Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Wed Jan 15 2025 Valid 12Z Sat Jan 18 2025 - 12Z Wed Jan 22 2025 ...Strong Arctic surge pushing into the central U.S. by the weekend and then expanding eastward... ...Overview... The latest models and ensembles still show a very amplified pattern featuring an eastern Pacific/far northwestern North America ridge and a deep Canada through lower 48 trough. In the early stages of this pattern, very cold air will drop into the central U.S. Then well below normal anomalies will spread into the South and East by early next week as the upper trough likely takes on a more positive tilt, while the northern/central Plains eventually moderate. Within the highly confident overall trough, guidance is coming closer to converging for details of a couple lower- predictability leading shortwaves--one aiding in potentially heavy rain over parts of the Southeast on Saturday and another that may produce some snow over the Rockies on Saturday and then a progressive wave with mostly light precipitation of various types near the East Coast on Sunday. Guidance is now starting to focus in on another shortwave digging into the West around next Monday, with considerable uncertainty for its evolution/surface pattern that may result in potential for winter weather threats across the southern tier during the first half of next week. ...Guidance/Predictability Assessment... Early in the period, guidance has finally improved its clustering for a leading sheared shortwave that originates from an upper low currently to the west of Baja California, and which should support weak waviness across the Southeast on Saturday. The same is also the case for trailing energy digging into the Rockies on early Saturday and continuing rapidly eastward after then. Recent guidance has been trending toward what had been the faster half of the spread, with the 12Z CMC still in catch-up mode after consistently being a slow extreme. However there is still some spread for the associated surface wave that should track from the Mid-Atlantic/Southeast to a position south or east of Nova Scotia by early Monday. The remainder of the forecast is much more contentious though. There is a decent signal for another well-defined shortwave to drop into the western half of the country by around next Monday, with a lot of model divergence and run-to-run variability for how this feature will evolve thereafter. Some of this is attributed to various ideas for yet another shortwave that could reach western North America by midweek (with a lot of spread for how it may pass around/through the mean ridge to the west). Overall the 12Z machine learning (ML) models suggest faster progression of the Monday shortwave than most dynamical guidance, albeit with a lot of detail differences, due to arrival of the trailing midweek energy. Meanwhile the very elongated 00Z GFS/ECMWF are not exactly a common theme either. The character of this energy will play an important role in the surface pattern across lower latitudes along with the coverage and amounts of what could be meaningful wintry precipitation over parts of the South. During the first half of the period a composite of the 12Z ECMWF, 12Z/18Z GFS, and 12Z UKMET provided a reasonable starting point for the forecast to account for common themes or lingering spread depending on the feature. Day 5 Monday started to introduce some 18Z GEFS/12Z ECens means. Rapidly widening spread for individual model runs favored increased ensemble mean weight by Days 6-7 Tuesday-Wednesday, reaching 60-70 percent total weight at that time. ...Weather/Hazards Highlights... The well-advertised surge of Arctic air dropping south over the eastern Rockies into the Plains by this weekend will bring increasing coverage of temperatures that are 15-35F or so below normal, with expansion of the cold air bringing highs down to 15-25F below normal across much of the East by Monday. The most extreme anomalies are likely to be over the central High Plains and vicinity during Sunday-Monday and over the far southern Plains by Tuesday. Coldest temperatures on an absolute basis should be over northern tier areas where some subzero highs and lows in the -20s are expected. Wind chills should be even colder and more dangerous. WPC has begun posting Key Messages regarding this cold outbreak. The northern-central Rockies/Plains should moderate considerably by Tuesday-Wednesday while cold weather will persist into Wednesday across the South and East. The West will see a period of cool but not extreme temperatures from the weekend into the start of next week, with near normal readings slowly beginning to push eastward from the West Coast. Ahead of the cold front progressing into the East by Saturday, expect an area of above normal temperatures that will extend to the East Coast into early Sunday. Approaching shortwave energy and a combination of fronts should start to generate some enhanced rainfall over the Lower Mississippi Valley and Southeast around Friday night (just before the extended forecast period starts). As this activity continues across the Southeast during Saturday and Saturday night, the supporting wavy surface front may become more east-west oriented, possibly supporting some training. Thus there is some potential for locally heavier rainfall during the Day 4/Saturday ERO period in the Southeast and Florida Panhandle. As rain occurred in that general vicinity yesterday, soil moisture percentiles are actually showing some above average anomalies there currently, but soils should dry out by late week. However latest guidance has improved its clustering, mainly focusing a potential band of relatively heavier rainfall across northern Florida and southern Georgia. Also some first-guess guidance now indicates a Marginal Risk level of threat, so the new Day 4 outlook has introduced such an area over the aforementioned region. Some rainfall should persist over parts of the Southeast, especially portions of Florida, into Sunday but with lesser rates. Much lighter precipitation of varying types will be possible farther north over the East during the weekend. Guidance is slowly in the process of converging toward the idea of a weak and progressive East Coast into western Atlantic wave on Sunday which could produce some light precipitation of varying types, with wintry types possible from the Mid- Atlantic/Appalachians northward. Probabilities of 0.25 inch liquid in the form of snow continue to reflect the fairly light nature of this event, with low coverage of 10 percent or greater values, though higher elevations like the Adirondacks have higher probabilities. Some locally heavier snow will be possible to the lee of the Great Lakes from the weekend into early next week, as well as across the Appalachians with upslope flow. Farther west, expect snow to move southward across the Rockies/High Plains into Saturday in association with the Arctic surge and digging shortwave energy. Upslope flow should help to enhance accumulations over some areas. Another shortwave may produce some snow over parts of the Rockies early next week. What this shortwave does will determine how much precipitation emerges over the southern tier from Texas eastward on the southern periphery of the overall upper trough and surface high pressure. However, the southern extent of the cold air at the surface means that if there is precipitation it would mainly be in the form of wintry precip, perhaps all the way to the Gulf Coast, and freezing rain/ice is an increasing concern for parts of the South given the possibility of a warm nose atop Arctic cold air. Southern tier snow potential shows up in the winter weather probabilities for 0.25 inch liquid in the form of snow, now 10-15 percent over an area just north of the western half of the Gulf Coast for the early Monday-early Tuesday period and about 10-35 percent across a much broader swath of the South into southern Mid-Atlantic early Tuesday into early Wednesday. Uncertainties in the setup aloft over the Southwest into next week make the pattern at the surface somewhat uncertain as well. However, there is a general trend for weaker winds over southern California into the weekend, but offshore flow may strengthen again by Monday as 850mb winds increase (dramatically in some models). Keep an eye on SPC fire weather forecasts for further information. 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