Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM 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 strong surface low development beginning on Thursday, producing areas of heavy rain and/or snow along with strong winds. ...Guidance/Predictability Assessment... The 00Z model guidance suite is still indicating some meaningful differences as early as Tuesday, both across the Gulf Coast with the GFS/GEFS slower than the other guidance, and the GFS is also a bit more amplified across the Upper Midwest, similar to its previous runs. By Wednesday, the differences with the GFS/GEFS become quite pronounced across the Great Lakes region and even become somewhat out of phase with the non-NCEP guidance by Thursday across the Northeast. There has also been a trend for the surge of arctic air to be centered farther west and including the High Plains of Colorado and Wyoming with a trend for even colder conditions by mid-late week. The forecast remains challenging for the second half of the forecast period regarding the eventual development of a potentially strong low pressure system across the eastern U.S. late in the week. The overall trend compared to yesterday has been for the main surface low to track closer to the Appalachians and less of a coastal low scenario that earlier solutions were portraying, and this generally means more rain than snow for the major cities along the Interstate 95 corridor, and snow confined to interior portions of the Mid-Atlantic and Northeast. However, the GFS is still maintaining the idea of a strong low near the Coast, but given the differences noted, this is not the preferred solution at this time. There has also been a slightly slower trend in the arrival of heaviest precipitation. An examination of the ICON and JMA models supports a solution closer to the ECMWF/CMC/ECENS for fronts and pressures, so the forecast was hedged in that direction for the entire forecast period, with more weighting towards the ECENS mean by Friday and into Saturday. For QPF, about half of the NBM was used along with a nearly equal proportion of the ECMWF/CMC/ECENS. ...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 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