Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Mon Nov 12 2018 Valid 12Z Thu Nov 15 2018 - 12Z Mon Nov 19 2018 ...Overview and Guidance/Uncertainty Assessment... The one point of agreement/consistency in the forecast is the building of an upper ridge toward the West Coast with a downstream mean trough prevailing over the eastern half of North America. Otherwise solutions have been showing meaningful adjustments/trending for aspects of the ridge and downstream trough as well as for eastern Pacific energy trying to push into the ridge. Over the East, guidance is stabilizing for the general idea of short range energy closing off an upper low over the southern Plains with the feature progressing northeastward during the first half of the medium range period--but there is still a lot of spread and variability for timing and surface details. For the late week system affecting the East the 12Z ensemble guidance depicted a very wide timing spread, sufficiently wide to include the 12Z UKMET that was on the slow side and the new 00Z run that has switched to one of the fastest solutions. The 12Z CMC was somewhat slower than the 12Z ECMWF and recent GFS runs. At the surface there is some semblance of clustering near the Mid-Atlantic/New England coast for the corresponding low track but still a meaningful number of solutions still offshore. Overall the 18Z GFS/12Z ECMWF provided the best majority input for yielding an intermediate timing along with a near-coast surface low track that accounts for westward trends over the past 1-2 days. Models/ensembles have been showing some detail uncertainties for evolution of surface low pressure over and northeast of the upper Great Lakes late in the week but the majority clustering from day to day has held fairly steady. GFS runs from the past couple days have tended to be on the northern side of the spread but the 00Z version has adjusted to the consensus over Lake Superior as of 12Z Fri. Farther westward, over the past day the guidance has trended slower/sharper with the upper ridge building toward the West Coast. The downstream effects are more western Canada shortwave energy dropping into the west-central U.S. (specifics varying considerably in the models/ensembles) which ultimately leads to a more rounded eastern mean trough, a reversal of yesterday's trend toward a more amplified eastern trough. Meanwhile guidance is continuing to suggest that approaching eastern Pacific trough energy will split as it nears the West Coast. The 18Z GFS/12Z ECMWF provided the best reflection of this idea within the framework of the means which have been showing less flow separation due to their averaging of many varied member solutions. The new 00Z GEFS mean has more of a separation signal than seen in previous GEFS or other ensemble mean runs, while the new 00Z GFS and recent CMC runs offer potential for a farther south track of southern energy. Based on forecast considerations through the period the 18Z GFS/12Z ECMWF served as the primary operational model input for the forecast, though during the first half of the period the 12Z CMC provided some modest account for remaining detail differences while staying within the consensus mean pattern. The forecast incorporated 18Z GEFS/12Z ECMWF ensemble means mid-late period, but kept their total weight to 50 percent or less given the desire to reflect some split in the flow near the West Coast. ...Weather Highlights/Threats... The system affecting the East will spread precipitation of various types northeastward during Thu-Fri. Best potential for at least some wintry weather will be over inland/higher elevation locations. Guidance spread for various aspects of this system keeps confidence lower than desired for timing as well as coverage/duration of the various precipitation types. Low pressure tracking over or near the upper Great Lakes late this week will bring an area of mostly light precipitation to the region. Behind this system expect cold cyclonic low level flow to generate an episode of lake effect snow. Meanwhile the combination of shortwave energy dropping out of Canada and low level upslope flow supported by high pressure building into the northern Plains may produce areas of snow over parts of the northern Rockies/High Plains around Fri-Sat. Trends continue to be toward less moisture reaching the Pacific Northwest with either light or no precipitation being most likely. Depending on the character of upper trough energy that may split offshore the West Coast, some light precipitation could reach California very late in the period but with below average confidence at this time. Locations from the West Coast to Intermountain West should see the most persistent warmth during the period with highs generally 5-15F above normal. The northern High Plains and vicinity should see a warm start to the period followed by a cooling trend into the weekend and then a warmer trend early next week. The South and East will tend to see below normal temperatures through the period. Best potential for minus 10-20F anomalies will extend from the southern Plains through much of the East on Thu and then with a reinforcing push of cold air progressing from the northern Plains into the East Sat-Mon. 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