US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 430 PM EDT Wed Sep 02 2020 Valid Saturday September 05 2020 - Wednesday September 09 2020 Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Southern Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Great Lakes, Mon-Tue, Sep 7-Sep 8. - Heavy rain across portions of the Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Sat, Sep 5. - Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains. - Flooding occurring or imminent across portions of the Mid-Atlantic, the Lower Mississippi Valley, and the Southern Plains. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Sat-Mon, Sep 5-Sep 7. - Excessive heat across portions of California, Sat-Tue, Sep 5-Sep 8. - Much above normal temperatures across portions of California, the Pacific Northwest, and the Northern Great Basin, Sat-Wed, Sep 5-Sep 9. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat-Mon, Sep 5-Sep 7. - Much below normal temperatures across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, and the Northern Great Basin, Tue-Wed, Sep 8-Sep 9. - Much below normal temperatures across portions of the Central Plains, the Central Rockies, the Northern Plains, the Southern Rockies, the Upper Mississippi Valley, and the Southern Plains, Wed, Sep 9. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sun-Tue, Sep 6-Sep 8. - Heavy rain across portions of mainland Alaska, Sun-Mon, Sep 6-Sep 7. - High winds across portions of mainland Alaska, Sun-Mon, Sep 6-Sep 7. Detailed Summary: The medium range period (Sept. 5-9) can be summarized by an expansive upper level ridge along the west coast of North America, an amplifying downstream trough over the Canadian Prairies and central U.S., and a far reaching ridge across the North Atlantic. The resulting temperature disparity across the Lower 48 will be quite pronounced with more autumn-like temperatures making their first appearance of the season in the northern High Plains and Rockies. A few locations may even have a shot of seeing patchy frost but most areas should, at minimum, experience much below temperatures. From a hazards perspective, it is the Desert Southwest and throughout California's Great Valley that stand out most with record breaking high temps likely this weekend and into early next week. The Northwest will also contend with hotter than normal conditions as daily highs make a run at triple digits in the Willamette Valley Sunday and Monday. How long the heat wave persists in the West is likely to be determined based on the position of an intense shortwave trough diving into the Plains early next week. Temperatures should cool down by day 7 in the Intermountain West but the heat may linger along the immediate West Coast. The dome of upper level high pressure over the West unfortunately leads to favorable conditions for ongoing wildfires to rage and potentially the start of new fires. There is the potential by the middle of next week for an upper low over the Great Basin that could minimize the fire weather risk. Expect wildfire smoke and poor air quality to continue throughout the period. As the pattern becomes increasingly amplified, the threat for heavy rainfall becomes more apparent across the Nation's Heartland. To start the period, a potent surface low tracking across the Midwest should trigger thunderstorms and showers that could contain excessive rainfall rates. As the upper level ridge builds across the West this weekend, a developing area of low pressure tracking across southern Canada will generate showers and thunderstorms across the Midwest and Great Lakes with heavy rain possible in the Upper Peninsula of Michigan. By the start of next week, an intense upper trough traverses the central U.S. and tap into Gulf moisture. This will lead to a swath of heavy rainfall that could extend from the southern Plains to the Great Lakes. There is also the potential for heavy rainfall in the High Plains and central Rockies, but model spread remains high due to the unclear position of the upper level disturbance. A heavy rainfall hazard may be required should guidance suggest the trough aloft sets up farther west. In fact, enough cold air is in place that the first measurable snowfall of the season could occur in the higher elevations of the Rockies. The theme of an amplified jet stream pattern also exists over the North Pacific, meaning Alaska will also contend with hazardous weather. A formidable storm system is forecast to track across the Aleutians Sunday and then reach the Bering Sea on Monday, resulting in soaking rainfall from the Aleutians to the Panhandle. This storm contains rich subtropical moisture, thus increasing the potential for excessive rainfall totals. Multiple inches of rainfall are possible (heaviest amounts in the higher elevations) along with gusty winds and rough seas along the coast. speaking of gusty winds, winds could become strong over southwest Alaska, thus leading to the issuance of a high wind hazard. Heavy rainfall may linger into next Tuesday but it depends upon how long the fetch of subtropical moisture can persist. The occluded storm system is forecast to take its time leaving northern Alaska Tuesday and Wednesday, keeping a mix of rain and snow showers in the forecast into mid-week. Mullinax