Extended Forecast Discussion NWS Weather Prediction Center College Park MD 112 AM EST Sun Dec 6 2020 Valid 12Z Wed Dec 9 2020 - 12Z Sun Dec 13 2020 ...Pattern Description... A closed upper low west of Baja California on Wednesday is expected to become absorbed by an amplifying trough over the Intermountain West by the end of the week. These height falls over the western High Plains will likely induce surface cyclogenesis, with the low expected to lift north towards the Great Lakes by next weekend. Meanwhile, the the upper trough initially over the East Coast region is progged to lift out with more of a zonal flow pattern emerging ahead of the Midwest trough. ...Model Guidance Evaluation... The deterministic guidance has come into better agreement regarding the development of the low pressure system evolving across the Plains for the latter part of the week, and there is a trend for a stronger system compared to yesterday. The 00Z CMC has trended closer to the guidance consensus, albeit slower with the low track compared to the faster ECMWF and GFS. The ensemble means are slightly farther east with the low, and the WPC forecast was weighted more heavily in that direction, while also taking into account the stronger magnitude of the ECWMF and GFS. Greater model uncertainty exists across the Pacific Northwest by next weekend with more substantial differences in timing of Pacific shortwave energy. The fronts and pressures forecast was primarily based from a GFS/ECMWF/UKMET/some ensemble mean blend through Friday, and primarily GEFS/EC mean by the weekend 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 with onshore flow and a couple of weakening storm systems approaching the region. This will likely lead to some moderate amounts for the Coastal Range and Cascades, with the best prospects for this late Friday and into Saturday. As the upper trough builds some across the Intermountain West, snow showers are expected to increase in coverage across the central/southern Rockies as colder air returns to the region. Farther east, rain will likely develop for portions of the Southwest and extending across the Southern Plains by Thursday as the Baja upper low/trough lifts out, and depending on the upper pattern's evolution and the potential for Gulf of Mexico moisture to be advected northward, rain is becoming more likely across the Midwest and Great Lakes region to close out the week, and perhaps some thunderstorms in the warm sector of the low pressure system. The greatest temperature anomalies during this forecast period are expected across the central and northern Plains on Wednesday, with highs running on the order of 15 to 25 degrees above normal. This will make it feel more like October for these areas with readings in the 50s and 60s. A return to reality will ensue by Friday 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 ten 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