Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Thu Dec 07 2023 Valid 12Z Sun Dec 10 2023 - 12Z Thu Dec 14 2023 ...Pattern Overview... Broad cyclonic mean flow aloft is forecast across the eastern two-thirds of the lower 48 through mid-next week, then the upper trough will gradually shift offshore. A vigorous shortwave moving from the southern Plains to the Northeast over the weekend will support a strong surface low pressure system that will bring significant precipitation from the Gulf Coast to the Great Lakes and Northeast. On Monday, the eastern system will push offshore, and an amplifying Pacific ridge will reinforce the mean trough over the East but give it a positive tilt. A few relatively weaker frontal systems will swing across the central and eastern U.S. next week, and models are starting to agree on an upper low forming over the Southwest Wednesday and Thursday. ...Guidance Evaluation/Predictability Assessment... Models are in fairly good agreement on the general upper level pattern, but there is still some uncertainty surrounding the system that will impact the eastern U.S. this weekend. Latest model guidance has been showing a leading low pressure center tracking from the southern Plains to the Great Lakes and a second wave of low pressure forming in the vicinity of the Mid-South and tracking across the Northeast. Even with changes in how models are depicting the structure of this system, there is still a consensus that there will be widespread moderate to heavy rainfall in the East. Through the rest of the period, there are some expected differences in timing of shortwaves, which will affect timing of surface features. Model agreement has been increasing overnight to show a closed upper low forming over the Southwest around Wednesday, which may help enhance precipitation mid-to-late next week. For this forecast package, the day 3-4 WPC model blend is comprised of deterministic model solutions with more weight on the ECMWF/GFS than the UKMET/CMC. The CMC continues to stray the farthest from the consensus, so it has the least weight in the blend. From day 5 onward, ensemble means were added in increasing amounts to help smooth differences between the deterministic runs. ...Weather/Hazards Highlights... Precipitation will be winding down in the Pacific Northwest on Sunday, and precipitation totals are not expected to be high enough to be hazardous. The CMC continues to show higher precipitation amounts than the other guidance, and if this solution comes to fruition, there could be isolated flooding in western Oregon and Washington. For now, no risk area was added to the Pacific Northwest in the Day 4 Excessive Rainfall Outlook, but we will continue to monitor model trends as new guidance becomes available. In the East, a broad area of significant precipitation will stretch from the Florida panhandle through the Mid-Atlantic to the Northeast on Sunday as a strong frontal system passes through. A Marginal Risk of Excessive Rainfall was maintained for this area in the Day 4 Outlook. The progressive nature of this system will help to limit rainfall totals. However, anomalous moisture and sufficient instability should still support rain rates high enough to produce some flooding concerns, and there may also be thunderstorm potential in the warm sector over the Southeast. Mixed precipitation and snow are forecast on the northwestern side of the system, which will likely impact higher elevations of the Appalachians and interior Northeast. In addition to potential precipitation hazards, elevated winds are forecast along the East Coast. The forecast trends drier for most of the nation through the rest of the medium range period, but a lingering frontal boundary will likely enhance precipitation chances in Florida and parts of the southern Plains and Southwest. Light precipitation will also return to the Pacific Northwest as a Pacific frontal system approaches mid-next week. In terms of temperatures, the forecast calls for well above average temperatures in the warm sector of the eastern frontal system Sunday and Monday. High temperatures will be 10-20 degrees above normal from South Carolina to the Northeast. Highs in a few locations in the Mid-Atlantic may challenge daily records. Temperatures will hover near for the rest of the nation through Tuesday, then above normal temperatures may develop over the northern Plains and Upper Midwest as a warm front lifts across the region. Dolan 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 forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages 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 https://www.wpc.ncep.noaa.gov/#page=ovw