Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Aug 18 2021 Valid 12Z Sat Aug 21 2021 - 12Z Wed Aug 25 2021 ...Weather Pattern Overview... A well defined upper level trough should be in place across the Pacific Northwest and the Upper Midwest to begin the forecast period Saturday, and an upper level ridge situated over the Northeast U.S. and also along the Gulf Coast. This Gulf Coast ridge will keep Grace tracking westward and remaining well south of the Texas/Mexico border. ...Model Analysis and preferences... Much of the model discrepancy continues to be focused across the Pacific Northwest and extending eastward to the northern Plains through the medium-range period, where multiple shortwaves are forecast to interact with a synoptic scale trough. The 18Z GFS remained more progressive than the model consensus with these shortwave passages, however the 12Z GFS aligned better and it was incorporated into the forecast in lieu of the 18Z run. With the next trough/upper low arriving by Sunday night/Monday across the Pacific Northwest, the 00Z GFS accelerates across southern Canada by Tuesday, whereas the ensemble means and the ECMWF/CMC suggest a slower progression. In terms of Tropical Storm Henri, the 00Z guidance has continued to adjust westward with the track, bringing the storm closer to eastern New England by early next week, particularly with the UKMET and GFS, so this is something that will need to be monitored very closely in the upcoming days. The WPC fronts/pressures forecast was derived from a blend of the 12Z ECMWF/ECENS/CMC/GFS was used as a baseline, and then a greater portion of the ensemble means for Tuesday and Wednesday. ...Sensible weather and potential hazards... Moderate to heavy rainfall is likely across portions of northern Minnesota and the northern Great Lakes on Saturday ahead of the developing low pressure system, with the majority of the rain happening before this forecast period. The next upper trough dipping into the Pacific Northwest should bring another round of rain/storms across the northern Plains into the Upper Midwest this weekend into early next week. Meanwhile, another corridor of enhanced showers and storms may materialize across portions of the Mid-South to the Tennessee Valley this weekend. Depending on the eventual track of Henri, some rain and wind may be possible for portions of New England early in the week. In the temperature department, expect rather warm conditions across the upper Midwest and northern New England while some high elevation snow for Wyoming and Montana is within the realm of possibility. Meanwhile, the upper high that is forecast to extend across the Deep South and Gulf Coast region will likely result in temperatures running a few degrees above average. Pleasantly cool conditions are expected from the northern Plains to the Upper Midwest with the trough in place. Hamrick 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