Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Thu Dec 26 2024 Valid 12Z Sun Dec 29 2024 - 12Z Thu Jan 02 2025 ...Heavy rain and mountain snow likely for the West Coast/Northwest early next week... ...Overview... Progressive flow with multiple shortwaves rotating around the southern side of a south-central Canada upper low will be in place next week, producing many low pressure/frontal systems across the lower 48. In the West on Sunday, ample moisture ahead of an eastern Pacific upper trough should produce possibly heavy rain continuing along the West Coast and snow over the northern half or so of the Rockies, along with some areas of strong winds. The Northwest looks to quiet somewhat into the workweek but modest rain and snow may still be expected, especially into midweek. Farther east, rain that could be heavy is likely over the Appalachians to Eastern Seaboard early next week, with freezing rain possible in northern New England. Another round of precipitation (including possible snow in the north-central Plains) is forecast to push from the north- central to eastern U.S. Monday-Wednesday. Warmer than normal temperatures for most of the lower 48 early next week will moderate closer to normal from west to east during the period. ...Guidance/Predictability Assessment... Recent model guidance maintains the theme of reasonable agreement for the overall pattern but with important detail differences that affect sensible weather. As the medium range period begins Sunday, one such issue is with the depth and orientation of a trough moving through the east-central to eastern U.S. ECMWF runs have shown this trough to be quite sharp/narrow, ending up more negatively tilted than other guidance and leading to a stronger deep-layer gradient feeding moisture into the Mid-Atlantic and Northeast that progresses rain/convection more slowly. The 00Z ECMWF has now trended broader and not as deep with the trough, which matches other models better, but the flow pattern with multiple shortwaves during the short range period interacting does lend low confidence that this is a solid trend. At least for now, the 00Z EC QPF timing and placement fortunately aligns better than the 12Z EC did. The CMC/UKMET were meanwhile on the faster side with low track/QPF progression while GFS runs seem like a fair middle ground. The next significant system in the series reaches the West Coast by early Sunday, with models reasonably agreeable on the trough and a somewhat elongated area of surface low pressure. Guidance presses the shortwave quickly across the central and eastern U.S. Monday-Tuesday while a couple of surface lows spin up in the High Plains and move east. Models showed mostly minor differences until Tuesday-Wednesday, when the 12/18Z GFS indicated a sharper trough tracking across the Northeast and leading to a quicker/farther northeast surface low in the western Atlantic by Wednesday. With the control run of the EC, EC mean, and AI/ML models as well as the CMC showing good agreement in the surface low position near New Jersey or so 12Z Wednesday, favored these rather than the GFS. The new 00Z GFS seems less sharp with the trough aloft but still faster with the eastern side of the trough and thus the low position. More shortwaves coming in from the West into the central U.S. show typical detail/timing differences. The pattern by Wednesday- Thursday may start to be more dominated by longwave features as an upper ridge axis forms over the West in response to Pacific troughing and behind the central-eastern U.S. trough, hopefully increasing predictability in future forecasts. The WPC forecast used a multi-model deterministic blend early in the period. Eliminated the GFS runs from the blend by Day 6 given the aforementioned issues, and gradually reduced the proportion of other deterministic models so that ensemble means made up half the forecast blend by Day 6 and more Day 7. ...Weather/Hazards Highlights... An atmospheric river will take aim at the West Coast over the weekend. The heaviest rain is likely on Saturday (now into the short range), but rain totals still appear sufficient to warrant a Marginal Risk for coastal Oregon to northern California in the Day 4/Sunday ERO, especially considering the increased sensitivity due to prior rainfall. Moisture spreading farther inland into the higher terrain will allow for heavy snow totals on Sunday in the Cascades and northern Rockies, with lesser amounts in the Sierra Nevada and central Rockies. Snow levels should be lowered by Sunday compared to earlier days. Precipitation should lessen across the Northwest Monday-Tuesday with lighter amounts expected. Precipitation could increase again Tuesday night into Wednesday but currently this round does not look to be as heavy as the weekend system. Downstream, the eastern U.S. can expect moderate to heavy rain on Sunday in the vicinity of a couple of frontal systems. While there is a fair amount of spread in the timing and evolution of heaviest rain, high probabilities for rainfall exceeding an inch on Sunday are in good agreement in the GEFS and EC ensembles across Pennsylvania given likely Gulf and Atlantic moisture inflow. Farther north into New York state, frozen ground/snow cover will lead to added sensitivity. Meanwhile farther south across the Appalachians to Piedmont, relatively more instability may overcome relatively drier to average antecedent conditions to produce convection with heavy rain rates on Sunday, continuing from Saturday-Saturday night's heavy rain in the Mid-South. Thus the Marginal Risk in the Day 4/Sunday ERO was expanded south in this forecast package to encompass these areas as well. Another threat will be the potential for freezing rain across Maine in particular with this system Sunday-Monday. A low pressure system will develop in the Plains by Monday with a shortwave ejecting from the West. Gusty winds are likely behind this low across parts of the Rockies and High Plains. Meanwhile north of the low track, snow is possible across the north-central Plains Monday and then will spread another area of precipitation across the East Tuesday-Wednesday. As with the leading system, wintry weather should be confined fairly far northward. However the extent of snow/ice over the Northeast around midweek will be sensitive to surface low track details that will take more time to be resolved. Much of the country will be well above average temperature-wise early next week. Low temperatures of 20-30 degrees above normal will be common in the Midwest to East on Sunday, with 50s for lows reaching all the way into the Ohio Valley. High temperatures should be maximized across the southern Plains on Sunday and especially Monday, with highs in the 70s and 80s across Texas that could break daily record highs. As multiple cold fronts track across the nation, more seasonable cool air should gradually take over as the week progresses. The northwestern to central U.S. is forecast to see near normal temperatures Monday-Tuesday, and cooler temperatures will spread across the eastern U.S. Wednesday- Thursday. By midweek and beyond the main area of above normal temperatures is forecast to be the southwestern U.S., but only by about 5-10 degrees. 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