Extended Forecast Discussion NWS Weather Prediction Center College Park MD 207 AM EST Sat Mar 8 2025 Valid 12Z Tue Mar 11 2025 - 12Z Sat Mar 15 2025 ...Potent system brings precipitation to the West Wednesday- Thursday and the central U.S. Friday... ...Overview... As the medium range period begins Tuesday, a leading southern stream compact closed upper low/shortwave will make its way into Southern California and vicinity. This should promote light to moderate precipitation spreading into the Southwest and eventually the Lower Mississippi Valley to Southeast Wednesday-Thursday as the upper shortwave moves eastward, forms a surface low, and taps into Gulf moisture. Upstream, there remains high confidence that a deep and phased upper trough will move through the eastern Pacific midweek into the West later week along with an atmospheric river, which will bring widespread rain and higher elevation snow across the West. A strong surface low is forecast to develop in the central Plains in the lee of the Colorado Rockies as this trough pivots east. Snow is possible on the backside of the low in the north-central U.S. while rain is likely in the Mississippi Valley. East of the main trough, bouts of upper ridging will allow for above normal temperatures across the central and eastern U.S. for much of next week. ...Guidance/Predictability Assessment... Model guidance is in generally good agreement on the broad scale while smaller scale features show varying degrees of spread. Models continue to vary with the timing of a Southeast upper/surface low pulling away as the period begins Tuesday, with the CMC on the slower side of the envelope. The first upper trough/compact closed low and the associated surface low entering southern California Tuesday show pretty good agreement on timing initially, though as they track quickly across the southern tier, ECMWF runs become slower than the rest of the guidance. The 12Z ECMWF was still within the ballpark of other models but the newer 00Z ECMWF has trended even slower. Future forecasts will continue to monitor to see if other guidance, including the 00Z runs of the AI/ML models, start to show this trend. This will affect an associated surface low track and timing as well, but newer 00Z guidance shows the surface low quite weak or dissipated by early Thursday. Then models agree in principle that deep troughing will move through the eastern Pacific and reach the West Coast by Thursday. One model issue is the potential for a closed upper low to be embedded within the trough, peaking in strength on Wednesday offshore of the Pacific Northeast. The 12Z/18Z operational models were overall on board with this upper low, which supported a deep surface low, but the 00Z GFS and CMC trended away from this deep low while the 00Z ECMWF kept it going. At least guidance agrees in the shift of the trough eastward with time, and spinning up a consolidated surface low in the central Plains. The 12Z CMC was east of consensus with the surface low but the 00Z looks more reasonable. The 12Z and 18Z GFS showed the trough becoming positively tilted by next Saturday as some energy is held back into the Four Corners, which was not favored as it was unlike the other models and the ensemble means. The WPC forecast began with a blend of the deterministic guidance favoring the 12Z ECMWF and the 18Z GFS, and increased the proportion of ensemble means to half Day 6 and more Day 7 as model spread increased. ...Weather/Hazards Highlights... On Tuesday, some modest lower elevation rain and higher elevation snow is forecast across southern California and the Southwest, likely remaining below excessive rainfall thresholds. Then the main atmospheric river will come into the West ahead of the deep eastern Pacific/West trough by late Tuesday or Wednesday. Wednesday should be the primary day for the AR to move through California, and a Marginal Risk is in place for the Day 5 ERO across lower elevation areas. The relatively fast movement of the AR could limit the flooding potential to stay below Slight Risk levels, but will continue to monitor for any localized enhanced areas that could flood, with particular concern for burn scars. There also remains a chance that the AR could come into northern California/far southern Oregon before 12Z Wednesday, so during the Day 4 ERO period, but continue to hold off on any ERO risk for Day 4 given the model spread on the timing. Higher elevation snow could be heavy across the Sierra Nevada with the AR. More broadly, areas of rain and higher elevation snow are forecast to spread across much of the West Wednesday-Thursday. Widespread high winds will also be a concern there. Farther east, rain is forecast to spread across the Lower Mississippi and Tennessee Valleys and Southeast Wednesday-Thursday ahead of the initial shortwave and a couple of frontal systems. Farther north there is some model spread with shortwaves and frontal systems that could cause precipitation. Generally model guidance is trending lower with precipitation chances, but there could be periods of light precipitation across the Great Lakes to Northeast. As the primary trough moves into the central U.S., there is high confidence in surface low development/consolidation in the central Plains by Friday with strong fronts. On the backside of the low, snow could spread across the central High Plains and into the Dakotas on Friday and perhaps into the Upper Midwest on Saturday. Rain chances emerge ahead of the cold front across the Mississippi Valley Friday, and heavier rain is likely by next Saturday with the current forecast showing it centered in the Tennessee Valley and vicinity. Winds are another possible concern with the deep low, and in the Plains behind the cold front. Above average temperatures are likely generally for areas east of the Rockies during the period. Temperatures of 15-20F above normal are forecast across the Ohio Valley to Northeast on Tuesday, with anomalies of that magnitude pushing a bit southward Wednesday with a backdoor front in the Northeast. Highs could reach the 60s and 70s. Temperatures are forecast to increase in the northern Plains to Midwest and Great Lakes Thursday and Friday, with anomalies reaching 20F to approaching 30F above average. Temperatures in the south-central U.S. could reach the 80s while localized areas like far South Texas can expect 90s, even approaching 100F. The Plains are likely to cool late week behind the cold front. Meanwhile, the troughing across the West along with increasing clouds and precipitation will promote below normal temperatures spreading across the region as the week progresses. Highs are forecast to be generally below average by 15-20 degrees while lows of 5-15 degrees below normal are likely. Tate 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