Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Tue Aug 17 2021 Valid 12Z Fri Aug 20 2021 - 12Z Tue Aug 24 2021 ...Weather Pattern Overview... An amplified upper trough/low lifting out of the northern Rockies should bring much cooler conditions eastward into the northern Plains following some late week enhanced rainfall across the upper Midwest to the northern Plains. Another 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. A broad upper-level ridge re-establishing across the Deep South will likely steer tropical cyclone Grace on a westerly track across the southern portion of the Gulf of Mexico. Post-tropical cyclone Fred should be departing coastal New England on Friday. Meanwhile, recent model guidance has raised the possibility for tropical storm Henri to track closer to the East Coast of the U.S. and New England this weekend into early next week than previous forecasts. ...Model Analysis and preferences... Much of the model discrepancy continues to be focused across the Pacific Northwest into the northern Plains through the medium-range period where multiple shortwaves are forecast to interact with a mean trough. The 00Z CMC/CMC mean lifts the amplified trough/low at a much slower pace than the ECMWF and GFS in the short-term period. This rendered the 00Z CMC solution incompatible with the ECMWF and the GFS early in the medium-range period so that the CMC were eliminated from the model blend. In contrast, the 00Z ECMWF and the 06Z GFS were in good agreement with each other. However, the 06Z GFS began to develop a frontal wave much more quickly over the northern Plains late this weekend than its ensemble mean. The 00Z ECMWF also showed this tendency but with less discrepancy than the 06Z GFS/GEFS solutions. The EC mean and the GEFS mean were in good agreement with one another through the medium-range. Therefore, the WPC fronts/pressures forecast was derived from a blend of 00Z ECMWF/ECENS with the 06Z GFS/GEFS. A larger proportion of the 06Z GEFS were incorporated starting on Day 5, trending toward mostly a blend of the 00Z EC mean and 06Z GEFS by Day 7. ...Sensible weather and potential hazards... Improving weather conditions are expected for the East Coast to close out the work week after the remnants of Tropical Storm Fred is forecast to exit New England on Friday. Some showers and storms are expected to remain over portions of the Mid-Atlantic and the interior eastern U.S. ahead of a lingering weak upper trough. Activities should diminish quite a bit by Monday as the trough weakens further. But a cold front moving through the Northeast on Tuesday could sustain the rain across New England. Meanwhile, moderate to heavy rainfall is likely across portions of North Dakota and Minnesota ahead of the developing low pressure system to end the work week. Potential exists for some heavy rain together with strong to severe thunderstorms across the upper Midwest on Friday. Another corridor of enhanced showers and storms may materialize across portions of the Mid-South to the Tennessee Valley this weekend owing to a weak front lifting across the region. 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. Kong/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