US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 446 PM EDT Wed Jul 28 2021 Valid Saturday July 31 2021 - Wednesday August 04 2021 Hazards: - Heavy rain across portions of the Lower Missouri Valley and Middle Mississippi Valley, Sat, Jul 31. - Heavy rain across portions of the Front Range and Central and Southern High Plains, Sat-Sun, Jul 31-Aug 1. - Heavy rain across portions of the Northern Rockies, Sat-Sun, Jul 31-Aug 1. - Heavy rain across portions of the interior Northeast, Sun, Aug 1. - Heavy rain across portions of the Southern Plains, Mid-South, and Tennessee Valley, Sun, Aug 1. - Heavy rain across portions of the Western and Central Gulf Coast, Lower Mississippi Valley, and the Southeast, Mon-Tue, Aug 2-Aug 3. - Heavy rain across portions of the Southeast, Sun-Wed, Aug 1-Aug 4. - Excessive heat across portions of the interior Pacific Northwest, Sat, Jul 31. - Excessive heat across portions of the Southern Plains and Mid-South, Sat-Sun, Jul 31-Aug 1. - Heavy rain across portions of mainland Alaska, Sat-Sun, Jul 31-Aug 1. Detailed Summary: Multiple threats for heavy rain across the CONUS and Alaska highlight the hazards outlook for the medium-range forecast period (Saturday, July 31 - Wednesday, August 4). The start of 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 significantly into the Southern U.S. A low pressure center over Canada will move to the east, with a trailing cold front moving to the southeast across the Plains and Midwest and into the Southeast along the northwesterly upper-level flow as the trough digs into the South. A second, quasi-stationary front will be draped from the Carolinas west into the Missouri Valley. Organized thunderstorms are expected to develop on Saturday (July 31) along the quasi-stationary front and any mesoscale boundaries produced from overnight convection across the Lower Missouri Valley and Middle Mississippi Valley. Heavy rain of 1 to 2 inches is forecast, with locally higher amounts of 3 inches and above possible. As the low pressure center over Canada moves eastward, a secondary low is forecast to develop over the eastern Great Lakes with a warm front extending to the southeast, moving to the northeast across the interior Northeast on Sunday (August 1). Increased moisture advecting northeast with the warm front will lead to the threat of showers and thunderstorms producing 1-2 inches of heavy rain across the area. Later in the period, 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, with additional reinforcement from the cold front moving in from the northwest and a closed low developing at the base of the upper-level trough. Daily rainfall totals between 1-2 inches and locally higher appear likely, particularly over the Carolinas. A weak low pressure center over the Carolinas along with the quasi-stationary front already in place will lead to showers and thunderstorms beginning Sunday (August 1). The cold front from the northwest will begin to move across the region, with the quasi-stationary boundary beginning to move further to the southeast towards the Florida Panhandle. The cold front will become quasi-stationary across the Southeast by Tuesday, continuing shower and thunderstorm chances through at least Wednesday (August 4). Further west, the cold front looks to move southward a bit faster, with showers and thunderstorms possible across the Southern Plains through the Mid-South and Tennessee Valley on Sunday and across the Lower Mississippi Valley and Gulf Coast on Monday (August 2) and Tuesday (August 3). The cold front is forecast to become quasi-stationary here as well, and if the front does not progress as far south as currently forecast the threat for heavy rain may continue into Wednesday along the Gulf Coast. Precipitation 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 around 1-2 inches of heavy rain are expected both Saturday and Sunday across portions of the Front Range of the Rockies as well as the Central and Southern High Plains. The cold front pushing in from the northeast may help to enhance this rainfall on Sunday. Showers and storms will continue into Monday as this front stalls along the High Plains, but an extension of the outlook area was not included for now given the lighter nature of the forecast rainfall in the current model guidance. Additionally, an anomalously high increase in moisture further north into central and southern Idaho will lead to the threat of heavy rain on both Saturday and 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. 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 precipitation increases. A combination of high temperatures in the upper 90s to low 100s along with dewpoints in the 70s will lead to widespread heat indices between 105 to 110, locally as high as 115, across portions of the Southern Plains and Mid-South Saturday and Sunday. There will be little relief overnight as lows dip only into the upper 70s to low 80s. This excessive heat is likely further east across the Southeast to southern South Carolina 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. Excessive heat is also a concern for the interior Pacific Northwest at the beginning of the period, with highs in low to mid-100s continuing through Saturday for northeastern Oregon, eastern Washington, and northwestern Idaho. In Alaska, a low pressure system moving northeast across the Arctic Ocean with a trailing cold front moving onshore over the western mainland will bring continued chances for significant heavy rainfall over both the southern Seward Peninsula and portions of the southwest mainland as well as portions of the western Brooks Range on Saturday. This will continue into Sunday as the front becomes stationary. Some of the heaviest rainfall may approach climatological extremes for these areas. 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. Putnam