Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 PM EST Fri Dec 13 2024 Valid 12Z Mon Dec 16 2024 - 12Z Fri Dec 20 2024 ...Overview... The pattern during early next week will be characterized by progressive shortwaves carrying a couple of cold fronts marching eastward from the central U.S. to the East Coast, with moderate to potentially locally heavy rain. Another shortwave will affect the West Coast/Northwest with a round of precipitation Monday-Tuesday. By the latter half of the week, a significant pattern shift is likely, with amplifying mean troughing covering the central-eastern states while ridging develops upstream over the West. This should promote warmer and drier conditions in the West and cooling temperatures in the East, and there may be an emerging nor'easter threat by late next week but with low confidence. ...Guidance/Predictability Assessment... Besides some relatively minor timing/surface detail differences, the models are still in good agreement the first part of next week as a shortwave moves through the Midwest into the Northeast, and another shortwave enters the West. By midweek upper ridging is forecast to form over the West Coast states, but a shortwave and surface low west of the ridge in the eastern Pacific is uncertain in placement and could be relatively strong. At first glance the ECMWF runs are quite aggressive and farther east than other models like the GFS, but most AI models cluster well with it. Preferred something like the EC mean for this feature, weaker than the control run of the EC but with similar positioning. This shortwave is one contributing factor to maintaining the central-eastern trough into later week. Meanwhile, details of the central-eastern trough diverge even before this feature arrives. Most notably, the 00Z ECMWF formed a closed low in the southern stream, causing an associated surface low to slow considerably and track farther inland compared to the phased trough and quickly moving surface low of the GFS runs. The CMC did show some amplification of the southern stream as well, but not to the degree of the ECMWF. Incoming newer 12Z models may be reaching some compromise on a southern stream closed low developing at some point not as far south as the 00Z ECMWF, though there is still some spread, and the lack of run-to-run consistency in models keeps confidence low. The forecast did trend toward more precipitation along the Eastern Seaboard than the previous forecast had, given the slowing low track through the western Atlantic. The WPC forecast was based on a 00/06Z model/mean blend through the period, with more deterministic models than means Days 3-4, even weighting of models and means Day 5, and more means than models Days 6-7 given the spread. ...Weather/Hazards Highlights... A cold front moving across the central and eastern U.S. Monday- Tuesday should be accompanied by some modest precipitation. Areas from the Lower to Middle Mississippi Valley into the Ohio Valley are likely to see the heaviest rain since rain amounts should decrease in the East Tuesday. Areas right near the Ohio Valley like southern Illinois/Indiana and northern Kentucky appear to have the best chance for rain totals over an inch Monday, but lack of instability may limit rain rates. Meanwhile farther southwest, instability should be higher, but any heavy rain amounts should be more scattered and there are ample placement differences in the models, as well as some timing differences that include heavier amounts taking place on Sunday rather than Monday. These areas generally have dry antecedent conditions in terms of soil moisture and streamflows, though some rainfall expected in the short term may temper the initial dryness. So there are some positives and negatives in terms of flash flooding ingredients and will hold off introducing an ERO risk area for Monday. Details of the surface pattern and corresponding precipitation coverage, amounts, and timing over the central/eastern U.S. become increasingly uncertain by mid next week, but models do continue to indicate another round of moderate to heavy rainfall emerging in the south-central U.S. to Mid-South. Precipitation may spread across the Eastern Seaboard on Thursday-Friday, with some wintry weather possible on the northwestern side of the low track. This forecast indicates some 10-30 percent probabilities of 0.25" QPF in the form of snow in the Interior Northeast especially in higher elevations, but continue to monitor forecasts because snow amounts and placement will be quite sensitive to the uncertain eventual low track. The northern half of the West Coast and into the Northwest should see another episode of rain and higher elevation snow on Monday lasting into Tuesday. After that, the West should see drier conditions underneath ridging, other than perhaps some moisture into western Washington rounding the ridge. Within an initial generally mild pattern across the lower 48, warmest anomalies should extend from the eastern slopes of the Rockies to the East. Expect a decent area of highs 10-15 degrees above normal and anomalies up to plus 15-25 degrees in places for morning lows. After Wednesday, temperatures should trend back to normal across the central-eastern U.S. with some below normal readings possible across the northern Plains as arctic high pressure nudges into the region. Out West, expect temperatures to trend warmer underneath persistent amplified ridging. Tate/Santorelli 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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