Extended Forecast Discussion NWS Weather Prediction Center College Park MD 441 PM EDT Tue Jul 13 2021 Valid 12Z Fri Jul 16 2021 - 12Z Tue Jul 20 2021 ...Heat expected to continue across the northern Great Basin to the northern Plains... ...Overview... Upper troughing settling into the Northeast will extend a front southwestward into the central Plains but with uncertain details regarding the speed and location of individual frontal waves. These details will be important in placing the pre-frontal rainfall axis, where bouts of heavy rain and training thunderstorms are possible across the Ohio Valley early this weekend and then further south and west early next week. Meanwhile, high temperatures in the 90s to triple digits are forecast to continue ahead of an upper-level trough edging into the Pacific Northwest. Over the Southwest, the Monsoon will help touch off some showers and storms around the upper high across portions of AZ/NM/CO. ...Guidance/Predictability Evaluation... Models and ensembles showed decent agreement on the the longwave pattern evolution across the U.S. through the medium range but continued to show noticeable uncertainty on predicting if/where/when frontal waves will lift across the Northeast this weekend. This introduced more uncertainty than desired regarding the pre-frontal heavy rain potential with the ECMWF being quite robust in the QPF amounts. Model differences are less significant regarding the upper trough approaching the Pacific Northwest. The WPC medium-range package was derived based on the consensus of the 00Z ECMWF/00Z EC mean, the 06Z GFS/06Z GEFS, and a smaller proportion from the 00Z CMC/CMC mean, trending toward their ensemble means for Days 6-7. ...Weather Highlights/Hazards... In the West, rising heights will maintain dry and warming conditions for the northern Great Basin through the northern Plains where high temperatures are forecast to be the 90s and into the 100s this weekend into early next week into early next week, which will be 10 to nearly 20 degrees above normal for this time of the year. 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/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 out of the Midwest to the East, showers and storms will be scattered to numerous, with locally heavier amounts where thunderstorms locally train. Temperatures will be near to above normal ahead of the front by a few degrees, with only a few to several degree drop in its wake. Areas of highest rainfall chances will slowly shift southward across the MS Valley, TN Valley, Appalachians, and mid Atlantic to the southeast. Kong Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Southern Plains, and the Ohio Valley, Sun-Tue, Jul 18-Jul 20. - Heavy rain across portions of the Central Plains, the Northeast, the Central Appalachians, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Fri-Sat, Jul 16-Jul 17. - Excessive heat across portions of the Northern Rockies and the Northern Great Basin, Sun-Tue, Jul 18-Jul 20. - Excessive heat across portions of the Northern Plains, Fri-Tue, Jul 16-Jul 20. 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