Southeast. The cold front will become stationary across the Carolinas and Georgia, leading to multiple days of heavy rain that look to continue through the end of the forecast period on Thursday (August 5). To the west, the cold front looks to move southward farther and faster, with showers and thunderstorms likely developing along the quasi-stationary front and ahead of the cold front across the Southern Plains through the Mid-South and Tennessee Valley on Sunday, and then across the Lower Mississippi Valley and Gulf Coast on Monday into Tuesday (August 3). The cold front is forecast to become quasi-stationary here as well, but the threat for heavy rain looks to shift just offshore into the Gulf. The threat for heavy rain may continue into Wednesday and Thursday along the Gulf Coast if the front does not progress as far south as currently forecast. Showers and storms associated with an active and expanding Monsoon, as well as an influx of moisture from the Gulf moving northward across the High Plains, will bring the threat of heavy rain to the Rockies and High Plains. Showers and thunderstorms producing 1-3 inches of heavy rain are expected to continue on Sunday across portions of the Front Range of the Rockies as well as the Central and Southern High Plains. Additionally, an anomalously high increase in moisture farther north into central/southern Idaho and eastern Oregon will lead to the threat of 1-2 inches of heavy rain on Sunday. It is worth noting that while the location and intensity of any additional heavy rain threats may not be consistent or widespread enough for outlook areas, model guidance indicates the potential for at least isolated instances of heavy rain elsewhere throughout the region as the Monsoon continues. Also, on Wednesday (August 4), a shortwave will round the base of the trough ahead of the western portion of the stationary front in New Mexico and Texas. The continued influx of moisture from the Gulf ahead of the front may lead to the chance for heavy rain with showers and thunderstorms developing across southeast New Mexico and west Texas. However, have opted to wait to introduce an outlook area for today until there is more consistency in the model guidance for heavier rain totals. In addition to the threat for heavy rain, excessive heat will also be a concern across the South before the cold front passes through and storms increase. A combination of high temperatures in the upper 90s to 100 degrees along with dewpoints in the 70s will lead to widespread heat indices between 105 to 110 across portions of the Southern Plains and Mid-South on Sunday. There will be little relief overnight as lows dip only into the upper 70s. This excessive heat is likely further east into the Carolinas as well. However, given the multiple heavy rain outlook areas already noted, opted not to include an outlook area for excessive heat to keep the focus on the heavy rain threat, although an outlook area is warranted. In Alaska, a low pressure system moving northeast across the Arctic Ocean with a trailing stationary front draped over the western mainland will bring continued chances for heavy rainfall over both the southern Seward Peninsula and portions of the southwest mainland as well as portions of the western Brooks Range on Sunday. Heavy rain may also effect additional areas outside of the noted outlook areas, but these outlook areas were chosen to highlight the locations where model guidance showed the most agreement for significant rainfall. Another storm system will move east across the Bering Sea with a warm front and cold front extending to the southeast over the eastern Aleutians and western Alaskan Peninsula on Tuesday and Wednesday. Showers triggered by these fronts may produce some heavier rain totals between 1-3 inches. However, the heavy rain threat looks to remain isolated and an outlook area will not be introduced for now. Putnam