US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 243 PM EST Thu Dec 16 2021 Valid Sunday December 19 2021 - Thursday December 23 2021 Hazards: - Heavy rain across portions of California, the Central Great Basin, and the Pacific Northwest, Mon-Tue, Dec 20-Dec 21. - Heavy snow across portions of the Central Great Basin and California, Mon-Wed, Dec 20-Dec 22. - Heavy snow across portions of the Central Great Basin and California, Tue-Wed, Dec 21-Dec 22. - Heavy snow across portions of mainland Alaska, Sun, Dec 19. - Heavy snow across portions of mainland Alaska, Sun-Mon, Dec 19-Dec 20. - Heavy snow across portions of mainland Alaska, Mon, Dec 20. - High winds across portions of mainland Alaska and the Aleutians, Sun, Dec 19. - High winds across portions of mainland Alaska, Sun-Mon, Dec 19-Dec 20. - High winds across portions of mainland Alaska, Tue-Wed, Dec 21-Dec 22. Detailed Summary: The majority of the active weather across the lower 48 during the upcoming day 3-7 period (Sunday, December 19 - Thursday, December 23) will be focused over the West. A highly amplified mean upper trough setting up off the Pacific coast will result in several rounds of coastal/valley rain and mountain snow, with northern and central California feeling the brunt of it. The first wave of low pressure will push through on Sunday and while it will bring some precipitation into northwestern California/southwestern Oregon, total amounts are not expected to be significant. A second and stronger system will move in during Monday and bring moderate to heavy rain and snow to much of northern California, mainly north of San Francisco (and lighter rains across Oregon/Washington). This feature will move south and east Monday night and Tuesday, shifting the primary axis of precipitation southward into central and north-central California. Rainfall amounts of 1-3 inches are possible during this 2-day period, with up to 1-3 feet of snow likely in the Sierra and Shasta/Siskiyous. The pattern over the West becomes more uncertain as we head into Wednesday and Thursday of next week. The general consensus is that unsettled weather will continue with precipitation spreading farther inland and probably southward; however overall total rain and snow accumulations are expected to come down due to a reduction in available deep Pacific moisture. The one area of agreement was across the central and northern Sierra, where more snow will pile up...at least through Wednesday. Elsewhere, there is increasing confidence that a low pressure area will form either in the eastern Gulf of Mexico of just off the southeast coast Monday night or Tuesday along a old stationary frontal boundary draped across the area. While this low has the potential to bring heavy rainfall to the Southeast, there is still a lot of uncertainty pinning down exactly where the greatest amounts are expected. Solutions range from the northern Florida Peninsula northward into the Carolinas, so will hold off placing a hazard area but continue to monitor. While precipitation placement is uncertain, the guidance generally agrees that within the axis of heaviest rainfall, 1-3 inch totals are likely. In addition, there may be a wind threat along the Carolina/Georgia coast depending on how much this system strengthens and how close to the coast it tracks. Across Alaska, impactful weather is likely for at least the first part of this medium range period under a highly amplified upper pattern. Late in the weekend will feature very strong southerly to southwesterly winds for the Aleutians and the western mainland ahead of a powerful low moving through the north-central Pacific. Sustained winds of 30 mph with gusts over 60 mph are possible. Pacific moisture within this strong flow will overrun relatively cold air over the mainland and bring the potential for widespread moderate to heavy snowfall Sunday into Monday. Heaviest snows will concentrate over the Seward Peninsula and northwestern Alaska/Brooks Range on Sunday, then as the cold front sweeps eastward, moderate to heavy amounts will push across interior sections on Monday. In addition to snow and wind, the air mass out ahead of the cold front is quite anomalously warm. Temperatures across western and northern Alaska will range up to 20-40 degrees above normal Sunday and Monday. This will result in readings rising high enough in parts of southwest Alaska to change snow over to rain. This poses a potential ice hazard; given the very cold ground temperatures, rain will likely freeze quickly on road surfaces. As the northern Pacific low crosses the Chukchi Sea and into the Arctic Ocean, strong westerly winds are expected to develop along the North Slope, potentially bringing local blizzard conditions into midweek. Elsewhere, there remains a threat for northerly gap/outflow winds Tuesday-Wednesday as a strong pressure gradient sets up between a developing Gulf of Alaska low and high pressure building in behind the previously discussed cold front. As was the case yesterday, the highest gusts are most likely between Kodiak Island and the Kenai Peninsula. Klein