Extended Forecast Discussion NWS Weather Prediction Center College Park MD 411 PM EDT Thu Aug 19 2021 Valid 12Z Sun Aug 22 2021 - 12Z Thu Aug 26 2021 ...Dangerous Henri to intensify to Hurricane status and threaten New England by Sunday... ...Overall Pattern and Weather/Hazard Highlights... Henri is forecast to gain strength over the western Atlantic and threaten New England Sunday into early next week, bringing a substantial threat for high winds/waves and heavy rainfall. Guidance has overall come in better agreement with Henri, but the overall blocky flow around the system and less predictable smaller scale interactions still offer a tricky forecast. The National Hurricane Center indicated during our 17 UTC collaboration that Hurricane Hunter recon aircraft now investigating Henri may shed additional light for subsequent guidance runs. Please refer to the latest information from the NHC on Henri. Otherwise, an active upper level pattern will be in place across the northern tier states and southern Canada with several well developed shortwaves moving through with development and slow translation of well defined and wavy fronts to pool moisture and instability. This offers broad potential for scattered to perhaps numerous showers and storms, some of which could be strong or severe where convective complexes develop as per SPC. Meanwhile, a building upper level ridge underneath will continue to build across the southern tier states and the northern Gulf of Mexico leading to hot and humid conditions for much of the south-central/southern states. The upper high/ridge is expected to slowly retrograde westward next week and allow for a mainly dry and increasingly hot West. ...Model analysis and preferences... The WPC medium range product suite is primarily derived from the NHC forecast for Henri and elsewhere a composite of reasonably well clustered model and ensemble guidance along with the National Blend of Models days 3-5 (Sun-Tue). In this preferred blend outside of Henri, leaned strongly on the GFS/ECMWF/Canadian/UKMET days 3-5 (Sun-Tue) and then the GEFS/ECMWF ensemble means into days 6/7 amid growing forecast spread. In particular, recent GFS runs have been more progressive/amplified bringing east Pacific upper trough energies into the Northwest later next week and that does not seem well supported by the pattern nor GEFS/ECMWF ensembles. The resultant WPC forecast suite maintains good product continuity. Hamrick/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-Tue, Aug 23-Aug 24. - Heavy rain across portions of the Middle Mississippi Valley and the Upper Mississippi Valley, Sun, Aug 22. - Heavy rain across portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - Severe weather across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Sun, Aug 22. - Flooding possible across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians. - Flooding occurring or imminent across portions of the Southeast, the Great Lakes, the Mid-Atlantic, and the Southern Appalachians. - 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 winds across portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - High significant wave heights for coastal portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Aug 23-Aug 24. - Heavy rain across portions of mainland Alaska, Sun-Mon, Aug 22-Aug 23. 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