Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Jan 14 2025 Valid 12Z Fri Jan 17 2025 - 12Z Tue Jan 21 2025 ...Strong Arctic surge pushing into the central U.S. by the weekend and gradually expanding eastward... ...Overview... By this weekend the guidance continues to show the mean pattern evolving into a very amplified regime consisting of a deep trough extending from Canada through the lower 48 and a ridge whose axis extends over and well northward from the eastern Pacific. As this occurs, very cold air will drop into the central U.S. with the most extreme anomalies likely to be during Sunday-Monday. The trough axis should nudge far enough east by next Tuesday to offer some moderation over the northern-central Plains while coldest anomalies become aligned more over the East and South. Within this highly confident large scale pattern, embedded shortwave uncertainties continue to pose issues for specifics of potentially enhanced rainfall over the South around the end of the week into the weekend as well as for East Coast into western Atlantic frontal waviness that could produce lighter precipitation of various types. Meanwhile expect an area of snow to push south through the Rockies/High Plains with the cold surge before tapering off next week. ...Guidance/Predictability Assessment... At the start of the period, a system forecast to track just north of Minnesota and Lake Superior will be the main feature of note. Among 12Z/18Z runs, recent trends and continuity (plus machine learning guidance) favored ECMWF runs and the 12Z UKMET which were in the middle of the spread. New 00Z runs seem to be narrowing the spread. Meanwhile guidance has come into better agreement for shearing shortwave energy that originates from a short-term upper low to the southwest of California, but a lot of spread has persisted for more significant shortwave energy digging into the West during Friday-Saturday. ML models showed a fair amount of spread as well but leaned away from the more extreme sides of the dynamical envelope (mostly fast GFS runs and slow/amplified CMC and CMCens runs). This led to a preference toward an average of the 12Z and 00Z/13 ECMWF runs along with the 12Z UKMET as well as the 12Z GFS that was closer than most other GFS runs. The intermediate solution for the shortwave led to weak waviness from the East Coast into western Atlantic during Sunday-Monday. ML models also depicted this wave potential but with a fair amount of spread for timing, but all keeping the wave fairly weak. New 00Z guidance offers some hints at convergence as the previously slow ICON trended toward the ECMWF runs and the CMC is not quite as extreme before (but still extreme versus other guidance). The UKMET has trended faster though. By late in the period there are various ides for shortwave details over the West, though not much support exists for the degree to which the 00Z GFS brings shortwave energy through the Pacific ridge. Also some GFS runs have tended to bring the ridge axis somewhat farther east than most other solutions. Finally, by next Tuesday the ML models cluster fairly well in the direction of ECMWF runs and 00Z ICON in bringing surface high pressure into the southern tier versus some other solutions that keep it somewhat farther north at that time. Combined considerations for the above features led to a preference consisting of relatively greater emphasis on the past two ECMWF runs, with minority input from the 12Z GFS/UKMET. The forecast introduced some ensemble mean input (18Z GEFS/12Z ECens) late in the period, reaching a total of 40 percent by next Tuesday. ...Weather/Hazards Highlights... The forecast is fairly consistent with respect to the Northwest seeing a moderate cooling trend on Friday followed by a pronounced surge of Arctic air dropping south over the eastern Rockies into the Plains by this weekend. Expect increasing coverage of temperatures that are 15-35F below normal, with expansion of the cold air bringing highs down to 15-25F below normal as far east as locations between the Mississippi River and Appalachians by Monday and continuing toward the East Coast by Tuesday. The most extreme anomalies are likely to be over the central High Plains and vicinity during Sunday-Monday, while coldest temperatures on an absolute basis should be over far northern areas where some subzero highs are possible in that same time frame. The northern-central Rockies/Plains should begin to moderate on Tuesday. 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 eventually beginning to push eastward from the West Coast. Ahead of the cold front progressing south/east from the Plains Friday onward, an area of above normal temperatures will progress from the central U.S. on Friday to the East Coast by early Sunday. The Upper Midwest may see lows that are 20F or more above normal early Friday, with plus 5-15F anomalies more common elsewhere within this area of relative warmth. By Friday night, approaching shortwave energy and a combination of fronts may start to generate some enhanced rainfall across the southern tier, over or east of the Lower Mississippi Valley. This activity should continue into the Southeast during Saturday and Saturday night with the supporting wavy surface front possibly supporting the potential for some training. As the event starts, guidance is clustering better for the shearing shortwave but generally does not suggest high enough rain rates to cause runoff issues. Then on Saturday there is potential for locally heavier rainfall but with a majority of it over areas that have dry antecedent conditions, plus with some model spread for specifics. Therefore the Days 4-5 Excessive Rainfall Outlooks currently do not show any risk areas, but will continue to monitor for any trends toward a combination of better clustering and higher rainfall totals. Some rainfall may persist over parts of the Southeast beyond Saturday. Much lighter precipitation of varying types will be possible farther north over the East during the weekend and possibly into early next week. There are still significant differences for potential frontal wave development, keeping confidence low for details of precipitation coverage and amounts. However what signals exist suggest that any waves should be fairly weak with relatively light precipitation (which could be of the wintry variety from the Mid-Atlantic/Appalachians northward). Probabilities of 0.25 inch liquid in the form of snow reflect this, with low coverage of 10 percent or greater values. Some locally heavier snow will be possible to the lee of the Great Lakes from the weekend into early next week. Farther west, expect snow to move southward across the Rockies/High Plains late this week into the weekend as part of the Arctic surge, with upslope flow helping to enhance accumulations over some areas. 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