Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM 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 especially in the early-middle part of the week. Early in the week there are separate forecast difficulties along the East and West Coasts. Over the East Coast/western Atlantic the models have varied considerably with important shortwave details affecting low pressure development which may consolidate/deepen into a system over or near the Canadian Maritimes. In general most guidance is signaling somewhat faster development than shown by the 12Z ECMWF/ECMWF mean, with new 00Z runs thus far appearing to show some improvement in clustering. The new 00Z ECMWF is trending toward this cluster. Meanwhile the precise details of how an eastern Pacific shortwave separates before reaching the West Coast affect the strength and track of a surface wave expected to move along or a little south of the western U.S.-Canadian border. The 12Z CMC was a notable extreme in its delay of flow separation and thus could not be used in the early-period blend. Lack of confidence in specifics near either coast led to a non-CMC 12Z/18Z model blend that also included modest ensemble mean input. New 00Z runs on average have trended stronger/northward for the incoming Pacific wave, a trend that extends into the northern Plains by Tue. By mid-late period a combination of GFS/ECMWF and GEFS/ECMWF means provided a good representation of the amplifying forecast pattern. New 00Z CMC/UKMET trends provide some added confidence in a vigorous storm system reaching the Upper Midwest/Upper Great Lakes by day 5 Wed, as their old runs were somewhat on the slower and weaker side. It will still take time to resolve the specifics of the upper trough, which will ultimately determine progression of the trailing front and any possible embedded waves. Meanwhile the western upper ridge axis should drift just far enough inland by late next week to begin allowing moisture to return to the Northwest. Differences in timing the arrival of this moisture are within typical guidance spread/error for that time frame. ...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. To the east of the Rockies by Tue-Wed, uncertainty over the exact strength and track of this system has held confidence down somewhat for precipitation coverage/type/amounts though new 00Z model runs may help to add some definition to the forecast. 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. Over the rest of the eastern half of the country expect rain to develop and spread eastward after early Wed. Some areas of rainfall may be moderate to heavy, including over the Northeast where Atlantic moisture could enhance amounts. Cold air may arrive quickly enough to generate some snow before the precipitation ends. Early in the week a system crossing the Great Lakes will produce mostly light precipitation with a brief period of lake effect possible as it is absorbed into larger scale cyclonic flow. Evolving low pressure near/offshore the East Coast may brush some areas with precipitation but confidence in specifics is still fairly low at this time. By Christmas Day some moisture may begin to work its way back into parts of the Pacific Northwest. 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. 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