Extended Forecast Discussion NWS Weather Prediction Center College Park MD 305 AM EDT Thu Jul 18 2019 Valid 12Z Sun Jul 21 2019 - 12Z Thu Jul 25 2019 ...Widespread Excessive Heat Threat Through The Weekend... ...Overview and Weather Highlights/Threats... A hot and humid airmass is expected to affect a large portion of the country into the beginning of this coming week as a broad ridge strengthens. Many locations within this excessive heat area will likely reach, or set new daily high minimum records and some maximum temperature records into Monday. Heat indices are forecast to be 105 to 115F+; therefore heat headlines will persist. This heat wave may become problematic for those working or recreating outdoors, individuals sensitive to heat, especially the elderly and animals. The intensity of the heat wave will reduce by early next week for a large portion of the country. As the ridge builds over the west-central portions of the country, a trough develops over the eastern one-third of the U.S. A cold front will pass through and eventually stall over the southern Mid-Atlantic, Southeast, Gulf Coast and Texas. This boundary will become a focus for widespread convection Tuesday and beyond. Moisture and instability pooling along this boundary may result in enhanced rainfall amounts and increase the risk for flash flooding. A last of a series of impulses will track east from the Canadian Prairies and into the northern tier of the U.S. this weekend. This pattern will be favorable for developing showers and thunderstorms capable of producing locally heavy convection/MCS activity over the northern Great Plains/Great Lakes Saturday night before activity focuses more on the cold front Sunday into next week. Locally excessive rainfall may result from the multiple rounds of rain/high rain rates over the northern tier. Guidance continues to suggest the max QPF occurs over the Upper Mississippi Valley and Great Lakes this weekend that spreads into the east-central and southeast states early next week with the trough amplification/cold front. The tail end of this cold front is likely to bring enhanced upslope flow to the central/southern Rockies as it sags south Sunday into next week. Expect renewed monsoonal convection over the Southwest under the developing ridge starting Monday. ...Guidance/Predictability Assessment... The greatest uncertainty continues to be with the strength and positioning of a western Canadian ridge this weekend and the associated effects on the strength and timing of the downstream trough over the eastern U.S. early next week. This forecast consisted of the 12Z ECWMF, 12Z ECWMF/NAEFS ensemble mean and the 18Z/00Z GFS, with the weighting of the means increasing day 5 through 7. Campbell/Jackson 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