US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 323 PM EDT Tue Aug 24 2021 Valid Friday August 27 2021 - Tuesday August 31 2021 Hazards: - Flooding occurring or imminent across portions of the Mid-Atlantic and the Northeast. - Flooding likely across portions of the Mid-Atlantic. - Excessive heat across portions of California and the Southwest, Fri, Aug 27. - Heavy rain across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Fri-Sun, Aug 27-Aug 29. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sun-Tue, Aug 29-Aug 31. - High winds across portions of mainland Alaska, Fri-Sat, Aug 27-Aug 28. - Much below normal temperatures across portions of mainland Alaska, Fri-Sat, Aug 27-Aug 28. - High significant wave heights for coastal portions of mainland Alaska, Fri-Sat, Aug 27-Aug 28. - Heavy rain across portions of the Alaska Panhandle, Sat-Sun, Aug 28-Aug 29. Detailed Summary: During the medium range (Friday, August 27th - Tuesday, August 31st) the upper-level pattern over the CONUS is forecast to at first be characterized by ridging in the east and troughing over the Northwest before a more zonal regime emerges towards the latter half of the period. Coupled with low pressure/frontal systems at the surface, this troughing aloft will aid in the development in multiple rounds of heavy rain over the Upper Midwest throughout the weekend. Beginning on Friday, a low pressure wave over Nebraska with an eastward extending warm front is expected to lift into the Upper Mississippi Valley. Abundant atmospheric moisture will likely be available in this region, allowing any convective precipitation that forms along and ahead of the northward moving warm front to produce heavy rain with accumulations between 2 and 3 inches in 24 hours from eastern Minnesota to the Michigan Upper Peninsula Friday into Saturday. In spite of relatively dry soils and near normal precipitation throughout the Upper Midwest, these high rainfall totals will make localized flash and urban flooding a concern. Saturday into Sunday the upper-level trough will propagate out of the Northwest and lift over the Northern Plains, ushering an occluded system at the surface from the Northern Plains into the Upper Midwest, bringing the region its second bout of heavy rain. Widespread accumulations of 1+ inches in 24 hours will be likely from the Dakotas to the Michigan Upper Peninsula, putting more stress on the already saturated soils from the previous days' rain. The aforementioned high pressure aloft over the eastern US will allow daily highs from the Central Plains to the Mid-Atlantic to sit 5 to 10 degrees above normal throughout the weekend, while daily lows will reach 10 to 15 degrees above average. Conversely, during the same time period upper-level troughing will likely keep high temperatures 5 to 10 degrees below normal in the Northwest. While not a large deviation from normal, highs in extreme southwestern California and the Desert Southwest are forecast to be sweltering, reaching between 110 and 115 degrees on Friday. Residents of these regions, which includes the Phoenix metro area, will be exposed to excessive heat, putting them at risk for heat-related illness. To lessen the impact of extreme heat, residents should stay indoors, seek air-conditioned buildings, avoid prolonged outdoor exposure, and stay hydrated. Shifting focus to the tropics, the National Hurricane Center is currently keeping an eye on an area of low pressure that could develop into a tropical depression over the weekend either in the northwest Caribbean Sea or southwest Gulf of Mexico. Current guidance suggests that a tropical cyclone may form and bring heavy rain to coastal Texas and Louisiana beginning Sunday night into Monday morning before moving further inland. While there is uncertainty in the models surrounding the exact location of the heavy rain along the western Gulf Coast, there is consensus that wherever the potential tropical cyclone hits there will be a significant amount of rain. At this time a broad heavy rain contour has been drawn in Texas and Louisiana to highlight the areas of potential impact, however, throughout the course of the week this area will likely be refined to match the most up to date guidance. Over Alaska, the first half of the medium range period is forecast to be active as two systems move in on the state simultaneously from both the north and the south. Focusing on the northwest, on Friday an occluded system is expected to pass north of Alaska over the Beaufort Sea as its trailing cold front propagates west to east through the state. Behind the front and ahead of a shortwave trough over the East Siberian Sea, high winds with gusts up to 30 knots will bring windy conditions to the western Far North and significant waves to the coastline extending from Point Barrow to the Kotzebue Sound on Friday and Saturday. In addition to driving the high winds and significant waves the cold front is forecast to drop temperatures 10 to 15 degrees across the central mainland throughout the weekend, keeping highs generally in the 50s and 60s and lows in the 30s at lower elevations. As the previously mentioned shortwave trough moves out of the East Siberian Sea and into the Far North over the weekend a moderate amount of mixed precipitation will also be possible over the Lisburne Peninsula and Brooks Range, however, it is not expected to be hazardous. To the south, an occluded system is forecast to move through the Gulf of Alaska on Friday before reaching the Alaska Panhandle on Saturday. As the system approaches the region and moves onshore it is expected bring heavy rain in excess of hazardous levels to the Panhandle, with accumulation totals between 2 and 2.5 inches in 24 hours likely throughout the weekend. Zavadoff