Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Thu Nov 29 2018 Valid 12Z Sun Dec 02 2018 - 12Z Thu Dec 06 2018 ...Overview... Mean troughing aloft expected to cover most of the western 2/3 of the Lower 48 late in the weekend will gradually shift eastward as energy within an initial east-central Pacific trough undercuts an eastern Pacific ridge. The resulting southern stream trough/embedded upper low should approach California by midweek while the leading ridge moves into the West. Meanwhile most guidance shows the northern part of the eastern Pacific ridge closing off a high for a time, complicating the downstream forecast to some degree, but good consensus exists toward the establishment of Canadian flow aloft extending into the northern half of the central-eastern U.S. by Tue-Thu. Details of individual shortwaves/associated surface fronts within this Canadian flow will have lower predictability though. ...Guidance Evaluation/Preferences... After showing considerable divergence over the past couple days, solutions have come into much better agreement for the system forecast to be over the Midwest/Great Lakes region as of early day 3 Sun. Current consensus appears to be somewhat closer to prior slower ECMWF/CMC runs versus the faster GFS. There is still some complexity to system evolution as dynamic support rapidly transitions from a central Plains/Midwest upper low to separate eastern Canada/Upper Great Lakes energy that accelerates best defined surface low pressure into the Canadian Maritimes by day 4 Mon. There is persistent guidance spread regarding specifics of the eastern Pacific ridge aloft/possible closing of an upper high near the Alaska Panhandle, and corresponding influence on shortwave energy emerging from the West. This continues the significant waffling that guidance has exhibited regarding whether meaningful surface development/precipitation will occur over the eastern states around Tue-Wed. In recent model runs (especially the latest 00Z version) the ECMWF has been trending back toward some hint of at least a suppressed surface wave after completely wiping out any low pressure for a couple days. On the other hand GFS runs had been the most persistent in holding onto a stronger/slower shortwave and greater surface development (like the current 00Z CMC) but the 06Z GFS has trended flat/progressive like some earlier ECMWF runs. The history of guidance thus far and broad/ill-clustered ensemble members for possible surface low pressure as of early Tue favor an intermediate solution, with the full array of guidance suggesting enough shortwave energy aloft to promote at least a modest surface reflection on Tue before stronger development occurs over the western Atlantic by Wed. For the southern stream upper low/trough approaching California, the majority consensus/average has provided fairly good continuity thus far. The CMC continues to be a prominent extreme relative to most other solutions, holding the feature farther westward by the latter half of the period. Even the CMC ensemble mean is much closer to the other models/means so confidence in the 00Z CMC is quite low. The updated forecast started with primary emphasis on the 00Z FV3 GFS/ECMWF with lesser input of some aspects of the UKMET/CMC for days 3-5 Sun-Tue. The blend incorporated about half total weight of the 06Z GEFS/00Z ECMWF means for days 6-7 Wed-Thu while keeping some FV3 GFS/ECMWF influence--but a little less of the FV3 GFS as its depiction of the arriving eastern Pacific feature differed from consensus somewhat. ...Weather Highlights/Hazards... The storm system tracking out of the Midwest/Great Lakes after early Sun will bring an axis of snow to the northwest of its track as well as northern New England. New England may see a changeover depending on how quickly warmer air arrives. Lingering moisture/waviness along the trailing front over the Southeast may give that region areas of enhanced rainfall early in the week. It will take additional time to see if ingredients may combine sufficiently to support any strong convection. Meanwhile cold flow around the departing storm should produce periods of lake effect snow. Central/northern parts of the East may see some rain/snow Tue-Wed but with continued significant uncertainty regarding coverage/intensity. Moisture ahead of the system approaching California should reach the West Coast during the first half of the week and then extend into the Southwest by Thu. Expect the best potential for greatest rainfall/high elevation snow totals with this system to extend from California into Arizona. Ahead of this system, initial upper troughing will promote areas of rain/mountain snow primarily focused over the Four Corners states on Sun. Forecast pattern evolution will lead to increasing coverage of below normal temperatures across the Lower 48 during the period. The East will see well above average readings in the warm sector of the initial Midwest/Great Lakes system, with min temps potentially exceeding climatology by at least 20F or more at some locations Sun into Mon. Farther west, high temperatures 10-20F below normal over the central/southern parts of the West into the central High Plains will expand eastward with time. One or more fronts dropping south from Canada may provide reinforcing surges of cold air during the latter half of the forecast period. 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