US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 943 AM EST Sat Dec 12 2020 Valid Tuesday December 15 2020 - Saturday December 19 2020 Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Tue-Fri, Dec 15-Dec 18. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Tue-Fri, Dec 15-Dec 18. - Heavy precipitation across portions of the Mid-Atlantic and the Northeast, Wed, Dec 16. - Heavy snow across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians, Wed-Thu, Dec 16-Dec 17. - High winds across portions of mainland Alaska, Mon-Tue, Dec 14-Dec 15. - Much below normal temperatures across portions of mainland Alaska, Wed-Fri, Dec 16-Dec 18. Detailed Summary: Throughout the medium range period (Monday, Dec 14 - Friday, Dec 18), a flurry of activity is forecast for both sides of the continental United States. On Tuesday, a low pressure/frontal system will propagate onshore from the northeast Pacific. The system itself, as well as subsequent upper level disturbances, will support a virtually continuous influx of moisture and precipitation hazards extending from Northern California through the Pacific Northwest throughout the period. Along the coastline and at lower elevations heavy rain will be the main threat, whereas heavy snow will be the primary concern at higher elevations along the Coastal Ranges and the Oregon and Washington Cascades. Since yesterday the models have trended toward lower and more uncertain precipitation totals in the Sierra Nevada, so it will not be included in the hazard area at this time. As this system moves along its eastward track it is expected to bring heavy snowfall to the Northern Great Basin, Northern Rockies, and potentially the Wind River Range/Tetons, Tuesday through Friday. In the neighboring Northern Great Plains, a series of warm fronts are expected to pass through the region throughout the medium range. This will help facilitate above normal, though non-hazardous, temperatures extending from Nebraska to the Canadian border Thursday and Friday. Looking to the east, a low pressure system and its associated cold front will quickly pass through the Mid-Atlantic and Southeastern seaboards on Monday. While rain will accompany this system, the heaviest of which will be along the Virginia/North Carolina border, it is not expected to be hazardous. Later in the period, multiple surface lows spread throughout the Ohio River Valley and Southeast will combine late Wednesday and deepen, developing into the second nor'easter of the season. Friday's model guidance has pointed towards a shift in the low track past Wednesday evening from a northeastward to eastward trajectory. This change in the low track could shift the forecasted snowfall and mixed precipitation areas south. Additionally, if this trend toward a more eastward trajectory persists a majority of the hazardous weather over the region will likely occur on Wednesday, with snowfall lingering in Northern New England into Thursday. As this event is still evolving with time, it will continue to be monitored in the coming days. In Alaska, a persistently strong high pressure system in the Arctic Ocean opposed by strong low pressure systems in the Gulf of Alaska will set up a strong pressure gradient over the mainland. This enhanced pressure gradient will produce high winds from Monday into Tuesday, which could cause blowing snow and the lowering of visibility. The main areas impacted will be the Far North and the Interior from the Seward and Lisburne Peninsulas in the west through Fairbanks in the east. Wednesday through Friday temperatures are expected to drop much below normal in the eastern Interior, reaching as low as -40F. Prolonged exposure to these temperatures can pose a risk to health and safety. Toward the end of the period there is a wide spread in model solutions concerning a low pressure system that could impact the Alaska Panhandle and southern Mainland. While this system has the potential to produce heavy precipitation and high winds, no hazards will be issued at this time due to the uncertainty associated with its location and intensity. Zavadoff