US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 319 PM EST Wed Dec 22 2021 Valid Saturday December 25 2021 - Wednesday December 29 2021 Hazards: - Heavy precipitation across portions of California and the Pacific Northwest, Sun, Dec 26. - Heavy snow across portions of California and the Pacific Northwest, Sat-Mon, Dec 25-Dec 27. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Sat-Tue, Dec 25-Dec 28. - Heavy snow across portions of the Great Lakes and the Upper Mississippi Valley, Sun, Dec 26. - Flooding occurring or imminent across portions of the Pacific Northwest. - High winds across portions of the Southern Rockies, the Central Rockies, the Southern Plains, and the Central Plains, Sun, Dec 26. - Much below normal temperatures across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Mon-Wed, Dec 27-Dec 29. - Much below normal temperatures across portions of the Northern Plains and the Northern Rockies, Sat-Wed, Dec 25-Dec 29. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Tue-Wed, Dec 28-Dec 29. - Heavy snow across portions of mainland Alaska, Sun, Dec 26. - Heavy snow across portions of mainland Alaska, Tue, Dec 28. - Much below normal temperatures across portions of the Alaska Panhandle, Sat-Tue, Dec 25-Dec 28. Detailed Summary: Over the lower 48 during the upcoming medium range period (Saturday, December 25 - Wednesday, December 29), persistent upper troughing will continue across the western states while ridging covers much of the Southeast. In between, a mean west to southwest polar jet will allow a series of fast-moving systems to transit across the central U.S. through the Northeast and Mid-Atlantic region. Under the upper trough, the weather across the West will remain unsettled, with the most significant precipitation again expected from southwestern Washington southward into northern and central sections of California. The heaviest amounts through the period will tend to favor the west-southwest facing slopes of the Cascades, Klamath mountains, coastal ranges of northern California and the Sierra. Some locations, especially across the Sierra are likely to see several feet of snow in total by the early part of next week. There are indications that the upper trough may start to push inland a bit by the middle of next week, reducing the onshore flow and resulting in an overall lessening of precipitation. Another very notable hazard for this period will be extremely cold temperatures that will impact the north-central and northwestern states. A well-entrenched Arctic air mass pushing southward into the far northern High Plains Friday will overspread much of central and eastern Montana and North Dakota this weekend. During the early-mid part of next week, it will build farther into the Plains and Upper Mississippi Valley, as well as spill westward over the Rockies and encompass a good portion of the Pacific Northwest. High temperatures in the core of the coldest air in Montana will average 30-40 degrees below normal through the period, which translates to highs near or below zero and lows between -15 and -25. By the middle of next week, low temperatures below zero will be common from western Minnesota westward across the Dakotas, Montana, and even into parts of northeastern Washington. The cold temperatures in the Pacific Northwest will allow snow levels to fall to sea level and likely bring accumulating snow into the major cities west of the Cascades, with the best threat for light-moderate amounts expected Sunday-Monday before drier air infiltrates the region. Elsewhere in the west, a short wave ejecting out of the western U.S. trough will spawn an area of low pressure to the lee of the south-central Rockies on Sunday, and combined with a strong mid-upper level jet will support locally windy conditions into the southern High Plains Monday. At this time, the greatest threat (winds gusting to 40-60 mph) will extend across southeastern Colorado, eastern New Mexico eastward into the Oklahoma and Texas Panhandles. Over the central and eastern states, a series of low pressure systems and associated fronts will impact the region. Periods of snow are expected on some days over the northern Plains, upper Mississippi Valley, Great Lakes region, and sections of the Northeast during the 5-day period. Looks like the best threat for potentially warning level snow amounts are across the northern half of Minnesota, northern Wisconsin and the western Upper Peninsula of Michigan during Sunday into early Monday. Periods of rain are likely farther south along and ahead of the transient fronts, but there's a lot of uncertainty in placement of the heaviest axis of rainfall. Probably the closest areas to watch during the early part of next week are from the Ohio Valley through the mid-Atlantic states. Across Alaska, the same Arctic air mass impacting the north-central and northwestern sections of the lower 48 will be deeply entrenched over the Alaska Panhandle through much of the period. Temperatures are likely to average well below normal, with the most anomalous readings likely across the southern half to two-thirds of this region. There are, however, some indications that temperatures will begin to moderate toward the middle of next week. Across the mainland, there appear to be two very good chances at significant snowfall during the period as a couple of systems move through the amplified upper ridge parked in the eastern Pacific. The models remain in decent agreement that a storm system and associated cold front will move quickly across interior sections of the mainland Sunday. Deep moisture preceding the front will support an area of heavy snow from the west coast of northern Alaska east-southeastward through the southern part of the Brooks Range and the Yukon-Tanana Uplands. With somewhat lower confidence, believe a second strong storm with another slug of deep moisture will bring another round of widespread precipitation to the mainland Tuesday-Wednesday. The details are still murky, but heavy snow may impact much of the same region as Sunday's storm, though this feature has the potential to spread heavy snow into the Alaska Range and South-Central Alaska by the end of the period. Klein