Extended Forecast Discussion NWS Weather Prediction Center College Park MD 156 AM EST Wed Nov 14 2018 Valid 12Z Sat Nov 17 2018 - 12Z Wed Nov 21 2018 ...Overview... The latest update to the forecast maintains fairly good continuity, with consensus ideas through the 12Z/18Z cycles not showing dramatic changes and later period spread similar to that from the past day. Continue to expect broad mean troughing downstream from a ridge that builds into western North America during the weekend. With typical differences/variability in the details, there is still agreement in principle that upstream eastern Pacific trough energy will split with some passing through the western Canada part of the ridge and the rest forming an undercutting trough/low that should progress into the Southwest U.S./northwestern Mexico by next Tue-Wed. A fair degree of uncertainty persists with the specifics of the next Pacific trough forecast to approach and possibly reach the West Coast by midweek. ...Guidance/Uncertainty Assessment... The forecast approach for this update was similar to recent cycles with a consensus model blend used early in the period, as the 18Z GFS and 12Z FV3 GFS/ECMWF/UKMET/CMC displaying some comparable aspects depending on the region/feature. Then the forecast eventually trended toward about a 2/3 tilt to the 18Z GFS/GEFS mean and 12Z FV3 GFS relative to the 12Z ECMWF/ECMWF mean. The key weaknesses in the 12Z GFS appeared to be a more rapid than consensus breakdown of the western ridge by day 4 Sun and a faster than consensus progression of the southern stream trough/upper low. The 18Z GFS was on the sharp side with the upper trough crossing the Great Lakes/Northeast Sun-Mon leading to surface low pressure to the east/northeast of Maine wrapping up much more strongly than in other solutions, favoring sufficiently low weight to downplay this aspect of the model run. The full range of model/ensemble solutions over the eastern Pacific/western North America would suggest a moderate or higher degree of uncertainty in forecast specifics already by day 4 Sun, as some lingering members did not show the flow separation which nearly all operational runs have displayed recently. The operational scenario is most likely given its general consistency in recent runs (minus track and timing differences for the southern low) but the modest trend over the past day toward a little more initial energy staying in the northern stream highlights the ongoing potential for further changes in some details. Then by the second half of the period the overall solution spread has remained similar to yesterday with the 12Z ECMWF/ECMWF mean still bringing upstream height falls into the West faster than the GFS/GEFS mean. Through the time of forecast preparation, multi-day trends and support from the CMC/CMC mean (which at times over recent weeks had actually been too flat/progressive when other guidance was indicating amplified patterns) favored leaning more toward the slower GFS/GEFS scenario. At least 1/3 inclusion of the ECMWF idea was still reasonable as upstream North Pacific positive height anomalies teleconnected to troughing that could be a little east of the GFS/GEFS. New 00Z guidance that has arrived so far is suggesting at least a more even balance of the faster/slower ideas as the CMC made a dramatic change to being between the GFS and 12Z ECMWF while extrapolation of the 00Z UKMET would lead to a farther east trough than the GFS as well. Elsewhere there was good continuity/clustering with the initial cold front pushing southward/eastward ahead of high pressure reaching the northern-central Plains during the weekend. Spread and variability so far yield lower confidence in specifics of a front and embedded wave(s) reaching the northern tier states by early next week and perhaps persisting over or near the Great Lakes/Northeast thereafter. ...Weather Highlights/Threats... Expect the forecast pattern during the Sat-Wed period to yield fairly low coverage of meaningful precipitation across the lower 48 states. Some rain will likely develop over parts of southern Texas by late Sun in association with a western Gulf Coast surface trough and approach of the cold front pushing southward through the Plains. Rainfall coverage/intensity may increase over the southern Plains and vicinity toward Tue-Wed as the southern stream upper system approaches. Also by next Tue-Wed the West Coast states should see some increase in moisture but with considerable uncertainty regarding the timing/coverage/magnitude of associated precipitation. Farther eastward the north-central Rockies/High Plains will see some early-period snow generated by shortwave energy aloft and low level upslope flow. Some of the moisture may extend eastward as light rain/snow. The Great Lakes region will see periods of lake effect snow with this activity tending to favor northern and eastern areas. The wavy front potentially settling over the area next week may promote some synoptic-scale snow as well. Over the course of the period the emphasis for coldest anomalies (some highs at least 15-20F below normal) should shift from the central U.S. during the weekend into the Northeast by Tue-Wed. Behind the cold air there will be a moderating trend that progresses from the Plains into east-central U.S. Anticipate warmest highs relative to normal over and near northern California/southern Oregon during the first half of the period with some plus 10-15F anomalies. 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