Extended Forecast Discussion NWS Weather Prediction Center College Park MD 208 AM EST Wed Dec 07 2022 Valid 12Z Sat Dec 10 2022 - 12Z Wed Dec 14 2022 ...Heavy coastal rains and mountain/inland heavy snow threats across the West/Rockies... ...Heavy rainfall threats over the Mississippi Valley/vicinity along with heavy snow/ice/frezzing rains threats over the north-central U.S.... ...Overview... A leading wavy front across the southern tier may promote some locally heavy rainfall late this week into the weekend. Guidance also agrees upstream to show a mean trough axis working inland from the West Coast through the weekend into early next week, with the deepest reinforcing energy arriving Sunday into Monday leading to closed low/trough formation, with the dynamic system lifting over the Plains Tuesday/Wednesday. This pattern would support a period of widespread and significant terrain-enhanced rain and mountain snow over the West from late this week through the weekend. This is increasingly expected to be followed by expanding/intensifying rainfall over the Mississippi Valley/vicinity eastward across the east-central U.S. along with possibly significant wrapback snow over the north-central U.S. early next week in association with deepening low pressure and corresponding fronts. Expect mostly below normal high temperatures over the West and near to above normal pre-frontal readings over much of the eastern half of the country. ...Guidance/Predictability Assessment... It remains the case that the most significant large-scale aspect of the forecast involves recent pronounced trends in guidance over the Pacific into Bering Sea/Alaska region leading to a similarly dramatic change for upper dynamics reaching the lower 48 mid-late period. This recent adjustment in the models has led to more pronounced digging of trough energy into the West Coast by Sunday and then a slower and more amplified depiction of the trough as it moves eastward. The ensemble means also continue to show this trend. Subsequent ejection continues to show a growing signal in support of a potentially significant central U.S. low pressure/frontal system, albeit slower system/QPF development and progression than continuity. The WPC medium range product suite was primarily derived from a blend of best clustered guidance from the 18 UTC GFS/GEFS and 12 UTC ECMWF/ECMWF ensemble mean along with WPC continuity and the 01 UTC National Blend of Models, applying the bulk of blend weights to the models into the weekend before transitioning to mostly these ensemble means for early-mid next week. ...Weather/Hazard Highlights... Lead system ejection out from the West may support moderate to heavy rainfall with some potential for runoff issues centered over the southern Plains and mid-lower Mississippi/Tennessee Valleys and into the Southeast this weekend as the upper energy interacts with Gulf moisture inflow into a wavy southern tier frontal system. Meanwhile expect renewed potential for heavy precipitation over the West through the weekend, including heavy terrain/mountain enhanced snows. Latest consensus suggests highest precipitation totals should be along the central West Coast into the Sierra Nevada. Moisture will spread inland as well and bring a period of enhanced rain/mountain snow to the Great Basin/Rockies. With recent guidance trends supporting a slower upper low reaching the Plains by next Tuesday and associated deepening low pressure/frontal system, confidence is gradually rising that Gulf inflow may help to enhance the northward flow of Gulf moisture to produce significant rainfall over the Mississippi Valley and vicinity. Some strong to severe convection is also possible, with the Storm Prediction Center already monitoring this potential. Moisture wrapping back into the northern tier may produce significant snow/ice/freezing rain. Probabilities for at least a quarter-inch of liquid equivalent in the form of snow over the northern Plains continue to increase. The West should see below normal highs through the period, with the most widespread readings of at least 10-15F below normal as the deep upper trough crosses the region early next week. Some of this colder air should spill into the High Plains by next Tuesday behind the expected Plains storm. On the other hand, southern/eastern areas will be near to above normal for temperatures. Anomalies should be greatest for morning lows, with some plus 15-25F anomalies across the southern tier and straying farther northward over some areas this Friday and again by next Tuesday/Wednesday. Schichtel WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml