US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 502 PM EDT Wed Aug 11 2021 Valid Saturday August 14 2021 - Wednesday August 18 2021 Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, the Central Appalachians, and the Tennessee Valley, Sun-Tue, Aug 15-Aug 17. - Heavy rain across portions of the Southeast, Sat-Sun, Aug 14-Aug 15. - Heavy rain across portions of the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Sat-Wed, Aug 14-Aug 18. - Heavy rain across portions of the Southwest, Sat-Tue, Aug 14-Aug 17. - Flooding possible across portions of the Central Plains and the Southeast. - Flooding possible across portions of the Southeast. - Flooding possible across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast. - Flooding occurring or imminent across portions of the Southeast. - Flooding likely across portions of the Southeast. - Flooding likely across portions of the Southeast. - Flooding likely across portions of the Southeast. - Excessive heat across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat, Aug 14. - Excessive heat across portions of the Northern Plains, Sun, Aug 15. - Heavy rain across portions of mainland Alaska, Sun-Mon, Aug 15-Aug 16. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Aug 17-Aug 18. Detailed Summary: The medium range forecast period (August 14th to 18th) features an active period of late Summer hazards, including excessive heat in the West, monsoon activity in the Southwest, and heavy rain in the Southeast/Mid-Atlantic associated with Tropical Storm Fred. Over this weekend, a deep upper-trough centered over the Great Lakes will lift northeast, while a similarly amplified trough migrates over the Pacific Northwest and weakens the strong ridge centered around the Four Corners. At the surface, the main players include a cold front forecast to stall across the Mid-Atlantic/Southeast by the end of the weekend, and a progressive cold front moving through the West beginning next early week. To the south, Tropical Storm Fred is expected to impact Florida over the weekend, and parts of the Southeast/Mid-Atlantic by early next week. Beginning Saturday, heavy rain is expected to envelop portions of the Mid-Atlantic and Florida. Over the northern heavy rain area, an initial round of thunderstorms packing intense rainfall rates are expected as the cold front associated with the Great Lakes trough interacts with a moist and unstable pre-frontal airmass. By the end of the weekend, the front is forecast to stall across the southern Mid-Atlantic/Southeast and spark additional thunderstorm development. While storms focus along the front to the north, Tropical Storm Fred should move into the Florida Keys by Saturday before turning north through the eastern Gulf of Mexico, producing heavy rain over Florida over the weekend. By Monday, Fred is expected to cross the Peninsula inland to the Southeast, pulling an influx of tropical moisture north as it weakens. However, the heavy rain threat will continue through at least midweek as tropical moisture surges north of the remnants of Fred and interacts with the stalled Mid-Atlantic/Southeast front. Moreover, a potential track for Fred along the eastern spine of the Appalachians could further enhance rainfall efficiency via orographic lift. All this being said, uncertainty remains regarding the track/timing details of Fred (and associated heavy rain); users are highly encouraged to monitor the latest NHC forecast information as the event draws near. Over the Southwest, the forecast remains on track for an uptick in monsoonal activity as above average precipitable water values surge into the region. The most active monsoon conditions are expected to abate beyond the 17th, although showers and thunderstorms below hazard levels tied to a weak upper low are likely. To the north in Montana, a progressive sweeping cold front will kick off showers and thunderstorms beginning Monday, although at this time the rainfall amounts do not appear to reach hazard levels. In fact, the activity will be beneficial in mitigating the ongoing drought over the region. Triple digit excessive heat is anticipated to linger for one more day over areas east of the Cascades on Saturday before the brutal heat relaxes by the end of the weekend with the arrival of the cold front. Before the relief arrives, however, heat indices ranging from 100-105 on Saturday support a very high heat risk to those outside. Accordingly, Excessive Heat Warnings cover the region until Saturday evening. Fortunately, post-frontal temperatures should be pleasant as cooler air moves in behind the front, and high temperatures 5-10 degrees below average are forecast by Tuesday. Ahead of the front in the Northern Plains, another shot of excessive heat is likely on Sunday beneath upper-level height rises forecast east of the Rockies. High temperatures 10-15 degrees above average are possible, with 100 degree heat indices before temperatures begin to moderate by Monday as the front approaches. Pleasant continental air behind the front, alongside clouds and precipitation provide refreshing temperatures beyond Monday, as temperatures 10-15 degrees below average are anticipated. For Alaska, another chance of heavy rain will overspread West-Central Alaska from Sunday-Monday as a frontal complex approaches. Much of Alaska is anticipated to persist in a wet period through next week, although the next risk of hazardous heavy rain is apparent from the 17th-18th from the East Kenai Peninsula to the Inside Passage. Ensemble guidance maintains reasonable agreement for higher rainfall totals there ahead of an approaching atmospheric river. Asherman