Extended Forecast Discussion NWS Weather Prediction Center College Park MD 128 AM EDT Wed May 15 2019 Valid 12Z Sat May 18 2019 - 12Z Wed May 22 2019 ...Guidance Assessment and Weather Highlights/Threats... ...System moving from the Plains Sat/Sun to Upper Ms Valley/Great Lakes Mon/northeast Tue... The latest deterministic models (18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian) have good clustering of the surface low forecast to be in Nebraska Sat morning, with gradual weakening of this low followed by development of a second low 00z Sun over KS that moves north to near the NE/IA border by 12z Sun morning. The slow frontal movement leads to potential for multiple rounds of showers/storms and locally heavy rain from the southern to the central Plains. See the Storm Prediction Center outlooks for the probability of severe convective weather. The WPC product suite primarily was derived from a composite ensemble mean solution with greater weighting on the ECMWF ensemble mean as its position is close to the multi-model consensus 12z Sun. By 12z Mon 20 May, excellent overlap of the 12z ECMWF Ensemble Mean and 18z GEFS Mean with the low pressure center supports a mean solution. Greater weighting on the means remains through mid week as the models/means divergence as the system crosses into the Upper MS Valley/Great Lakes/northeast and into the coastal and offshore waters by the middle of next week. ...Developing western US Mean trough over the weekend into the middle of next week... The models show a negatively tilted upper trough poised to move into the western US over the weekend and remain in place through the middle of next week. There is potential for a closed low to develop at the base of the trough in the southwest but the 12z ECMWF/18z GFS/12z Canadian global models are hundreds of miles apart on the low track/position by the middle of next week, lending decreasing confidence to the forecast. The 12z ECMWF was on the southeastern edge of the suite of solutions in the southern Plains with its 500 mb low position, so more weighting to the better clustered 12z ECMWF/18z GEFS Ensemble Means was used instead. Much of the western US to the northern high Plains will turn colder, with persistent areas of 10-20 egress below normal for high temperatures expected in the central to northern Great Basin to the northern High Plains. As a result of cold temperatures, terrain enhanced snows are possible in the ranges in the Pacific Northwest/northern CA/Great Basin/northern to central Rockies. Downstream, the persistence of western cyclonic circulations induces a mid level anticyclone over the lower MS Valley to the southeast. Underneath the upper ridge, much of the lower MS Valley eastward across the southeast, including the Carolinas, turn more summer like with above normal temperatures this weekend into the middle of next week. A stalled front leads to the persistence of showers/storms Sat over the FL Keys. Petersen 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