Extended Forecast Discussion NWS Weather Prediction Center College Park MD 217 PM EST Sat Dec 10 2022 Valid 12Z Tue Dec 13 2022 - 12Z Sat Dec 17 2022 ...Major storm system to bring heavy snow/blizzard conditions to northern-central Plains with a heavy rain/severe weather threat for southern areas next week... ...Overview... A major storm system will be exiting the Rockies by the start of the period on Tuesday, with high confidence for a deep low pressure system to emerge into the Plains. This system should bring a wide array of hazards including heavy rain/severe weather to the south and heavy snow/blizzard conditions across parts of the north-central Plains and into the Upper Midwest around midweek with some chance of freezing rain for some as well. This storm system should track steadily eastward bringing a heavy rain threat to parts of the Southeast and Mid-Atlantic (with some more uncertain snow potential farther north) later next week. ...Guidance/Predictability Assessment... Through around midweek, model guidance has been and continues to be well clustered for the overall pattern setup involving a vigorous surface low consolidating in the central Plains and shifting toward the Midwest. While there remain some minor wiggles in the timing of the surface low track and its associated fronts, with recent CMC runs on the faster side and UKMET runs a little slower through the 00Z cycle, they seem to be converging a little better for the 12Z cycle. By day 4/Wednesday and beyond, there has been quite a bit more model waffling with the evolution of the upper and surface lows particularly in the deterministic GFS and GEFS ensembles. Yesterday's 12Z, 18Z, and today's 00Z runs indicated a faster weakening and progression eastward of the upper low (and shunting of a surface low directly east) likely due at least in part to interaction with an upstream Pacific upper low that other models kept separate. However, the 06Z GFS and GEFS trended back toward other guidance with these features, and their 12Z runs appear even more in line. So hopefully the earlier GFS/GEFS runs were an anomaly and models stick with better agreement on the type of solution that has been most consistent in the ECMWF suite--namely, the upper low gradually tracking eastward late week which helps to form a surface low in the Southeast that looks to track as a nor'easter near the coast. The CMC had been faster with that surface low for a few runs now, but the 12Z came in slower. There is still uncertainty with the details of the exact track of this potential nor'easter, which will have notable implications for precipitation types across the East, but at least confidence in the pattern on a broad scale is rising especially given the recent 12Z models. The WPC forecast leaned toward the more agreeable 00Z ECMWF/EC ensemble mean and 06Z GEFS mean by the latter half of the forecast period, which was similar to the previous WPC forecast and now to the newer 12Z guidance. Upstream farther west, differences remain regarding an upper low closing off in the Pacific, but models (especially the 12Z suite) seem to be converging on a solution that keeps the upper low farther out into the Pacific. Additionally, periods of energy may dip south from the far northern stream and could interact with the central U.S. upper low. Uncertainty remains given the blocky pattern over the Pacific/western North America. The model blend favoring the 00Z ECMWF and EC mean and 06Z GEFS mean also worked reasonably well for this area. ...Weather/Hazard Highlights... As the upper trough emerges into the Plains and a surface low consolidates and deepens, deep ample moisture will spread ahead of a cold front to produce significant rainfall across the Lower/Middle Mississippi Valley into the Tennessee Valley/Appalachians and vicinity, steadily shifting east with time. The latest WPC experimental Excessive Rainfall Outlooks show Slight Risks across eastern Arkansas, southeastern Missouri, far west Tennessee/Kentucky, and northern Mississippi on Tuesday, with the Slight shifting east into the Tennessee Valley/southern Appalachians and northern parts of the Southeast on Wednesday where streamflows and soil moisture are above normal, which could enhance flooding potential. The Storm Prediction Center continues to monitor and highlight the potential for strong to severe convection across the lower Mississippi Valley Tuesday and the central Gulf Coast on Wednesday as well. 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 maxed out WPC Winter Weather Outlook probabilities, with some mixed precipitation/freezing rain potential in the transition zone as well as along the eastern edge of the precip shield near/north of the warm front. By the latter half of the week, current forecasts show precipitation is likely to spread across the East Coast along and ahead of the frontal system, with potential for wintry weather spreading across the Great Lakes region and Northeast. But precipitation types and areas of heaviest precipitation have a great deal of uncertainty especially considering possible coastal low development off the Mid-Atlantic coast late week. The West to High Plains should see below normal conditions through the period, with the most widespread readings of generally 10-20F below normal for high temperatures behind and underneath the upper low. On the other hand, temperatures will initially be above normal centered across eastern parts of the Plains and the Mississippi Valley Tuesday and shifting east. Anomalies will be greatest for morning lows, with some plus 15-25F anomalies across the southern tier through Thursday. Highs are forecast to be around 10-15F above normal for the central and then east-central U.S., locally higher ahead of the storm system. Much of the country looks to be near or below normal by next Friday and Saturday after the cold frontal passage as high pressure overspreads much of the country. Tate/Santorelli 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