US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 316 PM EST Fri Dec 17 2021 Valid Monday December 20 2021 - Friday December 24 2021 Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Fri, Dec 24. - Heavy precipitation across portions of California, the Central Great Basin, and the Pacific Northwest, Mon-Tue, Dec 20-Dec 21and Thu, Dec 23. - Heavy snow across portions of the Central Great Basin and California, Tue-Fri, Dec 21-Dec 24. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Wed-Fri, Dec 22-Dec 24. - Heavy snow across portions of mainland Alaska, Mon, Dec 20. - High winds across portions of mainland Alaska, Tue-Fri, Dec 21-Dec 24. - High winds across portions of mainland Alaska, Tue-Wed, Dec 21-Dec 22. Detailed Summary: As was the case yesterday, it looks like the most active weather over the lower 48 during this upcoming day 3-7 period (Monday, December 20 - Friday, December 24) will be focused over the West. A series of short waves moving through a persistent mean upper trough in the eastern Pacific will result in several rounds of coastal/valley rain and mountain snow, with generally the heaviest amounts extending from western Washington southward across central California. The actual details regarding timing and placement of heavy precipitation are challenging, as there is a lot of disagreement among the model solutions. The current thinking is that rain and snowfall at the beginning of the period (Monday into early Tuesday) will be on the relatively light-moderate side. Then during Tuesday, a plume of Pacific moisture will concentrate over northern and central California and bring locally heavy rainfall to the coast and moderate to heavy snow to the Shasta and northern Sierra. On Wednesday, precipitation will spread across western Washington and Oregon and push inland across the northern Intermountain Region. Precipitation totals over these regions are not expected to be significant. Over the northern and central Sierra, however, a combination of strong upslope flow and somewhat deeper moisture will bring continued heavy snowfall accumulations. By Thursday and Friday, the forecast becomes even less certain. There is at least general consensus that precipitation will continue to spread farther inland across the West, but it should be mostly unimpactful. Some higher elevations in Idaho, western Wyoming, northern Utah and western Colorado may see moderate snow accumulations over the 2-day period, but nothing out of the ordinary. The corridor from western Washington to central California should continue to see the heaviest precipitation under persistent onshore flow, with greatest amounts in favored westerly-southwesterly facing slopes. Once all totaled, the potential exists for quite a few feet of snow across the Sierra and Shasta/Siskiyou next week. Elsewhere, low pressure is expected to develop over the Gulf of Mexico late Monday or early Tuesday and quickly track across the Florida Peninsula, then potentially strengthen off the southeast coast. Showers and thundershowers are expected across much of the southeastern U.S. early in the week, with the most widespread coverage just north of the low track, probably extending from the Florida Panhandle across southern/central Georgia and into the Carolinas. While it's likely that parts of this region receive locally heavy rainfall, opted against a hazard area as much of this region has been on the dry side recently and the system is fast-moving. Across Alaska, a fairly potent cold front will be crossing the mainland on Monday. Strong southerly to southwesterly flow ahead of it will bring both deep Pacific moisture and very mild air into interior sections late this weekend into Monday. Heavy snows are possible in association with the front, concentrating along the Alaska Range and Denali National Park. Meanwhile, temperatures across western and northern Alaska will range up to 20-40 degrees above normal. 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. The primary northern Pacific low associated with the above-mentioned cold front will cross the Chukchi Sea into the Arctic Ocean and gradually weaken as it moves eastward north of the state. While it is weakening, there will be a strong pressure gradient to its south, which will bring high winds and potential blizzard conditions to the North Slope Tuesday into Wednesday. Elsewhere, there remains a threat for northerly gap/outflow winds next week as a strong pressure gradient sets up between a high pressure area building in behind the cold front and low pressure and troughing extending across the Gulf of Alaska. There's a lot of uncertainty with the handling of the low pressure in the Gulf, so opted to carry this hazard through much of next week even though it's unlikely to be a continuous problem through the period. Klein