Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 PM EST Fri Jan 17 2025 Valid 12Z Mon Jan 20 2025 - 12Z Fri Jan 24 2025 ...Strong Arctic surge will move from the Central U.S. into the South/East next week... ...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 progressing from the Central U.S. to the East early-mid next 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 gradually 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 just north. There is still uncertainty with the wintry precipitation amounts and axis though. Another, even more uncertain, round of precipitation is possible across the southern tier later next week. ...Guidance/Predictability Assessment... By the medium range time frame there is more typical model spread compared to the past couple of days, with detail differences early on and increasing spread in the medium to larger scale pattern as the period progresses. Early in the week, a shortwave tracking across the southern tier shows some spread that is minor but does create some sensible weather differences like amounts and type of precipitation. A multi-model blend favoring the GFS and ECMWF seemed like the best approach for an intermediate forecast. By midweek, the next potent shortwave will be digging into the northern Rockies or so while a possible compact closed low in the eastern Pacific may or may not get absorbed back into the mean flow. GFS runs and the GEFS mean from 00Z and 06Z maintained a brief ridge axis in between the initial trough in the East and the digging one in the west-central CONUS. This allows for more Gulf moisture to flow into the South for mid-late week, and GFS/GEFS based guidance and the NBM were aggressive in showing ample precipitation there. This WPC forecast did decrease precipitation amounts compared to the NBM for Wednesday-Thursday, but now the 12Z GFS and GEFS have trended toward more zonal flow and thus drier. This is more like the other guidance, so expect the next issuance of NBM and WPC QPF may decrease even more. However, 00Z/06Z GFS runs did seem to be a good middle ground with the trough axis moving slowly eastward into the Central U.S. Thursday-Friday and matched the ensemble means best compared to the more progressive 00Z CMC and much slower 00Z ECMWF (the ECMWF was slower than many of the EC-based AI/ML models as well). There was some minor model convergence in the newer 12Z runs toward a middle ground solution on trough timing, but still some notable differences. Overall the WPC forecast decreased the proportion of deterministic guidance as the medium range period progressed in favor of the ensemble means (60 percent means by Day 6 and 80 percent by Day 7) considering the increasing spread. ...Weather/Hazards Highlights... By Monday, a broad area of hazardously cold Arctic air will already cover the Central to Eastern U.S. with daytime highs at least 20F below normal for most. The coldest anomalies are likely to be over the High Plains and vicinity where highs could be 25-35F below normal. Coldest temperatures on an absolute basis though should be over northern Plains and Upper Midwest where highs well below zero and lows in the -20s are likely early in the week. Wind chills should be even colder and more dangerous. 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. A shortwave moving from the Four Corners through the southern tier Monday-Wednesday will provide support for precipitation across the South as moisture overruns a front or two in the Gulf. 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 show up in the winter weather probabilities for 0.25 inch liquid in the form of snow, 10-20 percent over an area just north of the western Gulf Coast for the early Monday- early Tuesday period and 10-50 percent across a much broader swath of the South into southern Mid- Atlantic early Tuesday into early Wednesday. 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 set to begin for this South winter storm this afternoon. Just to the south of the winter concerns, rain totals are increasing across Florida Monday and especially Tuesday. There may be enough confidence for a Marginal Risk in future cycles for Tuesday's ERO, but at this point, there is still uncertainty if this rain will cause flash flooding as some models are much more bullish than others with the rain amounts. It will also depend on rain rates and the potential for repeated rounds of rain over the same areas. Farther north, the pattern will be favorable for lake effect snow across/downwind of the Great Lakes especially Monday-Tuesday, with the highest amounts off of Lake Ontario. By mid-late next week, precipitation may increase across the South again including some winter weather, but models have shown considerable variations with that potential, so will continue to monitor. 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. Also of note, a pattern favorable for stronger offshore winds is possible across southern California and vicinity around Monday-Tuesday which will increase fire weather concerns. Continue to monitor Storm Prediction Center fire weather forecasts for further information. 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