US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 455 PM EDT Tue Sep 14 2021 Valid Friday September 17 2021 - Tuesday September 21 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, Fri-Sun, Sep 17-Sep 19. - 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 Northern Plains and the Northern Rockies, Sat, Sep 18. - 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. - Much below normal temperatures across portions of mainland Alaska, Mon-Tue, Sep 20-Sep 21. Detailed Summary: The medium range forecast period (Friday, Sept. 17 to Tuesday, Sept. 21) 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 an anomalously cool weather pattern across the Pacific Northwest beginning Friday, which is forecast to expand through the Great Basin and Northern Plains by early next week with the passage of the upper-trough and associated cold front. Cool temperatures 10-15 degrees below normal can be expected over the aforementioned areas as high temperatures in the 60's take hold behind the front. Chilly nighttime temperatures could drop into the 40's throughout the northern Great Basin and Northern Rockies, with localized cooler temperatures in areas of elevation. Ahead of the front, gusty winds are possible throughout the Great Basin and into parts of the northern/central Rockies. An additional High Winds area was added for Saturday over western Montana, where strong winds aloft may reach the surface to produce hazardous wind gusts. East of the approaching trough, widespread above-average temperatures will extend from the Southern Plains, across the Midwest, and into the Northeast. A focused area for anomalous warmth (15-20 degrees above average) will reside beneath the building upper-ridge from the High Plains to the Upper Midwest over the weekend, which equates to high temperatures into the upper 80's and low 90's. Afterwards, the warm pattern over the Central U.S. should moderate to around seasonal by early next week with the frontal passage. No highlighted areas were added to communicate the most anomalously warm temperatures over the North-Central U.S. since they are likely to fall below heat advisory criteria and are not expected to threaten many daily records. Beyond Monday, the bulk of the anomalous warm temperatures (10 degrees above average) should be relegated to the Upper Midwest, Northeast and Mid-Atlantic. Temperatures aside, an active wet period is likely over the Pacific Northwest beginning Friday ahead of a strong early season atmospheric river event. The aforementioned upper-level trough and associated cold front will interact with a plume of Pacific moisture to produce moderate to locally heavy rain across parts of western Washington and Oregon over the weekend. Widespread showers and moderate rain should also reach into parts of northern Idaho and northwest Montana this weekend. Locally higher rainfall amounts may be possible in areas where upslope enhancement occurs, and snow is possible over higher elevation areas in the Cascades and Northern Rockies. 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 locally heavy rainfall over the region through at least the weekend. Additional shower and thunderstorm activity is possible over the region going into next week as tropical moisture seeps into the Southeast, which will be monitored in subsequent Hazardous Weather updates. Over the Northern Tier, moderate to locally rainfall is possible as showers and thunderstorms form along and ahead of the advancing cold front late this weekend into early next week. However, a fair amount of uncertainty remains as to the placement and magnitude of highest accumulations. Coupled with dry antecedent conditions over the region, have decided to hold off on a Heavy Rain area for now. For Alaska, the next chance of active weather presents early next week as the extratropical remnants of current Tropical Storm Chanthu move east across the Gulf of Alaska, while a blast of cold air approaches behind a cold front surging south through Interior Alaska. Moist southwesterly flow ascending the terrain across the Panhandle will produce heavy rain, while snow/mixed precipitation appears more likely closer to Prince William Sound closer to the cold airmass. Accordingly, have a added a Heavy Precipitation area to account for the uncertainty for precipitation type, although this may need adjusted on future Hazardous Weather updates as forecast guidance still differs with placement of higher accumulations further north. Moreover, cold advection behind the front over Interior Alaska will keep temperatures 15-20 degrees below average beginning on the 20th, supporting the need for a Much Below Normal Temperatures area. Asherman/Snell