US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 308 PM EDT Mon Sep 21 2020 Valid Thursday September 24 2020 - Monday September 28 2020 Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Northern Great Basin, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Pacific Northwest, and the Ohio Valley, Thu-Sat, Sep 24-Sep 26. - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Mon, Sep 28. - Heavy rain across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Sun-Mon, Sep 27-Sep 28. - Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding occurring or imminent across portions of the Southeast. - Flooding likely across portions of the Southeast. - Much above normal temperatures across portions of California and the Pacific Northwest, Sun-Mon, Sep 27-Sep 28. - Much above normal temperatures across portions of the Central Plains, the Northern Plains, the Central Rockies, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Southern Plains, Thu-Fri, Sep 24-Sep 25. - Much above normal temperatures across portions of the Central Plains and the Northern Plains, Thu, Sep 24. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Fri-Sat, Sep 25-Sep 26. Detailed Summary: The medium range period (Thursday, Sept 24 - Monday, Sept 28) features a jet stream pattern that becomes increasingly amplified heading into the weekend and early next week. To start the period, remnants of Tropical Storm Beta look to pass over the Southeast late week causing heavy rainfall to envelope the region. Much of the region highlighted to be at risk, especially the southern Appalachians, could witness areas of flash, urban, and river flooding due to potentially excessive rainfall rates occurring across areas of overly saturated soils. Farther west, an upper level trough tracking into British Columbia will direct a swath of Pacific moisture at the Pacific Northwest. The Olympics and Cascades in particular can expect the heaviest rainfall totals due to prolonged upslope flow. While the dousing of heavy rain is sorely needed over the drought-stricken and wildfire plagued Pacific Northwest, burn scarred areas that witness heavy rainfall rates could be subject to localized flooding and debris flows. In the northern Plains and Midwest, abnormally warm conditions are expected as high temperatures looks to range between 10-20 degrees above normal. The heat is only expected to last a couple days at most before a more seasonally cool air-mass arrives this weekend and into next week. It is the latter half of the forecast period (this weekend into next Monday) where the jet stream pattern from the north Pacific to the eastern Canadian maritime becomes increasingly amplified. Latest ensemble guidance shows an intense subtropical high near California on Saturday arching north towards the Pacific Northwest on Sunday, then reaching southwest Canada by next Monday. This results in a dip in the jet stream pattern over the eastern U.S., forcing a cooler regime to develop in the Mississippi Valley and a wetter pattern along the East Coast by days 6-7. Temperatures are not cold enough to result in a much below normal temperature area at this time, but there is a signal for heavy rain over portions of the Northeast and the central/southern Appalachians. Meanwhile, under the expanding dome of high pressure aloft, temperatures are forecast to reach much above normal levels along the West Coast. Much above normal temps could expand more into the Pacific Northwest and Desert Southwest in future forecasts. In addition, strengthening high pressure over the Great Basin and Intermountain West may lead to increased wildfire risk due to dry conditions, low humidity levels, and the potential for high wind gusts. Southern California could contend with a Santa Ana wind event late this weekend into early next week should recent trends in model guidance persist. There is no let up in the active weather pattern across southern Alaska to start the medium range period as another potent storm system crosses the Gulf of Alaska and heads for the Upper Panhandle Friday into Saturday. A stream of atmospheric moisture will head for the southern Gulf Coast and the Upper Panhandle, leading to the development of heavy valley/coastal rain and mountain snow. There is a chance for more heavy rainfall in the Panhandle to close out the weekend and into early next week, but latest ensemble model trends suggest the ridge over the western U.S. may crest into the Panhandle by days 6-7. Combined with increased model spread in 500mb heights over the Gulf of Alaska, confidence in precipitation forecasts in the latter half of the period is low for the time being. Temperatures do not appear to reach hazardous levels but expect a warmer regime across the eastern two-thirds of the mainland while the western third and Aleutians witness below average temperatures. Mullinax