US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 429 PM EDT Tue Jul 28 2020 Valid Friday July 31 2020 - Tuesday August 04 2020 Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Southern Plains, and the Ohio Valley, Fri-Sat, Jul 31-Aug 1. - Heavy rain across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Tue, Aug 4. - Heavy rain across portions of the Southeast, Sun-Tue, Aug 2-Aug 4. - Heavy rain across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Sun, Aug 2. - Heavy rain across portions of the Mid-Atlantic, Fri, Jul 31. - Flooding possible across portions of the Central Plains and the Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Northern Plains. - Flooding likely across portions of the Central Plains. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Fri-Tue, Jul 31-Aug 4. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Sun, Aug 1-Aug 2. Detailed Summary: In the medium range period (Friday, July 31 - Tuesday, August 4), precipitation hazards will be the predominant concern during the period thanks to a long-wave upper trough stationed over the east-central U.S. and the ever present Bermuda ridge. The periods starts with an upper trough over the Middle Mississippi Valley and a stationary front from the Ohio Valley to the Mid-Atlantic. Ample moisture associated with a wave of low pressure along the front is expected to generate heavy showers and storms from the South Central U.S. and Mid-South on east to the Mid-Atlantic coast on Friday. As the frontal boundary over the Mid-Atlantic lifts northward as a warm front this weekend, the threat for heavy rain will move north towards the Northeast. There could be a threat for heavy thunderstorms over the northern Mid-Atlantic late Saturday into Sunday, but the amounts and where the heaviest activity takes shape remains unclear. Farther north, however, the warm front will lift as far north as the Northeast which will also be centered under the right entrance region of a jet streak over southeast Canada. This makes parts of the eastern Great Lakes and Northeast susceptible to periods of heavy rain and the potential for flash flooding. The forecast track of Potential Tropical Cyclone Nine will become an increasingly important in determining the weather hazards over not just the Southeast but much of the East Coast. The latest forecast shows the storm tracking towards South Florida but with the potential to track as far south and west as the Florida Straits and eastern Gulf of Mexico, or as far east as off the Southeast coast. Latest guidance suggests the threat for heavy rainfall begins to reach far southern Florida as early as the Saturday, but more likely sometime Sunday. There is also a threat for high winds but that is heavily dependent upon the strength of PTC Nine, which is far from certain at this time. How long the storm remains near Florida is up to the steering flow pattern between the long-wave trough in the east-central U.S. and the strong Bermuda ridge. Heavy rainfall could take shape over Florida into the first half of next week. Regardless, a SW to NE oriented jet streak and low level southerly flow should allow for tropical moisture to stream northward into the Southeast and Mid-Atlantic. The concern then would be for areas of heavy showers and storms capable of producing torrential downpours. Those with interests from the Gulf of Mexico up the East Coast should continue to monitor forecast advisories from the National Hurricane Center for the most up-to-date intensity and track forecasts. Meanwhile, an upper-level ridge will build across not only the western U.S but through much of western North America. Temperatures are expected to be above 110 degrees in the Desert Southwest through early next week as the center of the ridge stations itself over the region. Abnormally hot conditions should extend north through the Great Basin and into the Intermountain West, but current forecasts indicate temperatures falling just short of excessive heat criteria. The regions with the odds of perhaps contending with the most anomalously hot conditions could be in the rain shadow areas of eastern Washington and the northern High Plains. Elsewhere, the Southeast and Mid-Atlantic will continue to deal with heat indices ranging from 100-105 with some 110 degree heat indices also possible. Over Alaska, an occluded storm system associated with a remarkably deep upper trough for late July/early August in the Gulf of Alaska is forecast to advect copious amounts of moisture northward into the Panhandle and southeast mainland this weekend. While rainfall totals likely fall just short of heavy rain criteria in the Panhandle, it is the southeast mainland that could pick up anywhere from one to three inches of rainfall with locally heavier amounts possible. The pattern remain active into early next week as guidance hints at yet another potent upper trough and resulting frontal system near the Aleutians and Bering Sea making its way across the southern tier of the state early next week. There is likely to be plenty of moisture with this system as well, but details on amounts are still murky due to uncertainty in forecast track, speed, and intensity of the upper level system. Mullinax