Extended Forecast Discussion NWS Weather Prediction Center College Park MD 251 AM EDT Fri Aug 20 2021 Valid 12Z Mon Aug 23 2021 - 12Z Fri Aug 27 2021 ...Dangerous Henri likely to affect New England into early next week... ...Overview... The 03Z National Hurricane Center advisory for Tropical Storm Henri shows the system strengthening to hurricane status as it lifts northward over the western Atlantic during the rest of this week into the weekend. Henri should weaken somewhat and be near southern New England by the start of the medium range period early Monday, followed by a track to the northeast. Expect Henri to bring a substantial threat for high winds/waves and areas of heavy rainfall. Refer to the latest NHC products for further information on Henri. Meanwhile the large scale pattern will be fairly typical for late summer with southern Canada/northern U.S. flow aloft pushing along rainfall-focusing waves/fronts across the northern half of the lower 48 while mean ridging aloft prevails farther south and likely expands westward with time. ...Guidance Evaluation and Preferences... The main complexity in Henri's forecast lies in the short range as the system interacts with an eastern U.S. shortwave/upper low. The consolidated system should be near southern New England by early Monday, with subsequent opening of the upper low and northeastward progression of the surface system. Guidance still displays some significant differences for the precise definition and track at this time frame. The 18Z and new 00Z GFS appear to be the closest in principle to the official track provided in the 03Z advisory. Guidance has varied with the specifics of features within southern Canada/northern tier U.S. flow. The primary uncertainties lie with what becomes of an upper low that ejects from southwestern Canada Monday onward and then Pacific flow that may reach the Northwest after midweek, with neither one particularly stable and/or agreeable in the models over the past couple days. Most solutions now indicate the southwestern Canada upper low should open up with time and with moderate troughing to the south, in contrast to some earlier runs that held onto a more persistent upper low and more amplified trough. The 00Z CMC still holds onto a closed low for quite a while though. These differences continue to temper confidence in frontal position over the lower 48. For the trailing Pacific energy, the 12Z ECMWF/ECMWF mean have arrived at their solutions after trending away from Northwest ridging they had depicted a couple days ago. On the other hand GFS runs have been a fairly fast and amplified extreme with the upper trough by late in the week, with the 12Z CMC a bit slower but also rather amplified. The new 00Z GFS has trended favorably back closer to the means but the 00Z CMC/ECMWF have adjusted faster and flatter than their 12Z runs, highlighting the continued uncertainty. Away from Henri the initial forecast blend utilized a composite of 12Z/18Z models for the first half of the period. Lower confidence aspects of the GFS/CMC after midweek led to a steady phaseout of those runs, yielding a day 7 Friday forecast consisting of mostly the 18Z GEFS/12Z ECMWF means and a lingering minority component of the 12Z ECMWF. ...Sensible Weather/Hazard Highlights... Extending into early next week Henri will bring a threat for high winds/waves along with some bands of heavy rainfall over and near New England. The system should become extratropical by midweek as it continues northeastward. Surface waves/fronts crossing the northern U.S. and southern Canada will interact with leading moisture and instability to produce episodes of showers and thunderstorms that could be locally heavy. Currently the best potential for highest five-day totals is over and near the Upper Mississippi Valley and Midwest. Central and southern parts of the East may see areas of diurnally favored convection within a humid and warm to hot airmass. Monday-Wednesday should feature a broad area of above normal temperatures from the central-southern Plains into the Mid-Atlantic/Northeast. Scattered plus 10-15F anomalies will be possible mainly over the Mid-Mississippi Valley into Lower Great Lakes and more for morning lows versus daytime highs. Even away from the areas with highest anomalies, some daily record warm lows may be challenged. Highs should moderate later in the week but warm lows will likely persist over the eastern half of the country. Expect the highest heat index values to be over the Mid/Lower Mississippi Valley and along the Gulf Coast early-mid week. Westward expansion of upper ridging will promote increasing coverage of highs 5-10F above normal over southern parts of the West later in the week. The Northern Plains region will be the main area of cool temperatures with some highs up to 10-15F below normal, mainly Tuesday-Thursday. Parts of the Northwest will also be on the cool side early in the week. Rausch 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