Extended Forecast Discussion NWS Weather Prediction Center College Park MD 230 PM 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 this weekend into Monday... ...Heavy rainfall and severe threats over the Mississippi Valley/vicinity along with heavy snow/ice/freezing rain threats over the north-central U.S. Monday through Wednesday... ...Overview... Over the weekend, rain showers and thunderstorms are possible across the southeastern U.S. with a wavy front in the vicinity, while farther north a clipper system that seems stronger in the updated forecast may spread rain/snow across the Great Lakes region into the northern Mid-Atlantic and Northeast. But the main story during the medium range period will be upstream where guidance agrees upon 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... Recent model guidance continues to agree in principle with the large-scale aspects of the forecast including a trough that is still predicted to dig and amplify as it comes into the West by Sunday and tracks slowly eastward through midweek, along with a consolidating surface low pressure system. This shows relatively minor spread compared to some medium range forecasts at times. But minor differences include timing differences as shown by some west-east spread in models/ensemble members, but with a general trend of slightly slower movement east continuing. 12Z models seem to have come into somewhat better agreement with timing of the upper low through around day 5/Monday. As the upper/surface lows eject there is still a good signal for a significant central U.S. storm system. Regarding other aspects of the forecast, there has been some model waffling regarding the strength of a shortwave/weak surface low moving across the Great Lakes around Sunday. The 00Z/06Z guidance trended stronger with that (though the incoming 12Z model cycle is generally weaker) to support more precipitation including possible snow in the Great Lakes/northern Mid-Atlantic/Northeast compared to the NBM and previous forecast. Then behind the main upper trough/low there may be some brief West Coast ridging by around day 7/Wed but the overall flow across the Pacific is quite uncertain by then and may pose forecast challenges for the CONUS in the coming days. The WPC forecast was based on a blend of the 00Z/06Z deterministic guidance led by the ECMWF and GFS early in the period, with incorporation and gradual increase of the ensemble means through early-mid next week amid increasing (but still not too notable) model spread. ...Weather/Hazard Highlights... For the weekend, Gulf of Mexico moisture inflow combined with a wavy surface front and weak upper energy may support moderate to heavy rainfall across the southeastern part of the country. Localized flooding/runoff issues may be a threat depending on where the rainfall sets up. Farther north, precipitation likely including some snow is forecast to spread from the Great Lakes region into the northern Mid-Atlantic into southern parts of the Northeast Saturday night into Sunday. As the main upper trough digs across the West this weekend and shifts eastward through the first half of next week, multiple hazards are likely across the country. Heavy precipitation is initially likely across the West this weekend, including heavy terrain-enhanced snows across the Sierra Nevada and farther east into higher elevations of the Great Basin and Rockies. Lower elevation heavy rain is also possible this weekend particularly across coastal and interior parts of California in conjunction with a weak-moderate atmospheric river. Then as the upper trough/low approaches the Plains and a surface low consolidates and deepens, ample moisture will spread ahead of a cold front to produce significant rainfall across the Lower/Middle Mississippi Valley and vicinity beginning Monday and shifting east with time. Some strong to severe convection is also possible, with the Storm Prediction Center already monitoring this potential. On the backside of the low, cold temperatures and wrapback moisture will likely support significant snow in the Northern Plains/Upper Midwest as well as areas of ice in a transition zone. 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 for the first half of next week. Some of this colder air should spill into the High Plains by next Tuesday/Wednesday behind the expected Plains storm. On the other hand, southern/eastern areas will be near to above normal for temperatures. Anomalies will be greatest for morning lows, with some plus 15-25F anomalies across the southern tier through much of the period expanding northward at times across the Mississippi Valley. Highs are forecast to be around 10-15F above normal for the central and then east-central U.S., locally higher. Tate/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