US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 337 PM EST Tue Dec 15 2020 Valid Friday December 18 2020 - Tuesday December 22 2020 Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Sat-Mon, Dec 19-Dec 21. - Flooding possible across portions of the Pacific Northwest. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sat-Mon, Dec 19-Dec 21. - Heavy snow across portions of the Northern/Central Rockies, Sun-Mon, Dec 20-Dec 21. - High winds across portions of the Northern/Central Rockies and the Northern/Central Plains, Sat-Sun, Dec 19-Dec 20. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat, Dec 19. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Fri-Sat, Dec 18-Dec 19. - High winds across portions of mainland Alaska, Fri, Dec 18 and Mon-Tue, Dec 21-Dec 22. - Much below normal temperatures across portions of mainland Alaska, Fri-Tue, Dec 18-Dec 22. - Heavy precipitation across portions of the Eastern Aleutians and mainland Alaska, Mon-Tue, Dec 21-Dec 22. Detailed Summary: Throughout the medium range period (Friday, Dec 18 - Tuesday, Dec 22), hazards affecting the continental United States will occur mainly in the Northwest. From Saturday through Monday a series of upper-level troughs are forecast to drive multiple frontal systems and the moisture they carry onshore from the northeast Pacific, fueling a near constant flow of heavy precipitation to northern California and the Pacific Northwest. While lower elevations closer to the coast will likely feel this precipitation as heavy rain, heavy snow should fall at higher elevations along the Coastal Ranges and Oregon/Washington Cascades. As these systems move further inland, they are expected to produce heavy snow over the Northern Rockies and Wind River Mountains/Tetons over the weekend and into Monday. Moderately gusty winds may accompany the frontal system passing through the Pacific Northwest late Saturday into early Sunday, however these winds are anticipated to remain non-hazardous. Conversely, as the same system passes over the Rockies and into the Plains the winds will pick up, producing strong enough gusts in Montana, Wyoming, and portions of northern Colorado that they may be hazardous. To the East, a low pressure/frontal system is forecast to drop from the Central to the Southern Plains Friday into Saturday, bringing with it the potential to produce heavy rain along the Gulf Coast from eastern Texas to western Louisiana. Additionally, throughout the entire period temperatures in the Plains and Mississippi Valley are expected to be anywhere from 10 to 25 degrees above normal, with Sunday and Monday forecast to have the largest and most widespread departures. Though anomalous, these temperatures are not expected to be hazardous. Similarly to the Northwest CONUS, Alaska will have a full plate of hazards as well. On Friday, a strong high pressure system over the Arctic Ocean opposed by a deep low positioned in the Gulf of Alaska will set up a strong pressure gradient over the Mainland, creating the potential for a high wind hazard over the central Interior. The same low pressure system is forecast to bring heavy precipitation to the Panhandle and eastern Southcentral near the coast throughout the beginning of the weekend. At the start of next week another very deep low pressure system is forecast to move eastward through the Bering Sea. As upper level disturbances as well as the fronts associated with this system approach the western coastline, it is expected to bring high winds to the Southwest Mainland as well as heavy precipitation from the eastern Aleutians to the Kenai Peninsula. Throughout the period much of the state will likely experience much below average high and low temperatures. While this is not much of a concern for the highs, much below average lows can be hazardous, as they pose a risk to health and safety if there is prolonged exposure. The most severe departures from normal are forecast for the Central and Eastern Interior, where temperatures are expected to reach as low as -35 to -40F. Zavadoff