Extended Forecast Discussion NWS Weather Prediction Center College Park MD 201 PM EST Sat Dec 17 2022 Valid 12Z Tue Dec 20 2022 - 12Z Sat Dec 24 2022 ***Bitterly cold temperatures likely to settle over the northern Plains for much of next week, spreading south and east after Wednesday*** ***Potential for a significant winter storm over the Eastern U.S. late in the week*** ...Synoptic Overview... A significant pattern change is forecast to evolve across much of the continental U.S. for middle to late week as a pronounced upper trough digs southward across the central part of the nation, accompanied by an impressive arctic airmass settling southward across the Plains and then advecting eastward, with widespread subzero temperatures likely for the north-central states as the coldest airmass of the season arrives. This is mainly in response to a building West Coast upper ridge (+PNA pattern) and a negative arctic oscillation (-AO pattern) that allows arctic airmasses to drop southward from Canada. Most guidance suggests that the amplifying trough will support rapid surface low development beginning on Thursday, producing areas of heavy rain and/or snow along with strong winds. ...Guidance/Predictability Assessment... The main area of uncertainty during the medium range pertains to the timing of an amplifying upper trough over the central part of the country late next week. The last several runs of the EC and CMC have been in close agreement with respect to the evolution of the mean trough over southern Canada, it's transition into a cutoff low over the Midwest and its eventual wrapping back into southern-central Canada next weekend. Meanwhile, the GFS has consistently been weaker and farther to the east than the EC and CMC with the amplifying upper low. The latest trends have brought the negatively tilted trough axis closer to the coast and thus closer to the deterministic 00z EC/CMC solutions. While there's reasonable agreement between the last few deterministic EC/CMC runs, their ensemble means tell a different story. There's notable member spread with respect to the timing of the upper trough in the 00z ECE/CMCE over the central U.S. late next week, while the 06z GEFS is showing better run-to-run consistency. Thus, a general model blend of the 00z EC/CMC/UKMET and 06 GFS were utilized through day 4. The 06z GEFS was incorporated into the blend on day 5 to account for inconsistencies between model output of the upper trough/low over the central U.S.. The 00z ECE and CMCE were incorporated for days 6 and 7. ...Weather/Hazard Highlights... The main weather story that will make weather headlines next week will be the massive expanse of frigid temperatures from the Northern Rockies/Northern Plains to the Midwest and then farther south and east by Thursday and beyond. A couple of reinforcing cold fronts will usher in the coldest air of the season by a considerable margin with the expectation of widespread subzero readings for overnight lows from the Northern Rockies to the central/northern Plains and the Upper Midwest, with some lows approaching -30 degrees across portions of Montana and North Dakota. Although the frigid airmass will modify as it reaches the Ohio Valley and Mid-South, it will still be quite cold with widespread 0s and 10s for overnight lows by the end of the week, and this very cold weather is expected to have some staying power as the Climate Prediction Center outlook indicates high probabilities of below average temperatures persisting over much of the eastern half of the country for the day 6-10 period. As cold as it will likely be, it will be difficult to challenge daily record lows/cold highs due to this event overlapping with portions of the historic 1983 and 1989 cold waves. At the very least portions of the East may end up seeing the coldest temperatures for any December since 2017. As for the potential storm affecting the eastern U.S. late next week, the most common theme in the current guidance is that the strong upper trough amplification and potentially deep upper low would produce a vigorous surface system and a broad area of strong winds. Beyond that, a potential storm track anywhere between the Great Lakes and East Coast make it very difficult to outline where the heaviest snow is most likely. Right now, the central Appalachians into interior New England and upstate New York currently have the highest probability of significant snowfall. However, if trends continue going farther westward, then the greatest snow threat could reach as far back as the Midwest/Ohio Valley with more in the way of rain farther east along a strong cold front. Changes in future forecasts are very likely. Elsewhere, a shortwave tracking near the Gulf Coast along with an associated frontal wave over the northern Gulf of Mexico could produce a period of enhanced rainfall across portions of the Florida Peninsula on Tuesday. The Northwest should see multiple episodes of rain and higher elevation snow with highest totals over favored terrain. Confidence is moderate at best for day-to-day details regarding amounts and coverage given the dependence on exact orientation of the upper flow over the region. The gradual warming trend over the West as upper ridging builds toward the coast should lead to slowly rising snow levels with time. Hamrick/Kebede 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml