Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Sun Nov 25 2018 Valid 12Z Wed Nov 28 2018 - 12Z Sun Dec 2 2018 ...Pattern Overview... The medium range period begins on Wednesday with a large closed upper level low centered over New England with multiple embedded shortwaves pivoting around it, and quasi-zonal flow across the western U.S. to the central plains. The large closed low slowly exits the Northeast U.S. on Thursday, and a large scale trough is expected to move inland across the western U.S. by the end of the week. Once the trough axis reaches the Plains by the weekend, a developing low pressure system will likely bring widespread precipitation from the Gulf Coast to the Upper Midwest. Upper level ridging is expected to develop ahead of this developing storm system for next weekend. ...Model Guidance and Uncertainty Assessment... Deterministic models and the ensemble means are in relatively decent synoptic scale agreement on Wednesday across the continental U.S. With the quasi-zonal flow pattern across much of the continental U.S. on Thursday, the deterministic models are indicating some differences with regard to a subtle shortwave feature crossing the Plains, with the 00Z UKMET showing the greatest amplitude with that. The models remain in decent agreement regarding a Pacific storm system reaching California late on Thursday, with the 00Z GFS slightly faster. By the end of the medium range period next weekend, speed and amplitude differences are apparent as the upper trough from the western U.S. reaches the central plains, with the ECMWF and the EC mean being slower and more amplified, and the GFS and CMC more progressive with its trough axis. There is good model support for a downstream ridge east of the Mississippi River, with the ECMWF showing a rather pronounced ridge axis compared to the broader GFS ridge. Differences are even more apparent across southern Canada in the northern stream flow. Taking these considerations into account, the WPC forecast was based primarily on the ECMWF/GFS/UKMET through Friday, and then primarily NAEFS mean/EC mean with some deterministic GFS/ECMWF for the weekend. ..Weather Highlights/Threats... The departing storm system over the northeastern U.S. early in the forecast period is expected to produce lake effect snow on Wednesday, with heavy snow possible downwind of Lake Erie and Lake Ontario, and lighter snow for northern Michigan. The heaviest precipitation during this forecast period is expected from central California to western Washington as two Pacific storm systems are expected to move onshore and bring enhanced moisture into these areas, with several inches of rain possible for northwestern California and southwestern Oregon. Heavy snow is likely for the higher mountains from the Sierra Nevada to the central and northern Rockies, with 1 to 2 feet of snow possible for the highest elevations through Friday night. Another area of heavy rainfall will likely develop from the Gulf Coast to the Ohio Valley as the central plains surface low advects copious moisture northward next weekend. Temperatures are expected to be on the order of 10 to 20 degrees below average from the Midwest to the Appalachians for Wednesday and Thursday before a moderation trend ensues by the end of the week. Areas with fresh snow cover across the Midwest in the wake of the upcoming winter storm will likely have much colder overnight lows early in the forecast period, with a few subzero readings possible where winds subside and skies clear out. Temperatures should generally be within 5 degrees of climatological averages for the western third of the nation. D. Hamrick WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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