Extended Forecast Discussion NWS Weather Prediction Center College Park MD 202 PM EST Wed Jan 15 2025 Valid 12Z Sat Jan 18 2025 - 12Z Wed Jan 22 2025 ...Strong Arctic surge will push into the central U.S. this weekend and expand east early next week... ...Overview... A very amplified pattern is expected through the medium range period, with an eastern Pacific ridge and a deep trough extending through the lower 48. This pattern will allow Arctic air to surge south into the central U.S. this weekend, and the anomalously cold air mass will expand east and southeast early next week. 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 also starting to pick up on a late period wave that could create winter weather concerns in the southern U.S. mid-next week. ...Guidance/Predictability Assessment... Model guidance has come into good agreement on the overall pattern and shortwave details through the weekend. On Saturday, a sheared shortwave will push east-northeast across the southern tier while a second shortwave moves south across the Rockies. This secondary shortwave will move quickly east on Sunday, swinging across the South to the Mid-Atlantic. There are still minor differences in the timing of the second shortwave, but overall model agreement on these features is good. Model spread increases significantly next week as they try to resolve details of shortwaves in the West. Another shortwave will likely drop south into the western U.S. by Monday, but the subsequent evolution of this feature is uncertain. The evolutions will be influenced by timing and strength of another shortwave that will likely move into the Northwest mid-next week, and there is a lot of spread for how this feature may pass through the mean ridge to the west. However the pattern evolves, 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. For WPC's afternoon forecast, a blend of the GFS/ECMWF/CMC/UKMET were used through the weekend with heavier weight placed in the GFS and ECMWF which are the best clustered and are demonstrating the most skill with this weather pattern. For Monday-Wednesday, increasing amounts of the GEFS and ECENS ensemble means were added to the blend while the UKMET and CMC amounts were gradually decreased. ...Weather/Hazards Highlights... The well-advertised surge of Arctic air will bring hazardous cold to much of the lower 48 this weekend into early next week. Cold will spread from the Rockies and Plains on Saturday to the South and Midwest on Sunday and to the East and Southeast on Monday. Temperature anomalies are forecast to reach 15-35 degrees below normal, with the most extreme anomalies in the Plains. Coldest temperatures on an absolute basis should be over northern Plains and Upper Midwest where 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. Shortwave energy moving over the southern U.S. will push a low pressure system across the Southeast this weekend. The leading front may become east-west oriented, which would support training showers and thunderstorms that could produce locally heavy rainfall. Thus, there is a Marginal Risk of heavy rainfall for portions of the Southeast on Saturday (Day 4 ERO). This system should quickly move northeast ahead of an approaching cold front later this weekend, which will lead to a decreased risk of heavy rainfall in the Southeast for Sunday (Day 5 ERO). Precipitation will also extend well to the north ahead of the approaching cold front this weekend, with precipitation chances for most areas east of the Mississippi River and chances for wintry precipitation for much of the Northeast and central Appalachians. 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. Dolan/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