Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Dec 13 2023 Valid 12Z Sat Dec 16 2023 - 12Z Wed Dec 20 2023 ...A wet pattern is likely over the South/Southeast and potentially up the East Coast this weekend into next week... ...Overview... A southern stream upper low traversing the south-central to southeastern U.S. interacting with northern stream energy this weekend into early next week will provide support for a surface low to strengthen and spread widespread heavy rainfall across Florida on Saturday. Heavy rain may spread farther north into early next week, but its placement remains quite uncertain given model differences in the upper levels and the surface low track. Behind these features, dry upper ridging will build in the Interior West ahead of a mean eastern Pacific trough axis that should push periodic precipitation into the West Coast states. ...Guidance/Predictability Assessment... Model guidance has shown good agreement regarding the southern stream upper low located over the central U.S. on Saturday, but has varied quite a bit over the last several days with a northern stream shortwave and how it interacts and potentially combines with the southern stream feature as it dives southeast from the north-central CONUS. The differences aloft lead to considerable spread in the resultant surface low strength and track from model to model and run to run. After the surface low develops in the Gulf, it looks to move across Florida this weekend with at least some model/ensemble agreement. But then given the differences aloft, the low tracks after that vary greatly in the deterministic models with ensemble members showing some spread as well. Fortunately the 12Z/18Z ensemble means were clustered fairly well in showing a low tracking northeastward into the western Atlantic through Monday/Tuesday, though with slower movement compared to the previous forecast. The 12Z ECMWF appeared the closest to the ensemble mean low track through Monday or so. CMC runs show a similar track but faster moving, and the 12Z UKMET is similar as well but likely too strong. The GFS runs have been outliers in showing surface lows much farther west/inland, leading to widespread precipitation farther west than the non-NCEP guidance that is better clustered with the ensemble means. So the GFS solutions were not favored. The newer 00Z ECMWF switches back to a faster low track (looking similar to the CMC), so not only is there this west-east spread, north-south spread with timing is an issue. By Tuesday-Wednesday there is ample uncertainty in whether the low just meanders over the western Atlantic or rushes north, with perhaps additional low pressure forming over the Atlantic. The WPC forecast favored a blend of the 12Z ECMWF with the GEFS and EC ensemble means through the forecast period, with small proportions of other guidance early on, increasing the percentage of ensemble means to half by Day 5 and more by 6-7 given the model spread. This blend also worked for eastern Pacific to West Coast troughing, though the 12Z ECMWF pulled a closed upper low farther west of other guidance by Tuesday-Wednesday. The 00Z EC seems more reasonable, though is actually still on the western side of the guidance envelope as the 00Z GFS and CMC trended east too. ...Weather/Hazards Highlights... Ample moisture with precipitable water values likely well over the 90th percentile will be in place across Florida as the southern stream upper low enhances lift and develops a surface low. These factors will lead to widespread heavy rainfall of 2-4 inches with locally higher amounts likely. A Slight Risk has been raised for Day 4/Saturday in the Excessive Rainfall Outlook, as rainfall of this magnitude with some high rain rates could cause instances of flash flooding. Heavy rain and flooding may be a concern for the Lower Mississippi Valley as well on Saturday in the vicinity of another frontal system. A Marginal Risk continues to cover this potential as recent models have trended up with rainfall onshore, though there is still a chance that most could fall offshore in the central Gulf. By Sunday, increasing model spread makes rainfall totals and placement very uncertain. The WPC precipitation forecast is reflective of the model preference for a low moving northeastward into the western Atlantic, focusing rainfall from Georgia into the Carolinas into the Mid-Atlantic. A Marginal Risk is planned as a starting point across these areas for Day 5/Sunday. Slight Risk-level impacts are certainly possible but the uncertainty in placement of heavy rain was too large to define a higher risk area at this time, but the ingredients are there and an upgrade is likely at some point. Heavy rain could spread into the the Northeast on Monday depending on the surface low track. Precipitation chances will be increasing over the weekend for California as shortwave energy supporting a possible atmospheric river approaches. Rain amounts and rates on Sunday could be sufficient to cause some flooding concerns, especially given some instability could reach coastal areas, so a Marginal Risk is in place in the ERO. Precipitation looks to persist across California and reach the Pacific Northwest into Monday-Tuesday, including some higher elevation snow, though confidence in the details remains somewhat low. The Interior West to the Plains will be the dry spot over the lower 48 through the first half of next week. The northern two-thirds of the lower 48 should see near to above normal temperatures this weekend into early next week, with temperatures 10-20F above average periodically affecting the northern/central High Plains into the Upper Midwest. The West can also expect generally above normal temperatures underneath upper ridging. Cooler than normal temperatures will be limited to the southern Plains (in terms of highs) eastward into the Southeast this weekend, moderating closer to normal by early next week, though Florida could still see below normal highs 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