Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EST Sat Dec 14 2024 Valid 12Z Tue Dec 17 2024 - 12Z Sat Dec 21 2024 ...Overview... The medium range period begins Tuesday with a shortwave shifting through the Northeast as another enters the West Coast. The latter will be responsible for a significant pattern shift as it amplifies across the Central-Eastern U.S. mid to late week and ridging develops upstream over the West. This should promote warmer and drier conditions across the West, and cooling temperatures and precipitation for the East, with a potential emerging nor'easter threat late week, but with low confidence. ...Guidance/Predictability Assessment... The overall synoptic trend is for a singular amplified trough to traverse the CONUS during the medium range period. The GFS/GEFS suite trended closer to a slower pattern with a cutoff low developing over the Four Corners region. Otherwise, there's a general consensus for the aforementioned synoptic trend. A general model blend consisting of the deterministic 00z EC/CMC/UKMET and 06 GFS were used through day 4. The ensemble means were introduced on day 5 and increased in weighting through day 7. ...Weather/Hazards Highlights... A lingering surface front across the Mid-South should be accompanied by some modest precipitation, but doesn't at this time appear hazardous. Details of the surface pattern and corresponding precipitation coverage, amounts, and timing over the central/eastern U.S. become increasingly uncertain by Wednesday though, but models do continue to indicate another round of moderate to heavy rainfall emerging in the south-central U.S. to Mid-South. There remains enough uncertainty to preclude any sort of risk areas on the Day 5/Wednesday Excessive Rainfall Outlook at this time, but there is some general agreement on enhanced rainfall across portions of the southern-central Appalachians where soil moisture is above normal. 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 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 Southwest/Rockies into to the East on Tuesday. 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 a warming trend out West. As troughing gets established across the East late week, an arctic high will spread below normal temperatures from the Northern Plains/Upper Midwest into much of the East by next weekend. Kebede/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