Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Dec 29 2024 Valid 12Z Wed Jan 01 2025 - 12Z Sun Jan 05 2025 ...Overview... Guidance is consistent in depicting an amplified western ridge and eastern trough pattern by late this week, followed by an eastern Pacific trough passing through the West into the central U.S. while ridging rebuilds off the West Coast. This pattern/evolution will favor colder temperatures from the northern Plains through much of the East and above normal readings from the Great Basin/Southwest into far southern Plains. Coastal low pressure will bring rain and snow to the Northeast around midweek, followed by multiple days of lake effect snow. The Pacific Northwest and northern Rockies will see episodes of rain and mountain snow mid-late week, with moisture eventually pushing somewhat south/east as the upper trough approaches/crosses the West. By next weekend the trough may support a developing low pressure system over the Plains with possible north-central U.S. snow and south-central U.S. rain. ...Guidance/Predictability Assessment... Model guidance remains agreeable that upper troughing will be in place across the central and eastern U.S. for the latter half of next week. Latest guidance continues to show gradual consolidation for the surface low tracking from over/near New England as of early Wednesday into eastern Canada thereafter. The current majority shows the best defined low pressure inland over southern New England as of 12Z Wednesday and then reaching a little east/northeast of Maine by early Thursday (as supported by the machine learning models). Consensus/continuity provide high confidence in the forecast of mean cyclonic flow in the system's wake. There is still the signal for a weak wave progressing from the Plains through or near the Tennessee Valley and into the western Atlantic Thursday into Saturday, though if anything the wave has become a little less defined today. Most dynamical support for this feature comes from shearing Pacific energy which looks to have low predictability a few days out in time, so occasional adjustments may well continue. In the West, the upper ridge axis seems well represented in the guidance Wednesday-Friday before an eastern Pacific trough approaches. However there is continued spread and variability for along/off-shore surface details that will affect the precipitation forecast over the Pacific Northwest. Then the eastern Pacific trough continues to show spread in its timing and depth, affecting the forecast from West Coast into the Plains. As of Friday, ECMWF runs continue to be on the southern side of the spread for the core of the upper trough and associated surface low but the overall guidance spread has been getting trimmed more on the northern side--bringing the overall guidance mean southward somewhat. ECMWF/ECens runs have been on the amplified side of the spread as the trough pushes through the West next weekend, but 06Z/12Z GFS and recent machine learning model trends suggest a leaning toward the more amplified half of prior spread at least (especially favoring discounting the very flat earlier CMC runs and 00Z GFS). There are also significant timing differences, with the 12Z CMC and new 12Z ECMWF on the slower side while most ML guidance (minus the 00Z AIFS mean) strays faster than most dynamical guidance--even showing a closed upper low over the Midwest as of early Sunday. Prefer an intermediate timing given the significant spread that currently exists. A 00Z/06Z model composite (with more 00Z ECMWF/06Z GFS emphasis) represented the best ideas of guidance early in the period. Then the 12Z/28 ECMWF compared a little better to other dynamical guidance from the East Pacific through the West, while the 00Z CMC compared increasingly poorly as well. By late in the period a blend among the 12Z/28 ECMWF and 06Z GFS plus the 00Z ECens/06Z GEFS means provided a good dynamical/ML model compromise along with reasonable continuity. ...Weather/Hazards Highlights... Gradually deepening low pressure tracking near the New England coast will bring precipitation to the Northeast around midweek. Overall expect coastal and lower elevation rain, while higher elevation areas of the Interior Northeast such as the Adirondacks and the Green and White Mountains will see accumulating snow. There may be greater coverage of rain early in the day, followed by expansion of snow as colder air fills in right behind the low. 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 to central Appalachians. Farther south, parts of the western/central Gulf Coast could see one or more periods of light rain. Moist inflow into the West should get renewed by Wednesday and continue through late week, producing additional rounds of precipitation in the Pacific Northwest and at times farther east into the northern Rockies. Precipitation amounts from day to day and in total for any specific location vary considerably in the models as there is a fair amount of spread for system/frontal specifics. There is still a subset of model guidance suggesting potential for a period of relatively heavier precipitation along the West Coast near the California/Oregon border on Wednesday. However other solutions vary in placement/coverage and magnitude. The combination of forecast totals and guidance spread continue to favor not introducing any risk areas in the Day 4 ERO. 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. By Day 5 (Thursday through Thursday night), Pacific Northwest precipitation should shift or expand northward ahead of a better defined system approaching by very early Friday, with details and magnitude again not favoring any ERO risk area at this time. The upper trough nearing the West Coast as of early Friday will continue onward thereafter, most likely reaching the central U.S. by next Sunday. Rain and higher elevation snow should spread from the Northwest and northern California into the northern-central Rockies during Friday-Saturday, with continued uncertainty for southward extent and duration in light of guidance spread for upper trough amplitude and timing. Surface low pressure supported by the trough will develop over the Plains next weekend. While the details remain nebulous, precipitation chances look to increase generally across the Plains and Mississippi Valley from Saturday into Sunday. This precipitation would be snow in the north- central U.S. given the cold temperatures, with rain in the south. There may be a transition zone of ice in between the rain and snow, and some locations could transition from snow to rain or vice versa. The central Plains into Middle Mississippi Valley appears most likely for transitioning precipitation types at this point. Keep an eye on future forecasts as details are likely to change. Behind this system, another front could bring more precipitation to the Northwest next weekend but with low confidence in specifics for now. Generally, the mean upper ridge over the West will promote above normal temperatures from the Great Basin and Southwest to the southern/central Plains through the period, and the mean trough in the central/eastern U.S. will lead to cooler than normal temperatures from the north-central U.S. into the Midwest to Eastern Seaboard (after one more warmer than average day in the Northeast on Wednesday). The northern Plains is forecast to see the most anomalous temperatures, around 10-20F below normal for highs and around 5-15F below normal for lows. This translates to highs barely reaching 0F and lows between -10F and -15F or so in some areas. Meanwhile highs will reach well into the 70s in the Desert Southwest, and temperatures could warm into the low 80s in southern Texas by next weekend. 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