Extended Forecast Discussion NWS Weather Prediction Center College Park MD 252 PM EDT Fri Aug 20 2021 Valid 12Z Mon Aug 23 2021 - 12Z Fri Aug 27 2021 ...Dangerous Henri to affect New England into early next week... ...Overview... Tropical Storm Henri is forecast to move into Southern New England late Sunday into Monday to start the medium range period. Though it will weaken over land before pushing into Atlantic Canada on Tuesday, it will bring a heavy rainfall and flooding threat along with strong winds and waves to the region. 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. Western Atlantic ridging, partially responsible for Henri's rather unusual northward track, will meander in place between Nova Scotia and Bermuda as the remnants of Henri skirt around its northern periphery. ...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. There still remains some west-east uncertainty which will affect the likely asymmetric rainfall distribution which may interact with the terrain back to eastern New York and across New England. Please see the latest National Hurricane Center for the latest information. A GFS/ECMWF/Canadian blend was close to the earlier forecast position when the forecast was made. 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. These differences continue to temper confidence in frontal position over the lower 48, but a blended solution comprising of a plurality ECMWF and minority GFS/Canadian weighting offered a reasonable starting point amid the wavering of solutions. Increased ensemble mean weighting for the last days of the period where the recent GFS runs were either questionably strong or weak (and displaced) over the East/southeastern Canada. ...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 around Tuesday as it continues slowly northeastward. Rainfall could be locally heavy and enhanced by the terrain. 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 in the vicinity of a wavy frontal boundary. 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, especially in the wake of Henri. 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 where values may rise to over 110F. Westward expansion of upper ridging will promote increasing coverage of highs 5-10F above normal over southern parts of the West and across much of California 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 in the wake of a Pacific front under somewhat persistent upper troughing. Fracasso/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