US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 500 PM EDT Tue Jul 27 2021 Valid Friday July 30 2021 - Tuesday August 03 2021 Hazards: - Heavy rain across portions of the Upper and Middle Missouri Valley, Fri, Jul 30. - 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 High Plains, Sat-Sun, Jul 31-Aug 1. - Heavy rain across portions of the Southern Plains, Lower Mississippi Valley, and Tennessee Valley, Sun-Mon, Aug 1-Aug 2. - Heavy rain across portions of the Lower Mississippi Valley and Southeast, Sun-Tue, Aug 1-Aug 3. - Excessive heat across portions of the Southeast, Fri, Jul 30. - Excessive heat across portions of the interior Pacific Northwest, Fri-Sat, Jul 30-Jul 31. - Excessive heat across portions of the Southern Plains and Lower Mississippi Valley, Fri-Sun, Jul 30-Aug 1. - Excessive heat across portions of the Southern Plains, Sat-Mon, Jul 31-Aug 2. - Heavy rain across portions of mainland Alaska, Sat-Mon, Jul 31-Aug 2. Detailed Summary: There are several threats for heavy rain and excessive heat ahead in the medium-range forecast period (Friday, July 30 - Tuesday, August 3). The start of the period will feature a ridge over the western and central portions of the U.S. and a trough over the Midwest and Northeast that will begin to dig deeper into the Southeastern U.S. A cold front will move southeast across the Plains and Midwest along the northwesterly upper-level flow on the northeastern side of the ridge, with a second, stationary front draped from the Carolinas west into the Missouri Valley. Organized showers and thunderstorms are expected to develop at the northern extent of a plume of higher moisture ahead of the cold front and along the stationary front on Friday (July 30) over the Upper Missouri Valley, producing heavy rain totals over an inch as they move to the southeast overnight. Additional organized thunderstorms will likely develop again on Saturday (July 31) ahead of the synoptic fronts and along any mesoscale boundaries produced from the overnight convection, bringing another threat of heavy rain of about 1 to 2 inches to the Lower Missouri Valley and Middle Mississippi Valley. 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 stationary front, with additional reinforcement from the cold front moving in from the northwest. Daily totals between 1-3 inches and higher appear possible. The cold front moving southeast from the Midwest will meet the stationary front already in place across the Carolinas and slow its easterly progression from Sunday (August 1) through Tuesday (August 3). There are also chances for additional storms further to the southwest across the Central Gulf Coast along a surface trough. The cold front looks to be a bit faster/more progressive further west with its southern movement, with the heavy rain threat focused between Sunday through Monday (August 2). Further west, precipitation associated with an active and expanding Moonsoon will bring the threat of heavy rain to portions of the Rockies and High Plains. Showers and thunderstorms producing around 1-2 inches of heavy rain are expected both Saturday and Sunday across the Front Range of the Rockies and Central High Plains from southeastern Wyoming south into northern New Mexico. A backdoor cold front moving in from the northeast may help to enhance this rainfall on Sunday. The heavy rain may 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 more isolated nature of heavier rain totals in the current model guidance. Additionally, an anomalously high increase in moisture further north into central Idaho will lead to the threat of heavy rain on both Sunday and Monday. In addition to the threat for heavy rain, excessive heat will also be a concern across the South before the eastern trough begins to dig deeper south and precipitation increases ahead of the series of fronts. A combination of high temperatures in the upper 90s to 100 along with dewpoints in the low 70s will lead to heat indices between 105 to 110 along the coast of the Carolinas on Friday. Similar high temperatures and dewpoints contributing to heat indices between 105 to 110 across the Lower Mississippi Valley will continue from Friday through Monday. Finally, high temperatures will increase across the Southern Plains, specifically southern Oklahoma and north Texas, as the upper-level ridge shifts south. A combination of temperatures in the low 100s along with dewpoints around 70 will lead to afternoon heat indices in the mid 100s, with little relief overnight as lows dip only into the upper 70s to low 80s. Excessive heat is also a concern for the interior Pacific Northwest at the beginning of the period, with highs in mid 100s in the Rogue Valley, in the low 100s in eastern Washington and Northwestern Idaho, and in the upper 90s for the Willamette Valley. In Alaska, a stationary front oriented northeast to southwest across the northwest mainland will lead to continued chances for significant heavy rainfall over both the Seward Peninsula and portions of the western Brooks Range on Saturday through Monday. Some of the heaviest rainfall may approach climatological extremes for these areas. Additional chances for heavy rain look possible further to the southwest, particularly ahead of a low pressure system approaching from the west over the Bering Sea. However, the amounts and placement of any significant rain totals is more uncertain in the model guidance and an outlook area will not be included at this time. Putnam