Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Sun Dec 15 2024 Valid 12Z Wed Dec 18 2024 - 12Z Sun Dec 22 2024 ...Overview... The pattern should be turning much more amplified during the medium range period as mean troughing becomes established over the East, and strong ridging builds over the West. A couple of shortwaves will help reinforce the Eastern trough during the period, including an initial one late this week which should allow a low pressure system to lift from the Mid-South to the Northeast with potential snow on the north side of the track. The next shortwave digging down the west side of the trough should bring with it a fresh blast of arctic air to the East by next weekend, while the West turns progressively warmer. An atmospheric river may bring a period of heavy rain/mountain snows to parts of the West Coast by next weekend as well. ...Guidance/Predictability Assessment... Latest models and ensembles continue to show generally good agreement on the overall synoptic pattern but plenty of uncertainty in the details. Models are trending closer together with the initial shortwave off the Southeast Coast and more northerly/inland with associated low pressure off the Northeast Coast, but there does remain some uncertainty in the timing. The next shortwave should move through the Midwest into the East late week into next weekend with plenty of uncertainty in the details. Upstream, ridging builds over the West then eventually shifts eastward as the next shortwave moves into the West Coast next weekend. Generally good agreement on this setup, but uncertainty in the timing and evolution of energy as it progresses inland. The WPC blend for tonight leaned more heavily on the ECMWF, with minor contributions from the CMC and UKMET early period since the GFS was too slow/different with the Eastern trough. With increasing uncertainty, used increasingly more ensemble means mid to late week and next weekend. ...Weather/Hazards Highlights... A cold front shifting through the East mid week will be accompanied by modest precipitation. Moderate to heavy rainfall is possible across portions of the Tennessee/Ohio Valley into the central Appalachians but low rainfall rates and the overall progressive nature of the front should limit any sort of excessive rainfall/flash flood threat. Some wintry weather is possible on the northwestern side of the surface low which tracks near the Northeast Coast, with highest probabilities in the higher terrain. Behind this system, westerly flow across the still relatively warm lakes should allow for some lake effect snows, reinforced by the next shortwave through the region late week/next weekend. Meanwhile, out West, conditions should dry out by the medium range period, except for some increased precipitation associated with weak shortwaves into the Northwest. Increasing precipitation looks likely by next weekend with the next Atmospheric River into the region. 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. 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