US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 509 PM EDT Thu Oct 07 2021 Valid Sunday October 10 2021 - Thursday October 14 2021 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sun-Mon, Oct 10-Oct 11. - Heavy precipitation across portions of the Central Rockies, the Central Plains, the Northern Plains, and the Northern Rockies, Tue-Wed, Oct 12-Oct 13. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Sun, Oct 10. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Southern Plains, the Upper Mississippi Valley, and the Northern Plains, Tue, Oct 12. - Heavy rain across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Tue-Wed, Oct 12-Oct 13. - Heavy snow 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, Oct 12-Oct 13. - Heavy snow across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Mon-Tue, Oct 11-Oct 12. - Severe weather across portions of the Central Plains, the Lower Mississippi Valley, and the Southern Plains, Sun, Oct 10. - Severe weather across portions of the Southern Plains, Tue, Oct 12. - Flooding possible across portions of the Great Lakes. - Flooding possible across portions of the Great Lakes. - Flooding occurring or imminent across portions of the Southeast and the Tennessee Valley. - Much below normal temperatures across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Northern Plains, the Northern Rockies, the Southern Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Tue-Thu, Oct 12-Oct 14. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Mon-Wed, Oct 11-Oct 13. - Heavy precipitation across portions of the Alaska Panhandle, Tue-Wed, Oct 12-Oct 13. - Heavy rain across portions of the Alaska Panhandle, Tue-Wed, Oct 12-Oct 13. - High winds across portions of mainland Alaska and the Aleutians, Sun-Tue, Oct 10-Oct 12. Detailed Summary: The medium range forecast period (Sunday, October 10th - Thursday, October 14th) features heavy rain and severe weather over the Central/Southern Plains to close out the weekend, before a strong storm system swings across the Four Corners into the Plains beginning Monday. A plethora of hazardous weather, including winter precipitation, severe weather, heavy rain, and cool temperatures are likely across the West and Central U.S. with the impending system. Hazardous weather in Alaska also remains active with another shot of high winds along the Alaska Peninsula beginning Monday, and additional heavy rain/mixed precipitation through midweek. Beginning Sunday, heavy rain and severe weather are likely across the Central and Southern Plains ahead of a lead shortwave trough and frontal system ejecting eastward out of the Central Rockies. Strong southerly flow ahead of the frontal system will focus moisture and instability ahead of the cold front, supporting thunderstorms with high rainfall rates from northern Missouri through southern Oklahoma. SPC also continues to highlight Sunday for severe weather potential as well along the Oklahoma Ozarks into Northeast Texas where favorable instability and wind shear could overlap. The shortwave trough and frontal system should exit east toward the Midwest by Monday leaving lingering but benign rain chances overhead. Our attention then turns to the Western U.S. where a strong upper-trough is forecast to dive south from the Pacific Northwest toward the Great Basin and close off by midweek. Ahead of the trough, a strong cold front will point a plume of Pacific moisture toward the Cascades, leading to another shot of heavy elevated mixed precipitation through Monday. The passage of the closing low and front will siphon cool Canadian air southward beginning Monday to produce anomalously cool temperatures across the Western U.S. with daytime highs 25-30 degrees below normal possible. This would translate to daytime high temperatures in the 40s and 50s over the Intermountain West. With the anomalously cool airmass overhead, overnight low temperatures may drop below freezing overnight, supporting the addition of a Much Below Normal Temperatures area from the 12th-14th across much of the Intermountain West where vegetation impacts may be highest. Vegetation concerns aside, the influx of cold air coupled with dynamic cooling should also drop snow levels throughout the region, and moderate to heavy mountain snows appear likely beginning Monday over potions of the Northern Rockies/Wasatch Range. As the closed low lifts northeast toward the Plains by midweek, the emergence of a deformation zone on the west side of the low will extend heavy snow chances toward the Central Rockies Tuesday, with mixed precipitation in lower elevations in the Rockies. Another strong influx of Gulf Moisture is expected to aid in heavy rain production north and east of the upper wave from the Northern Plains southward, where some areas over the Central U.S. may be more vulnerable to additional rainfall given expected accumulations in the short term period and Sunday. Meanwhile, more severe weather is anticipated Tuesday by SPC over the Southern Plains within the warm sector of the emerging Colorado Low. Of note is the WPC preference toward a slower solution of the system lifting out of Colorado, which could enhance rain/snowfall due to a longer influx of Gulf Moisture. For a more technical discussion of models blends and preferences, refer to the latest WPC Medium Range Forecast Discussion. The deepening low pressure will support gusty winds over the Plains as well Tuesday and Wednesday, which will be monitored in subsequent Hazards updates. Over Alaska, a series of low pressure systems will produce high gusty winds across the Aleutians and Alaska Peninsula through midweek. The next batch of active precipitation should arrive along the southern Alaska coastline beginning Monday with the arrival of a strong system southwest from the Gulf of Alaska. Some uncertainty remains with the placement of higher totals along the coastline, but have maintained continuity for the previous Heavy Precipitation/Heavy Rain areas based on ensemble consensus. A High Wind area may be necessary for the 12th along the Southeast Alaska Panhandle as the low deepens, but uncertainty regarding the track of the low precludes an area for now. Asherman