FXUS02 KWNH 110701 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM EST Wed Dec 11 2024 Valid 12Z Sat Dec 14 2024 - 12Z Wed Dec 18 2024 ...Continuing to monitor a potentially significant storm system to affect the Pacific 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... For the system nearing the West Coast by early Saturday, 12Z ECMWF/CMC runs made a significant detail adjustment to trend toward the general evolution of the GFS. This involves leading shortwave energy supporting a northern surface low that tracks toward Vancouver Island while trailing energy digging underneath produces a frontal wave (albeit with some lingering timing spread). There is still spread for depth within the 970s-980s mb, with 12Z machine learning (ML) models averaging in the lower half of the 980s mb. The ML models have consistently been advertising a fairly progressive solution, and some from the 12Z/10 cycle actually showed a low track to the north of Vancouver Island. A blend of latest operational runs provides good continuity in the forecast. After this system moves inland, guidance continues to diverge with recent ECMWF/ECens runs leaning somewhat on the slower side of the spread as the upper trough continues across the lower 48 as well as with the next trough likely to reach into the West by next Tuesday. By Wednesday the ML models support sufficient progression to produce some surface waviness over the Plains per the GFS/GEFS and hinted at by the 12Z CMCens. However there is little support for surface low pressure as deep as GFS runs. Also by Wednesday significant differences arise over east-central North America, with 12Z ECMWF/CMC runs showing an upper trough over the Upper Midwest/Great Lakes while the GFS/GEFS show ridging. ML models generally favored something in the range of flat westerly or southwesterly flow to modest ridging--favoring leaning away from the ECMWF/ECens mean. The trough/upper low crossing the East continues to look too slow in the UKMET. Other models cluster better during the weekend but still diverge farther eastward, in particular with the ECMWF pulling the shortwave energy/upper low farther south. The early part of the forecast started with half GFS weight (split between the 12Z/18Z runs) with the 12Z ECMWF/CMC comprising the rest. Then the forecast split the ECMWF component between it and the ECens mean to dampen ECMWF details over the western half of the country. Tuesday-Wednesday removed the ECMWF and increased CMCens/GEFS means while decreasing GFS input some. The end result yielded a Plains surface pattern compatible with the general theme of the ML models and was closest to the GEFS mean. ...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. Better clustering of guidance for how this system will evolve has continued into the 00Z model cycle, favoring sufficiently fast movement of the associated atmospheric river to keep rainfall totals from being too extreme. However there is still enough rainfall potential for the Day 4 Excessive Rainfall Outlook valid on Saturday to maintain a Marginal Risk area over parts of northern California and far southwest Oregon. For now this area is unchanged from continuity as most ensemble guidance and some model runs maintain support, but other models offer potential for future adjustments once clustering improves again. Some moisture should continue across the West into the weekend and heavy snow is possible in the higher elevations of the Cascades, Sierra Nevada, and northern Rockies. 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 aside from the UKMET (suspiciously slow with the upper low) 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. 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. Meanwhile the Mid-Atlantic may see a fairly wet Sunday. Another round of precipitation is likely to develop across the east-central U.S. early next week as the system near the West Coast on Saturday pushes eastward and 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. Highest rainfall totals may be near the western Gulf Coast and Lower Mississippi Valley, but some important details are yet to be resolved. Expect another Pacific system to produce additional rain and higher elevation snow across the West during the first part of next week as well, and then produce another area of precipitation over the central U.S. toward Wednesday. 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. 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 $$