Extended Forecast Discussion NWS Weather Prediction Center College Park MD 152 PM EST Fri Dec 18 2020 Valid 12Z Mon Dec 21 2020 - 12Z Fri Dec 25 2020 ...Heavy Northwest/northern Rockies precipitation to taper off after early next week... ...Overview and Model Guidance Evaluation/Preferences... Progressive mean flow aloft early in the week will steadily transition toward a more amplified regime that becomes dominated by a large scale western upper ridge and eastern trough late next week. This evolution will lead to some very noticeable temperature changes, generally toward colder conditions. There is reasonably good agreement and continuity for the overall pattern forecast but some embedded uncertainties in the early-middle part of the week in the western/central states and late in the week in the East. Trends in the Northeast were for a weaker or faster to weaken system through the Great Lakes Mon to Tue, resulting in the offshore initially weak frontal boundary to deepen into a robust system over the Atlantic Canada. Off coastal Washington, guidance has trended toward a more defined system pushing eastward along the U.S.-Canadian border Mon-Tue into the Great Lakes Wednesday. Thereafter, guidance has trended or continues to signal a wrapped-up/multi-low pressure system over the Eastern states with a track northward into the Northeast by next Fri (Christmas Day). Amplitude of the upper pattern and evolution/relative strength of the northern/southern stream components will determine how the system evolves (one or more areas of low pressure) and where it tracks (along or near the Appalachians or closer to the coast). This will ultimately dictate the rain/snow line and other details which are fuzzy at the moment. However, the amplified upstream pattern certainly suggests the potential for a robust system. A blend of the non-00Z Canadian guidance (through the 06Z cycle) was utilized as a starting point which maintained good continuity but with edited details on the systems noted above. By the latter periods, Canadian fell back in line toward the larger cluster with the 06Z GFS/00Z ECMWF and ensemble consensus with low pressure(s) in the East. Expect shifts in the frontal configuration as those shorter wavelength features have limited predictability much less than 7 days out. Also by the end of the week, system in the Pacific will move closer to the coast with an increase in moisture to at least coastal WA/OR. ...Weather/Threats Summary... The most prominent storm system next week will move into the extreme northern parts of the West next Mon and then likely continue across the northern Plains through Upper Great Lakes by Tue-Wed. This system will produce locally moderate to heavy coastal rain and mountain snow along favored terrain in the Pacific Northwest and the northern Rockies early next week. This storm may also produce areas of brisk to strong winds. Cold cyclonic flow after the system's passage should produce areas of snow to the lee of the Great Lakes. Some lighter rain/snow may come ashore coastal WA/OR around next Friday. As the southern portion of the front moves into the lower MS Valley Wednesday, increased Gulf moisture will promote an expanding area of rainfall across the Southeast. Some areas of rainfall may be moderate to heavy, depending on the system characteristics/dynamics/moisture influx. This area of precipitation is forecast to lift northeastward late Thu into Fri/Christmas with modest rainfall ahead of the cold front and some snowfall to the west/northwest of the front/low(s). Amounts may be modest/appreciable and the winter aspects have been accordingly depicted probabilistically in our Winter Weather Outlook. Cold air may arrive quickly enough to generate some snow before the precipitation ends for some locations -- a white Christmas remains possible for some areas that currently have no snow cover. Most of the country should see near to above normal temperatures early next week. Expect the Plains to see the greatest anomalies, plus 10-20F for highs and up to plus 20-25F for lows (the latter in northern areas). As the storm system tracks across the northern tier and into Canada while the upper pattern amplifies, the above normal temperatures will moderate a bit as they head into the East while moderately below normal readings spread into the West and Plains. By late in the week a majority of the eastern half of the country should see highs 5-15F below normal while the northern High Plains should return to above normal and the West moderates to a mix of above/below normal readings (despite the upper ridging, due to local inversions/stable conditions). Fracasso/Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml