Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EST Thu Dec 08 2022 Valid 12Z Sun Dec 11 2022 - 12Z Thu Dec 15 2022 ...Heavy coastal rains and widespread mountain/inland heavy snow/wind threat across the West/Rockies into Sunday/Monday... ...Deep storm genesis/blizzard threat with heavy snow and transition areas of ice/freezing rain for the north-central U.S. Monday past midweek... ...Excessive rainfall and severe weather threat to spread eastward from the south-central U.S. Monday past midweek... ...Overview... Rain showers and thunderstorms are possible across the southeastern U.S. with a wavy front in the vicinity this weekend, while farther north a potent shortwave will spread rain/snow from the Great Lakes region into the northern Mid-Atlantic and Northeast before tracking offshore to develop a deep Atlantic storm threat off the Northeast toward the Canadian Maritimes next week. However, the main story during the medium range period will be upstream where well clustered guidance have now locked in on depiction of an amplified trough to work inland from the West Coast through the weekend, with the deepest reinforcing energy arriving Sunday into Monday leading to closed low/trough formation over the Intermountain West. The dynamic system will lift vigorously over the central U.S. Tuesday-Thursday. This pattern would support a period of widespread and significant terrain-enhanced rain and mountain snow over the West through the weekend into early next week. 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 significant wrap-back snow over the north-central U.S. early-mid next week in association with deep storm/possible blizzard genesis and corresponding fronts. This system should also cause 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... Model and ensemble forecasts continue to be well clustered for the main surface/upper low considering the medium range timescale, bolstering forecast confidence. The storm system keeps trending a little slower in the 00Z/06Z runs and new 12Z runs compared to previous forecasts, leading to an upward trend in precipitation on the wraparound northwest side of the low as well as a slower arrival of precipitation. Farther upstream, differences in the overall flow become considerable by Tuesday and beyond over the Pacific--after what looks to be a narrow ridge behind the central U.S. upper low, placement differences abound with a Pacific upper low with the GFS suite generally farther west with the low compared to the farther east EC/CMC. This leads to differences in the pattern across the western U.S. by day 7/Thursday and could cause forecast issues in the upcoming days as the low may drift toward the CONUS. The WPC forecast was based on a multi-model 00/06Z deterministic blend early with increasing amounts of the GEFS and EC ensemble means for days 5-7 given increasing model spread especially in the Pacific/West. ...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. Farther north, precipitation will likely include some plowable snow to spread from the Great Lakes through the northern Mid-Atlantic and the Northeast Sunday, with highest WWO probabilities now over the beautiful Catskills. 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 imminent across the West this weekend/Monday, especially 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 especially by Tuesday 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 wrap-back moisture will support significant snow/winds in the Northern Plains/Upper Midwest as shown by steadily increasing WPC Winter Weather Outlook probabilities as well as areas of ice/freezing rain in a transition zone. This also shows some wintry weather potential spreading across the Great Lakes region and Northeast in favorable downstream theta-e advection over a pre-cooled airmass. The West should see below normal conditions through the period, with the most widespread readings of at least 10-15F below normal for high temperatures 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-Thursday 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. Schichtel/Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php 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