Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1217 AM EDT Fri Jul 17 2020 Valid 12Z Mon Jul 20 2020 - 12Z Fri Jul 24 2020 ...Hot temperatures for the Northeast/Mid-Atlantic through early next week... ...Overview and Weather/Hazard Highlights... A relatively strong and broad subtropical ridge is forecast to remain in place across the CONUS southern tier during the extended forecast period, while an active and progressive setup persists across the northern tier. The strongest positive 500 hPa height anomalies are initially expected to be in place across the Mid-Atlantic and Northeast Mon-Wed. The ridge will promote above average temperatures, with highs forecast to be 5 to 15 deg F above normal across portions of the Mid-Atlantic and Northeast Mon-Wed. Portions of the southern Mid-Atlantic region may see temperatures approach or reach 100 deg (with heat indices in the 105-110 range), especially on Mon. Isolated record high temperatures are possible during that time frame, with record high minimum temperatures likely more commonplace. Farther west, a wave of low pressure is forecast to develop along a frontal boundary and move across the Central/Northern Plains, Midwest, and Great Lakes Mon-Wed. This low pressure system along with a mid/upper-level shortwave are expected to produce an area of enhanced shower/thunderstorm potential. Model guidance shows the strongest signal for areas of heavy rainfall across the Midwest/Great Lakes Mon into Tue, where 1-2 inches of rain are possible (with locally higher amounts). As the system reaches the eastern U.S. Wed-Thu, scattered to numerous showers and thunderstorms are expected along much of the Eastern Seaboard, with locally heavy rain possible. By Thu-Fri the next shortwave is likely to be moving from the Rockies into the Northern Plains, accompanied generating another potential wave of low pressure, and areas of convection across the Midwest along a lingering surface front. Lastly, a broad area of moisture and convection is forecast to drift westward across the Gulf of Mexico next week, and may bring some increase in shower/thunderstorm activity to portions of the central/western Gulf Coast by late next week. ...Guidance/Predictability Assessment... Overall, model/ensemble guidance showed relatively good consensus at the larger scales, but deterministic guidance did show some notable differences on the finer-scale details of the forecast. The 12Z ECMWF was the preferred deterministic solution during this forecast cycle. The ECMWF was used heavily during the first half of the extended forecast period. Emphasis on ECENS/GEFS ensemble means was gradually increased through time to account for gradually increasing spread/uncertainty on the details, with majority weight placed on ensemble means during days 6-7 (Thu-Fri). The GFS showed some differences from the overall consensus, particularly with respect to the shortwave/low pressure crossing the Great Lakes/Northeast Wed-Fri - showing a more amplified solution for this system, resulting in a somewhat slower progression relative to the consensus of other guidance. Thus, the emphasis on the ECMWF along with ensemble means. 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