Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1248 AM EDT Sun Jul 19 2020 Valid 12Z Wed Jul 22 2020 - 12Z Sun Jul 26 2020 ...Overview and Weather/Hazard Highlights... The flow pattern across North America is forecast undergo some amplification during the extended period. Models/ensembles show consensus that a ridge axis should gradually strengthen by late in the week across the central U.S./Canada, with positive 500 hPa height anomalies forecast by the GEFS to extend from the Gulf of Mexico to the Arctic. Both upstream and downstream of the ridge axis, anticyclonic flow and upper-level troughing will be favored through time, across the Pacific Northwest, as well as New England and the Canadian Maritime provinces. A wave of low pressure is forecast to move from the Great Lakes to the Northeast Wed-Thu, with a trailing cold front moving from the Ohio Valley to the Eastern Seaboard, and a trailing stationary/warm front lingering across the Central/Northern Plains and Midwest. The wave of low pressure and cold front are expected to bring scattered to numerous showers and thunderstorms, with locally heavy rain to areas from the Great Lakes to the Ohio Valley and East Coast Wed-Thu, with locally heavy rains possible. This front will also bring an end to the intense heat across the Mid-Atlantic with Wed-Thu the last days where temperatures should reach into the mid-90s. A more significant round of convection and potential heavy rainfall appears possible across the Northern Plains and Upper Midwest Wed night through Fri along the trailing stationary/warm front, as additional mid/upper-level shortwave energy traverses the region. Latest guidance shows a decent signal for fairly widespread 1-2 inch rainfall amounts in those areas, with the potential for localized 3+ inch amounts (driven largely by convective-scale processes with low predictability in the medium range). Convective activity associated with this shortwave is forecast to spread farther east next weekend, perhaps becoming somewhat more scattered/disorganized in nature across the lower Great Lakes/Ohio Valley and Northeast. Farther south, a broad surface trough is expected to propagate westward across the Gulf of Mexico Wed-Fri, increasing the potential for scattered to numerous showers and thunderstorms across central/western portions of the Gulf Coast. In the Southwest, the seasonal monsoon should begin to ramp up as the central U.S. ridge intensifies late in the week, with guidance suggesting that convection across the Four Corners states should become more widespread by late in the week. ...Guidance/Predictability Assessment... Overall, model guidance was relatively agreeable through much of the extended forecast period, particularly at large scales. A heavily deterministic blend including the 12Z ECMWF/18Z GFS was used initially during days 3-4 (Wed-Thu), with a gradual increase in emphasis on ensemble means (ECENS/GEFS) through time, and majority weight placed on ensemble means by days 6-7 (Sat-Sun). This blend served as a reasonable compromise to smooth out various timing/amplitude differences on various features, while showing good larger scale consensus, and maintaining relatively good WPC forecast continuity. 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