Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 PM EST Wed Dec 11 2024 Valid 12Z Sat Dec 14 2024 - 12Z Wed Dec 18 2024 ...Atmospheric river could cause heavy rain and snow in the West Coast/Northwest by this weekend... ...Overview... Most guidance continues to show a sequence of shortwaves approximately 2-3 days apart from the Pacific through the lower 48 from the weekend into the first half of next week. The character of the pattern may begin to change toward the middle of next week as the last shortwave of the series reaches the western-central U.S., with larger scale ridging potentially building into the West Coast. Each shortwave's associated surface system should produce episodes of rain and higher elevation snow over portions of the West and then development of precipitation over the southern Plains and expanding across the east-central U.S. then continuing into portions of the East. Guidance finally agrees better for the system approaching the Pacific Northwest by early Saturday but there are other aspects of the forecast that still show a fair amount of spread. The forecast pattern will lead to mostly near or above normal temperatures during the period, with only localized areas of below normal readings. ...Guidance/Predictability Assessment... At the start of the forecast period Saturday, troughing with embedded vort maxes over the eastern Pacific and a shortwave/embedded upper low atop the central Plains are both reasonably agreeable in the model guidance. The former will support a deep surface low in the eastern Pacific toward Vancouver Island, generally within the 970s-980s mb early Saturday and the newer 12Z models are in even better agreement with placement. As that western trough continues eastward/inland, the 00Z ECMWF becomes a slow outlier even by Sunday compared to the preponderance of other guidance including the EC-based AI/ML models. The 12Z EC is now more consistent with the consensus. Troughing is forecast to continue into the central U.S. Monday and lift as it pushes into the Northeast. There is some timing spread with this (00Z GFS faster, 00Z ECMWF slower, and the 06Z GFS in between), and differences in the orientation of the trough/possible upper low and how that affects the associated surface low(s) in the Great Lakes and/or farther north in Canada. Meanwhile, the aforementioned upper low over the central U.S. is mostly agreeable though the 00Z ECMWF remained farther south than other guidance. Then there is another round of troughing pushing across the eastern Pacific early next week and into the western/central U.S. Tuesday-Wednesday. The 00Z CMC is slow with this even in the Pacific, and the 00Z EC ends up separating a southern stream low into the Southwest unlike guidance consensus. The GFS runs and AI models are more agreeable with the ensemble means in a more phased trough making it to the Plains by Wednesday. Downstream, this leads to the ECMWF showing troughing in the East by midweek whereas ridging to the east of the trough is more favored. The WPC forecast favored the 00Z/06Z GFS in terms of deterministic runs given the aforementioned issues with the ECMWF and CMC. Increased the proportion of the GEFS and EC ensemble means as the period progressed to over half by Day 6-7 though as model spread increased in general. ...Weather/Hazards Highlights... The storm likely nearing the Pacific Northwest by early Saturday will bring the potential for areas of significant rain and higher elevation snow to locations along the central and northern West Coast. Brisk to strong winds may be possible at least offshore. Guidance continues to indicate a fairly progressive track of the atmospheric river, but rain rates and totals could be high enough to support localized flooding. A Marginal Risk remains in place for Day 4/Saturday. Minor adjustments from continuity were to modify the risk area out of southwestern Oregon while expanding to the south toward the Bay area due to the faster progression of the AR. Some moisture should continue across the West over the weekend and heavy snow is possible in the higher elevations of the Cascades, Sierra Nevada, and northern Rockies. Another round of precipitation is likely to come into the West early next week. A shortwave with an embedded upper low should track through the Midwest and Great Lakes/Ohio Valley during the weekend. This feature and associated surface wave/frontal system should spread precipitation from the Mississippi Valley eastward, possibly including some areas back into eastern Texas as well. Multi-day trends have been favoring somewhat higher rainfall totals within an axis from eastern Texas into the Ohio Valley on Saturday, but consensus of guidance still indicates that system progression and relative lack of instability should keep rain rates below excessive thresholds. Most of this region has neutral to dry ground conditions as well. Modest to perhaps locally heavy rain progresses into the Mid-Atlantic Sunday. Parts of the Upper Midwest into Northeast could see some wintry precipitation at times as the moisture shield progresses eastward. Probabilities for 0.25 inch liquid in the form of snow remain below 30 percent in most areas, except with higher probabilities in the Adirondacks. Then into the workweek, another round of precipitation is likely to develop in the central and eastern U.S. as another low pressure/frontal system pulls in moisture. This currently looks to be a warmer system producing mostly rain, though wintry weather may be possible in the far northern tier. There may be more instability with this system as well, for higher rainfall rates and totals compared to Saturday's system. Though this is currently out of the ERO timeframe, flooding potential will have to be monitored in future forecasts. Much of the lower 48 should see near to above normal temperatures during the period, with the warmest anomalies for highs (10-15F above normal) forecast to be over the central/east-central U.S. Sunday-Monday and near the East Coast by Tuesday ahead of a couple cold fronts. Some days could feature some pockets of morning lows up to 15-20F above normal. Localized areas with slightly below normal highs should include some areas near the East Coast on Saturday, the central West Coast from the weekend into Monday, and the far northern tier Tuesday-Wednesday. Tate/Rausch 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