Extended Forecast Discussion NWS Weather Prediction Center College Park MD 119 AM EST Mon Dec 27 2021 Valid 12Z Thu Dec 30 2021 - 12Z Mon Jan 03 2022 ...Bitterly cold temperatures to continue Thursday-Saturday across portions of the Northern Plains... ...Overview... A blocky and amplified ridge/upper high anchored over the northern Pacific should finally begin to break down and weaken next weekend. Before that though, renewed troughing (and chilly weather) will continue across the Western U.S., including a southern stream closed low near southern California Thursday-Friday. An initial northern stream shortwave will send one system across the Great Lakes/Northeast late this week, but by the weekend a stronger system (and northern Pacific ridge weakening) should finally allow for a pattern shift as amplified troughing shifts into the central U.S. and the East. ...Guidance/Predictability Assessment... The latest models and ensembles continue to show good agreement with the large scale pattern through much of the period, but with lingering uncertainties in the smaller scale details. The 12/18z guidance continues to indicate good consensus for closed low development off the California coast, with just some mainly minor questions remaining in terms of timing and how quickly the energy ejects into the Southwest and merges with a stronger shortwave from the north. After this, again, some timing and strength variability with the amplified trough as it slides into the Central and eventually Eastern U.S., and then bigger differences on surface low placement across the Northeast next Sunday and Monday. Given the day 6-7 time frame though, these differences are still well within the normal spread. WPC relied on the deterministic models for days 3-4, eliminating the UKMET by day 5 as its still a bit faster with the closed low and shortwave into the Southwest mid period. Given growing uncertainties late period, favored some incorporation of the ensemble means, but still able to maintain at least 50% of the deterministic GFS and ECMWF even at day 7. This also maintained good consistency with yesterdays WPC forecast. ...Weather/Hazards Highlights... A cold front into the East on Thursday will allow for some lingering showers across mainly the Southeast. Precipitation chances increase again in roughly the same region by the weekend as the next cold front makes its way eastward. Another well defined cold front diving south and east this weekend should result in another round of widespread moderate to locally heavy rainfall for the Tennessee Valley/Southern Appalachians, and into the lower Ohio Valley on Saturday. Expect rainfall with this system to gradually press East on Sunday, with the best chance for organized rains along and south of the front across parts of the Southeast. Expect some wintry weather in the north side of the precipitation shield from the Midwest to the Great Lakes and the Northeast as well. Another round of heavy mountain snows is possible for portions of the Pacific Northwest/northern Rockies on Thursday ahead of a shortwave into the Northwest. Farther South, the likely closed low will direct increased moisture into parts of southern California and into the Four Corners region through Friday or Saturday, with snow likely in the higher terrain, especially the Colorado Rockies as the northern stream and southern stream systems merge. The next frontal system towards the West should bring yet another round of heavy mountain snows to the Pacific Northwest by this weekend. Bitterly cold temperatures should continue through at least Saturday particularly for the northern Plains with daytime highs (and a lesser extent overnight lows) 20 to 35 degrees below average. Some areas of northern North Dakota and Montana may stay below 0F even into next weekend, despite temperatures moderating slightly as the week progresses. The addition of gusty winds could result in dangerously cold wind chills, especially for northern parts of the High Plains where wind chill values could dip well below 0F for many. Temperatures are also expected to be below normal by about 5-15 degrees across the rest of the western U.S. through Sunday. Meanwhile, mild to warm weather is expected across the south-central U.S. and northeastward through next week, where many places from the southern half of the Plains to the Ohio Valley/Mid-Atlantic could be 15-25 degrees above normal through Saturday. More daily record high minimum and maximum temperatures are likely. A potential pattern shift may finally bring values back closer to normal, possibly below average in the south-central U.S. by Sunday and the East by next Monday. Santorelli Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml