Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 PM EST Wed Dec 25 2024 Valid 12Z Sat Dec 28 2024 - 12Z Wed Jan 01 2025 ...Unsettled weather for the West Coast with periods of heavy rain and high elevation mountain snow... ...Overview... Guidance continues to show a large scale pattern that features a rapid succession of Pacific systems moving into/through the West and feeding into a mean upper trough of moderate depth aligned near the Mississippi Valley. A leading upper ridge initially near the East Coast will drift into the western Atlantic. This pattern will lead to areas of heavy precipitation along the northern- central West Coast through the weekend and snow over the northern half or so of the Rockies, along with some areas of strong winds. After a brief lull, precipitation should rebound again over the Pacific Northwest toward midweek. In addition a couple systems should spread precipitation (mostly rain) across the eastern half of the country. Some of this activity may be on the heavy side. Expect most of the Lower 48 to see well above normal temperatures from the weekend into early Monday, followed by a rapid west-to- east progression of near or slightly below normal highs. ...Guidance/Predictability Assessment... The latest models and ensemble means agree well for most aspects of the overall pattern but there are persistent and important detail differences, especially affecting system evolution over the eastern U.S. from the weekend into early next week and for the second system over the East by Tuesday-Wednesday. Looking at individual systems in sequence, the 00Z CMC was a slow extreme with the leading wave forecast to reach the Upper Midwest by early Saturday. The new run has adjusted faster toward consensus. Then guidance diverges for the exact character of the next shortwave reaching the Plains Saturday and lifting northeastward thereafter. ECMWF runs have been on the sharper and slower side among the operational models, ultimately leading to a stronger deep-layer gradient feeding moisture into the Mid- Atlantic and Northeast. However the GFS runs and GEFS/ECens means fit into the general idea of a negatively tilted shortwave, and most of the 00Z machine learning (ML) models showed a sharp shortwave like the ECMWF. CMC/UKMET runs continue to be more rounded with the overall trough into the 12Z cycle but the GFS has nudged a bit in the ECMWF direction and the 12Z ICON made a pronounced shift to the ECMWF scenario. One other detail of note is that by early Monday the ML models favored the surface system's parent low tracking into or near Ontario while a triple point wave reaches the Mid- Atlantic. ECMWF runs tracked the parent low a little farther south but the new 12Z version has trended in the ML model direction. The next significant system in the series reaches the West Coast by early Sunday (with a somewhat elongated area of surface low pressure), with the 00Z CMC already on the slow side at that time. Thereafter an average among the GFS/ECMWF and the ensemble means has held up rather well for timing over the past day or so. Meanwhile the new 12Z CMC compares a lot better with this cluster versus the slow 00Z run. By early Tuesday there is continued north- south spread, ranging from the Ohio Valley to Tennessee Valley (GFS leaning north, ECMWF leaning south). Then the system reaches the eastern Great Lakes/Northeast by Wednesday, with guidance split regarding inland or coastal surface low dominance. During the Tuesday-Wednesday time frame the ML models are noticeably weaker with this system versus the dynamical guidance. There have been at least a couple cases of a weak/suppressed bias in the ML models for waves near the East Coast, so preference sides with the dynamical cluster for the time being. The next potential shortwave reaching the West by Monday-Tuesday is weaker than its predecessors, with GFS/GEFS runs tending to be most enthusiastic for its strength over the Southwest and beyond. ML models have been mixed but latest runs favor leaning toward a more subdued presentation of this feature. Yet another system or two should reach or approach the Pacific Northwest by Tuesday- Wednesday, with typical detail/timing differences. Guidance preferences led to an early period blend consisting of 60 percent ECMWF (split between the 00Z/25 and 12Z/24 runs) and 40 percent GFS (00Z/06Z). Then the forecast started including ensemble means, more 00Z ECens relative to 06Z GEFS, with total ensemble weight reaching 40 percent by next Wednesday. ...Weather/Hazards Highlights... Multiple rounds of moderate to heavy rainfall will likely impact the West Coast through the remainder of this week and into the weekend as multiple frontal systems approach the coast, with the signal for heavy rainfall becoming strongest on Saturday concentrating more near the California/Oregon border with time. Flooding will be possible in rivers and streams, with more flashy responses possible where burn scars exist and possibly near the steep terrain of the coastal mountain ranges and into the foothills of the Cascades. The combination of rainfall expected through Friday night and a more concentrated heavy QPF signal in some guidance focused over far southwest Oregon and far northwest California on Saturday continues to favor a Slight Risk area over that region in the Day 4/Saturday Excessive Rainfall Outlook. While QPF is less the following day, increased sensitivity due to prior rainfall supports maintaining the Day 5/Sunday Marginal Risk for western Oregon and northwest California as introduced in the previous cycle. Moisture spreading inland across the West will likely produce heavy snow over the higher/highest terrain of the Cascades and Rockies, as well as portions of the Sierra Nevada. Expect the heavy snow potential to continue through Sunday. Precipitation over the Pacific Northwest should trend lighter early next week, with Rockies snow tapering off by Monday or Tuesday. Tight pressure gradients may result in some periodically gusty winds for some locations over the Pacific Northwest, Interior West, plus the northern/central Rockies and High Plains. Precipitation should trend lighter over the Pacific Northwest around Monday into Tuesday but then increase again late Tuesday/Tuesday night through Wednesday. Downstream, multiple frontal systems will bring rain and thunderstorm chances to the eastern half of the country. Some moderate to heavy rainfall will be possible within a general area from the lower half of the Mississippi Valley into the Mid-Atlantic and Northeast. The wetness of the some most recent guidance in and near Tennessee and the southernmost Appalachians showing local amounts in the 3-5" range led to the introduction of a Day 4/Saturday Marginal Risk area. Heaviest rainfall shifts into the vicinity of Pennsylvania and New York state on Day 5/Sunday with a Marginal Risk area again depicted. Across northern areas, frozen ground/snow cover will lead to added sensitivity. There is still a fair amount of uncertainty over rainfall specifics, but the overall pattern favoring a period of Atlantic inflow suggests potential for significant totals. The next system may bring a band of rain/snow to the north-central Plains early in the week and then 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. Above normal temperatures will prevail over much of the country during the weekend and into early Monday. The warmest anomalies of plus 20F or greater will likely be for morning lows from the Upper Midwest into the South/Southeast during the weekend, possibly challenging daily records for warm lows in some areas. Warmest min temperature anomalies should be more confined to the Upper Mississippi Valley to New England early next week. Also the southern Plains could see temperatures reach 15-25F above normal on Monday in the warm sector of a system tracking by to the north. Near to slightly below normal high temperatures should spread across the West into the northern Plains on Monday, expanding through the rest of the Plains and into east-central U.S. Tuesday- Wednesday. Rausch/Roth 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