US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 443 PM EDT Tue Aug 10 2021 Valid Friday August 13 2021 - Tuesday August 17 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 Southwest, Sat-Tue, Aug 14-Aug 17. - Severe weather across portions of the Great Lakes, the Mid-Atlantic, the Northeast, the Central Appalachians, and the Ohio Valley, Fri, Aug 13. - Flooding possible across portions of the Central Plains and 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 likely across portions of the Southeast. - Flooding likely across portions of the Southeast. - Excessive heat across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Fri-Sun, Aug 13-Aug 15. - Excessive heat 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 Southeast, the Southern Plains, and the Ohio Valley, Fri, Aug 13. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri, Aug 13. - Heavy rain across portions of mainland Alaska, Sun-Mon, Aug 15-Aug 16. Detailed Summary: The medium range forecast period (August 13th to 17th) is characterized by an active period of late-Summer hazards around the U.S.: heavy rain, dangerous heat, tropical activity, and severe weather. The general pattern to begin the weekend will feature a deep upper-level trough swinging through the North-Central U.S., flanked by ridging over the Mid-Atlantic and Great Basin. By early next week, the pattern is forecast to shift with the western ridge weakening in response to height falls associated with troughing over the western and central sections of the country. At the surface, the main features will be a refreshing cold front forecast to push from the Midwest and Central Plains towards the East Coast before stalling over the Southeast and Deep South, as well as Potential Tropical Cyclone Six currently located South of Puerto Rico. On Friday, the aforementioned cold front is forecast to extend across the Great Lakes into the Central Plains and focus thunderstorm development as it interacts with a moist, unstable airmass. Accordingly, moderate to locally heavy rainfall is likely across the Northeast into the Ohio Valley, although confidence in widespread heavy rain is low. However, the Storm Prediction Center has highlighted a chance of severe weather on Friday across the Northeast and Ohio Valley, primarily for locally strong to severe wind gusts with storms that develop along and ahead of the front. As the cold front nears the Mid-Atlantic Saturday, it will spur more thunderstorms capable of packing intense rainfall rates before stalling across the Mid-Atlantic and Southeast on Sunday. Meanwhile, the current forecast track of Potential Tropical Cyclone Six suggests heavy rain impacting Florida beginning Saturday, before spreading north toward the Peninsula by Sunday. As Six moves inland early next week, heavy rain chances overspread the Southeast as tropical moisture feeds inland and interacts with the stalled front. However, a fair amount of uncertainty remains regarding the track and timing of Six as it nears the coast, in part due to forecast upper-troughing over the Southern Plains on days 5-6. Users are encouraged to monitor the latest forecast track and intensity information from the National Hurricane Center as Six approaches the U.S. In the Southwest, an uptick in monsoon activity is expected across parts of Arizona and southwest New Mexico this weekend into early next week. Total precipitable water amounts near 2 inches are possible and would produce a ripe environment for scattered thunderstorms and heavy rainfall. The Heavy Rain area over the Southwest was extended until the Tuesday given the consensus for continued monsoon activity in the region. Another hazard to contend with for certain parts of the country will be excessive heat. Once again, an anomalous upper-level ridge over the Pacific Northwest will produce a scorching heat wave between Friday and Sunday. Highs into the upper 90s and low triple digits will impact highly populated regions east of the Cascade mountains through Saturday, as well as the interior Northwest through the end of the weekend. The mid-August heat will also be felt farther south into northern California and Nevada. Temperatures are expected to reach upwards of 20 degrees above average and break several daily high temperature records. The good news is that this heat wave is expected to relax by early next week as above average temperatures shift into the northern High Plains. For the Mid/Deep South, lingering hot weather and oppressive humidity is forecast on Friday before a cold front brings much-needed relief. Heat indices should exceed 100 degrees both days and approach 100-105 during the peak heating hours of the afternoon. These high heat indices may continue across the Gulf Coast through Saturday, but temperatures are currently forecast to remain close to average. Last, but certainty not least, excessive heat remains a concern throughout the densely populated Mid-Atlantic and Northeast between Richmond, VA and Portland, ME on Friday. Heat indices are expected to reach near 105 degrees, with low temperatures only dipping into the mid-70s. For Alaska, heavy rain associated with a potent atmospheric river event is anticipated to end the week over parts of the southeast mainland and northern panhandle. An impressive surge of moisture that reaches southwest toward ongoing typhoon activity in the west-central Pacific could lead to multiple inches of rain across both coastal and mountainous regions. Another chance of heavy rain will overspread West-Central Alaska beginning Sunday as a frontal complex approaches. The Heavy Rain area may need extended further north in subsequent updates, but have kept the area relatively confined given uncertainty regarding the northern extent of higher rainfall totals. Asherman/Snell