Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Wed Nov 28 2018 Valid 12Z Sat Dec 01 2018 - 12Z Wed Dec 05 2018 ...Overview... The forecast continues to present some significant uncertainties within a multi-day mean pattern evolution that shows better consensus. The more agreeable larger scale evolution depicts broad mean troughing aloft that should support an expanding area of below normal temperatures over the Lower 48, with eastern Pacific/northwestern North America mean ridging eventually leading to more Canadian flow reaching the northern tier states. Meanwhile separating trough energy over the eastern Pacific should form an upper low that may approach California by the middle of next week. The major question marks involve timing of the Mississippi Valley/Great Lakes system during the weekend and exact details of the eastern Pacific ridge aloft. The latter will affect the timing/coverage of moisture that would reach the West Coast after Mon as well as specifics of downstream flow and character of a system that could affect the eastern U.S. toward Tue-Wed. ...Guidance Evaluation/Preferences... After good clustering as of early day 3 Sat, models/ensemble members still rapidly diverge for the system initially over the eastern Plains/Mississippi Valley region. Solutions still occupy similar parts of the envelope from yesterday, with GFS-based guidance on the fast side and the 00Z ECMWF/ECMWF mean/CMC on the slow side. The forecast for this system is still very sensitive with fairly small scale details along either side of the Canadian border having a significant influence on system progression. Fast GFS progression corresponds to that model being quickest to amplify Ontario shortwave energy into the upper Great Lakes. The 00Z FV3 GFS and CMC mean provide added support for timing that is slower than operational GFS runs while also maintaining potential for a little more progression than the slow end of the spread. There continues to be somewhat better clustering as low pressure reaches the vicinity of the Canadian Maritimes by day 5 Mon. Starting with yesterday's (Nov 27) 12Z cycle, some guidance--most notably the ECMWF--has been depicting closure of the northern part of the eastern Pacific ridge into a high. This changes the details of shortwave energy amplifying into the western half of the Lower 48, making it somewhat more rounded and leading to less eastern U.S. surface development until height falls aloft approach the East Coast and support offshore development. The 00Z ECMWF backed off a bit from its very flat 12Z/27 run, while in various forms GFS runs (operational and FV3)/GEFS means/CMC means still show some surface of development west of the Appalachians as of early day 6 Tue, and more broadly there still appears to be enough signature of a shortwave aloft to have some evidence of a surface reflection. That said, the signal is certainly weaker now than in yesterday's 00Z-06Z cycle. Thus for the deterministic forecast prefer to weaken the system a bit but still maintain its presence. Consensus still looks best for the eastern Pacific/West Coast region. There is a moderate degree of spread for the upper low that forms well offshore California and approaches the state late in the period. The one extreme solution by day 7 Wed is the 00Z CMC that strays to the southwest of other solutions and thus significantly delays arrival of any moisture. Even with the detail uncertainties, teleconnections relative to the best defined features provide reasonable support for some degree of increasing moisture over California and relatively dry conditions over the Pacific Northwest. From day 3 Sat into day 5 Mon the primary note is that the 00Z FV3 GFS served as the GFS input for the multi-model blend. For the rest of the forecast the blend started with some 00Z FV3 GFS/ECMWF weight with the rest consisting of the 06Z GEFS/00Z ECMWF means. ...Weather Highlights/Hazards... The storm system tracking northeastward from the eastern Plains/Mid-Upper Mississippi Valley during the weekend will bring an array of significant weather to the eastern half of the country. Showers and thunderstorms in the warm sector of the system will produce some areas of heavy rainfall and potentially strong/severe convection. Consult SPC outlooks for the latest information on severe threats. Locations to the northwest of the surface low track may see a band of heavy snow along with brisk to strong winds. Low pressure/upper troughing that initially moves into the West will produce areas of rain and higher elevation snow. As shortwave energy ejects eastward there is considerable uncertainty over coverage/intensity/type of precipitation over the eastern half of the country by early-mid week. Southern stream troughing/upper low approaching California later in the period will likely bring an area of rain and mountain snow by Tue-Wed. Expect high temperatures of 10-20F above normal from the Gulf Coast to Great Lakes during the weekend, ahead of the strong storm system affecting the eastern half of the country. Some morning lows may exceed 20F above normal. Much colder weather will settle in across the western U.S. courtesy of the amplifying upper level trough and then expand eastward. Anomalies of 10-20F below normal for highs will expand from the Great Basin through the Rockies and into the Plains. Much of the Lower 48 should see below normal temperatures by the middle of next week. 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