Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 AM EST Tue Dec 12 2023 Valid 12Z Fri Dec 15 2023 - 12Z Tue Dec 19 2023 ...A wet pattern is likely over the South/Southeast... ...Overview... A southern stream upper low traversing the south-central to southeastern U.S. during the medium range period will be a primary driver of the sensible weather, including rain and snow in the southern Plains late this week. Rain will potentially spread into the Southeast into the weekend and early next week, though with more uncertainty as models struggle to agree on a possible Gulf surface low forming, in part due to differences with possible interaction of a northern stream shortwave. Ahead of this possibility, moist easterly flow into southern Florida could cause localized heavy rain into Friday. Fairly progressive mid-upper flow in the northern stream is likely with a series of shallow troughs moving through the north-central and northeastern U.S., while mean ridging builds in the West ahead of eastern Pacific troughing. ...Guidance/Predictability Assessment... Model guidance continues to be agreeable with the southern stream upper low system into Friday, but model spread rapidly increases into the weekend. Models have been waffling over the last few days with the strength and track of a northern stream shortwave likely moving southeast through the northern Plains Friday. This feature may combine with the southern stream upper low in the east-central U.S. in some way, and most models from the 12/18Z model suite and now the incoming 00Z have trended toward some sort of combination. GFS runs indicate the northern stream energy dropping south to become part of the upper low, while the ECMWF and CMC create more of a phased trough for a time before additional southern stream energy comes in to help close off a low (with different timing between the two). These differences aloft create ample differences in models and ensemble members for a resultant surface low track and strength. A day ago, GFS runs (with some GEFS member support) were the fastest and farthest north to bring the low track across Florida. Now the GFS runs have slowed but still look to be on the west side of the spread (resulting in more QPF onshore in the Southeast). Meanwhile the 12Z and newer 00Z CMC switched to the faster side of the spread in bringing a low north. The 12Z ECMWF seemed to be a good middle ground in track/timing but the 00Z ECMWF is even stronger and faster in bringing the low north as a nor'easter. Confidence is very low in the low track and QPF given the models are in such flux. The northern stream shortwave is currently over the northeastern Pacific, a classic data-devoid region that leads to uncertainties in the models downstream. Elsewhere, upper ridging seems well predicted in the West behind these features and ahead of large scale eastern Pacific troughing. The timing of the troughing edging toward the West Coast early next week with an initial shortwave perhaps entering the lower 48 Monday-Tuesday still shows some spread but nothing too egregious for the Days 5-7 timeframe. The WPC forecast started with a blend of deterministic guidance favoring the ECMWF and GFS, but with lessening amounts of the deterministic models in favor of the EC and GEFS ensemble means, reaching over half ensemble means by Days 6-7. Expect changes to the forecast that could be significant in future cycles. ...Weather/Hazards Highlights... The upper low traversing the southern U.S. combined with ample moisture advecting northward will lead to widespread rain across Texas as the medium range period begins Friday. Instability may be somewhat limited except right near the Gulf Coast, but the dynamical support the upper low provides could lead to heavy enough rain to cause some flooding issues, so a Marginal Risk is in place in the Excessive Rainfall Outlook for the eastern half of Texas and into western Louisiana on Friday. There may be some lingering snow on the northwestern side of the precipitation shield in the Texas Panhandle, but most of the snow chance has phased into the short range/Thursday now. By Saturday there is increasing uncertainty with how much rain falls onshore versus offshore in the central Gulf. This iteration of the WPC forecast trended down in rain totals into the Lower Mississippi Valley and Southeast as model guidance is reduced with amounts other than right along the coast. A Marginal Risk is planned for Saturday for coastal areas of Louisiana where the best chance of heavy rain is (coincident with some instability) but this may need to be expanded if models trend north, or removed if they trend south. The same type of forecast issue is seen across Florida, where there is uncertainty with placement of heavy rain on the southern side of a strong surface high in easterly flow. For Friday a Marginal Risk remains in place across south Florida to the Keys where the best model compromise for heavy rain is, but there is potential that precipitation could focus north or south. This low confidence continues further into Saturday as the upper levels and surface low track still show model spread. The general consensus is that some heavy rain should be spreading north by Saturday, so a Marginal Risk reflects this. Precipitation reaching farther north along the Eastern Seaboard into early next week is quite uncertain at this time. Elsewhere, a clipper system should spread rain and snow to the Upper Midwest/Upper Great Lakes on Friday and perhaps lingering into Saturday. Light Pacific Northwest precipitation is also possible. By early next week, eastern Pacific upper troughing should sneak energy and perhaps an atmospheric river into California and spread precipitation there, but there remains uncertainty in timing when this will occur, which affects the precipitation amounts. The Interior West is forecast to stay dry. The northern two-thirds of the lower 48 should see near to above normal temperatures late week 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 late week, 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