Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Wed Dec 09 2020 Valid 12Z Sat Dec 12 2020 - 12Z Wed Dec 16 2020 ...General Weather Pattern... Guidance agrees upon broad cyclonic mean flow aloft across much of the lower 48 and southern Canada, to the south of an upper low expected to meander just north/northwest of Hudson Bay during the Sat-Wed period. This pattern will likely promote fairly progressive features and a lack of Arctic air masses due to most flow originating from the Pacific. Leading shortwave energy emerging from the Plains on Sat will support low pressure that should track from the Middle Mississippi Valley through the Great Lakes into southeast Canada. Much less certain is the role of initial western energy aloft that could generate a Southeast U.S. wave along the front trailing from the anchoring northern low. Meanwhile, two separate Pacific storm systems should affect the Northwest with additional rain/mountain snow and gusty winds. Expect some moisture to extend into northern California. Energy from the leading system may produce a wavy front reaching the eastern half of the country by next Wed. ...Model Guidance Evaluation and Preferences... Some moderate timing/track spread develops for the weekend Midwest/Great Lakes system from about late Sat onward. An intermediate solution looks reasonable at this time. The consensus favors a stronger system than forecast by the 00Z ECMWF but perhaps a bit weaker than recent GFS runs (which have already weakened some from 24 hours ago and beyond). System strength will depend on precise shortwave details that may take into the short range time frame to resolve. The most common theme from guidance for trailing energy that starts over the Northwest U.S. on early Sat is that it should be somewhat sharper and perhaps a bit slower than the GFS but not slow and closed like the past two ECMWF runs through the 00Z cycle. The ECMWF scenario is within the envelope of a few ECMWF/CMC ensemble members but currently too low in probability to represent in a deterministic forecast. The new 12Z UKMET/CMC have nudged closer to the GFS idea. The 12Z ECMWF has adjusted faster and more open aloft so now the model spread is not as dramatic as before. Rapid divergence of solutions for North Pacific details around Sun-Mon ultimately leads to a wide variation of possibilities for flow aloft over the West by Tue-Wed. The GFS differs the most from other guidance, being quite fast and strong with a shortwave reaching the West Tue-Wed. GEFS means through the 06Z run shared hints of the GFS idea but the 12Z run appears to be trending closer to the ECMWF mean. Based on the above considerations the forecast blend started with an operational model composite for days 3-4 Sat-Sun, followed by increasing emphasis of the 06Z GEFS/00Z ECMWF means while reducing 00Z ECMWF/06Z GFS/00Z CMC input starting at their respective times when significantly differing from the majority of guidance. ...Weather/Threats Summary... The system tracking northeast from the Mid Mississippi Valley this weekend will bring wintry precipitation that is most likely to extend from Iowa through the central Great Lakes and northern New England. Some of these areas could see several inches of snowfall. In the warm sector ahead of the trailing cold front a corridor of showers and some thunderstorms will extend from the Gulf Coast to the Ohio Valley and Northeast states. The progressive nature of the front should keep rainfall totals from becoming excessive. There is low confidence for precipitation coverage/amounts over the South and parts of the East from the weekend into early next week due to wide guidance spread for a possible trailing frontal wave. The Pacific Northwest and parts of northern California will be in a rather unsettled pattern through the middle of next week with a couple of systems moving onshore and then continuing through the West. Expect coastal rain and mountain snow this weekend with another episode early-mid week. Periods of gusty winds will be possible as well. Highest precipitation totals for the overall period should focus along favored terrain of the coastal ranges and Cascades of the Pacific Northwest and extending into extreme northern California. Lesser amounts of mostly snow will extend into the northern-central Rockies. Continued progression of the leading western system could bring a wavy front with accompanying area of precipitation into the East by the middle of next week. The warm sector ahead of the initial Midwest/Great Lakes system will bring temperatures up to 10-20F or so above normal over the East from the weekend into early Mon. The core of coolest anomalies behind the cold front, up to 10-15F below normal, should progress from the central Rockies/Plains into the southern Plains/Lower Mississippi Valley Sat-Mon. Otherwise temperature anomalies should be within 10 degrees on either side of normal, with a tilt toward slightly below normal readings over the South/East through at least Tue and above normal readings over the northern Plains most of the period. Pacific flow should keep the West generally near normal for highs and near to above normal for lows. Rausch/Hamrick 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