Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Mon Aug 16 2021 Valid 12Z Thu Aug 19 2021 - 12Z Mon Aug 23 2021 ***Tropical Depression Grace expected to strengthen back to tropical storm status over the Gulf of Mexico*** ***Emerging North-Central U.S. Heavy Rain Threat ahead of cold front and upper trough*** ...Weather Pattern Overview... A well defined upper level trough with potential for a cut-off low should be in place across the Intermountain West and Northern Rockies to begin the forecast period Thursday, and an upper level ridge situated over the Southeast U.S. in the wake of Tropical Storm Fred. The upper trough then reaches the northern Plains and drives a cold front and surface low eastward, with a building ridge across the Northeast states. A more robust ridge axis developing over the Gulf Coast region will tend to steer Tropical Storm Grace farther westward across the Gulf compared to the more eastward track of Tropical Storm Fred. ...Model Analysis and preferences... The 00Z model guidance suite has come into better agreement for the beginning of the period compared to the 12Z/18Z guidance. Although still progressive with the northern trough, the GFS slowed down from its 18Z run and closer to the model consensus. The past two runs of the CMC have been more amplified with the western to northern Plains trough, and is slower with the arrival of the next trough across the Pacific Northwest by the end of the forecast period. There still remains significant model spread with the path of Grace, with latest trends suggesting a more southern track owing to the strength of the ridge to its north. The WPC fronts/pressures forecast was primarily derived from a 12Z ECMWF/ECENS/UKMET blend along with a smaller portion of the 18Z GFS and previous WPC continuity for the end of the week, and then primarily ECMWF/ECENS/GEFS mean for next weekend. ...Sensible weather and potential hazards... Residual tropical moisture from what will become the remnants of Fred will fuel scattered to numerous showers and thunderstorms across the Northeast U.S. on Thursday prior to exiting the region, along with some lingering showers on Wednesday. Some instances of flooding will be possible, particularly across areas that have received above average rainfall already. Moderate to heavy rainfall is also becoming increasingly likely across portions of the central/northern Plains to close out the work week, with the potential for some scattered 1 to 3 inch totals where mesoscale convective complexes develop. Another corridor of enhanced showers and storms may materialize across portions of the Mid-South this weekend owing to a slow moving front in the vicinity. In the temperature department, expect readings to be up to 20 degrees below normal across portions of the northern Rockies and western portions of the northern Plains for the end of the week, and some high elevation snow for Wyoming and Montana is within the realm of possibility. Except for some slightly above average temperatures across the Great Lakes and New England, no serious heat waves are currently expected. 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