Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 PM EST Thu Dec 17 2020 Valid 12Z Sun Dec 20 2020 - 12Z Thu Dec 24 2020 ...Heavy precipitation pattern for the Northwest into the snowy northern Rockies... ...Overview and Model Guidance Evaluation/Preferences... Quasi-zonal progressive flow will persist through the extended period, allowing a steady stream of Pacific moisture to trek inland with several embedded disturbances. This will increase the risk for heavy rainfall along the coast and higher snowfall in the mountains. Strong 200+ kt jet off Japan will aide in the split flow over the central Pacific, with the northern branch directed into the Pacific Northwest into next week. While the CMC and ECWMF are deeper/more amplified for the beginning of the forecast, they trend back toward the means and the 00/06Z GFS solutions by the middle and end of the period. The ECWMF ensemble mean and the GEFS mean show the progressive pattern but are closer to the parent GFS. Therefore the preferred blends used the means lightly to start off and then increased to 50% percent by the end of the period. ...Weather/Threats Summary... The Pacific Northwest will be in a wet pattern through the extended period as multiple rounds onshore moisture accompany frontal systems tracking inland. Coastal rain and mountain snows can be expected, with local areas of enhancement in the favored upslope regions. At times the precipitation intensity may be locally moderate to heavy this weekend and again early next week. Some the higher accumulations are expected to be in the favored upslope elevations of the Olympics, Coastal Ranges, and Cascades-- where several inches are likely. Enhanced snows will spread inland to the Northern Rockies during the holiday week along with very windy conditions along and east of the Divide. Some of the guidance has been showing an area of modest snowfall that extends further in the Northern Plains while others do not; therefore, WPC continued to be conservative. An amplified trough will progress east of the Rockies which will increase moisture over the Lower Mississippi Valley and Gulf states as the surface cold front advances. Showers and thunderstorms will likely initiate along and ahead of the boundary and may be locally moderate to heavy in intensity as it spreads toward the Southeast coast. In general, temperatures will be near to above normal over the Plains but may cool down by the end of the forecast as cool air filters out of the Rockies. Further north, multiple fronts will pass through the northern states. Although these systems look to have limited moisture associated with them, parts of the Great Lakes region may have snow with some lake enhancement east of the lakes. Campbell 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