US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 345 PM EST Fri Jan 01 2021 Valid Monday January 04 2021 - Friday January 08 2021 Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Mon-Thu, Jan 4-Jan 7. - Heavy precipitation across portions of the Central Great Basin, California, and the Southwest, Mon, Jan 4 and Wed-Thu, Jan 6-Jan 7. - Heavy precipitation across portions of the Southeast, the Southern Appalachians, the Mid-Atlantic, and the Central Appalachians, Thu-Fri, Jan 7-Jan 8. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Mon-Thu, Jan 4-Jan 7. - Heavy snow across portions of the Central Rockies, the Northeast, the Central Great Basin, the Northern Rockies, and the Northern Great Basin, Mon-Tue, Jan 4-Jan 5. - Flooding possible across portions of the Southeast, the Middle Mississippi Valley, the Mid-Atlantic, and the Ohio Valley. - Flooding likely across portions of the Pacific Northwest. - High significant wave heights for coastal portions of California and the Pacific Northwest, Mon, Jan 4 and Wed-Thu, Jan 6-Jan 7. - Heavy precipitation across portions of the Alaska Panhandle and mainland Alaska, Tue-Thu, Jan 5-Jan 7. - Heavy precipitation across portions of the Alaska Panhandle, Tue-Wed, Jan 5-Jan 6. Detailed Summary: The upper level pattern from the North Pacific to the North Atlantic will be undergoing substantial changes during the medium range period (Mon, Jan. 4 to Fri, Jan. 8). As the week progresses, a blocking ridge pattern begins to take shape over the North Atlantic with anomalously high 500mb heights extending west towards the Davis Strait. Meanwhile, a retracting North Pacific jet streak will force heights to build over western Canada and an upper low to deepen in the Gulf of Alaska. The resulting weather pattern across the Lower 48 goes from one of transitory waves at the start of the week to a larger scale blocking pattern by the start of next weekend with much above normal heights over Canada and an increasingly active storm track from the South-Central U.S. to the East Coast. The impressive jet streak stretching from central China to the North Pacific is the reason for the onslaught of rain and mountain snow along the West Coast. Powerful low pressure systems swinging through the Gulf of Alaska are tapping into a rich reservoir of sub-tropical moisture and directing them at the Pacific Northwest. Copious amounts of rain are forecast along coastal sections of the Northwest and northern California, with potentially feet of snow falling across the Sierra and Cascade mountains on Monday and again Wednesday into Thursday. Significant waves and even some strong winds could lead to coastal hazards across the Northwest during this time period, especially around mid-week. Heavy snow is also forecast from the northern Rockies of Idaho and Montana on south to the Tetons and Wasatch this upcoming week. Precipitation may linger into the end of the week in the Northwest, but upper level ridging building in and should begin to scale back the influx of Pacific moisture into the Northwest. To the east, the forecast calls for an area of low pressure to weaken over the eastern Great Lakes Sunday night and a potent area of low pressure to develop in the Gulf of Maine the first half of the week. Operational model guidance remain at odds on the track, longevity, and thermodynamic profiles along the New England coast with varying locations of where the 850mb low tracks. The CMC and GFS are closer to one another while the Euro is more offshore. The same can be said for the 700mb low tracks as well. High pressure stationed to the north though, supplying the region with enough cold air to generate heavy snowfall. The coastal areas could become located beneath the deformation axis and any slight change closer to or away from the coast could mean drastic changes in snowfall totals. Interests in New England will certainly want to keep an eye on the forecast throughout the weekend as snowfall totals are likely to be heavy under the deformation axis and with disruptive conditions to travel possible. Meanwhile, an upper level trough responsible for the heavy rain and mountain snow to start the week heads east reaching the central Plains by mid-week. At the same time, the low pressure system producing heavy snow in New England heads slowly east. Confluent flow over southeast Canada forces high pressure to strengthen as it begins to inch closer to the northern Great Lakes and New England late week. In addition, an intensifying jet streak over the Gulf Coast that in turn would position the favorable left-exit region over the TN Valley and towards the Mid-Atlantic, allowing for precipitation associated with an area of low pressure to breakout from the Mid-South to the Mid-Atlantic. There is still a great deal of uncertainty on this setup late week (timing/intensity of the central U.S. trough, strength/positioning of intense upper low east of Nova Scotia, strength of confluent flow over southeast Canada) but the setup does support the opportunity for disruptive wintry weather from parts of the OH/TN Valleys to the East Coast. For now, I have chosen to introduce a heavy precipitation area over the southern and central Appalachians but this forecast is subject to change. Should the central U.S. trough become increasingly flatter in the coming days, less precipitation and wintry impacts would ensue. If the trough were to deepen, however, the potential for a more impactful late week winter storm would become more apparent. In Alaska, an active storm track will be the story as wave upon wave of low pressure tracks through the Gulf of Alaska. Ensemble means are pointing to a sub 960mb low tracking south of the southwestern peninsula by Monday night and a rich supply of moisture aimed at the southern coast and the Alaskan Panhandle. Heavy mountain snowfall would transpire along with periods of rain in the valleys and along the coast. Gusty winds and rough surf along the coasts are also a concern but and may even challenge thresholds, but most ensemble guidance keeps winds just shy of high wind criteria for now. The second half of the week should lead to more rain and snow along the south-central coast but exact amounts are lower confidence at this time. Milder temperatures return to much of the interior locations later in the week as mild Pacific air races in ahead of these storm systems. Mullinax