Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sat Mar 18 2023 Valid 12Z Tue Mar 21 2023 - 12Z Sat Mar 25 2023 ...Another strong storm with heavy rain/snow and wind threats to impact California... ...Overview and Guidance/Predictability Assessment... Guidance is still showing broad cyclonic mean flow aloft with an overall axis along and then inland from the West Coast, while an upper ridge meanders from the northwestern Caribbean into the Gulf of Mexico and then over Cuba and the Bahamas. The system forecast to head into the West on Tuesday-Wednesday with an atmospheric river containing some moisture of tropical origin has held fairly steady in the guidance over recent runs, aside from detail differences within the typical guidance spread for a few days out in time. However the latest guidance behavior for the system's evolution east of the Rockies as well as southern Canada/northern U.S. flow and associated surface reflection have lowered confidence in forecast specifics over the eastern two-thirds of the country compared to yesterday. Still, it appears likely there will be heavy rainfall over some areas along with snow potential over some northern locations. The next Pacific system dropping into the western mean trough Friday-Saturday should be weaker and less amplified than leading one. For the system entering the West by midweek, there are still some differences among the models for exact timing and track of the upper low by early day 4 Wednesday. An average of latest guidance looks reasonable for resolving those issues given typical error ranges at that time frame. After midweek the models and means have become increasingly divergent over the past day with how this system will evolve. Latest GFS/GEFS runs have trended noticeably weaker from 24-48 hours ago with the core of the upper trough reaching the northern tier and associated surface low over the eastern half of the country. Now the 00Z GFS shears out the northern energy while placing more emphasis on the southwestern part of the upper trough--ultimately leading to a trailing strong surface system that reaches the Great Lakes about 24-36 hours later than the primary system that the majority cluster has been depicting. The last couple CMC runs have been very different among each other and the model/mean majority, with the new run actually taking a cue from the GFS in holding back some of the upper trough energy to yield a Friday-Saturday surface low that tracks farther south than the GFS. Meanwhile the details of flow from central Canada into the northern tier U.S. and continuing eastward remain another significant uncertainty. Recent trends have been for energy digging into the far northern tier by midweek to be slower, ultimately leading to an Upper Midwest into eastern Canada wave per the GFS/GEFS mean/ECMWF/UKMET majority. The 12Z ECMWF mean was somewhat eastward and looking less probable given the distribution of latest solutions. The new 00Z CMC is more extreme with its southwest elongation of this energy, to the point of influencing the leading dynamics emerging from the West and accelerating the associated system well ahead of other guidance. Solutions for the shortwave energy expected to reach the West Coast late in the week have been consolidating on the more shallow side of the prior guidance spread, with most emphasis over the Northwest. There is another modest impulse that may stream into the West underneath the leading feature. For the updated forecast based on 12Z/18Z cycle input, guidance comparisons were sufficiently favorable to start with an operational model blend on day 3 Tuesday and with minimal incorporation of the 18Z GEFS/12Z ECMWF means by day 4 Wednesday. Thereafter the CMC/UKMET strayed from the other models means in various ways, leading to a 18Z GFS/GEFS and 12Z ECMWF/ECens blend as the best way to yield a coherent evolution within the guidance spread. ...Weather/Hazards Highlights... A strong upper trough digging toward the West Coast, along with an associated low pressure/frontal system, will support a plume of enhanced moisture and yet another Atmospheric River event into parts of California and the southwestern U.S. around Tuesday. Expect a period of moderate to heavy lower elevation/coastal rain and heavy high elevation snow, especially in the Sierra Nevada. Rainfall totals should be less extreme than this past week's events while snow levels remain lower, somewhat reducing the flooding/runoff risk in comparison. However, forecast rainfall totals over favored terrain along with existing wet ground conditions support a Marginal Risk in the day 4 (12Z Tuesday-12Z Wednesday) Excessive Rainfall Outlook for areas from the lower elevations of the Sierra Nevada to the southern California coastal ranges, in addition to favored terrain over central Arizona. The overall array of guidance seems looks similar to the past 12-24 hours, supporting continuity for the aforementioned risk areas. Meanwhile expect significant snowfall across parts of the Great Basin and central Rockies into midweek, with lighter amounts to the north. Farther east, a leading system supported by northern stream energy may spread some light to moderate precipitation from the Plains/Midwest into the Great Lakes/Northeast. The trailing front that stalls may interact with moisture arriving from the ejecting western system to begin generating heavier rainfall over the Midwest around Wednesday night per some guidance, but other solutions have very different ideas. This area may require monitoring for future trends/guidance consolidation but the current guidance spread and neutral to dry soil conditions over the region of interest favor no risk area for now in the day 5 (12Z Wednesday-12Z Thursday) period. Right after this time frame, there should be a broader area of moderate to heavy rain and thunderstorms developing over areas from the south-central Plains east and northeast into the lower Great Lakes, Ohio Valley, and Tennessee Valley. Latest spread in the guidance seems to keep lowering confidence for specifics of this event, but there still appears to be a decent probability for some heavy rainfall in general. There may also be potential for severe weather. Check latest Storm Prediction Center outlooks as details of this system's evolution become more clear. Northern areas may see one or more opportunities for snow, but again with moderate or lower confidence in specifics. The next arriving Pacific system should bring the best focus for rain and mountain snow to the Pacific Northwest with somewhat lesser totals reaching California and eastward into the northern-central Rockies. The mean trough aloft over the West will keep the region chilly through the period, with fairly broad coverage of highs at least 10F below normal through the period. On the other hand, most of the eastern half of the country will see a warming trend to above normal temperatures after the Southeast starts out with lows 10-15F below normal early Tuesday. By late in the week many locations in the East should be at least 10-20F above normal, with locally higher anomalies for morning lows. Frontal progression will trim the western portion of this warmth by Saturday. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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