Extended Forecast Discussion NWS Weather Prediction Center College Park MD 207 AM EST Wed Jan 1 2025 Valid 12Z Sat Jan 04 2025 - 12Z Wed Jan 08 2025 ...Broad chances for impactful wintry weather across the north- central U.S. to Mid-Mississippi and Ohio Valleys to the East Coast Saturday-Monday... ...Overview... An upper trough moving across the Interior West into the High Plains over the weekend will spin up a surface low over the southern Plains by Sunday. These features will combine with moisture to produce precipitation in much of the central and eastern U.S. into early next week. On the southern side, heavy rain and thunderstorms are likely over parts of the Lower Mississippi and Tennessee Valleys. Meanwhile colder than average temperatures in place farther north will lead to a broad area of possibly impactful snow accumulations in the north-central Plains to Mid- Mississippi Valley and Ohio Valley this weekend, perhaps stretching into the Mid-Atlantic Monday. Freezing rain and sleet are likely in the transition zone, which could be hazardous as well. Then additional energy diving into the West next week should lead to broad upper troughing and near to below average temperatures for most of the lower 48. ...Guidance/Predictability Assessment... The primary 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 generally 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 12Z GFS was farthest south while the 18Z and new 00Z have both bumped north. The new 00Z GFS also slowed down compared to its 12/18Z runs while the new 00Z ECMWF trended even faster. CMC runs have been in between the GFS and EC camps on timing, as was the 12Z UKMET (though the UKMET ended up farther north than consensus by Monday). Generally, an intermediate solution in between the slower and faster models seems like the wisest forecast, so the WPC forecast was in the middle by incorporating the CMC/UKMET and ensemble means blended with the GFS/EC. This also matched the previous forecast low track reasonably well. By Tuesday-Wednesday, models diverge with whether or not energy diving south across the West will pull off into a closed low somewhere near the eastern Pacific/California/Mexico border. The 12Z CMC and ECMWF created this low while GFS runs did not, but the 00Z GFS run has tended toward having a closed low by Wednesday, though much farther west of consensus. The blend was over half ensemble means by this forecast period, which were more phased (did not show the closed low) as expected, but will continue to monitor for future forecasts. ...Weather/Hazards Highlights... Modest precipitation including higher elevation snow is forecast across the Northwest Saturday as an upper trough provides support. Some snow will likely stretch into the northern Plains. As the surface low consolidates into Sunday, precipitation is expected to push across the eastern half of the U.S. with good dynamical support. Rain across the southern tier (Lower Mississippi Valley to Tennessee Valley and Southeast) may be heavy and strong thunderstorms are possible as well. Have a Marginal Risk delineated on Day 5/Sunday in the ERO as a starting point. Cold air in place will ensure snow across some areas farther north. Probabilities for "plowable" snow/sleet continue to increase for parts of the Mid- Mississippi Valley into the Ohio Valley peaking on Sunday. Then there is a relatively broad area that may see mixed precipitation types, transitioning between multiple p-types and/or seeing snow and sleet, most likely over parts of the central Plains into Middle Mississippi Valley to the Tennessee and/or Lower Ohio Valley. By Monday some snow and ice probabilities look to come across the Appalachians into the Mid-Atlantic and southern parts of the Northeast, but with low confidence in the amounts. 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. Elsewhere, lake effect snow is likely to continue Saturday in cold brisk flow ahead of the main system, tapering off on Sunday, but chances may get renewed by Tuesday and beyond after the low exits. The Northwest could see additional rounds of light to moderate precipitation Sunday. Some light precipitation could occur in the Intermountain West and Rockies into next week, with some uncertain chances for precipitation into the southern Plains and Gulf Coast as well. 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 temperatures at times not reaching or 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 next week, especially if areas are snow covered and hold temperatures down. Meanwhile, initial upper ridging in the western U.S. will allow for above normal temperatures in the Intermountain West to southern Plains over the weekend. Highs in the 80s are likely in south Texas ahead of a cold front. Temperatures are forecast to cool to near normal in the Southwest and below normal in the southern Plains into the workweek. 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