Extended Forecast Discussion NWS Weather Prediction Center College Park MD 205 PM 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 up the East Coast 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 into the Interior West ahead of a mean eastern Pacific trough axis that should push periodic precipitation into the West Coast states. ...Guidance/Predictability Assessment... There is fairly good agreement on the two streams of energy across the Midwest and the South, the interaction of which should induce cyclogenesis across the southeast Gulf of Mexico on Saturday which tracks across Florida into Sunday. There remains however a lot of uncertainty in the details of this interaction, and also with additional weaker energy in the Gulf. The models struggle run to run and model to model on the interaction between these three features which have huge implications for exact surface low track/placement and heavy rainfall potential across the Southeast and into the Mid-Atlantic and the Northeast as well. Based on the 12z guidance for today though, there does seem to be some trend towards a surface low track closer to the coast (or possibly even inland) across the Southeast and Mid-Atlantic. By Monday-Tuesday, additional energy dropping in from the Midwest looks to reinforce some kind of troughing still off the Southeast coast, with some support for surface low pressure to linger off the coast into next week. The 06z GFS favored this solution most, but its new 12z run today backed off on this idea. The CMC and ECMWF show the low racing northward up the East Coast. The various ensemble guidance offers a lot of spread all the way up the East Coast, but both the EC and GEFS means do support some sort of weak area of low pressure lingering off the coast. The WPC forecast for today trended mostly towards the ensemble means the second half of the period, with some blending of the previous shift continuity too, but this is a very low confidence forecast and highly depending on exact stream/energy interactions which could take until the short range period to fully resolve. Out West, there is a lot of uncertainty with troughing off the West Coast, but the 12z guidance for today does show better support for a cut off upper low deepening as it moves towards the California coast mid-next week. Lots of questions still on placement of this feature and effects on heavy rainfall potential across parts of California as well. ...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 continues on 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 far western Atlantic, focusing rainfall from Georgia into the Carolinas and the Mid-Atlantic but the latest 12z guidance shows better support for heavier rainfall even farther inland. For the day 5 ERO, there was enough agreement (and support from WFOs ILM, MHX, and AKQ) for a slight risk across far eastern North Carolina to southeast Virginia. A Marginal Risk surrounds this extending from northeast Florida, across much of the Southeast, and into the eastern Mid-Atlantic region where the inland extent for heavy rain impacts are much more uncertain. Heavy rain could also spread into the the Northeast on Monday depending on the surface low track, with models as of now showing some agreement on this potential across eastern New England. 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. Santorelli/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