Extended Forecast Discussion NWS Weather Prediction Center College Park MD 219 PM EST Wed Dec 06 2023 Valid 12Z Sat Dec 09 2023 - 12Z Wed Dec 13 2023 ...Pattern Overview... Guidance shows broad cyclonic mean flow aloft persisting over the eastern two-thirds of the lower 48 through most of the medium range period. Initially, a deep upper-level shortwave will emerge from the West late this week, supporting a wavy surface front that should cross the eastern half of the country during the weekend into early Monday. Significant precipitation is likely from the Gulf Coast to the Great Lakes and into the Northeast as this system progresses, which could cause flooding concerns. Meanwhile, the Pacific Northwest will be dealing with another surge of Pacific moisture on Saturday that may result in scattered instances of flooding. By next Monday-Wednesday, an amplifying Pacific into Northwest U.S. ridge will help to reinforce the mean trough over the East but with a positive tilt that will lead to more benign weather over most areas. Incoming Pacific energy may eventually form a trough or upper low over the southwestern states. ...Guidance Evaluation/Predictability Assessment... The models and ensembles continue to show good general agreement on the larger scale aspects of the forecast but differ on some important details. During the weekend, recent guidance runs have been changing the specifics of how shortwave energy is distributed within the overall upper trough crossing the eastern half of the country--leading to differences at the surface. Instead of prior consensus that had depicted a dominant Plains through Great Lakes into eastern Canada system, the latest clustering has toned down the strength of the leading wave crossing the Great Lakes somewhat but then shows additional frontal wave development over the Appalachians into New England and southeastern Canada. The scale of shortwave details involved is small enough to have low predictability 4-5 days out in time so additional changes are certainly possible. However the overall pattern evolution maintains the idea that this system will produce of a broad area of significant precipitation and potential for some brisk to strong winds. Farther upstream, recent trends (minus the 12Z CMC) have been less amplified and/or farther north with energy that drops into southern Canada by early next week, leading to a bit of a slower trend with southward frontal progression over the central U.S. by next Tuesday. Back toward the West Coast, the models vary considerably with small-scale/low-predictability shortwave details that will affect the duration and coverage of significant precipitation over the Pacific Northwest during the weekend. Latest CMC runs have been on the deep/slow side of the spread with its primary shortwave. This leads into the middle of next week when CMC runs are likewise farthest west with an upper low that could form over the Southwest/California. Recent trends have been leaning toward at least some degree of flow separation, though the closed low has been new enough in the guidance not to go completely to in the updated forecast based on 00Z/06Z runs. New 12Z guidance (including the GFS/GEFS, albeit with a slight timing delay) maintains the closed low potential. A composite of 00Z/06Z operational runs for the first half of the period followed by incorporating some 06Z GEFS/00Z ECens means again provides a good reflection of latest guidance ideas with a combination of continuity or trends depending on the feature/region. The blend excluded the 00Z CMC late due to greater differences from consensus. ...Weather/Hazards Highlights... Expect another surge of Pacific moisture to impact the Pacific Northwest on Saturday, and the region will be particularly sensitive to any additional rainfall after receiving significant rainfall earlier in the week. The Day 4 Excessive Rainfall Outlook continues to show a Marginal Risk of Excessive Rainfall for western Washington and Oregon into far northwestern California with an embedded Slight Risk area along the coast. However note that most of the new 12Z guidance is reducing the heavy rain potential in far northwest California. The outlook maintains the aforementioned areas since this trend is not quite unanimous and there has been some variability in heaviest axis thus far, but persistence/consolidation of the trend in the next cycle would favor a northward adjustment to the southern extent of risk areas. In addition to heavy rainfall, widespread moderate to locally heavy snow will be possible in the higher elevations of the Cascades and northern Rockies as moisture pushes inland. Precipitation will lighten up across the region thereafter but with considerable uncertainty over how quickly. The majority of guidance trends light enough on Sunday to preclude any risk area for Day 5, but the heavy extreme (via stronger and slower upper shortwave in the CMC) would have locally significant totals extend into Sunday. Will continue to monitor guidance for any change to the current lighter majority cluster. Some of the wave details have been changing for the front crossing the eastern half of the country this weekend, but there continues to be a good signal for this overall system to spread a broad area of moderate to heavy rainfall from the Gulf Coast to the Great Lakes and eastward toward the East Coast. The Day 4 Excessive Rainfall Outlook maintains a Marginal risk area from the Gulf Coast to parts of the Mid-Mississippi and lower Ohio Valleys, while Day 5 holds onto a Marginal Risk area from the Florida panhandle through the Mid-Atlantic and into much of the Northeast. Primary adjustment in this afternoon's update was to account for a slightly faster trend in the guidance for both days. 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. Current snow cover over parts of the Northeast could also exacerbate any runoff issues whether it melts before or during the rainfall event. In addition to potential flooding, thunderstorms will likely threaten areas along the Gulf Coast and the Lower Mississippi Valley on Saturday. The Storm Prediction Center has highlighted an area centered on East Texas and the Ark-La-Miss where severe thunderstorms are expected. Some snow will be possible in the northwestern side of the precipitation shield from the north-central Plains/Midwest into the Great Lakes, but details remain uncertain due to guidance variability for surface wave specifics. Some snow may also fall after system passes, mainly over the Great Lakes into the Appalachians. Finally, brisk to strong winds are likely to develop over portions of the Rockies and Plains on Saturday as the pressure gradient tightens between the wavy front and strong Interior West high pressure. Strong winds will also be possible in the warm sector along the Mid-Atlantic into New England coasts Sunday and perhaps into Monday (northern areas). Otherwise, parts of the southern Plains and far Southeast (especially Florida) may see increasing rainfall toward midweek, aided by moisture beginning to interact with a front and establishment of moist easterly flow respectively. An approaching frontal system may bring light precipitation into the Pacific northwest by next Wednesday. In terms of temperatures, much of the West will experience below normal temperatures in the wake of the upper-level shortwave on Saturday, followed by a rebound to near or slightly above normal readings late weekend into early next work week and perhaps a slight decline by midweek. Above to well above normal temperatures will prevail in the warm sector ahead of the wavy front crossing the eastern half of the country during the weekend, with widespread plus 10-25 degree anomalies. A few locations over or near the northern Mid-Atlantic could challenge daily record highs on Sunday. Temperatures will return to near normal after the system passes, aside from some below normal highs near the Gulf Coast and moderately above normal readings over the northern/central Plains early next week. Rausch/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