US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 341 PM EST Wed Dec 08 2021 Valid Saturday December 11 2021 - Wednesday December 15 2021 Hazards: - Heavy precipitation across the Pacific Northwest from the Cascades westward, and into portions of northern California, Sat-Sun, Dec 11-Dec 12. - Heavy precipitation from southwestern Oregon, across much of northern and central and California, and into portions of southern California and Nevada, Mon-Tue, Dec 13-Dec 14. - Heavy precipitation across portions of the central Great Basin and into the higher elevations of the Southwest, Tue, Dec 14. - Heavy precipitation across portions of Utah, Arizona, and Colorado, Wed, Dec 15. - Heavy rain across portions of the southern Appalachians, Sat, Dec 11. - Heavy snow across portions of the northern Rockies, Sat-Sun, Dec 11-Dec 12. - Flooding possible across portions of the central Appalachians and the upper Ohio Valley. - High winds across portions of the lower Great Lakes, and northwestern Montana, Sat, Dec 11. - High winds across northern New England, Sat-Sun, Dec 11-Dec 12. - High winds across portions of eastern Wyoming, Sat-Sun, Dec 11-Dec 12. - Much above normal temperatures from across the Central Plains to the Midwest, Tue-Wed, Dec 14-Dec 15. - Heavy snow across portions of southwestern Alaska, Mon-Tue, Dec 13-Dec 14. - High winds across portions of southwestern Alaska, Alaska Peninsula and the eastern Aleutians, Sat-Sun, Dec 11-Dec 12. - Much below normal temperatures for the southern half of Alaska including the Alaska Peninsula, Sat-Mon, Dec 11-Dec 13. Detailed Summary: An intensifying low pressure system will likely be centered over the upper Great Lakes Saturday morning as it gets ready to exit quickly into southeastern Canada by the beginning of the medium-range period. Any wintry precipitation remaining over the upper Great Lakes should quickly taper off Saturday morning. Meanwhile, warm and moist air converging ahead of the trailing cold front will likely support heavy rainfall mainly over southern Appalachians into Saturday morning. As the front moves further into the East Coast later on Saturday, the threat of heavy rain should generally decrease, although pockets of heavy rain are possible with some of the embedded thunderstorms. Model guidance since yesterday had trended toward faster progression of the front through the East Coast on Saturday, together with a more intense low pressure system moving across southeastern Canada. This resulted in an increasing threat of high winds first to emerge behind the cold front later on Saturday on the lee side of the lower Great Lakes, which will quickly overspread northern New England from west to east early on Sunday. These winds should gradually weaken as the storm moves farther away through the Canadian Maritimes later on Sunday. Behind the exiting low, a rather pronounced upper ridge is forecast to build and extend across the western U.S. toward the Great Plains. This pattern will encourage downslope winds to develop on the lee side of the northern Rockies under strong pressure gradient during the weekend. The highest threat of strong Chinook winds should appear in northwestern Montana during the day on Saturday, with another of enhanced threat across eastern Wyoming. The threat of high winds should decrease by Sunday although eastern Wyoming could see these strong downslope winds to continue a bit longer on Sunday. Meanwhile, amplified troughing digging down off the West Coast of the U.S. will likely bring the next wave of heavy coastal rain and heavy mountain snows through much of the medium-range period. Moderate to significant rain/mountain snow is expected to be in progress across northwestern Washington Saturday morning before steadily spreading down through the West Coast during the next few days. The Cascades, and especially the Sierra Nevada, will likely see big snowfall totals as this heavy precipitation event unfolds through the first half of next week. Some of the heavy snowfall will likely impact portions of the northern Rockies this weekend as well. By early next week, the associated upper trough is forecast to move onshore and into the southwestern U.S. This will increase the threat of heavy precipitation to move into portions of the central Great Basin and across the higher elevations of the Southwest on Tuesday, followed by the Four-Corners region on Wednesday. Models and ensemble show uncertainty with the timing of the upper trough moving through the Southwest early next week. Depending on the strength and timing of the trough, the central to southern Rockies into the High Plains could be impacted by high winds next Wednesday. As the amplified trough works its way through the Southwest, an anomalous warm ridge of high pressure is forecast to build over the mid-section of the country. A large area of well above normal temperatures is forecast to emerge over the central U.S. and then spread toward the Midwest and upper Midwest midweek next week where a swath of moderate to heavy snow is forecast to fall on Friday into Saturday. The area highlighted represents regions with temperatures more than 25 degree above normal for this time of the year. In Alaska, a large cyclone lingering near the south coast of Alaska will continue to sustain the threat of high winds across eastern Aleutians, the Alaska Peninsula and into portions of coastal southwestern Alaska through early Sunday morning before subsiding considerably as the cyclone drifts farther south and weaken. Prior to the departure of the cyclone, much below normal temperatures will overspread the southern half of mainland Alaska, where daytime highs 20-30+ degrees below normal are expected. By early next week, model guidance continues to indicate another significant storm to enter the Bering Sea and then impact parts of the Aleutians with rain and strong gusty winds, followed by snowy and windy weather across southwestern Alaska. Although models are still resolving the details, it appears that portions of southwestern Alaska could be impacted by a foot of heavy snow later on Monday and into Tuesday. This situation will need to be further monitored. Kong