Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Fri Mar 13 2020 Valid 12Z Mon Mar 16 2020 - 12Z Fri Mar 20 2020 ...Heavy rain threat over the Southern Plains next week... ...Guidance/Predictability Assessment... Guidance agrees in depicting mean troughing aloft over the eastern Pacific and western U.S., between strong ridges over east-central portions of the Pacific as well as the Gulf of Mexico and northwestern Caribbean. Within the mean trough the dominant feature will be an upper low that should be just off the northern California coast as of early Mon and then progress inland during the latter half of the week. Upstream North Pacific energy rounding the mean ridge will likely drop southward offshore from the West Coast next Thu-Fri with some potential influence on progression of the leading upper low. Meanwhile cyclonic Canadian flow aloft brushing the northern tier will support periodic surges of cold air into northern areas. The upper low tracking into the western-central U.S. and associated low pressure will support a broad area of precipitation from California into the east-central U.S. over the course of next week, with the potential for a significant snowfall over parts of the Northern Plains region by late next week. Over the last few model runs the primary trend for the upper low initially near California has been toward a somewhat westward/slower solution. At least through mid-period differences are generally within typical error ranges for the valid time but based on 12Z/18Z data the GFS strayed a bit faster than most other models/means. Thus the updated forecast leaned 75 percent toward the slightly slower consensus. By Thu-Fri there are some detail question marks regarding how the energy coming into the West ultimately evolves as it reaches the Plains. However the model/mean majority currently shows better than average agreement with vigorous low pressure reaching the central/north-central Plains by late in the week, supporting a blend of models/means (somewhat more ECMWF/ECMWF mean versus GFS/GEFS mean) late in the period. Farther west there is considerable spread for the energy dropping southward over the eastern Pacific late next week. Overall the ECMWF and GEFS/ECMWF means provided the best intermediate depiction of this energy--more offshore than the 12Z CMC and slower than the 12Z/18Z GFS--though the ECMWF mean could still be too close to the coast with the core of its upper trough. The 00Z CMC has favorably adjusted away from its 12Z run while the 00Z GFS may still be on the fast side. The 00Z ECMWF trended a bit faster to match the 00Z GEFS mean by early Fri. ...Weather/Hazard Highlights... The upper low initially near the West Coast will first support enhanced rain and higher elevation snow over favored terrain in the Sierra Nevada and ranges near the southern California coast. Precipitation will then spread into the Interior West and Rockies with highest totals likely over central Arizona and to a slightly lesser degree in the Great Basin and central Rockies. Dynamics aloft reaching the central U.S. should support Plains cyclogenesis during the latter half of the week with enough cold air present on the north side of the system to bring the potential of significant snowfall to parts of the Northern Plains. At the same time areas from the Southern Plains northeastward should see an episode of heavy rainfall with embedded convection. Some locations over/near the Southern Plains could also a separate period of moderate-heavy rainfall earlier in the week as a front lingers over the region. The combination of these two events, with highest totals currently most likely over parts of Texas/Oklahoma/Arkansas, may lead to flooding issues. The upper low tracking near the California coast and then into the West will bring a multi-day period of below normal highs to the southwestern U.S. with some readings 10-20F below normal. Some locations may challenge daily records for coolest highs. Central Montana will be another focus for cold weather during the week with highs 10-20F below normal. Some of this air will spread farther south through the north-central High Plains by Thu-Fri. Interior Northwest locations will be below normal early in the week. Ahead of the storm expected to develop over the Plains Thu-Fri, expect warmth to expand and become more pronounced over the South and East with time. By next Fri there should be a broad area of plus 10-25F anomalies for morning lows and plus 10-20F anomalies for highs. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml