Extended Forecast Discussion NWS Weather Prediction Center College Park MD 118 AM EDT Wed Jul 14 2021 Valid 12Z Sat Jul 17 2021 - 12Z Wed Jul 21 2021 ...Heatwave expected to continue across the northern Great Basin to the northern Plains... ...Weather Highlights/Hazards... In the West, a developing and persistent closed anticyclone from the central Rockies with ridging towards the northern Plains will lead to a heatwave across these regions. High temperatures are forecast to be the 90s and into the lower 100s at lower elevations this weekend into early next week, which will be 10 to nearly 20 degrees above normal. Temperatures in the Southwest will be slightly below normal due to increased cloud cover and higher rainfall chances. Showers and storms will be scattered across AZ/NM/southern CO during the afternoon and early evening. Below normal temperatures may also develop in the southern high Plains as northerly flow advects cooler air aloft across the area. As the cold front moves slowly south down the MS Valley, OH valley, central to southern Appalachians, mid Atlantic, and then the southeast,showers and storms will be scattered to numerous, with locally heavier amounts where thunderstorms locally train. High temperatures are forecast to be a few degrees below normal early next week in the southeast due the clouds and showers in the vicinity of the front, and modest cooling. ...Guidance/Predictability Evaluation... Models and ensembles showed decent agreement on the the longwave pattern evolution across the U.S. through the medium range, with persistent ridging in the interior west leading to downstream trough in the eastern US. One source of uncertainty was predicting where frontal waves will develop along the front from the Ohio Valley to the Appalachians and Northeast this weekend. This led to uncertainty regarding the rain potential with the 12-00z ECMWF showing bands of heavier QPF amounts north of the front in the northeast. The WPC medium-range package was derived based on the consensus of the 00Z-12z ECMWF/12Z EC mean, and the 18Z GEFS mean. Petersen Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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