US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 403 PM EDT Wed Aug 21 2019 Valid Saturday August 24 2019 - Wednesday August 28 2019 Hazards: - Heavy rain across portions of the central Gulf Coast, the northern Plains into the upper Midwest, as well as portions of the Southeast into the southern Appalachians, Sat-Sun, Aug 24-Aug 25. - Heavy rain across portions of the upper Midwest into upper Great Lakes, Mon-Tue, Aug 26-Aug 27. - Heavy rain across portions of the Midwest into the Upper Mississippi Valley, Tue-Wed, Aug 27-Aug 28. - Heavy rain possible over the Florida Peninsula, Sat-Tue, Aug 24-Aug 27. - Flooding possible across portions of the central Plains and the middle Mississippi Valley. - Flooding occurring or imminent across portions of the Southeast and the northern Plains. - Excessive heat across portions of western Oregon, Tue-Wed, Aug 27-Aug 28. - Excessive heat across portions of interior valleys of California, Sat-Wed, Aug 24-Aug 28. - Excessive heat across portions of southern Texas, Sat-Sun, Aug 24-Aug 25. - Excessive heat across portions of northern Texas into southern Oklahoma, Sun-Mon, Aug 25-Aug 26. - Much above normal temperatures across the southern High Plains, Sun-Mon, Aug 25-Aug 26. - Heavy rain across southeastern Alaska, Sat-Sun, Aug 24-Aug 25. - Heavy rain across portions of the southern mainland Alaska, Wed, Aug 28. Detailed Summary: A general zonal upper-level flow pattern across the U.S. mainland is forecast to become more amplified by the middle of next week. In terms of temperatures, summer heat is expected to continue across the southern tier while a couple of significant intrusions of cool air are anticipated for the northern Plains eastward into the northeastern U.S. In terms of rainfall, latest model guidance shows an increasing signal for heavy rain to impact portions of the Dakotas eastward into the upper Midwest during the weekend ahead of a developing low pressure system. The heavy rain threat will then shift toward the upper Great Lakes by next Monday, and down into the Midwest Tuesday and Wednesday ahead of a strong cold front. There are some uncertainties regarding the strength of the low pressure system and how fast the cold front will advance into the Plains and the Midwest. These factors will influence the locations of heavy rain in this region. Farther south, lingering tropical moisture will keep a good chance of showers and thunderstorms from the central Gulf Coast, across the interior Southeast, and then toward the southern Mid-Atlantic during the medium-range period. Additional moisture associated with a tropical wave will lead to a higher chance of heavy rain over the central Gulf Coast region during the weekend. Heavy rain will also be possible during the weekend across the Carolinas into the interior Southeast and southern Appalachians, as the boundary of a cool Canadian air mass edges southward across the Mid-Atlantic. Global models have begun to indicate a possible tropical cyclone forming off the southeast U.S. coast during the weekend. This could increase the chance of heavy rain over the Florida Peninsula even though we are ruling out the direct impacts of heavy rain along the Carolina coast early next week. Over the western U.S., global models indicate that an upper-level ridge will begin to build from the eastern Pacific into the West Coast early to middle of next week. This will lead to a heat wave to build into the West Coast, especially across the interior valleys where excessive heat is possible. Farther south and east across the southern High Plains, down slope winds will warm temperatures to more than 10 degrees above normal Sunday into Monday, although these temperatures may not reach excessive heat criteria. However, excessive heat is forecast for southern Texas for the weekend as afternoon heat indices are expected to exceed 110 degrees. In addition, excessive heat is becoming more likely over northern Texas and southern Oklahoma ahead of an approaching cold front Sunday and Monday. Across southeastern Alaska, heavy rain is expected through the weekend as an occluded cyclone (the partial remnants of Tropical Storm Krosa from the western Pacific last week) pushes onshore. Daily rainfall amounts could reach 2 inches in locations across parts of the Alaska Peninsula. This will be welcome rainfall for the drought-stricken region. However, the dry lands over the complex terrain could make it more susceptible to flash flooding. The next occluded cyclone is forecast to bring heavy rain farther north across the southern coast of mainland Alaska by next Wednesday. Finally, global models are indicating the possibility of a weakening tropical cyclone passing at some distance south of Hawaii by early to middle of next week. Impacts appear to be minimal at this point in time. Kong