Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1136 AM EDT Sun Jul 21 2019 Valid 12Z Wed Jul 24 2019 - 12Z Sun Jul 28 2019 ...Overview and Guidance/Predictability Assessment... Amplified upper-level pattern expected to be in place through much of the medium range, with persistent upper ridging centered across the Four Corners region/Southwest and an upper trough, transitioning to broad cyclonic flow, across the eastern half of the CONUS. A cold front is expected to be off the Eastern Seaboard on day 3 (Wed), with the trailing end of the front extending from the Florida Panhandle/northern Peninsula to South Texas. Models show consensus that this front should slow or occasionally stall as a few waves of low pressure move along it through late in the week, very slowly moving the front further south/east, with the trailing portion of the boundary gradually washing out. Farther west, shortwave energy reaching the Pacific Northwest on day 3 is should move a cold front across the northern Great Basin/northern Rockies, reaching the northern High Plains by day 4 (Thu). Models handle this a bit differently compared to yesterday starting on day 5 (Fri), with the consensus now keeping the draped polar cold front the dominant feature, with the Pacific front washing out across the Upper Midwest by day 6 (Sat). Finally, additional shortwave energy is expected to reach the Pacific Northwest late next week. Deterministic solutions showed sufficient consensus during days 3-4 to use a multi-model blend (00Z ECMWF/UKMET/CMC/06Z GFS) as a forecast starting point. Starting on day 5, timing/amplitude differences began to grow with the shortwave crossing the Northern Plains/Upper Midwest, and with the additional shortwave reaching the Northwest late in the week. With both of these, ensemble means supported a slower progression than the GFS and even the ECMWF, somewhat closer to the CMC. Thus, the forecast during days 5-7 was based heavily on the ECENS/GEFS ensemble means along with a minority component of the CMC/GEM global. ...Weather Highlights/Threats... The frontal boundary off the Eastern Seaboard into the Gulf of Mexico is expected to focus areas of showers and thunderstorms from the coast of the Carolinas south to Florida. Models suggest the greatest potential for locally heavy rains will be across the northern Florida Peninsula Wed-Fri. 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 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 (5 to 10 deg F below average for both max and min temps) are expected across a large area from the Southern Plains to the Southeast/Mid-Atlantic in the wake of the cold front moving off the Eastern Seaboard. Temperatures are expected to gradually moderate back towards normal next weekend. Ryan 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