Extended Forecast Discussion...amended NWS Weather Prediction Center College Park MD 237 AM EST Fri Dec 16 2022 Valid 12Z Mon Dec 19 2022 - 12Z Fri Dec 23 2022 ***Bitterly cold temperatures likely to settle over the northern Plains for much of next week*** ***Potential for East Coast winter storm to develop late in the week*** ...Synoptic Overview... A significant pattern change is expected to evolved across much of the continental U.S. next week as a pronounced upper trough builds in across the eastern half 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. 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. Things get interesting towards the end of the week as a potential East Coast storm develops, and the details of this will be dependent upon the phasing of both southern and northern stream shortwave energy, and when this takes place. If the low develops close enough to the coast, impactful rain and snow could materialize from the Mid-Atlantic to New England. ...Guidance/Predictability Assessment... The 00Z model guidance suite has a good overall depiction of the weather pattern on Monday, with the main differences observed with the timing of the upper trough/closed low across southwestern Canada and its arrival over the far northern Plains. By Tuesday, the southern stream trough over Texas is more amplified with the GFS and slightly faster with the ECMWF, and the GFS remains slightly more amplified going through Wednesday. Going into Wednesday night, the second lobe from a portion of the polar vortex starts dropping south across the Upper Midwest, and the axis of the trough is farther east with the CMC and ECMWF, whereas the GFS is sharper and slower. This will have implications on the eventual development of the coastal low near the East Coast, with the GFS having a more offshore track. The WPC fronts/pressures was primarily based on a 40% ECMWF/20% GFS/20% CMC/20% ECENS blend for Monday and Tuesday, and then the proportion of the ECENS and GEFS means were increased going through the remainder of the week to account for the increasing model uncertainty. Future forecast updates will provide additional clarity on the eventual East Coast storm evolution and potential impacts. ...Weather/Hazard Highlights... The main weather story that will make weather headlines next week will be the massive expanse of frigid temperatures from Montana/Northern Plains to the Midwest and even the Ohio Valley. 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 beyond this forecast period. The potential for an East Coast winter storm late in the forecast period also bears close monitoring. The overall upper level pattern is generally supportive of a winter storm developing in this region, although model differences and run-to-run variability limit confidence on the expected placement of heaviest snow. A low track just off the coast would favor significant snowfall for the Interstate 95 corridor, whereas a track along the coast or just inland would likely involve a wintry mix to rain scenario for those same general areas and snow farther inland. 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 southern Texas and Louisiana on Monday, and then into Florida 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 but there is a general signal that totals should increase toward Wednesday-Thursday. 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