Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 PM EST Sat Dec 28 2024 Valid 12Z Tue Dec 31 2024 - 12Z Sat Jan 04 2025 ...Overview... Mean upper troughing is likely to form across the central and eastern U.S. Tuesday-Wednesday and last through at least the rest of the week, with some embedded shortwaves producing low pressure/frontal systems. Guidance continues to show a primary surface low tracking through the Ohio Valley Tuesday and across the Northeast Wednesday-Thursday accompanied by precipitation of varying intensity depending on location. Cold cyclonic flow in the system's wake should support multiple days of lake effect snow through the latter half of the week. Upstream, an upper ridge will build over the West Coast and then the Interior West by Thursday- Friday, leading to temperatures warming above normal there while the East cools under the trough. Eastern Pacific energy/troughing that may move inland by Friday or Saturday will lead to increasing precipitation chances across the Northwest for the latter half of the week. ...Guidance/Predictability Assessment... Dynamical and machine learning (ML) models continue to agree upon the large scale pattern transitioning to an amplified western ridge/eastern trough regime, with leading intensifying low pressure tracking from the Ohio Valley on Tuesday to northeast or north of New England later in the week, and a Pacific upper trough pushing into the West by next Saturday. Within this consensus evolution, finer scale question marks persist for low pressure affecting the Northeast around midweek and features bringing moisture to the Pacific Northwest Wednesday-Thursday. More notable differences exist with the eastern Pacific into western U.S. upper trough and associated surface pattern Friday-Saturday. Guidance has converged well for the surface low forecast to track over the Ohio Valley on Tuesday. Then specifics continue to be a bit ambiguous around Tuesday night and early Wednesday with respect to the relative emphasis of New York/Lake Ontario versus coastal low pressure centers before guidance clustering improves toward a coastal New England track late Wednesday and continued progress to just east/northeast of Maine by Thursday and farther north/northeast thereafter. Dynamical guidance currently tilts a little more toward a coastal low as of 12Z Wednesday while the ML models are still mixed. Timing and strength differences after that time are generally typical for the time frame involved, though consensus suggested that the 00Z CMC was a little too slow (adjusting favorably in the new 12Z run) and the 06Z/12Z GFS possibly a little too weak. Behind this system, over the past day or so guidance has been gradually improving its signal for a weak wave progressing from the Plains through or near the Tennessee Valley and into the western Atlantic Thursday into Saturday. Most dynamical support for this feature comes from shearing Pacific energy which looks to have low predictability by 5-7 days out in time, so changes in specifics are certainly possible in future runs. Over the eastern Pacific/western U.S., dynamical and ML guidance continues to vary for surface details near the Pacific Northwest coast around midweek or so, as upper dynamics shear out and rising heights aloft eventually support northeastward progress of a mean warm front. There is no clear clustering for this part of the forecast. By Friday, the 00Z ECMWF becomes a southern extreme for the better defined upper trough and corresponding East Pacific surface low, with the best cluster of ML models favoring a surface low track near 50N as of 12Z Friday per the 00Z ECens mean. Then guidance diverges considerably for amplitude of the upper trough moving into the West. Latest ECMWF runs are on the amplified side of the spread (leading to greater southward extent of precipitation) while in contrast the CMC runs actually maintain ridging over the southern half of the West. ML models vary as well but do not show any support for the CMC. Prefer to lean away from the ECMWF as well (especially the 00Z run which also strayed slow aloft and for the surface low), given that passage of the trough through a longer term mean ridge does not seem to support the strong side of the spread. A blend consisting of the 00Z ECMWF and 00Z/06Z GFS with lesser weight of the 00Z UKMET/CMC represented consensus or preferred intermediate solution, depending on the feature, during the first half of the period. Then the forecast rapidly transitioned toward more ensemble mean input (reaching 60 percent for Saturday, split between the 06Z GEFS/00Z ECens) while switching ECMWF input toward the 12Z/27 run and phasing out the 00Z CMC. This approach yielded good continuity with most aspects of the forecast. ...Weather/Hazards Highlights... A low pressure system tracking from the Ohio Valley into the Northeast will spread precipitation across those regions Tuesday- Wednesday. Precipitation should fall as rain for most of the Ohio Valley to the coastal areas/lower elevations of the Northeast. The rain rates and rain amounts look to stay mainly under excessive thresholds. Some moderate to locally heavy precipitation may fall in the urban corridor (most likely Philadelphia to New York City) later Tuesday to Tuesday night though, and will continue to monitor since urban land use can be vulnerable to flash flooding. Snow is likely in the higher elevation areas of the Interior Northeast like the Adirondacks and the Green and White Mountains, and probabilities for 0.25 inch or more of liquid equivalent in the form of snow remain 30-70 percent there on Wednesday. In the wake of this system, expect multiple days of cold and brisk flow across the Great Lakes and Northeast, with lake effect snow persisting through the latter half of the week, perhaps extending into Saturday over some locations. Meanwhile shortwave energy and a weak frontal wave may produce a band of light snow late this week somewhere within an area encompassing the Midwest/Ohio Valley/central Appalachians. The Pacific Northwest will see a relative break in precipitation on Tuesday after a steady train of atmospheric rivers into the region. But by Tuesday night or Wednesday moist inflow may get renewed there and rounds of precipitation are likely to continue through late week and at times farther east into the northern Rockies. Precipitation amounts from day to day and in total vary considerably in models as there is a fair amount of spread for system/frontal specifics. During the Wednesday-Wednesday night period covered by the Day 5 Excessive Rainfall Outlook, latest guidance is starting to suggest more potential for some moisture focus along the West Coast near the California/Oregon border. The combination of forecast totals and guidance spread do not seem to warrant a risk area yet. However this region will be sensitive due to recent/ongoing rainfall and will monitor guidance in future cycles for any trends toward clustering in the wetter end of the envelope. Even with a better defined feature arriving around Friday and pushing into the West on Saturday, there are differences with the southward extent of moisture focus due to spread for surface low track over the Pacific on Friday and then the amplitude of the supporting upper trough by Saturday. The eastern U.S. can expect one more day of above average temperatures (by 10-15F) on Tuesday, before upper troughing pushes along a series of cold fronts that gradually cool temperatures to near normal on Wednesday and gradually below normal into late week. High temperatures by Saturday are forecast to be around 10-15F below normal for the Ohio Valley to Appalachians and Mid-Atlantic while lows should be 5-10F below average. Colder than normal temperatures will also impact the north-central U.S., and lows could reach 10-15F below zero over northern North Dakota and Minnesota by Friday and/or Saturday. Meanwhile, the amplifying upper ridge over the West will promote warming, with temperatures generally 5-10F above average increasing in coverage by the second half of the week. Locally higher anomalies are likely in the Southwest and highs could reach well into the 70s. Highs of 5-15F above normal may reach into the southern High Plains by next Saturday. 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