Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1248 AM EDT Sun Jul 21 2019 Valid 12Z Wed Jul 24 2019 - 12Z Sun Jul 28 2019 ...Overview and Guidance/Predictability Assessment... The pattern is amplified days 3-4 with the high amplitude ridge in the southern Rockies and high amplitude eastern US trough. The overall trend on days 5-7 is for the western ridge to drift west across the southwest and for deamplification of the eastern US trough. The models now show a well defined west-east ridge over the southwest Atlantic that builds across Florida into the Gulf of Mexico next weekend. The northern stream wave train shows potential for a few waves progressing across southern Canada and the northwest, northern Plains, upper MS Valley, and Great Lakes. The 12z ECMWF flipped on the timing and phasing of the 500 mb wave train. The 12z ECMWF Ensemble Mean maintained continuity, which was preferred over the large changes in the operational run. For example, most ECMWF ensemble members at 12z Sat have a 500 mb ridge in MT while the 12z operational run had a trough. The mean had a 500 mb ridge in MT Sat morning 27 Jul, which also matched up well with the GEFS and NAEFS Means and 12z GFS. A multi-model/ensemble mean blend (12Z ECMWF/12z ECMWF Ensemble Mean/12z CMC/18Z GFS/18z GEFS Ensemble Mean) was used as a basis for the forecast. Greater weighting was given to the ECENS/GEFS ensemble means to maintain continuity and to mitigate model to model and run to run differences. ...Weather Highlights/Threats... The frontal boundary expected to stall off the Eastern Seaboard through the medium range is expected to focus numerous showers and thunderstorms. The most widespread convection and the greatest potential for areas of heavy rainfall should occur from eastern/coastal North Carolina southward across the coastal plain to the Florida Panhandle Tue-Thu. Showers/storms may lessen in coverage/intensity by late next week as the surface front washes out. The shortwave/frontal system moving from the Pacific Northwest to the Northern Plains/Upper Midwest should bring an increase in shower/storm chances to the Upper Midwest by late next week and across the Great Lakes next weekend. Meanwhile, daily scattered storms will be possible across the central/southern Rockies as some monsoonal moisture manages to spread into the upper ridge. Below average temperatures are expected across a large area in the wake of the cold front moving off the East Coast Tue. Temperatures are expected to gradually moderate back towards normal next weekend. The persistence of the western Ridge leads to temperatures gradually increasing in interior parts of WA/OR/northern Ca and spreading across ID into western MT next Sat 27 Jul and Sun 28 July. 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