Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Sat Dec 01 2018 Valid 12Z Tue Dec 04 2018 - 12Z Sat Dec 08 2018 ...Cold pattern more typical of mid-winter over most of the contiguous U.S. next week... ...Overview... Northeast Pacific into western Canada mean ridging aloft will promote a downstream mean trough across the rest of Canada and into the central/eastern U.S. This pattern will support an extended period of cold weather over a majority of the Lower 48 with periodic reinforcement from fronts dropping southward from Canada. The mean trough may flatten/broaden to some extent during the latter half of the period but fairly dramatic trough amplification forecast over the Pacific around 140-150W late in the period will likely encourage persistence/building of the western North America mean ridge and in turn promote troughing farther east. At the same time an upper low offshore California will steadily move toward the Southwest U.S. and likely open up, with the resulting trough forecast to reach the southern Rockies/Plains and northern Mexico late in the period. This system will be a significant precipitation producer from portions of California and Arizona eastward across the southern Plains and eventually into the Southeast. By the latter half of the period cold air already in place should lead to some wintry weather in the northern part of the moisture shield. Issues regarding the degree of northern stream interaction will provide ongoing uncertainty over the ultimate track of associated low pressure and northern extent of precipitation. ...Guidance Evaluation/Preferences... During days 3-5 Tue-Thu an operational model consensus including ideas from the 06Z GFS and 00Z ECMWF/UKMET/CMC represents consensus well for significant features and accounts for detail differences sufficiently small in scale to have low predictability at those time frames. Adjustments in latest guidance are fairly modest for the system approaching California and the clipper forecast to reach near Lake Superior by day 5 Thu. Behind the clipper system guidance trends are still nudging the forecast toward a slightly faster southward progression of cold air over the Plains/Rockies. By days 6-7 Fri-Sat the models and ensembles diverge with various aspects of the forecast. Recent ECMWF runs have been rather aggressive to bring eastern Pacific shortwave energy into the West Coast ridge, eventually leading to faster timing for the system developing near the Gulf Coast. Ensemble means and the 06Z GFS trend versus the 00Z run provide support for leaning away from the ECMWF scenario for Fri-Sat. On the other hand the 00Z FV3 GFS shows more southwestward elongation of eastern Pacific trough energy, leading to slower progression downstream. Leading height falls over the Plains do not differ as much from consensus so the 00Z FV3 run is only slightly slower with the surface low versus the 06Z operational GFS by day 7 Sat. Interestingly latest GFS runs represent an adjustment away from a more ECMWF-like solution through the 12Z/30 cycle. The overall amplitude of flow evolution appears to favor weakening and/or northward deflection any energy nearing the West Coast by next Sat. The ECMWF also strays to the fast side for the clipper system, with GFS runs having trended slower toward the ensemble means. Based on these considerations the latter part of the forecast adjusts toward half or more total weight of the 06Z GEFS/00Z ECMWF means with some lingering minority input from the 06Z GFS/00Z CMC. ...Weather Highlights/Hazards... The dominant system of interest from the precipitation perspective will start to bring moisture (rain and high elevation snow) into California Tue-Wed and Arizona Thu, followed by areas of heavy rainfall likely developing over the southern Plains late in the week with eastward progression thereafter. Over southwestern areas the highest totals should be over favored coastal locations and along the Sierra Nevada range as well as favored terrain in Arizona. Farther eastward the best signal for heaviest rainfall extends from about the eastern half of Texas and Lower Mississippi Valley. Cold air to the north of developing low pressure near the Gulf Coast should support an area of snow and possibly other wintry precipitation types in the northern part of the moisture shield--currently most likely over parts of the central Plains/Mid Mississippi Valley. The Great Lakes and vicinity will see episodes of lake effect snow with the best activity likely in the wake of the clipper system forecast to track near Lake Superior around Thu. Much of the contiguous U.S. will see below normal high temperatures through the period. Coldest temperatures versus normal should be behind the front dropping south through the Plains mid-late week with some highs at least 15-20F below normal. Extreme southern locations ahead of the system progressing eastward from offshore California may see one or more days of above average min temps due to clouds/precipitation. 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