Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 PM 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 through late this week will continue to support western U.S. upper trough reinforcement and chilly weather. A separate piece of energy within the larger scale trough should track a little offshore southern California Thursday-Friday. Expect the pattern to change after Friday as a new upper ridge builds farther west, leading to a deepening trough over the Northeast Pacific and renewed moisture feed into the Northwest U.S. Meanwhile this pattern shift will kick out the late week western trough that will ultimately support a widespread area of precipitation across the eastern half of the country in association with organized low pressure/fronts. A much weaker wavy frontal system will depart from the East late this week. ...Guidance/Predictability Assessment... The spread among the 00Z/06Z models and ensembles, as well as the preferred forecast approach and resulting system evolution, are fairly similar to the previous cycle. An operational blend looks good early in the period with a gradual increase of the 06Z GEFS/00Z ECMWF means later in the forecast (reaching 40 percent total by day 7 Monday) helping to tone down lower-predictability differences. Through the period there are various embedded detail uncertainties within a more agreeable mean evolution. On Thursday-Friday differences are noticeable with the evolution/timing of the energy off southern California, with the 12Z GFS/UKMET/CMC all suggesting the 00Z ECMWF may be too slow and closed as northern stream energy digs down from the north. The 12Z ECMWF has indeed trended somewhat faster. Either way the overall pattern returns to fairly good agreement by Saturday. Then differences arise with how energy will be distributed within the overall upper trough crossing the eastern half of the country, and thus with surface evolution. Thus far ECMWF runs have been either sufficiently phased or diffuse aloft to yield a fairly consistent moderate surface development that eventually becomes deeper over or near the Canadian Maritimes. GFS runs have been quite variable, tending to show stronger development over the northeastern quadrant of lower 48 but also somewhat more flow separation that would support a trailing wave (the latter hinted at more in the new 12Z ECMWF). The 12Z GFS has backed off with the depth of primary low pressure but still has a pronounced trailing wave. A model/mean blend with the model component tilted somewhat more in the ECMWF direction provided good continuity while awaiting more confident adjustments in system evolution. On the other hand, for now the models and means show better than average agreement relative to typical days 6-7 forecasts for the upper troughing and corresponding storm system off the Pacific Northwest coast by Sunday-Monday. ...Weather/Hazards Highlights... A wavy cold front over the East on Thursday will allow for some lingering showers across mainly the Southeast, along with light precipitation over parts of the Northeast (snow in northern New England). By Friday night through the weekend developing low pressure and associated fronts will spread a broad area of precipitation across parts of the Plains and especially the East. Best potential for moderate to locally heavy rainfall will be ahead of the cold front, approximately from the south-central Mississippi Valley through the Lower Ohio Valley/Tennessee Valley and into the Southern Appalachians. Expect some wintry weather in the northern side of the precipitation shield--from parts of the northern/central Plains through the Midwest, Great Lakes, and Northeast. Strength and track of low pressure as well as any frontal waves will determine the precise details of placement and totals. Another round of heavy mountain snow is possible for portions of the Pacific Northwest/northern Rockies on Thursday ahead of a shortwave reaching the Northwest by early Friday. Farther south, the upper trough/possible closed low off southern California will direct increased moisture into the southwestern U.S. and Four Corners region through Friday or Saturday. Snow is likely in the higher terrain, especially over the Colorado Rockies with the merging of northern and southern stream upper systems. Flow ahead of the next frontal system approaching the Pacific Northwest should bring yet another round of heavy mountain snow to the region by the weekend. Lower elevation rain could become moderate to heavy from the Pacific Northwest into northwestern California by early next week. Bitterly cold temperatures should continue through at least Saturday particularly for the northern Plains with daytime highs (and to 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 likely to be below normal by about 5-15 degrees across the rest of the western U.S. through Sunday. Meanwhile, expect mild to warm weather across the south-central U.S. and northeastward through Saturday and areas near the East Coast into Sunday. 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. Also by Monday parts of the High Plains may see temperatures rebound to moderately above normal. Rausch/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