US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 325 PM EDT Wed Aug 25 2021 Valid Saturday August 28 2021 - Wednesday September 01 2021 Hazards: - Flooding occurring or imminent across portions of the Mid-Atlantic. - Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Sat-Sun, Aug 28-Aug 29. - Severe weather across portions of the Central Plains, the Upper Mississippi Valley, and the Great Lakes, Sat, Aug 28. - Heavy rain across portions of the Lower Mississippi Valley, Sun-Wed, Aug 29-Sep 1. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Mon-Wed, Aug 30-Sep 1. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat, Aug 28. - High winds across portions of mainland Alaska, Sat, Aug 28. - Much below normal temperatures across portions of mainland Alaska, Sat, Aug 28. - High significant wave heights for coastal portions of mainland Alaska, Sat, Aug 28. - Heavy rain across portions of mainland Alaska, Wed, Sep 1. Detailed Summary: During the medium range (Saturday, August 28th - Wednesday, September 1st) the upper-level pattern over the CONUS is forecast to at first be characterized by ridging in the east and troughing over the northern Rockies/Plains 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 Saturday, a low pressure wave over South Dakota with an eastward extending warm front is expected to lift into Minnesota. Abundant atmospheric moisture will likely be available in this region, allowing any convective precipitation that forms along and ahead of the warm front to produce heavy rain with accumulations between 1 and 2 inches in 24 hours from northern Wisconsin to the Michigan Upper Peninsula 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 Rockies and lift over the Northern Plains/Upper Mississippi Valley, ushering a cold front at the surface through the Upper Midwest. As storms organize along this frontal boundary and supply the region with its second bout of heavy rain, antecedent favorable environmental conditions such as enhanced wind shear could allow for some of these storms to become severe. Widespread accumulations of 1 inch with isolated higher amounts in 24 hours will be possible along the Minnesota/Wisconsin border, putting more stress on the already saturated soils from the previous days' rain. The aforementioned high pressure aloft over the eastern US will allow highs around the Great Lakes and 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, upper-level troughing will likely keep high temperatures 5 to 10 degrees below normal in the northern Rockies/Plains on Saturday. By the end of the period daily highs will return to or close to normal while lows will still hover marginally above normal. Shifting focus to the tropics, the National Hurricane Center is currently keeping an eye on an area of low pressure that is expected to develop into a tropical depression or tropical storm over the weekend either in the northwest Caribbean Sea or southwest Gulf of Mexico. Current guidance suggests that the potential tropical cyclone could bring heavy rain and gusty winds to coastal Louisiana beginning as early as Sunday night. As the storm moves closer to the coast, areal coverage of the associated high winds and rainfall will likely spread out in both directions, extending from eastern Texas to Mississippi by Monday evening. Once inland, the tropical cyclone's impacts may advance north into Arkansas Tuesday and the Tennessee and Ohio Valleys by Wednesday. While there is still a large amount of uncertainty surrounding the exact path the storm will take there is consensus that wherever the potential tropical cyclone ends up there will be a significant amount of rain. At this time a broad heavy rain contour has been drawn along and inland of the Gulf Coast to highlight the areas of potential impact, however, throughout the course of the week this area will likely be adjusted and refined to match the most up to date guidance. Over Alaska, the beginning of the medium range period is forecast to be active as two systems impact the state simultaneously from both the north and the south. In the northwest a low pressure wave and its associated shortwave trough over the East Siberian Sea is expected to encroach on two areas of high pressure over the central part of the state on Saturday causing the pressure gradient over the western Far North to tighten. As a result, high winds with gusts up to 30 knots are expected to impacting the region and contribute to the development of significant waves on the coastline extending from Point Barrow to the Kotzebue Sound. As the shortwave trough moves out of the East Siberian Sea and into the Far North on Saturday night a moderate amount of mixed precipitation will also be possible over the Lisburne Peninsula and Brooks Range, however, it is not anticipated to be hazardous. To the south, an occluded system that is forecast to move through the Gulf of Alaska late this week will reach the Alaska Panhandle on Saturday. As the system continues its approach and moves onshore it is expected bring heavy rain the Panhandle and the southeastern mainland, with accumulation totals in excess of 2 and, in some areas, 2.5 inches in 24 hours. Though the brunt of the rain is expected to fall on Saturday, some late rainfall may linger into the early hours of Sunday morning. During the latter half of the period another occluded system is expected to impact the state, this time moving through and bringing moderate amounts of rain to the Aleutian Islands early next week as it makes its way toward the mainland. Once the system reaches the Alaska Peninsula, rainfall totals are forecast to ramp up significantly, drenching the upper Peninsula, Kodiak Island, the Kenai Peninsula, and areas surrounding the Cook Inlet with 2 to 3+ inches of rain in 24 hours on Wednesday. Temperature-wise, at the beginning of the period daily highs and lows are expected to be 10 to 15 degrees below normal across much of the mainland due to a cold front that is forecast to pass through the state on Friday. On Sunday, however, temperatures will begin to steadily climb, bringing highs and lows back or close to normal by Tuesday. Zavadoff