Extended Forecast Discussion NWS Weather Prediction Center College Park MD 414 PM EST Tue Dec 06 2022 Valid 12Z Fri Dec 09 2022 - 12Z Tue Dec 13 2022 ...Overview... Most guidance shows a mean trough axis near the West Coast persisting through the weekend, with the deepest reinforcing energy arriving by Sunday. Then a developing consensus carries the overall trough toward the Plains as a potential embedded upper low closes off. This pattern would support a period of significant terrain-enhanced rain and mountain snow over the West from late this week through the weekend, followed by expanding/intensifying rainfall over the Mississippi Valley and vicinity along with possibly significant snow over the northern tier early next week in association with deepening low pressure and corresponding fronts. Meanwhile a leading wavy front across the southern tier may promote some locally heavy rainfall late this week into the weekend. Expect mostly below normal high temperatures over the West and near to above normal readings over the eastern half of the country. ...Guidance/Predictability Assessment... 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 adjustment (starting in some of yesterday's 12Z models and reinforced/extended in the 00Z-06Z runs) 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 are also trending toward the latest operational model runs that close off an upper low by next Tuesday, which would support significant central U.S. low pressure/frontal system. The 00Z ECMWF mean has this upper low and the GEFS mean has achieved a closed low in its new 12Z run. Present clustering of the GFS runs/00Z ECMWF and the ensemble means offers some degree of confidence, but recent volatility and full ensemble spread (as well as a 12Z CMC run that eventually strays about a day slower with the western trough, with a slower trend in the new 12Z ECMWF too) suggest there may be some refinements yet to come. As for other features, the ensemble means offer reasonable support for latest GFS/ECMWF runs for a trailing upper trough/surface system nearing the West Coast by next Tuesday. On the leading side of the initial West Coast mean trough, a compact Midwest system as of early Friday should shear out as it continues eastward while trailing energy that ejects from the trough should progress eastward/northeastward with some uncertainty over the precise distribution of energy. CMC runs are at odds with consensus as they become much deeper and more closed with this energy near the East Coast by Sunday-Monday. The updated forecast started with a 00Z/06Z operational model composite, traded the minimal CMC component for the 06Z GEFS/00Z ECMWF means by mid-period, and then gradually increased total ensemble mean weight to 40 percent by day 7 Tuesday to account for typical detail uncertainty in the otherwise agreeable 06Z GFS/00Z ECMWF runs. ...Weather/Hazard Highlights... A compact upper shortwave emerging from the Midwest along with lead frontal moisture may spread some rainfall across the lower Great Lakes and central Appalachians/Mid-Atlantic on Friday. Most amounts should be in the light to moderate range. Some locally heavier activity may be possible but confidence is not great in light of model spread and variability for magnitude and location of highest totals. There is also some potential for a swath of snow/ice in the cooler northern periphery of this precipitation shield. Initial energy digging southeastward from the Gulf of Alaska should continue to work across the West Coast into the West Friday and renew moderately heavy precipitation chances into the Northwest and California to include a threat for enhanced terrain/mountain snows that will spread over an unsettled and cooled Intermountain West/Rockies. System ejection may support moderate to heavy rainfall with some potential for runoff issues centered over the southern Plains and mid-lower Mississippi/Tennessee Valleys during the weekend as the upper energy interacts with Gulf moisture that could feed into a wavy southern tier frontal system. Expect renewed potential for some heavy precipitation over the West late Friday through the weekend, including terrain/mountain enhanced snows. Latest consensus suggests highest precipitation totals should be along the central West Coast into the Sierra Nevada. Moisture will likely spread inland as well and bring a period of enhanced rain/mountain snow to the Great Basin/Rockies. With latest guidance trends supporting a slower upper low reaching the Plains by next Tuesday and associated deepening low pressure/frontal system, confidence is gradually rising (though is certainly not high yet, given recent variability in the guidance) 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. Probabilities for at least a quarter-inch of liquid equivalent in the form of snow over the northern Plains have increased somewhat in this cycle. Continue to monitor forecasts as refinements in specifics are likely. 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. Rausch/Schichtel 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 Hazards: - Heavy rain across portions of California and the Pacific Northwest, Fri-Sat, Dec 9-Dec 10. - Heavy rain across portions of the Mississippi Valley, the Tennessee Valley, the Great Lakes, the Southeast, the Southern Plains, and the Ohio Valley, Mon-Tue, Dec 12-Dec 13. - Heavy snow across portions of California, the /Northern Central Great Basin, the Pacific Northwest, and the Northern Great Basin, Fri-Sun, Dec 9-Dec 11. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Fri-Sat, Dec 9-Dec 10. - Heavy snow across portions of the Central Great Basin and the Northern Great Basin, Sat-Sun, Dec 10-Dec 11. - Heavy snow across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Tue, Dec 13. - Severe weather across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Mon, Dec 12. 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