US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 446 PM EDT Wed Sep 15 2021 Valid Saturday September 18 2021 - Wednesday September 22 2021 Hazards: - Heavy rain across portions of the Southern Plains, the Lower Mississippi Valley, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sat-Sun, Sep 18-Sep 19. - Flooding possible across portions of the Lower Mississippi Valley. - High winds across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, and the Pacific Northwest, Sat-Sun, Sep 18-Sep 19. - High winds across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Southern Rockies, the Southern Plains, and the Southwest, Mon, Sep 20. - High winds across portions of the Northern Plains and the Northern Rockies, Sat, Sep 18. - High winds across portions of the Central Plains and the Northern Plains, Tue, Sep 21. - 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, Mon-Tue, Sep 20-Sep 21. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Sep 20-Sep 21. - Heavy rain across portions of the Alaska Panhandle, Mon-Tue, Sep 20-Sep 21. - High winds across portions of mainland Alaska and the Aleutians, Sat, Sep 18. - Much below normal temperatures across portions of mainland Alaska, Mon-Tue, Sep 20-Sep 21. Detailed Summary: The medium range forecast period (Saturday, Sept. 18 to Wednesday, Sept. 22) features an amplifying upper-level pattern as a strong trough establishes over the Intermountain West, while a downstream ridge over the Four Corners builds as it migrates toward the Mid-Atlantic. Accordingly, expect Fall-like temperatures to spread across the Pacific Northwest, Four Corners, and northern High Plains by midweek with the passage of the upper-trough and associated cold front. Very cool temperatures 15 to 20 degrees below normal are possible as mild Canadian air settles in behind the front, with high temperatures in the 50's and 60's anticipated from the Pacific Northwest to the northern High Plains. Chilly nighttime temperatures could drop below freezing from the Upper Colorado Basin to the northern Rockies early next week, especially in areas of elevation and low lying valleys. Appreciable ensemble probabilities for sub 32 degree overnight temperatures on the 20th, coupled with favorable climatology for a regional first freeze date prompted the introduction of the Much Below Normal Temperatures area to begin next week. Ahead of the front, gusty winds are possible throughout the Great Basin and into parts of the northern/central Rockies beginning Saturday, which will spread toward the Rockies by midweek as the upper-trough and frontal system crosses the Continental Divide. Before the cool air reaches the Central U.S. however, a plume of very warm air (15 to 20 degrees above average) will overspread the Northern Plains and Upper Midwest within a warm advection regime beneath the building upper-ridge, which equates to high temperatures in the upper 80's and low 90's. Despite the unseasonable warmth in the forecast, will continue to keep a highlighted area off the map as highs will likely fall below heat advisory criteria. Past Tuesday, most anomalous warm air should be relegated to the northeastern quadrant of the CONUS, given the anticipated progression of the frontal system and cool high pressure behind it. Temperatures aside, an active wet period is likely over the Pacific Northwest as a strong early season atmospheric river event is anticipated to last through the weekend. Moderate to locally heavy rainfall is possible across western Washington and Oregon through Monday as the aforementioned upper-level trough and frontal system ushers in an impressive Pacific moisture plume. The rain should be mostly beneficial for the region in combating drought conditions, and dry soils should limit widespread flood concerns. However, isolated flooding issues are possible in areas where enhanced rainfall rates tied to favorable terrain can materialize. As the moisture plume advects further downstream, widespread showers and moderate rain should fall over portions of eastern Washington, northern Idaho, and northwest Montana this weekend. Similar to above, the rain should be largely beneficial to partially abate drought conditions, although isolated flood issues remain possible. Moreover, snow remains possible over higher elevations in the Cascades and Northern Rockies through the weekend. Along the western Gulf Coast, tropical moisture left over from Nicholas may interact with a coastal trough and weak upper-low to maintain a threat of heavy rainfall over the region through at least the weekend. Confidence in the specifics of a heavy rain threat is low as these forcing mechanisms appear weak, but with a tropical airmass overhead and saturated antecedent soils in the wake of Nicholas and Ida, it would not take much to cause more flood problems in the region. Over the Northern Plains/Upper Midwest, showers and thunderstorms are possible ahead of the eastward surging front beyond Sunday. Some forecast guidance highlights higher convective totals ahead of the front, but too much spread remains regarding rainfall amounts and placement to introduce a Heavy Rain area. The progressive nature of the system should limit the overall heavy rain threat as well. For Alaska, the next chance of active weather presents on Saturday over the Gulf of Alaska and Aleutians as the approach of a deep low pressure system produces strong gradient wind gusts in the area, although no hazardous heavy rain is anticipated. Early next week moisture streaming off current Tropical Storm Chanthu will interact with a strong approaching low pressure area moving across the Gulf, while a shot of cold air approaches behind a cold front surging south through Interior Alaska. Moist southwesterly flow ascending the terrain across the Alaska Panhandle will produce heavy rain, while snow/mixed precipitation appears more likely closer to Prince William Sound closer to the cold airmass. Cold advection behind the front over Interior Alaska will also keep temperatures at least 10 to 15 degrees below average beginning on the 20th. Guidance has struggled with the magnitude and duration of the cold air that materializes, but current forecast temperatures well below average still support a Much Below Normal Temperature area. Asherman