Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1234 AM EST Mon Dec 7 2020 Valid 12Z Thu Dec 10 2020 - 12Z Mon Dec 14 2020 ...Weather Pattern... A weakening closed upper low near the Desert Southwest on Thursday is expected to become absorbed by an amplifying northern stream trough over the Intermountain West by the end of the week. These height falls over the western High Plains are forecast to induce surface cyclogenesis, with the low expected to lift northeastward towards the Great Lakes by Saturday. Meanwhile, the upper trough that will be over the East Coast region during the short range period is progged to lift out with a broad upper ridge axis developing ahead of the Midwest storm system and its cold front. Out West, a Pacific storm system is expected to move across Washington and Oregon by the end of the weekend. ...Model Guidance Evaluation... The deterministic guidance is in decent agreement regarding the development of the low pressure system across the Plains for the latter part of the week, and the trend over the past couple of days has been for a slightly stronger storm system. The 12Z CMC differs from the consensus since it is slower with the low track compared to the faster ECMWF/UKMET/GFS, which was favored for this forecast. The ensemble means are indicating the potential development of a triple point low over the interior Northeast by late Sunday and into Monday, although there is greater model spread in its eventual development. There remains a fair amount of model uncertainty across the northeast Pacific regarding both a lead disturbance and a stronger system following behind it for the weekend, with the CMC and ECMWF stronger with the parent surface low and taking it farther north, whereas the 00Z GFS is weaker and closer to the Pacific Northwest. The fronts and pressures forecast was primarily based from a GFS/ECMWF/UKMET/some ensemble mean blend through Saturday, and primarily GEFS/EC mean for Sunday and Monday to account for lower confidence in forecast details. ...Sensible Weather and Potential Hazards... In terms of precipitation, the heaviest rain and mountain snow is forecast for the Pacific Northwest this weekend ahead of a strong Pacific storm system and onshore flow, with 1 to 3 inches of rainfall and heavy snow for the higher elevations. As the upper trough builds some across the Intermountain West, snow showers are likely across the central/southern Rockies on Thursday and into early Friday as colder air returns to the region. Farther east, another area of noteworthy precipitation is expected as rain develops from the southern Plains and lifts northeast to the Great Lakes region, and perhaps some thunderstorms in the warm sector of the low pressure system, although rainfall totals are not expected to be excessive given the progressive nature of this storm system. The greatest temperature anomalies during this forecast period are expected across the Plains and Midwest states on Thursday, with highs running on the order of 10 to 20 degrees above normal. This will make it feel more like late October/early November for these areas with readings well into the 50s for many of these areas, and 40s across the Upper Midwest. A return to reality will ensue by the weekend and beyond as a more typical airmass for December settles across the region with highs close to climatological averages. For the East Coast region, a moderation trend is anticipated after the colder weather in the short range period, with highs up to 15 degrees above normal ahead of the next storm system. 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