Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Tue Nov 27 2018 Valid 12Z Fri Nov 30 2018 - 12Z Tue Dec 04 2018 ...Overview... The forecast period from this Fri through next Tue will be active with two dominant systems of interest. One vigorous shortwave over the West as of early Fri will progress into the Plains/Mississippi Valley by Sat and possibly close off an upper low, accompanied by a strong surface low pressure system. Guidance diverges regarding system progression after early Sat. Another fairly strong system will reach the West Coast by early Sat. Associated upper troughing should amplify/broaden over the West during the weekend, followed by ejection of leading energy which will may generate another Great Lakes storm system by next Tue. During the first half of next week the specifics of the eastern Pacific pattern become somewhat uncertain with individual models/ensembles differing over how flow may separate within the overall trough that will be heading into the upper ridge that approaches the West Coast. ...Guidance Evaluation/Preferences... One major uncertainty in the forecast involves the progression of the strong system reaching the eastern Plains/Mississippi Valley as of early Sat. There is decent clustering up to that time but then a pronounced divergence due to differences in flow details along/north of the Canadian border. One cluster including the ECMWF/CMC/UKMET shows some east-west upper ridging to the north, helping to hold the Plains/MS Valley system rather far westward. In contrast the GFS/GEFS mean show very little of this northern ridge and thus become quite progressive. Interestingly the 00Z GFS/ECMWF from yesterday (Nov 26) were much more similar and positioned near the middle of the current spread. An intermediate solution looks best here as the 06Z GFS has trended a bit slower than the 00Z run while the 00Z ECMWF mean/CMC end up being faster than the operational 00Z ECMWF. Upstream troughing seems to support somewhat greater progression over eastern North America than seen in the 00Z ECMWF as well. From the large scale perspective the next system, reaching the West Coast as of early Sat, is actually better behaved through the forecast period. Some detail differences aloft lead to noticeable spread at the surface over the Interior West and Plains around Sun-Mon but by day 7 Tue there is better than average agreement (for a day 7 forecast) among the models/means for surface low pressure in the Ohio Valley/Great Lakes. The other potentially significant uncertainty arises late in the period with respect to eastern Pacific trough energy heading into the upper ridge that approaches the West Coast early next week. Models and individual ensemble members become quite diverse with possible ideas for how the trough energy will separate and affect the West Coast ridge. The current array of guidance suggests the 00Z ECMWF may pull of its southern closed low to far southwestward while bringing too much shortwave energy through the northeastern Pacific. Latest GFS/CMC runs align well with the 06Z GEFS mean while other ensemble means show a less pronounced hint at flow separation. In these flow separation cases the operational guidance tends to lead the ensemble means so prefer to maintain some operational influence, with the GFS/CMC preferred over the ECMWF. The forecast blend employed a multi-model operational blend during the first half of the period to represent consensus or intermediate solution as appropriate, followed by a transition to about half model/half ensemble mean to account for typical increase in detail uncertainty at extended time frames. Initial mass fields/grids were adjusted a bit more toward operational guidance near the West Coast toward the end of the period, with slower progression of height falls/moisture approaching that region. ...Sensible Weather Highlights/Threats... The system tracking from the West into the Plains early in the period will bring areas of snow to mountainous terrain, tapering off after late this week, and multiple hazards farther eastward. The central Plains and Upper Midwest have the best potential to see a significant snowfall while heavy warm sector rainfall is possible from the Gulf Coast region northward/northeastward. There is also potential for severe weather over parts of the South so check SPC outlooks for the latest information in that regard. The system will likely be intense enough to produce fairly strong winds at some locations. Some precipitation of various types will likely reach New England by Sun. The next system in the series will bring rain and higher elevation snow into the West from late Fri/early Sat onward through the weekend. Then early next week expect another episode of potentially heavy snow from the central Plains northeastward along with heavy rainfall within portions of the warm sector. Pattern evolution/system progression will favor expansion/strengthening of negative temperature anomalies from the West into the Plains over the course of the period. Expect high temperatures to decline to at least 10F below normal over the Great Basin by Sat, with minus 10-20F anomalies for highs persisting over that region into next week while spreading across the Rockies and into the Plains. On the other hand the storm track will promote above normal temperatures over the South and East with some locations seeing morning lows 20F or more above normal one or more days. Such readings may approach or exceed daily records for warm lows. 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