Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Jan 18 2025 Valid 12Z Tue Jan 21 2025 - 12Z Sat Jan 25 2025 ...Hazardous cold temperatures early week east of the Rockies should gradually moderate with time... ...A winter storm is likely Tuesday in the South... ...Overview... Models and ensembles continue to show a very amplified mean pattern into Tuesday 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 east of the Rockies 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 and gradually move east. Systems on the southern periphery of this pattern will cause precipitation from the Southern Plains to Southeast. A winter storm across the southern tier is still on track for Tuesday, which could produce snow/sleet close to the western/central Gulf Coast across the Southeast and Carolinas, with notable freezing rain possible for the Florida Panhandle and southern Georgia. There is still uncertainty with the wintry precipitation amounts and axis though. Rounds of mainly precipitation are possible in the northern tier through the week and then uncertain troughing moving into the Northwest could produce light to moderate precipitation into late week. ...Guidance/Predictability Assessment... Model guidance shows general agreement on the large scale as the forecast period begins Tuesday, with a positively tilted trough from the Midwest to Southwest. The shortwave on the southern side will pivot quickly eastward to provide forcing for the winter storm in the South while the northern side of the trough moves more slowly through the Great Lakes. Meanwhile the precipitation types and amounts are more variable and will be much harder to resolve until closer to the event. Models have waffled with how far inland the wintry precipitation would reach but this forecast trended toward a slight inland expansion. A multi-model blend seemed to serve as a good starting point for the WPC forecast. Upstream, the next potent shortwave or two will dig into the northern Rockies by Wednesday and drop south and eastward with time to produce a larger longwave trough. Models continue to vary with the depth of this energy, and the details of the trough (and potential for a brief ridge in between this trough and the eastern one) will be main factors in determining another possible round of Gulf Coast states precipitation that could spread north over at least parts of the Eastern Seaboard. This remains quite uncertain as models continue to vary. Behind this trough, an upper ridge across the West Coast shows good model agreement through Wednesday until quickly increasing model differences form on its northern side in western Canada. The GFS/GEFS suite has been persistent in maintaining ridging well to the north, while the ECMWF and CMC and most of their ensemble members as well as many of the AI/ML models indicate that the northern side would be suppressed by shortwave energy. The latter cluster brings troughing into the Northwest by Friday digging south into the Great Basin by next Saturday, while the GFS/GEFS show slower and weaker troughing and becomes out of phase with the non- NCEP solutions. This forecast leaned away from the GFS/GEFS type solutions but heavily used the EC ensemble mean in the forecast blend by late period to not go too far to the EC/CMC operational runs either. Favoring this timing pushed the front more quickly across the northwestern quadrant of the lower 48 Friday-Saturday compared to the previous forecast, but future additional changes may be needed given the uncertainty. ...Weather/Hazards Highlights... By Tuesday, an expansive area of hazardously cold Arctic air will be in place basically from the Rockies eastward, with low temperatures that are below average by 20-30F in the Rockies to Plains to Midwest. Actual temperatures will be in the -20s in the northern tier, and the Midwest could even see subzero highs. Wind chills should be even colder and more dangerous. The Northern/Central Plains should rapidly warm by Tuesday afternoon and reach near to above normal values for the latter half of the week. Meanwhile the coldest temperature anomalies (20-30 degrees below normal) will migrate into the Ohio Valley toward the western/central Gulf Coast midweek and into the Eastern Seaboard Thursday, leading to some daily record low maxes and mins. Temperatures are forecast to gradually moderate late week. The West on the other hand can expect near normal temperatures gradually warming to slightly above normal, before trending colder again by Friday or Saturday. 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 as shown by the winter weather probabilities for 0.25 inch liquid in the form of snow/sleet, with a trend in this forecast to overspread areas farther inland. Additionally, freezing rain/ice remains a concern given the possibility of a warm nose atop Arctic cold air, which is most likely in northern Florida into southern Georgia and coastal South Carolina. WPC Key Messages are ongoing 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/axis, 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, before gradually tapering off midweek and beyond. Snow showers are possible into the Northern Plains Tuesday-Wednesday which could perhaps spread across parts of the Midwest later week. Depending on pattern evolution, by mid-late next week, precipitation may increase across the South again including some winter weather, perhaps spreading north across the Eastern Seaboard. But models have shown considerable variations with this potential and will still need to be monitored. The mean upper ridge over the West Coast through midweek will keep conditions dry. Unfortunately a pattern favorable for stronger offshore winds is possible across southern California and vicinity into Tuesday, which will increase fire weather concerns. Continue to monitor Storm Prediction Center fire weather forecasts for further information on this Critical Risk. Later in the week, upper troughing is likely to move through the Northwest (albeit with timing differences) and produce light to moderate precipitation. Tate/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