Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Thu Jan 2 2025 Valid 12Z Sun Jan 05 2025 - 12Z Thu Jan 09 2025 ...Winter storm to bring heavy snow and hazardous ice from the Central Plains to the Mid-Atlantic... ...Overview... A winter storm is likely on Sunday from the Central Plains to Mid- Mississippi and Ohio/Tennessee Valleys and the Mid-Atlantic, lasting into Monday for eastern areas. Heavy snow is possible, along with significant sleet and freezing rain in the Mid-South perhaps into the Southern Appalachians. On the southern side, heavy rain and thunderstorms are likely over parts of the Lower Mississippi and Tennessee Valleys and Southeast. The weather is forecast to quiet down in terms of precipitation after this system, aside from some relatively light lake effect snow and a couple of areas of light precipitation possible in the western and central U.S., but colder than average temperatures are likely to persist through the week in most areas east of the Rockies. ...Guidance/Predictability Assessment... The first model diagnostics concern during the medium range period is with the surface low across the central and eastern U.S. and its track and timing, naturally related to the upper trough's track and timing. ECMWF runs, including some AI/ML models and ensemble members, have consistently been on the faster side of the guidance. GFS runs have been on the slower side, but with more north/south variability (causing p-type differences). The 18Z GFS was particularly far south, and preferred the 12Z GFS track instead. The 12Z model cycle (GFS, UKMET, and CMC) did trend faster with the low track, and the WPC forecast followed with a quicker low, precipitation timing, etc. compared to the previous forecast. However, the newer 00Z models have now diverged again somewhat, as the ECMWF is even faster while the GFS/CMC/UKMET are similar to or a touch slower than their 12Z runs. So clearly there is some lingering spread in the timing of the low, affecting precipitation timing. At least the track of the low itself is generally converging in latitude despite the timing differences. However, even small shifts north or south will cause notable p-type differences so continue to monitor for any forecast changes. Then, models continue to support broad troughing in the central and eastern U.S., while energy diving south through the western part of the trough over the West shows more spread, especially by the time it reaches the Southwest. Models are split, with the non- NCEP guidance separating an upper low in the southern stream, while GFS runs do not show stream separation though have some energy lingering on the southwestern side of the trough. The WPC forecast took an intermediate approach in showing some southern stream troughing pulling off into northern Mexico without fully separating an upper low at this point, but this may need to be adjusted in future forecasts. The WPC forecast was based on a blend of the 12Z GFS/UKMET/CMC/ECMWF early in the period. Gradually reduced the proportions of the deterministic models in favor of the ensemble means as the forecast progressed, reaching half ensemble means by Day 7. ...Weather/Hazards Highlights... An upper trough in the central U.S. Sunday will produce a consolidating surface low in the Southern Plains. Precipitation is expected to push across the eastern half of the U.S. with this low and good dynamical support aloft. Cold air in place will ensure widespread snow that could be heavy across some areas of the Central Plains, Mid-Mississippi Valley, and Ohio Valley peaking on Sunday. Snow could push into the Mid-Atlantic Sunday night and/or Monday depending on the speed of the low track. Just to the south, there is a relatively broad area that may see mixed precipitation types, transitioning between multiple p-types and/or seeing snow and sleet/freezing rain. Potentially significant sleet and freezing rain are possible across the Ozarks east to the Tennessee Valley. The exact surface low track, which is still uncertain, will play a significant role in precipitation amounts, types, and timing. So keep an eye on future forecasts as details are likely to change. WPC is issuing Key Messages for this winter storm. Meanwhile farther south in the warm sector, severe weather and heavy rain that could cause isolated flooding are possible across the Lower Mississippi Valley to southern Tennessee Valley and Southeast. A Marginal Risk still seems prudent for the Day 4/Sunday ERO due to high rain rates, but fast movement of the storms should temper flooding potential. Lake effect snow may continue into Sunday, and then become rejuvenated with cold and brisk flow behind the main low pressure system. Most areas are forecast to see relatively light snow accumulations. Light to modest precipitation including higher elevation snow is expected across the Northwest on Sunday, and light amounts may be possible through the week in the Rockies, and eventually the Northern Plains with a weak surface low. Farther south, there are some uncertain chances for precipitation in the Southwest to Southern Plains and Lower Mississippi Valley. With the cold air in place from Texas eastward, some areas could see wintry precipitation, so this potential will continue to be monitored. Colder than normal temperatures are likely across the central and eastern U.S. underneath rounds of upper troughing and with cold surface high pressure systems. Notable cold is possible in the northern tier, with even high temperatures barely reaching 0F and lows between -10F and -20F in some areas. This should not be record breaking, but the cold could still be hazardous as wind chill values may dip below -20F to -30F. Temperature anomalies of 10-20 degrees are also likely across parts of the northern/central Plains to Middle Mississippi and Ohio Valleys to Appalachians into much of next week, especially if areas are snow covered and hold temperatures down. Meanwhile, initial upper ridging in the western U.S. will allow for slightly above normal temperatures in the Intermountain West, and 10-20 degrees above normal anomalies in the south-central U.S. Sunday will cool to below normal behind a cold front by Monday. The upper pattern in the West is uncertain by midweek next week, causing temperatures to be in question, but there may be a trend toward warming in for parts of the West. 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 (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