Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Tue Dec 24 2024 Valid 12Z Fri Dec 27 2024 - 12Z Tue Dec 31 2024 ...Unsettled weather for the West Coast with periods of heavy rain and high elevation mountain snow... ...Overview... The large scale flow pattern will feature a rapid succession of Pacific systems moving into/through the West and feeding into a mean upper trough of moderate depth aligned over the Plains/Mississippi Valley, while an upper ridge initially near the East Coast drifts 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. In addition a broad area of precipitation (mostly rain) will spread across the eastern half of the country with some enhanced totals possible. For the most part, this should be a mild pattern for the Lower 48 with much of the country experiencing above normal temperatures. ...Guidance/Predictability Assessment... Guidance agrees fairly well for the overall pattern through at least the first half of the period, with some typical embedded detail issues that have lower predictability a few days out in time. Of particular note, the 06Z GFS looked excessively weak/south for the eastern Pacific system affecting the Pacific Northwest as of early Saturday while the new 12Z UKMET is on the deep/eastern side of the spread for low pressure reaching the Great Lakes by early Sunday. At that time there is a fair amount of spread for the structure of the next system affecting the Pacific Northwest. By late in the period, three primary uncertainties arise. These involve the upper dynamics/surface system reaching the Mid- Atlantic/Northeast and relative strength versus the leading surface ridge, evolution of a trailing system reaching the Plains or east- central U.S. by early Tuesday, and character of flow over the West. Dynamical and machine learning (ML) solutions are mixed over the Northeast, with ML guidance also mixed for how much upper troughing may reach the Southwest (with the GFS and GEFS ensembles tending to be most enthusiastic among the dynamical solutions, though the 06Z GFS looks suspect with its closed low). As for the system in- between valid early Tuesday, GFS/ECMWF runs are running a bit on the strong side relative to the ML model envelope while there is a minority of dynamical guidance that is notably slower than the primary clustering of GFS/ECMWF runs and ensemble means. The new 12Z ECMWF has trended a bit slower than prior recent runs as well, so this potential trend will be worth monitoring. A composite of 00Z/06Z operational model guidance provided a good representation of consensus or intermediate ideas for the first half of the period, with some 06Z GEFS/00Z ECens guidance incorporated thereafter to account for increasing detail uncertainties. ...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. A Marginal Risk of Excessive Rainfall is in effect for coastal Washington, Oregon, and northern California in the Day 4 Excessive Rainfall Outlook valid Friday/Friday night. 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 favors a Day 5 Slight Risk area over that region. This was coordinated with the MFR/Medford OR and EKA/Eureka CA forecast offices for the previous issuance and support remains in the latest cycle of guidance. 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 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. Downstream, multiple frontal systems will bring rain and thunderstorm chances to the Plains, Mississippi Valley, and Southeast, with moisture spreading into the East late this weekend into early next week. 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. Currently the combination of guidance signals and neutral to dry antecedent ground conditions over the east-central states do not appear to support a risk area in the Days 4-5 EROs but will continue to monitor for any changes. Any wintry weather should be limited to locations close to the Canadian border. A majority of the central/eastern U.S. should see well above normal temperatures from late this week into early next week. Expect the warmest anomalies to be for morning lows across the east-central part of the country Friday-Sunday, in the plus 20-30F range and 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 highs reach 15-25F above normal on Monday in the warm sector ahead of a system reaching the High Plains by early in the day. Readings closer to climatology will broaden eastward from the West Coast early next week, reaching the Plains by Tuesday/New Year's Eve. At that time some highs may drop to slightly below normal over and near the Rockies. 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