Extended Forecast Discussion NWS Weather Prediction Center College Park MD 306 AM EST Thu Dec 14 2023 Valid 12Z Sun Dec 17 2023 - 12Z Thu Dec 21 2023 ...A wet pattern is likely for the Eastern Seaboard into next week... ...Overview... A surface low is forecast to track along or near the Eastern Seaboard during the first part of next week, spreading widespread precipitation to the East, including some heavy rainfall causing flooding concerns. However, amounts and placement of the heavy rain remain quite uncertain given model differences in the upper levels and the surface low track. Behind these features, dry upper ridging will build into the Interior West and shift into the Plains ahead of a mean eastern Pacific trough axis that should push periodic precipitation into the West Coast states. ...Guidance/Predictability Assessment... Model guidance has struggled for days regarding the evolution and track of the East Coast surface low due to differences aloft as northern stream energy dives southeastward across the Midwest and interacts with a southern stream upper low. Recent guidance still shows considerable spread, leading to lower than average confidence in the sensible weather forecast details like QPF, especially considering this is now a Day 3-5 forecast. GFS runs have been persistently farther west/inland with the low track compared to the non-NCEP guidance for Sunday-Tuesday. Still did not favor a solution as far west as the GFS, but the other guidance has continued its trend in shifting the low west likely staying onshore rather than in the western Atlantic, compared to an offshore track a day ago. Thus the WPC forecast for the fronts/pressures and QPF/ERO have shifted inland as well. The 12/18Z ensemble means seemed to be lagging a bit in terms of still showing an offshore track of the low around Monday. There may be a couple reasons for this...with surface high pressure coming in behind the low, more low pressure will be seen farther east, and also perhaps because the lows offshore can gain strength more quickly than onshore lows due to less friction, skewing the average east/offshore. Certainly the ensemble members themselves have been showing a lot of spread. Not only is there this west-east spread among models, there is ample north-south spread in the timing of the low too in the deterministic models and the ensemble members. The 12Z CMC seemed like an outlier in showing a fast low track northward, but the newer 00Z CMC seems more reasonable. Through Sunday-Monday the newer 00Z ECMWF/GFS/UKMET seems reasonably similar to their previous runs, but by late Monday into Tuesday there is a marked trend toward pivoting the low farther inland across the Lower Great Lakes or so, so this will continue to be monitored. Additional low pressure may form and track across the Atlantic (likely farther east) into the middle part of next week. Upper ridging west of these features seems well handled by the model guidance. Farther upstream, an initial upper low in the eastern Pacific will send shortwave energy east into the West early in the workweek, which ends up showing some differences in the amplitude of shallow troughing in the north-central U.S. by around midweek (GFS runs with more troughing). Meanwhile models show considerable energy dropping southeast from the northern stream and forming a closed low in the Pacific. There are some timing and placement differences with the upper low but nothing too egregious for the late medium range period. The WPC forecast was based on the 12/18Z guidance that was available at forecast production time. More specifically, a blend of the 12Z ECMWF/UKMET/GFS/CMC deterministic guidance along with a bit of the GEFS and EC ensemble means was used early in the period. With time, gradually transitioned toward favoring the ensemble means by more than half Days 6-7. ...Weather/Hazards Highlights... The surface low and deep troughing aloft will support heavy rainfall that could be widespread on Sunday and Monday. As mentioned there are significant differences in the placement and amounts of the rainfall. This forecast continues the trend toward more rainfall farther west in the eastern U.S. on Sunday. The Slight Risk in the Excessive Rainfall Outlook was expanded westward as well to reflect the latest model trends, and now covers much of the Carolinas into southern Virginia, where many models have multiple inches of rain. Instability will likely be maximized near the coast for some higher rain rates, but there is enough dynamical support for lift that heavy rainfall causing instances of flooding is possible inland as well. Then by Monday, the low lifting quickly northward should spread rain (and perhaps snow on the backside, though this is a fairly warm system overall) into the Northeast. A Marginal Risk is in place for the potentially heavy rainfall. An upgrade to a Slight Risk may be needed especially for Maine given the wet antecedent conditions and thus sensitivity to heavy rain there, but in collaboration with the Caribou and Gray, Maine NWS offices, held off for now. Precipitation should pull away on Tuesday but lingering coastal showers could continue. Precipitation chances will be increasing next week for California as shortwave energy supporting a possible atmospheric river approaches. Rain amounts and rates on Sunday and Monday could be sufficient to cause some flooding concerns, especially given some instability could reach coastal areas, so Marginal Risks are in place in the ERO. Precipitation looks to shift a bit north Monday compared to Sunday and reach the Pacific Northwest into Monday-Tuesday, including some higher elevation snow, though confidence in the details remains somewhat low. Meanwhile the High Plains to the Mississippi Valley will be the dry spot over the lower 48 through much of next week. Areas from the West into much of the Plains will see warmer than normal temperatures next week underneath upper ridging, with temperatures 10-20F above average periodically affecting the northern/central High Plains into the Upper Midwest. Meanwhile the northeastern U.S. should see above normal temperatures early in the week but moderating back toward normal by Tuesday behind the low pressure system. The Southeast including Florida looks to be the main cooler than average spot next week behind multiple frontal passages. 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 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