Extended Forecast Discussion NWS Weather Prediction Center College Park MD 155 AM EST Fri Nov 09 2018 Valid 12Z Mon Nov 12 2018 - 12Z Fri Nov 16 2018 ...Overview... A sharp/amplified flow pattern will prevail at the start of the week, featuring a West Coast ridge and central Canada through southern Rockies/High Plains trough. As incoming Pacific energy filters through the western ridge, the downstream trough should progress eastward/northeastward and ultimately support a storm system that develops near the East Coast and likely becomes quite strong by the time it reaches the Canadian Maritimes. By late in the week most guidance shows the return of a western ridge/eastern trough configuration but one that is somewhat broader and less amplified than expected on Mon-Tue. This pattern will bring much below normal temperatures from the central into eastern U.S., along with snowfall potential to some locations during the first half of the week to the west of the developing East Coast system. ...Guidance/Uncertainty Assessment... From about Tue onward models and ensemble members diverge regarding progression of the southern half of the overall upper trough that supports East Coast/Canadian Maritimes system development. Looking at guidance through the 18Z cycle, the 12Z ECMWF/UKMET/CMC to varying degrees all showed some shortwave energy hanging back far enough westward to produce more meaningful/slower waviness along the front that trails the primary eastern system compared to GFS/GEFS mean runs. Among the 12Z ECMWF ensembles there was only 10-20 percent support for a solution as slow as the operational 12Z ECMWF and even less for the 12Z UKMET. The combination of tepid ensemble support for the slow scenario and lack of continuity in the past few ECMWF runs favored trending the forecast more toward the GFS and GEFS/ECMWF means by mid-late week. Arrival of the 00Z UKMET/CMC provide seem to reinforce the greater probability of the GFS and GEFS/ECMWF mean scenario. As for the primary East Coast storm, the primary consideration among 12Z/18Z solutions was to exclude the UKMET as it showed a track well east of other solutions off the New England coast. Other models displayed coastal vs inland track differences that would have a significant influence on precipitation type at some locations, but overall differences appeared to be within typical error bounds for forecasts 4-5 days out in time thus favoring an intermediate track at this time. The most notable discrepancy upstream is with latest CMC runs being much faster than other guidance with pushing eastern Pacific shortwave energy through the western ridge aloft. Over the past several days CMC and/or CMC ensemble mean runs were last to catch onto the sharp/amplified pattern evolving in the short range and typical biases argue against the current CMC idea. By day 7 Fri there is reasonable agreement among most models/means that the incoming Pacific energy will amplify into moderate east-central U.S. troughing while the western ridge rebuilds to some degree. The trough is consistent with teleconnections relative to the negative height anomalies seen in most D+8 multi-day means over the northern Atlantic. The embedded energy may support a modest clipper type system late in the week. Based on the above guidance considerations the updated forecast blend started with mostly 12Z/18Z GFS and 12Z ECMWF runs on Mon but then steadily decreased ECMWF input to zero after Wed while gradually increasing 18Z GEFS/12Z ECMWF mean weight. GFS weight also declined with time but more gradually, still at 20 percent by day 7 Fri. ...Weather Highlights/Threats... Amplified upper troughing that progresses from the central through northeastern states Mon-Wed along with the circulation behind the leading East Coast storm system (leading to a period of breezy to windy conditions) will bring much below normal temperatures from the central U.S. into the East during that time frame. Some highs may be at least 15-25F below normal while anomalies for morning lows may be only a few degrees less extreme. Eastern U.S. storm evolution that includes a wavy warm front lifting northeast from the Gulf of Mexico will likely bring areas of heavy rainfall from parts of the Gulf Coast northeastward. Extent/axis of cold sector snowfall will be sensitive to storm track. Currently expect the Appalachians/western New England and points westward to see the best potential for any snow. Cold cyclonic low level flow will support periods of lake effect snow behind the system. Late in the week a clipper system may bring some moisture to parts of the Great Lakes region. As the upper pattern broadens somewhat the cold temperatures over the East should moderate late in the week but still remain below normal. Expect the western states to see an expansion of above normal temperatures Mon-Wed with above normal readings likely to extend through late week. Plus 5-10F anomalies should be common with localized higher values possible. After snow departs from the south-central Rockies/High Plains Mon, precipitation will be confined to the Pacific Northwest and extreme northern Rockies. Primary focus for rain and higher elevation snow will be with shortwave energy/associated surface front reaching the Northwest around late Tue-Wed and then with upstream troughing aloft/frontal system that may approach by Fri. 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