Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Nov 15 2018 Valid 12Z Sun Nov 18 2018 - 12Z Thu Nov 22 2018 ...Overview... Upper ridging forecast to build into western North America from the late weekend into early next week will promote downstream mean troughing, leading to warmth over parts of the West and well below normal temperatures over central and eastern states. As the strongest energy within the mean trough reaches the Northeast and eastern Canada there will be potential for cyclogenesis from near the Northeast U.S. Coast into the Canadian Maritimes. There is decent continuity in principle regarding undercutting Pacific trough/upper low energy nearing the southwestern U.S./northwestern Mexico by Tue with continued progression thereafter. However there is a lot of spread/uncertainty over the character of the next trough reaching the eastern Pacific with possible influence from flow even farther west. These issues begin to affect the western U.S. forecast by later Mon-Tue and then the west-central/central U.S. by Wed-Thu. Aside from typically low predictability shortwave details there is decent consensus for the eastern mean trough through at least Wed. ...Guidance/Uncertainty Assessment... Over the eastern Pacific/western U.S., the overall consensus for leading southern stream shortwave/embedded upper low nearing California and northwestern Mexico as of early Tue is consistent though individual solutions continue to vary for exact timing as well as track of the upper low center. This favors a multi-model compromise for at least the first half of the period. The majority of guidance suggests this feature will weaken as it progresses across the southern U.S. through Thu the occasional stray solution holds onto a stronger depiction to the end of the period. Behind this feature the models and ensembles continue to show significant divergence for important features. The past couple ECMWF runs have been surprisingly fast/strong with amplifying eastern Pacific shortwave energy that the model ultimately brings through the western mean ridge and into the Plains by late in the period. This evolution is likely aided by an upstream shortwave also stronger than in other guidance. The 12Z ECMWF mean had a much more subdued hint of the ECMWF ideas and provided the best forecast input for representing the minority probability of the ECMWF scenario. On the other extreme the GFS runs since at least the 12Z cycle have been separating the trough in a manner not seen in other guidance--leading to dramatic differences in downstream flow. Like the ECMWF mean relative to its operational run, the GEFS means have been hinting at some slowness/separation (but to a greater degree in the new 00Z run). The 12Z/18Z FV3 GFS runs clustered more similarly with the 12Z CMC/CMC mean and ECMWF mean, with this intermediate group of solutions providing the foundation for the forecast that maintains better than average continuity in spite of the wide solution envelope. Farther eastward a model blend represents consensus ideas well for a cold upper trough crossing eastern Canada and northeastern quarter of the CONUS during the first half of the period along with the leading surface front. Details of potential low pressure development from near the northeastern U.S. coast into the Canadian Maritimes Mon-Tue will be sensitive to specifics of energy aloft. At this time GFS runs are the weakest with this evolution. Behind this system there is a reasonable signal for a wavy front persisting over and west-northwest from the upper Great Lakes, but confidence in details is low given combined influence from Canadian flow as well as the increasing uncertainty to the west. Based on the current array of guidance and continuity, the days 3-4 Sun-Mon part of the forecast incorporated ideas from the 18Z GFS/FV3 GFS along with the 12Z ECMWF/UKMET/CMC. Day 5 Tue was the transition for replacing the ECMWF with the ECMWF mean and removing the 18Z GFS. The rest of the forecast used about 1/3 ECMWF mean with the remainder consisting of the 18Z FV3 GFS/12Z CMC and 18Z GEFS mean/12Z NAEFS mean. ...Weather Highlights/Threats... Over the West there is decent confidence in dry weather from Sun through about Mon night. After that time the increasing divergence in solutions for flow aloft lead to lower than average confidence for the timing/coverage/magnitude of precipitation. Locations near the central and northern West Coast should have the best potential for seeing some rain and possibly high elevation snow. The southern stream trough/shortwave nearing California by early Tue should generate little if any rainfall. As this feature continues eastward it should lead to a late-period expansion of rainfall that will likely tend to focus over/near southern Texas during early next week. The southern Texas activity will be aided by a cold front pushing southward through the area and a persistent western Gulf Coast surface trough. Some pockets of heavier rainfall will be possible within this area of moisture. Periods of lake effect snow should be most favored over northern and eastern portions of the Great Lakes. Some of the snow over this area may also be focused near one or more frontal boundaries/embedded waves. During Sun-Mon expect coldest temperature anomalies of 10-20F or so below normal to be most common from the southern Plains into Midwest/Great Lakes. Then as the core of upper troughing adjusts into the Northeast, that region should see coldest readings versus normal Tue-Thu, 10-25F below normal. Such anomalies may challenge daily record values at some locations. Confidence remains above average for plus 5-15F high temperature anomalies along/inland from the central West Coast during the first part of next week. Then there is greater than average uncertainty in the temperature forecast over the West given the divergence in guidance aloft. Rausch 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