Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 AM EDT Thu Aug 19 2021 Valid 12Z Sun Aug 22 2021 - 12Z Thu Aug 26 2021 ...Henri to intensify to Hurricane status and threaten parts of New England by Sunday and Monday... ...Overall weather pattern... A rather active upper level pattern will be in place across the northern tier states and southern Canada with several well developed shortwaves moving through. A building upper level ridge will continue to build across the southern tier states and the northern Gulf of Mexico, and the core of the upper high is expected to retrograde westward through early next week. Meanwhile, Henri is expected to come uncomfortably close to eastern New England by late Sunday and into Monday with gusty winds and heavy rainfall possible. ...Model analysis and preferences... Models and ensembles are initially in good overall synoptic scale agreement across most of the continental U.S. on Sunday. Noteworthy differences become apparent by Tuesday across the north-central U.S. with a strong shortwave perturbation near the Canadian border, with the GFS indicating a considerably faster solution than the better clustered CMC/ECMWF/ECENS, and the GFS is also faster than its ensemble mean. The GFS is also stronger with a trough building near the West Coast by next Thursday and is out of phase by having a ridge over the Upper Midwest compared to the other guidance. Taking these factors into account, the WPC fronts/pressures forecast was primarily derived from a non-GFS blend, although some of the 18Z GEFS mean was incorporated by the second half of the forecast period. In regards to Henri, the main factor determining its eventual track near New England will be the strength of the blocking upper ridge over eastern Canada and a weak upper low over the Mid-Atlantic region. The trend over the past 48 hours has been farther west and this brings the storm very close to Cape Cod owing to a stronger ridge to the northeast. The 00Z UKMET is the farthest west over New England, whilst the ECMWF still maintains a weaker storm by the time it reaches this region. ...Sensible weather and potential hazards... Bad weather is becoming more likely for eastern New England by Sunday afternoon as Henri approaches from the south and comes close enough to produce heavy rain, strong winds, and coastal flooding from Rhode Island to southern Maine. It is important to note that future adjustments in the track can be expected, and the impacts will become more predictable in the days ahead. The National Hurricane Center has the latest information pertaining to this storm. Elsewhere across the continental U.S., amplified shortwave troughs will steadily progress from the Pacific Northwest to the north-central U.S., and then the Great Lakes next week, and these will lead to the potential for scattered to perhaps numerous showers and storms, some of which could be strong or severe where convective complexes develop. In the temperature department, highs are expected to be about 5-15 degrees below normal from the Northern Rockies to the Northern Plains, and up to 10 degrees above normal for much of the central and southern U.S. that will be under the influence of the upper ridge, along with plenty of humidity. 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