Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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 down somewhat during the next workweek but modest rain and snow will still be possible, with most of any focused activity tending to be over western Washington. 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) should push from the north-central to eastern U.S. Monday-Wednesday. Expect a trend toward a larger scale western ridge/eastern trough pattern by mid-late week. Warmer than normal temperatures for most of the lower 48 early next week will moderate closer to normal, if not slightly below in some spots, from west to east during the period. ...Guidance/Predictability Assessment... Within the generally agreeable pattern evolution there are some persistent detail issues with individual systems/features, with a relative majority of guidance favoring maintenance of a solution that tilts toward a compromise or ECMWF/ensemble mean scenario overall. This provides fairly good continuity from previous forecast, with only modest refinements to reflect latest consensus ideas/trends. The updated forecast started with a 60/25/15 weight of the 00Z ECMWF/GFS/UKMET for the first half of the period and then transitioned toward the 00Z ECMWF and 06Z GEFS/00Z ECens- CMCens means. Guidance is still having some trouble in resolving a Mississippi Valley into Northeast shortwave Sunday-Monday, with resulting spread for structure and timing of the associated surface low pressure/frontal system. Machine learning (ML) models have been consistent in favoring a moderately sharp and slow shortwave-- though not as slow as the 12Z/25 ECMWF run, with the 00Z ECMWF and nearby solutions providing the best starting point. The 00Z GFS was a bit slower than the 06Z run so it and the UKMET provided a modest compromise with the ECMWF to account for ongoing timing spread. The 00Z CMC's system structure compared poorly to other guidance and the new 12Z run at least looks better. Dynamical and ML guidance agree on a somewhat elongated system reaching the Pacific Northwest on Sunday, with rapid progression of the shortwave leading to Plains low pressure by Monday. Low pressure should continue rapidly along and reach the Northeast U.S./Canadian Maritimes by Wednesday-Thursday. An intermediate solution looks good over the east-central U.S. with continued north-south spread over the Ohio/Tennessee Valleys and some opposing clustering/trends that include ML models leaning to the central-southern part of the spread but the 12Z ECMWF adjusting a little northward from prior runs. Thereafter, GFS runs still lean a bit fast with the system while guidance is still split over the relative emphasis on eastern Great Lakes versus coastal low pressure as of early Wednesday. Due to the favored overall timing/evolution of recent ECMWF runs the blend reflected a little more coastal low emphasis, but the new 12Z ECMWF has adjusted to a more even emphasis. By early Thursday there is fairly good support from the ML models and a decent number of dynamical models/means for the surface low to reach a position over or a little north/east of Maine. ML models have trended stronger with this system over the eastern half of the country versus 24 hours ago as a whole, supporting operational runs in principle. GFS runs/GEFS ensembles continue to be more pronounced than other guidance with shortwave energy that reaches the West by around Tuesday. This difference results in the GFS/GEFS producing much more southern tier precipitation over the central/eastern states by late in the forecast period. ML models strongly support the non- GFS/GEFS guidance which shows more eastern U.S. upper troughing and mostly dry conditions across the southern tier, aside from perhaps some light western Gulf Coast activity per the 00Z ECens mean. Notable QPF adjustments from the 13Z NBM for this cycle included a partial trend to improve definition of the north-central Plains into Midwest band of rain/snow per a composite of operational models, adjusting late-period southern tier QPF toward a less expansive version of the 00Z ECens mean (which maintains non-GFS focus per continuity), and more operational model input over/near the Great Lakes late in the period to improve Lake Effect definition. ...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. Expect precipitation to trend lighter across the Northwest Monday-Tuesday. Precipitation could increase again from Tuesday night onward but currently these episodes generally do not look to be as heavy as the weekend system. What areas of enhanced activity that do occur are most likely to be over western Washington. 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 persistent spread for system evolution and timing/magnitude 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. Heaviest ML model rainfall also fits this pattern. Thus the Marginal Risk in the Day 4/Sunday ERO stays fairly close to the prior cycle's continuity. Another threat will be the potential for freezing rain across Maine in particular with this system Sunday into Monday. A low pressure system will develop in the Plains by Monday with a shortwave ejecting from the West. Strong and 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 with probabilities for at least 0.25 inch liquid in the form of snow maximizing around 30-35 percent over parts of southern South Dakota. A broader area of precipitation will spread across the East Tuesday-Wednesday. Wintry weather should be confined fairly far northward as with the prior system, but precipitation totals over the East should be lighter with this storm. 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. Cold cyclonic flow behind the storm system will likely produce a period of lake effect snow. 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 (especially in southern parts of the state). As multiple cold fronts track across the nation, more seasonable cool air should gradually take over as the week progresses. Expect the northwestern to central U.S. to see near normal temperatures Monday-Tuesday, and cooler temperatures will spread across the eastern U.S. Wednesday-Thursday. Upper troughing may support highs up to 5-10F below normal in a few areas between the northern Plains and Appalachians by Thursday. On the other hand, by that time the upper ridging that builds into the West will likely raise that region's temperatures up to 5-10F above normal and especially over the Southwest. Rausch/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