Extended Forecast Discussion NWS Weather Prediction Center College Park MD 105 AM EDT Thu Jul 15 2021 Valid 12Z Sun Jul 18 2021 - 12Z Thu Jul 22 2021 ...Heatwave from the northern Great Basin to the northern Rockies/northern Plains... ...Weather Highlights/Hazards... A potent upper ridge building from the Great Basin to the northern Rockies/northern Plains this period will lead to a heatwave across these regions. Temperatures are forecast to be the 90s and lower 100s at lower elevations this weekend into next week, which are 10-20 degrees above normal. Expect some record values will be observed. The above normal area expands east into MN and WI next week as well. 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/southern UT during the afternoon and early evening. The monsoon season has been more active than last summer in AZ and the favorable conditions are forecast to persist across AZ. The GFS forecasts a moisture plume from the Gulf of California north into AZ, with above normal precipitable water values of 1.5-1.75 inches persisting through Thu 22 July supporting the monsoonal convective activity. A wavy and slow moving front will move south down across the Plains, MS Valley, OH Valley, Appalachians, Mid-Atlantic, and settle over the South/Southeast this period, with scattered to numerous showers and thunderstorms, with locally heavier amounts where thunderstorms locally train. High temperatures are forecast to fall below normal into early next week due the clouds and showers in the vicinity of the modestly cooling front. ...Guidance/Predictability Evaluation... The bulk of guidance seems in good agreement through early next week in a pattern with overall above normal predictability, due to both the large, sprawling anticyclone out west, plus slow movement of the eastern US upper trough. The WPC medium range product suite was primarily derived from the best clustered 12 UTC ECMWF/12z ECMWF ensembles. Minor timing differences exist with the upper trough in the east and anticyclone out west, but these differences are less than usually observed days 5-7. The favored WPC blend generally maintains good continuity. 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