Extended Forecast Discussion NWS Weather Prediction Center College Park MD 358 PM EDT Wed Aug 18 2021 Valid 12Z Sat Aug 21 2021 - 12Z Wed Aug 25 2021 ...Henri to strengthen to a Hurricane and threaten New England this weekend... ...Model Analysis and preferences... There is quite a bit of guidance variance with the ultimate track of strengthening Henri over the western Atlantic, but overall there has been a westward more progressive guidance trend over the past few runs that offers an increasing threat for maritime interests and New England this weekend. Please refer to the latest NHC advisories for updated forecasts on Henri. Elsewhere, models and ensembles are in much better general agreement with the 12 UTC GFS now in even better in line with other models, ensembles and the National Blend of Models over much of the nation days 3-7, bolstering forecast confidence. A composite blend is preferred to smooth out the less predictable smaller scale differences consistent with uncertainties. ...Sensible weather and potential hazards... Strengthening Henri looms over a highly settled Western Atlantic and the latest NHC track increasingly threatens New England this weekend with potential for enhanced winds/waves and rainfall. Again, please refer to the latest NHC advisories for updated forecasts. Upstream, amplified upper trough energies will steadily progress from the Northwest to the north-central U.S. then Great Lakes/Midwest this weekend into next week and spread potential for moderate amounts of lead convection/rainfall that may produce more local heavier amounts. Some high elevation snows are possible in this pattern for a cooled Northern Rockies with the approach and passage of the aforementioned upper troughs and associated frontal system and low developments and progression across this broad region. Meanwhile underneath, an upper ridge/high that is forecast to extend across the south-central U.S/South will support a protracted period of hot and humid temperatures over this broad region. Schichtel 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 Hazards: - Heavy rain across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Aug 23-Aug 25. - Heavy rain across portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - Flooding possible across portions of the Southern Plains. - Flooding occurring or imminent across portions of the Southeast and the Great Lakes. - Flooding likely across portions of the Southern Plains. - Excessive heat across portions of the Southeast, the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Sun-Tue, Aug 22-Aug 24. - High significant wave heights for coastal portions of the Northeast, Sat-Mon, Aug 21-Aug 23. - Heavy rain across portions of mainland Alaska, Sun, Aug 22. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Aug 23-Aug 24. 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