US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 300 PM EST Wed Dec 15 2021 Valid Saturday December 18 2021 - Wednesday December 22 2021 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sat, Dec 18. - Heavy rain across portions of California, the Central Great Basin, and the Pacific Northwest, Mon-Tue, Dec 20-Dec 21. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat, Dec 18. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Mon-Tue, Dec 20-Dec 21. - Heavy snow across portions of the Northeast, the Northern Rockies, and the Northern Great Basin, Sat, Dec 18. - 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, Sat-Sun, Dec 18-Dec 19. - High winds across portions of mainland Alaska, Sat-Mon, Dec 18-Dec 20. - High winds across portions of mainland Alaska, Tue-Wed, Dec 21-Dec 22. Detailed Summary: The medium range period (Saturday, December 18 - Wednesday, December 22) over the continental U.S. will feature a progressive pattern across the central and eastern states, while an amplified mean upper trough sets up off the Pacific coast and brings unsettled weather into the western states. Over the central and eastern states, the main feature of interest will be a fairly strong cold front advancing across the region during this upcoming weekend. For the most part, rainfall along and ahead of it is not expected to be heavy as it will be moving steadily eastward, however along the trailing end in southeastern Texas, a combination of deeper moisture and enhanced lift from an upstream trough may bring some isolated heavy amounts. Farther north, a slightly strengthening area of low pressure leading the cold front will push across the Ohio Valley and northern Mid-Atlantic region on Saturday. There's enough cold air in place across upstate New York and parts of southern New England to support a swath of accumulating snow along and north of the low's track. It is questionable as to whether snowfall amounts will reach winter storm warning criteria, but there was a general upward trend from yesterday's guidance, so opted for a hazard area today. Elsewhere, there are indications that a low pressure area may form along the above mentioned frontal system and track from the eastern Gulf of Mexico across the Florida Peninsula early next week. The models have had a tough time converging on a solution, but at the moment, there is a threat for some locally moderate to heavy rain across this region Monday into Tuesday. Given the lack of agreement and the likelihood that whatever forms moves pretty quickly, did not feel a hazard area was necessary, but we will monitor this over the coming days. Moving onto the western states, it will be a very active period as multiple systems move onshore out of the eastern Pacific upper trough. A potent storm system entering the Pacific Northwest on Saturday will lead to potentially heavy snow across the Olympic and Cascade mountains, with heavy rain possible along the Washington and northern Oregon coastline. Moisture from this system will spread eastward and bring locally heavy snowfall to the higher elevations across northern Idaho and far northwest Montana Saturday and Saturday night. Then as the trough amplifies, there is increasing confidence that it will begin to tap deeper Pacific moisture and focus widespread moderate to heavy precipitation across northern and central California beginning Sunday and continuing through Tuesday. Across lower elevations, rainfall amounts of 1-3 inches are likely, with locally higher amounts, while several feet of snow are again possible over the Sierra and Shasta/Siskiyous. There is a chance that this system will shift farther south during mid-week and bring potentially heavy rainfall into parts of southern California and spread precipitation into the southwestern states, but the current amount of uncertainty precludes adding a hazard area at this time. Across Alaska, impactful weather is likely as the upper level pattern becomes significantly amplified during this medium range period. Low pressure crossing the Bering Sea late this week will weaken and move into western Alaska early Saturday. While the system is weakening, westerly to northwesterly winds on the southern side will bring isolated gusty winds into southwestern Alaska. Meanwhile, very strong southerly to southwesterly winds developing between a powerful low moving through the north-central Pacific and high pressure south of the Aleutians will impact the central and western Aleutian Islands through the weekend. 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. As the northern Pacific low moves across the Chukchi Sea and into the Arctic Ocean, strong westerly winds are expected to develop across the North Slope, potentially bringing local blizzard conditions into midweek. Elsewhere, there are indications that a strong low may spin up in the Gulf of Alaska late in the period as a strong short wave crests and then descends down the eastern side of a strong upper level ridge. The main impact from this feature may be strong northerly gap/outflow winds that develop over southwestern and south-central Alaska Tuesday into Wednesday, with highest gusts possible between Kodiak Island and Kenai Peninsula. Klein