Extended Forecast Discussion NWS Weather Prediction Center College Park MD 151 AM EST Thu Dec 15 2022 Valid 12Z Sun Dec 18 2022 - 12Z Thu Dec 22 2022 ...Bitterly cold temperatures likely to settle over the northern Plains from Sunday onward... ...Overview... The overall upper pattern looks to become less amplified at the start of the medium range period on Sunday, with an upper low (the same one responsible for the northern Plains blizzard earlier this week) sheering east towards the Canadian Maritimes. Behind that, the flow becomes mostly zonal for a few days, aside from a weak shortwave moving through the southern tier states. By mid to late next week, another upper low drops in from western Canada and into the northern Plains, which could kick off an active period of weather, though with still quite a bit of uncertainty. In general the forecast confidence is highest with respect to this pattern supporting well below normal temperatures across much of the northern Rockies to northern Plains with this cold pattern likely to persist well beyond the end of the medium range period, per Climate Prediction Center outlooks. ...Guidance/Predictability Assessment... The forecast over and near the Northeast into Monday still has some complexity to it aloft, with east-west elongating energy drifting toward Nova Scotia and then followed by upstream flow (some combination of the parent upper low and/or surrounding energy possibly closing off another low over the Canadian Maritimes. The better consensus keeps the low closer to the coast than the UKMET shows, and suggests the evolution aloft by Monday could ultimately pull back the surface low a bit to a position just northeast of Maine. Models and ensembles continue to vary widely with the evolution of an upper trough digging into western Canada early next week, but are beginning to show a general trend toward a more persistent upper low over western Canada that would support fairly low heights extending into the Northwest for a time. The ECMWF and CMC are stronger/deeper with this low compared to the GFS and UKMET (through day 5), but there is ensemble support for that. The overall trough axis should begin to press eastward by around midweek, possibly bringing an upper low into the northern Plains. The 12z/Dec 14 CMC was much quicker than the ECMWF and the weaker GFS with its southward progression into the Midwest next Thursday, but the new 00z run came in slower and weaker (likely in part due to a stronger upper low depiction over southern Canada earlier in the period). Despite the uncertainty, there is support for surface cyclogenesis maybe around the north-central U.S. next week. The WPC forecast for tonight used a non-UKMET (it just seemed too flat with the flow early on) blend for days 3-5. For 6 and 7, the GFS and CMC were phased out (due to issues outlined above) favoring a blend of the ECMWF with the ensemble means. The ECMWF seemed closest in line with the ensemble means and was used for some added system definition within a very uncertain late period pattern. This maintained good continuity with the previous WPC forecast as well. ...Weather/Hazard Highlights... Heavy snows across New England associated with a nor'easter moving northward should be coming to an end by Sunday. Favorable westerly flow on the backside of the main upper low will continue to support some lake effect snows over downwind areas. A shortwave tracking across the southern tier could influence specifics of moisture that may spread across and north from the Gulf Coast late weekend into next week with some indications for widespread moderate to heavy rain, mainly along the coast. Flow to the south of a deepening western Canada trough/upper low may produce some terrain-enhanced precipitation over the Northwest/northern Rockies from Sunday onward. Confidence is currently fairly low for amounts and southward extent (whether moisture reaches as far south as portions of California and the Great Basin) due to guidance spread for important specifics of the upper pattern. Snow levels could be fairly low over parts of the Northwest depending on pattern specifics. Potential cyclogenesis in the central U.S. could bring another threat of snow to parts of the Plains and Upper Midwest, with rain farther south, but any sort of guess on amounts or impacts is impossible at this point given the spread. 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 higher across eastern 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. The West will tend to see readings 5-15F below normal during the weekend followed by some moderation over central/southern areas. The East should be coolest on Sunday with some highs up to 10-12F below normal, with slight moderation to single-digit negative anomalies early-mid week. 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, 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