Extended Forecast Discussion NWS Weather Prediction Center College Park MD 128 AM EST Sat Jan 18 2025 Valid 12Z Tue Jan 21 2025 - 12Z Sat Jan 25 2025 ...Hazardous cold temperatures early week across the South and East should gradually moderate with time... ...Confidence increasing for a southern tier winter storm Monday- Tuesday... ...Overview... Models and ensembles continue to show a very amplified mean pattern early next week consisting of a deep upper trough over much of the lower 48 and an eastern Pacific ridge. Confidence remains high in a surge of Arctic air which should be in place across the eastern half of the nation by early week due to this trough aloft and cold high pressure at the surface. This trough should lift north and east as the week progresses for gradually moderating temperatures, but troughing is forecast to reload in the Central U.S. for later week. Systems on the southern periphery of this pattern will cause precipitation from the Southern Plains to Southeast. There is increasing confidence in a winter storm across the southern tier, which could produce snow/sleet close to the western/central Gulf Coast across the Southeast and Carolinas, with notable freezing rain possible from the Florida Panhandle and just north. There is still uncertainty with the wintry precipitation amounts and axis though. A strong shortwave into the Northwest late next week should bring a round of generally light to moderate precipitation to that region. ...Guidance/Predictability Assessment... The models and ensembles show more than typical model disagreement, even on the large scale. There is some agreement on a general pattern but the mid to larger scale differences are very apparent by late period and would have some notable impacts on sensible weather, especially across the South. There is increasing agreement on a shortwave shifting through the Deep South Tuesday- Wednesday, and increasing confidence in significant wintry impacts as far south as the Gulf Coast/northern Florida. Plenty of uncertainty in the details and exact precip types, much of which will take until the short range to resolve fully. A multi-model blend seemed to serve as a good starting point for the WPC forecast. Behind this, the next potent shortwave will dig into the northern Rockies and drop south and eastward with time. There are considerable differences as early as Thursday both with timing and amplitude of this shortwave. Through the 12z/18z model runs, the UKMET was the fastest with this shortwave and the ECMWF was the slowest/deepest. Preferred a solution closer to the GFS/CMC which were better clustered with the ensemble means as well. This results in a slightly flatter wave and thus less precipitation over southern locations for late week. The next shortwave enters the Northwest around Friday and there are also notable differences in the timing of this as well. The CMC is the quickest with this, while the ECMWF and GFS show reasonable timing consistent with the ensemble means. The late period WPC blend heavily favored the means, given increasing uncertainty not only with this system, but also the evolution of the central U.S. trough as it shifts eastward next weekend. ...Weather/Hazards Highlights... By Tuesday, an expansive area of hazardously cold Arctic air will be in place from the Southern Plains to the Midwest and South/East with daytime highs at least 20F below normal for most. The coldest anomalies are likely to be over the Southern tier and Great Lakes/Midwest where highs could be 20-30F below normal. Coldest temperatures though should be over the Upper Midwest where highs near zero and lows in the -20s are likely on Tuesday. Wind chills should be even colder and more dangerous. Compared to the short range period, the northern- central Rockies/Plains should moderate considerably by Tuesday- Wednesday (though with a potential for gusty winds) while anomalously cold weather may persist into Wednesday and possibly to some extent Thursday across the South and East. The West on the other hand can expect near normal temperatures gradually warming to slightly above normal before trending colder again by next weekend. A shortwave moving through the southern tier Tuesday-Wednesday will provide support for precipitation across the South as moisture overruns fronts in the Gulf of Mexico. Given the cold airmass in place, the majority of the precipitation will be wintry, perhaps all the way to the Gulf Coast. Southern tier snow potential continues to increase in the winter weather probabilities for 0.25 inch liquid in the form of snow. Additionally, freezing rain/ice remains a concern given the possibility of a warm nose atop Arctic cold air, which is most likely in the Florida Panhandle into southern Georgia and coastal South Carolina. WPC Key Messages are in effect for this South winter storm. Just to the south of the winter concerns, moderate to heavy rain is possible across central Florida on Tuesday. A Marginal Risk may be needed in future cycles for Tuesday's ERO, but at this point, there is still uncertainty on QPF totals, instability, and rain rates. Farther north, the pattern will be favorable for lake effect snow across/downwind of the Great Lakes into Tuesday, with the highest amounts off of Lake Ontario. Depending on pattern evolution, by mid- late next week, precipitation may increase across the South again including some winter weather, but models have shown considerable variations, and generally have trended lighter/away from the potential, but will still need to be monitored. The mean upper ridge in the West Coast will keep precipitation amounts low and limited to the Pacific Northwest. Farther inland the northern/central Rockies could see light snow at times with uncertain snow showers also possible farther east into the High Plains. Additional rounds of precipitation may move into the Northwest late week. Also of note, a pattern favorable for stronger offshore winds is possible across southern California and vicinity around early week which will increase fire weather concerns. Continue to monitor Storm Prediction Center fire weather forecasts for further information. Santorelli 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