Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM 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 Plains/east-central U.S. Friday-Saturday... ...Overview... A leading southern stream compact closed upper low/shortwave will progress from offshore southern California early Tuesday and continue eastward with time, spreading light to moderate precipitation over the Southwest into midweek and then the Lower Mississippi Valley into Southeast Wednesday-Thursday as flow ahead of associated low pressure taps into Gulf moisture. This system will weaken thereafter. Upstream, confidence remains high in a deep and phased eastern Pacific upper trough reaching near the West Coast by around early Thursday along with a leading surface low/frontal system and atmospheric river. This system will bring widespread rain and higher elevation snow across the West in the mid-late week time frame. Most guidance suggests the upper trough will close off an embedded upper low over the Plains toward the end of the week, supporting strong surface low development from the central High Plains into Upper Midwest Friday-Saturday. Expect rainfall to increase in coverage/intensity over and just east of the Mississippi Valley by Friday-Saturday, while snow is possible to the northwest of the surface low track. Meanwhile areas of strong winds will be possible and the Storm Prediction Center is monitoring severe weather potential. The Pacific trough crossing the West will bring below normal highs to that region while the leading ridge (and possibly a weaker one farther east) should promote above normal temperatures over the central/eastern U.S. ...Guidance/Predictability Assessment... Latest guidance maintains the recent theme of reasonable agreement for the large scale pattern evolution but with some differences for embedded details. These differences involve the forecast of the leading southern tier system, mid-late week evolution over the Northeast and vicinity, a system affecting the Pacific Northwest around Wednesday (ahead of the large scale Pacific upper trough), and then the deep central U.S. system toward the end of the week. Some spread also arises for East Pacific/western U.S. flow by the end of the period next Saturday. Dynamical and machine learning (ML) models maintain some spread for the leading southern tier system. Based on guidance from the 00Z/06Z cycle, the ECMWF was on the slow side but could be used as part of a compromise. The new 12Z ECMWF run adjusted faster to this compromise, while the 12Z GFS nudged faster from its prior intermediate solution. Meanwhile guidance has been showing difficulty in resolving the evolution of small-scale shortwave energy off the West Coast Wednesday-Thursday, affecting low pressure forecast to track just off the Pacific Northwest coast. ECMWF runs have been the most persistent with energy spinning up a concentrated and deep upper low with corresponding deep low pressure. In contrast, latest CMC runs have a more open wave and thus faster/weaker low pressure. ML guidance is split (versus all runs being weak yesterday) but the cluster that develops a stronger low is more in line with the GFS/00Z UKMET that are around 980 mb versus the deeper 00Z ECMWF. The new 12Z ECMWF has adjusted to this intermediate idea that was favored. As the upper trough moves inland across the West and into the Plains, the consensus of dynamical model/ensemble and ML guidance suggests that the 00Z ECMWF may be a little on the slow side, and indeed the 12Z ECMWF has adjusted faster toward the majority. Accordingly, the new ECMWF is deeper at the surface. The ML model average for central High Plains into Upper Midwest low pressure is generally in the lower half of the 980s mb, additionally making the 00Z ECMWF questionably weak. Time will tell about the stronger side of the spread. The deep 00Z CMC over the Upper Midwest on Saturday trended a few mb higher in the 12Z run, while GFS runs have been inconsistent but averaging in the deeper half of the spread. A model/ensemble blend with manual adjustment to support low 980s depth Friday-Saturday looks reasonable and maintains continuity. As for the expanding rainfall shield ahead of this system, ML guidance for Saturday shows the axis of heaviest rain may extend farther south toward or to the Gulf Coast than GFS/GEFS runs suggest. Recent days of guidance has been at odds for the pattern over the Northeast and vicinity around Thursday, with a split among the stronger shortwave/surface low pressure of the ECMWF/ECMWF mean versus the flatter GFS cluster aloft with more high pressure. Into the 00Z/06Z cycle today, remaining support for the ECMWF scenario appeared to be waning somewhat, with the next closest theme coming from some ML models that had weak waviness over the Ohio Valley/Mid-Atlantic. Continuity favored holding onto an intermediate solution with a hint of surface waviness until confidence improved. The new 12Z ECMWF has tempered the signal for such a wave, so probable adjustments would be toward higher surface pressures and less precipitation. A blend approach provides a good starting point for the eastern Pacific/West Coast late week, with the majority of guidance generally favoring a flat mean ridge aloft over the East Pacific toward the West Coast. Forecast preferences led to a starting blend consisting of the 06Z GFS/00Z ECMWF/00Z UKMET for the first half of the period, followed by a transition to some 06Z GEFS/00Z ECens inclusion along with splitting the GFS component between the 00Z/06Z runs. ...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 (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. New guidance favors no change to the Days 4-5 EROs at this time. 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. Toward the end of the week another frontal system may bring some rain and mountain snow of varying intensity to the Pacific Northwest and northern California. 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 the latest model trends are showing decreasing precipitation potential/amounts across the Great Lakes to Northeast. As the primary upper 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, possibly extending as far south as the Gulf Coast. Winds are another possible concern with the deep low, and in the Plains behind the cold front. Currently the guidance shows the potential for highest wind speeds over the southern Rockies/High Plains around Friday. Monitor Storm Prediction Center outlooks for the latest information regarding potential severe weather threats. Above average temperatures are likely over most areas east of the Rockies during the period. Temperatures of 15-20F or so 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 (continuing into Saturday over the Great Lakes/Upper Ohio Valley), 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 except over northern areas, while lows of 5-15 degrees below normal are likely. Rausch/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