Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Thu Jan 16 2025 Valid 12Z Sun Jan 19 2025 - 12Z Thu Jan 23 2025 ...Strong Arctic surge will push into the central U.S. this weekend and reach the East by early next week... ...Overview... Models and ensembles continue to show a very amplified mean pattern consisting of a deep trough from northeastern Canada through the lower 48 and an eastern Pacific ridge extending well northward. Thus confidence remains high in the forecast of Arctic air dropping southward over the central U.S. this weekend and expanding to include much of the East by early next week. By midweek most of the Plains should see moderating temperatures while the cold will likely linger over the South and East. In contrast, guidance is having a lot of difficulty in resolving important embedded shortwaves that will play an important role in determining the coverage and amounts of potential wintry precipitation across the South next week, with some lingering issues also continuing for a progressive wave tracking off the East Coast on Sunday. ...Guidance/Predictability Assessment... Very early in the forecast, spread persists for the details of a shortwave reaching the eastern half of the country by Sunday along with its accompanying wave that should track off the East Coast and most likely reach a position just south or east of Nova Scotia by early Monday. The relative majority of guidance (also accounting for the 12Z machine learning models) clusters within the realm of the ECMWF/UKMET/ICON for track and timing. In contrast to recent GFS tendencies to be on the faster side for the shortwave and surface feature, latest runs have adjusted slower than other guidance. Meanwhile the latest CMC runs are a deep and western extreme with the surface low (leading to heavier Northeast precipitation than other guidance), though their upper dynamics finally look a lot better versus consensus than in recent days when they were excessively slow/amplified with the originating energy over the West. Detail confidence drops precipitously behind this feature, as models show wide variations among each other and from run to run for important shortwaves. The first one should dig into the West by next Monday, with its eventual evolution affecting how much wintry precipitation may spread across the southern tier early-mid week. Through the 12Z/18Z model runs, there was a prominent clustering toward greater southwestward elongation of this shortwave versus the GFS. This was due to the majority not dampening the northern side of the upstream ridge as much as GFS runs. GEFS means have a GFS flavor but show some nudge toward a little more trough amplitude. Meanwhile the 12Z ML models also show more elongation than the GFS but with faster timing than most of the dynamical models. The past couple UKMET runs appeared closest to the ML model ideas. The new 00Z ECMWF has introduced a complication in that it now looks more like a slightly slower version of the GFS. As for eventual effects across the South, the overall array of guidance has supported a moisture shield somewhere in the realm of the prior two ECMWF runs (12Z and 00Z/15) and the GEFS/ECens means--between the extremes of the relatively dry GFS and heavy/northward CMC. While some aspects of guidance actually trends a little closer together as the next bundle of shortwave energy drops into the mean trough by Wednesday-Thursday, significant differences develop regarding whether/how much flow splits over the east-central Pacific. This does not bode well for the forecast immediately after early Thursday. Based on the above considerations, the early part of the updated forecast started with the 12Z ECMWF/12Z UKMET/12Z GFS in order from more to less weight. The 12Z GFS was little more palatable than the 18Z run where differences existed. The blend started to introduce the 18Z GEFS/12Z ECens means by Day 5 Tuesday and reached 60 percent total mean weight by the end of the period, with 30/10 lingering percentages of the 12Z ECMWF/GFS respectively. This approach helped to provide the best continuity possible under the circumstances while waiting for better eventual agreement for details. ...Weather/Hazards Highlights... By Sunday a broad area of hazardously cold Arctic air will already cover the central U.S. and should expand to cover much of the east by Monday. Expect many areas to see highs at least 20F below normal, with the coldest anomalies likely to be over the High Plains and vicinity where highs could be 30-40F below normal. Coldest temperatures on an absolute basis should be over northern Plains and Upper Midwest where subzero highs and lows in the -20s are likely. Wind chills should be even colder and more dangerous. WPC is posting Key Messages regarding this cold outbreak. The northern-central Rockies/Plains should moderate considerably by Tuesday-Wednesday (though with a potential for high winds) while cold weather may persist into Wednesday and possibly to some extent Thursday 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 and then slightly above normal readings slowly beginning to push eastward from the West Coast. The wave forecast to track quickly off the East Coast on Sunday may produce a band of mostly light precipitation before its departure. Some wintry weather is possible over parts of the Mid-Atlantic and southern New England. Although there is an intermediate majority cluster for this system in the guidance, there are still a couple solutions on the extreme ends of the envelope so continue to monitor forecasts. Farther west, a shortwave digging into the West early next week may produce some scattered snow depending on its evolution. What this shortwave does after Monday 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. The southern extent of the cold air at the surface means that a majority of precipitation could be of a wintry variety, perhaps all the way to the Gulf Coast. Freezing rain/ice is also an increasing concern for parts of the South given the possibility of a warm nose atop Arctic cold air. Southern tier snow potential continues to show up in the winter weather probabilities for 0.25 inch liquid in the form of snow, now 10-20 percent over an area just north of the western Gulf Coast for the early Monday-early Tuesday period and 10-40 percent across a much broader swath of the South into southern Mid-Atlantic early Tuesday into early Wednesday. Some precipitation potential lingers thereafter, but through this whole period confidence in details is below average due to considerable model differences and variability. Uncertainties in details aloft over the Southwest into next week make the surface pattern uncertain as well. However, there is some potential for a period of relatively stronger winds over parts of California and vicinity around Monday-Tuesday. Continue to monitor Storm Prediction Center fire weather forecasts for further information. 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 /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