US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 518 PM EDT Fri Jun 18 2021 Valid Monday June 21 2021 - Friday June 25 2021 Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Mon-Wed, Jun 21-Jun 23. - Heavy rain across portions of the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Northeast, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Tue, Jun 22. - Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Mon, Jun 21. - Flooding possible across portions of the Southeast and the Lower Mississippi Valley. - Flooding possible across portions of the Southeast and the Lower Mississippi Valley. - Flooding possible across portions of the Southeast and the Lower Mississippi Valley. - Flooding possible across portions of the Southeast and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Plains. - Excessive heat across portions of the Southern Plains, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Mon-Wed, Jun 21-Jun 23. - Excessive heat across portions of California and the Southwest, Mon, Jun 21. - Excessive heat across portions of the Central Plains and the Southern Plains, Wed-Thu, Jun 23-Jun 24. - Much above normal temperatures across portions of the Central Great Basin, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Mon-Wed, Jun 21-Jun 23. - Heavy rain across portions of mainland Alaska, Wed-Sat, Jun 23-Jun 26. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri-Sat, Jun 25-Jun 26. Detailed Summary: The medium range hazards forecast period (June 21st-June 26th) features the moderation of excessive heat in the west, while Potential Tropical Cyclone Three produces one more round of heavy rain over the Carolina Coastal Plain before drifting off to sea by Wednesday. As the tropical system exits, showers and thunderstorms are likely over the Upper Midwest, and then eastward from New England into the Southeast as a cold front interacts with leftover deep moisture. Precipitation hazards shift to the Alaska coast by Wednesday as a deep low approaches from south of the Aleutians. After a week of blistering hot temperatures in the Western U.S., relief is on the way for the Great Basin and Southwest in the form of slightly cooler (albeit still warm) temperatures. By the 21st, the ridge axis migrates east as a closed low meanders off the California coast, while a trough pivots across the Midwest. Starting Monday, temperatures finally moderate to around average in California, the Great Basin, and Desert Southwest as the upper low inches closer toward land. Unfortunately, cool anomalies over parts of the west come at the expense of above average temperatures holding in the Pacific Northwest before spreading into the Northern Rockies and Plains later next week. With heat index forecasts less than 100, and widespread low impacts per NWS HeatRisk guidance, decided against an excessive heat area across the aforementioned areas for mid-late next week. An Excessive Heat area was placed over portions of the Central and Southern Plains for the 23rd-24th, however, where heat indices of 105 and higher are possible beneath the newly located high pressure center. Otherwise, the main changes to the current hazards in the west were to trim the Excessive Heat area in California, Arizona, and Nevada to mainly the valleys where the hottest temperatures reside. No significant changes were made to the Excessive Heat area in Texas for the 21st-23rd. By the morning of the 21st, the remnants of Potential Tropical Cyclone Three are forecast to reside over the Carolinas, bringing one last chance of heavy rainfall before it exits into the Atlantic. A sweeping cold front will follow behind the tropical remnants and produce heavy showers and thunderstorms in the process, first over the Upper Midwest with lingering heavy rain the morning of the 21st. The prefrontal airmass will be characterized by plentiful moisture with ensemble consensus precipitable water anomalies 1.5-2 standard deviations above normal. High ensemble probabilities of exceeding 1 inch of rainfall, coupled with concerns regarding inundated soils and rivers (particularly in the Ohio Valley and areas south) factored into the addition of a large heavy rain area on the 22nd. Afterwards, a push of dry continental air behind the front scours out the remaining tropical moisture across the Eastern U.S., which suppresses the heavy rain threat over the region for the time being. Some heavy rain is possible over the Midwest as convection initiates along and ahead of a lifting warm front, which will be monitored in future updates. While the CONUS settles down for the time being, in Alaska multiple rounds of heavy rain appears likely from the Kenai Peninsula and further south the 23rd-26th as a deep low approaches the Western Aleutian chain. An additional Heavy Rain area was added over the Southeast Alaska coast for the 25th-26th as well, where warm advection and orographic ascent support ensemble consensus QPF over an inch there. Asherman