Extended Forecast Discussion NWS Weather Prediction Center College Park MD 125 PM EDT Tue Aug 03 2021 Valid 12Z Fri Aug 06 2021 - 12Z Tue Aug 10 2021 ...Overview... The pattern switches to troughing in the Northwest and broad ridging over the central and eastern CONUS through this weekend. A drier pattern overall is expected with rainfall focused mostly along the Canadian border, across the Upper Midwest, with some along the Gulf Coast and into the Southwest. Temperatures will return to above normal in the East while the Northwest stays below normal under the trough. ...Guidance/Predictability Evaluation... The main features to track among guidance are a shortwave trough over the northern Great Basin Friday morning that shifts east to the northern Great Lakes through Sunday night and low pressure centered over BC/Alberta Friday/Saturday and how a reinforcing shortwave trough/low interacts with it. There is decent agreement with these features among global guidance on Friday. The 00Z UKMET is the slowest with the Great Basin trough, so is was removed from the blend on Day 4. Regarding the western Canada low, the 00Z ECMWF has the strongest reinforcing low from the Gulf of Alaska and maintains that low, shifting it to WA by Sunday/Day 5. Meanwhile the 06Z/12Z GFS has the most interaction with the Gulf of Alaska low and the western Canada low with both shearing with general troughing over the Canadian Prairies by Day 5 that does extend west to the PacNW. The 00Z CMC is essentially a more progressive version of the ECMWF. Given the deterministic differences, and similarities of the 00Z ECENS/06Z GEFS means, went heavier toward an ensemble mean solution, particularly for Days 6/7. ...Weather Highlights/Hazards... Stalled frontal boundary along the Carolina Coast dissipates Friday into Saturday as the responsible upper trough weakens. Rainfall will shift out of the northern Rockies to the Upper Midwest and Great Lakes for this weekend ahead of a shortwave trough south of a main low over BC/Alberta and then lingers into next week over the same area ahead of the parent trough that ejects east across the Canadian Prairies. Rainfall could be heavy at times Saturday night through Monday night over portions of the Upper Midwest. Monsoon rains persist, but look to be suppressed south/limited to AZ through the medium range period. Temperatures will be about 5-15 degrees cooler than normal in the Pacific Northwest behind the lead cold front Friday through this weekend, potentially continuing into next week depending on the low track from the Gulf of Alaska. Ahead of this lead cold front, temperatures will be 5 to 12 degrees above normal over the Plains Friday and this weekend before expanding to the East Coast Monday. A ridge over the Southeast sets up by Sunday and looks to persist well into next week bringing an extended heat wave from of the Lower Mississippi Valley to the Midwest and Mid-Atlantic. Jackson 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