Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 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 from Sunday onward... ...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... Favorable westerly flow on the backside of the main upper low will continue to support some lake effect snows over downwind areas for multiple days in the wake of the departing nor'easter over southeast Canada. A shortwave tracking across the southern tier along with an associated frontal wave over the Gulf of Mexico could produce a period of enhanced rainfall near the Gulf Coast and into Florida during Monday-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. Also around that time, upper trough amplification over the Plains-Mississippi Valley and associated low pressure/frontal system could bring another threat of snow to parts of the Plains and Upper Midwest into Great Lakes, with rain farther south. Lower latitude waviness late in the period, possibly evolving near the Southeast coast by Thursday, could also bring some moisture to parts of the South and East. Confidence in any specifics is low at this time, given the variability that guidance has had for pattern details over recent days. The most widespread and extreme cold anomalies will likely be across the northern Plains from Sunday onward, with daytime temperatures as cold as 30-40 degrees below normal (locally even colder across parts of Montana), equating to widespread and bitterly cold single digit temperatures with some below zero readings across eastern Montana and North Dakota. Morning lows could fall to at least 10-20 degrees below zero (locally colder in spots). Surrounding areas should see less extreme anomalies of 10-20F below normal. After midweek expect the cold air to surge farther south and east, bringing highs down to 10-25F below normal over the southern half of the Plains and an increasing portion of the Mississippi Valley by next Thursday. The East will be chilly to finish the weekend and moderate for a time, before starting to trend colder again mid-late week. The West will tend to see readings 5-15F below normal through Sunday and then central/southern areas should see a warming trend. Moderately above normal readings will likely become more common over the southern two-thirds of the West by Wednesday-Thursday as an upper ridge builds toward the West Coast. 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