US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 322 PM EST Mon Dec 13 2021 Valid Thursday December 16 2021 - Monday December 20 2021 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Fri-Sat, Dec 17-Dec 18. - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Thu-Fri, Dec 16-Dec 17. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Sun, Dec 18-Dec 19. - Heavy snow across portions of the Northern Rockies, Sat, Dec 18. - High winds across portions of the Great Lakes, Thu, Dec 16. - Heavy precipitation across portions of mainland Alaska, Sat-Sun, Dec 18-Dec 19. - Heavy snow across portions of mainland Alaska, Sun, Dec 19. - High winds across portions of mainland Alaska and the Aleutians, Sat-Sun, Dec 18-Dec 19. - Much below normal temperatures across portions of mainland Alaska, Thu, Dec 16. Detailed Summary: The medium range period (Thurs Dec. 16 - Mon Dec. 20) can be summarized with a parade of upper level disturbances tracking into the Northwest and entering the Intermountain West, while strong upper level ridging persists in the Southeast. Temperature-wise, it is hardly going to feel like Christmas is under two weeks away in the East as temperature departures Thursday, Friday, and Saturday range between 15 to 25 degrees above normal on average, with some spots reaching 30 degrees above normal at times. That said, these temperatures are not deemed to be "hazardous" given the time of year, so have chosen to not add in a "Much Above Normal" temperature area. Switching gears, a powerful cyclone tracking through Ontario Thursday morning will be responsible for strong winds in portions of the upper Great Lakes. With wind gusts in excess of 50 mph possible, chose to issue a High Wind area to highlight the wind threat Thursday morning. The storm's quick movement north and east throughout the day should lead to winds diminishing below threshold levels by Thursday afternoon. The Northwest will witness yet another potent Pacific storm system slam into the region this weekend. Heavy mountain snow is expected with periods of rain (heavy at times) in the valleys and coastal areas. Heavy snow is forecast to push inland into the Northern Rockies late Saturday into Sunday as the storm system moves inland. The area most at risk for precipitation hazards during the period is from east Texas and the mid-Mississippi Valley to the Upper Ohio Valley. A frontal boundary tracking through these regions initially on Thursday stalls out by Friday morning. Strong riding to the southeast and a long wave troughing pattern over the West and southeast Canada is resulting in a strengthening jet streak, placing its diffluent right-entrance region atop these regions late Thursday and into Friday. GEFS precipitable water guidance suggests +2-2.5 STDs are expected with some CAPE available, especially in the ArkLaTex area where rainfall rates could become excessive. Antecedent rainfall over the last week in the Ohio and mid-Mississippi Valleys have made these areas more vulnerable to flooding, even with minor levels of instability, largely due to the potential of training showers and thunderstorms. A trailing upper level feature near Baja California keeps the pattern active into the weekend in south Texas where heavy rain and thunderstorms are possible. When, where, and how deep this trough gets will determine whether additional heavy rainfall areas are necessary in the Deep South into early next week. In Alaska, frigid temperatures look to stick around for Thursday in the eastern mainland, specifically just north of the Alaska Range and south of the Yukon Flats. Meanwhile, a dramatic pattern change is on tap as a jet extension emanating out of East Asia leads to the formation of a powerful storm heading for the Bering Sea this weekend. The pressure gradient over the Aleutians and far western Alaska tightens substantially as these areas becomes sandwiched between the powerful cyclone, and a formidable anti-cyclone in the Gulf of Alaska. The result is a robust wind field with gusts surpassing 50 kts. In addition, a slug of deep subtropical Pacific moisture (precipitable water values of 2-3 STDs above normal) arrives ahead of the approaching warm front. This makes for a favorable set up for periods of snow, which combined with the high winds could result in blizzard conditions for all locations within the High Wind threat area. A handful of locations may receive enough precipitation to pick up over a foot of snow, including parts of the Brooks Range, the southern tier of the Seward Peninsula, and the southwest mainland. Hazardous travel conditions, drifting snow, and whiteout conditions are all possible. Mullinax