Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1058 AM EST Thu Nov 8 2018 Valid 12Z Sun Nov 11 2018 - 12Z Thu Nov 15 2018 ***Below normal temperatures for much of the U.S. with some early season snow possible from the Great Lakes to interior New England*** 16Z Update ---------- The deterministic models and their respective means are in above average agreement on the synoptic scale for the beginning of the forecast period owing to the high amplitude pattern in place, with an upper ridge off the West Coast, a trough over the north-central U.S., and a broad subtropical ridge over the southern Gulf of Mexico. The complexity in the forecast relates to the amount of eventual phasing between the initial northern stream and southern stream shortwave perturbations within the large scale synoptic trough axis, and these differences become apparent by day 5 on Tuesday. The operational GFS becomes faster with the large East Coast storm system compared to the model consensus, but given a faster trend in the ECMWF over the past 24 hours, the GFS scenario certainly seems plausible. Elsewhere across the continental U.S., the CMC becomes out of phase with the upper level pattern across the northwestern U.S. by Tuesday since the shortwave over the northeast Pacific becomes much more progressive than the model consensus. The WPC forecast incorporated more of the EC mean by Tuesday, and then also some of the GEFS mean by the end of the forecast period. The previous discussion follows below for reference. D. Hamrick ...Guidance/Uncertainty Assessment... GFS/ECMWF and GEFS/ECMWF ensemble solutions are well clustered into early next week, but become increasing varied with smaller scale system details into next mid-late week when a composite blend of ensemble means is preferred. This solution maintains good WPC continuity. ...Weather Highlights/Threats... A storm track to the northeast Pacific/Gulf of Alaska maintains an amplified ridge axis aloft along/offshore western North America into early next week. Downstream clipper low track and favorable dynamics will squeeze out a swath of snow in cooled air of the Great Lakes/Northeast. Meanwhile, potent weekend shortwave digging through the Intermountain West/Rockies will favor a lead snow burst over the south-central Rockies Sunday to spill into the Plains as aided by cold post-frontal upslope flow. The upper trough will amplify over the central U.S. early next week with increasing northern and southern stream phasing to the lee of the amplified but gradually inland shifting upstream ridge. Lead wave/frontal genesis and deep moisture return will fuel heavy rain/convection expansion northward across the east-central then eastern U.S. Moisture feeding back into a sharpening baroclinic zone meanwhile presents an organized snow threat through the Appalachians/Ohio Valley, with ample lake effect enhancement lee of the Great Lakes. There is continued uncertainty with respect to the lead coastal/inland low track and western extent of the main precipitation shield/cold air with recent ECMWF runs offering a more inland track than more coastal tracks from recent GFS runs. A larger percentage of GEFS and ECMWF ensemble members favor the coastal solution but the minority subset of more inland solutions cannot be rules out considering the amplitude and westward position of the main mid-upper level trough. Ensemble means offer a decent compromise at this point. Overall, there is a threat for heavy Great Lakes/OH Valley/Appalachians to interior Northeast early season snows. Well upstream, Pacific trough energy and height falls start to work into the Northwest early-mid next week as the lead/amplified ridge aloft gradually/finally shifts inland. This supports an increased chance for precipitation including modest elevation snows. Schichtel 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