US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 312 PM EST Fri Dec 24 2021 Valid Monday December 27 2021 - Friday December 31 2021 Hazards: - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, the Ohio/Tennessee Valley, Wed-Fri, Dec 29-Dec 31. - Heavy rain across portions of the Great Lakes, the Middle Mississippi Valley, the Central Appalachians, and the Ohio Valley, Tue-Wed, Dec 28-Dec 29. - Heavy snow across portions of the Central Great Basin and California, Mon, Dec 27. - Flooding occurring or imminent across portions of the Pacific Northwest. - Much below normal temperatures across portions of the Pacific Northwest, the Northern Rockies/Great Basin/Plains, Mon-Fri, Dec 27-Dec 31. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Tue-Wed, Dec 28-Dec 29. - Heavy precipitation across portions of the Alaska Panhandle, Wed, Dec 29. - Heavy snow across portions of mainland Alaska, Tue, Dec 28. - Much below normal temperatures across portions of the Alaska Panhandle, Mon-Tue, Dec 27-Dec 28. Detailed Summary: Over the lower 48 during the upcoming medium range period (Monday, December 27 - Friday, December 31), upper troughing will remain deeply entrenched over the western states, resulting in a prolonged period of below normal temperatures and unsettled weather. Meanwhile, ridging and mostly dry/mild conditions will predominate across much of the Southeast. Separating these different air masses will be an active set of frontal boundaries stretching across the central and eastern U.S which will feature several waves of low pressure as short waves rotate through and eject out of the western trough. One of the big stories this period will be the extremely cold weather over the north-central and northwestern states. Arctic air will take hold of the northern High Plains this weekend and likely persist through the end of next week. This prolonged cold outbreak will be centered over central and eastern Montana, but will build south and eastward across the Dakotas and upper Mississippi Valley during next week. The bitterly cold air will also spill westward over the Rockies and encompass the Pacific Northwest and parts of the Intermountain Region. High temperatures in the core of the coldest air (Montana/North Dakota) will remain at or below zero through the entire week, with lows dipping into the 20s and 30s below zero. In the Pacific Northwest, temperatures will range from 10-20 degrees below normal, resulting in many of the major cities west of the Cascades remaining below freezing much of the week, along with the threat for periods of light snow or snow showers. High temperature records may be set in the Pacific Northwest between Monday and Wednesday as a result of the anomalously cold air. Downstream of the western trough axis, the second of a series of frontal lows will form in the lee of the Colorado Rockies Monday night and promote gusty winds to its north across the Front Range, South Dakota, eastern Wyoming and northern Nebraska on Tuesday. The low that rides along this front is not expected to be very strong, so while some snow is likely in the Upper Mississippi Valley/Great Lakes region, amounts should generally be light. Farther south, there will definitely be a better influx of Gulf of Mexico moisture into the front than the previous system, and strong frontal convergence ahead of the low will result in widespread rainfall breaking out from the mid-Mississippi Valley eastward into the Mid-Atlantic states on Tuesday, which will then shift southward with the front Wednesday and persist into Thursday. In the West, while unsettled and showery weather is likely through much of next week, the general trend will be that the bulk of the precipitation shifts southward as the core of the trough begins to slide inland, but leaves behind a southern stream system near or off the coast. The highest likelihood for significant amounts will be with a system moving onshore Sunday into Monday. Heavy snow is expected from the Oregon Cascades southward across the Sierra Nevada range, with rains mostly confined to coastal locations and lowest elevations inland as snow levels with this feature are expected to be quite low. The majority of heavy snow over Oregon will fall Sunday into early Monday, then shift mostly into the Sierra Monday-Tuesday where one-week total accumulations will top several feet. During the middle to latter part of next week, there are some indications that the aforementioned southern stream trough will bring heavy precipitation into southern California and the Southwest, but the high degree of uncertainty precludes drawing any hazard areas at this time. Across Alaska, Arctic air will bring much below normal temperatures to the Panhandle through early next week. By mid-week, however, the coldest air will begin shifting inland, leading to temperatures moderating closer to normal levels. Across the mainland, there appear to be a very good chance at significant snowfall during the period as a system moves through the amplified upper ridge parked in the eastern Pacific. A deep area of low pressure will bring a slug of deep moisture into the Interior and a renewed threat of heavy snow across portions of the southern Seward Peninsula into the Nulato Hills as well as the Denali on Tuesday. This feature has the potential to then spread heavy snow into the Panhandle on Wednesday. Kebede/Klein