Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sat Aug 21 2021 Valid 12Z Tue Aug 24 2021 - 12Z Sat Aug 28 2021 ...Dangerous Henri to affect New England into early next week... ...Overview... The 03Z National Hurricane Center advisory for Henri shows the system strengthening to hurricane status in the coming hours as it tracks toward New England and arrives on Sunday--bringing heavy rainfall and flooding threats along with strong winds and waves. By the time the medium range period starts early Tuesday, Henri is likely to be much weaker and in post-tropical form as it begins to track away from New England. Refer to the latest NHC products for further information on Henri. Meanwhile guidance continues to depict a typical late-summer pattern as southern Canada/northern U.S. flow aloft pushes along rainfall-focusing waves/fronts across the northern half of the lower 48 and mean ridging aloft prevails farther south, likely expanding westward into the Southwest with time. Strong western Atlantic ridging that is partially responsible for Henri's rather unusual northward track will meander in place between Nova Scotia and Bermuda as the remnants of Henri continue around its northern periphery. ...Guidance Evaluation and Preferences... Latest guidance agrees upon the upper system associated with Henri opening up by early Tuesday and lifting northeastward thereafter. Latest GFS runs stray a bit to the faster side of the spread though, even compared to the GEFS mean. Varying degrees of uncertainty continue for features carried along by Northeast Pacific through southern Canada/northern CONUS flow. Over the past day solutions have displayed some improved clustering for the evolution of the upper system tracking north of Montana/North Dakota on Tuesday and reaching north of the Great Lakes by Thursday. However by day 7 Saturday GFS runs become fast/amplified with the trough that reaches the Canadian Maritimes so that model may be quick with its high pressure that builds into New England. Farther west the models and individual ensemble members still suggest a fair amount of uncertainty with shortwave details from the Pacific into western North America. Over the past day or so there has been a moderately faster trend for the overall trough that crosses the Northwest during the latter half of the week and brings height falls into the northern Plains. In the 12Z cycle the CMC became somewhat amplified with the upper trough late in the period though its surface pattern compared reasonably to consensus. The ECMWF became rather slow and sharp, and while the new 00Z run adjusted a bit faster it is still on the slow/sharp side. The new 00Z CMC looks more like the means. The 00Z GFS continues recent tendencies to be a little fast with leading height falls but recent trends allow for that scenario to be possible. A composite of 12Z/18Z model runs represented the most common themes of guidance for significant features during the first half of the period. The increasing divergence of operational model details beyond early Thursday continues to favor a rapid increase of 18Z GEFS/12Z ECMWF mean input later in the period, reaching 70 percent total by day 7 Saturday. ...Sensible Weather/Hazard Highlights... Expect Henri's threats for high winds/waves along with some bands of heavy rainfall over and near New England (with some terrain enhancement possible) to be mainly in the short range period from this weekend into Monday. Henri will likely be weaker and in post-tropical form as the system progresses east-northeastward away from New England Tuesday onward. 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. Over the past day the guidance has been fairly consistent in highlighting the best potential for highest five-day totals over and near the Upper Mississippi Valley and Midwest. This region will see an early frontal passage, followed by the front stalling to the south (Mid-Mississippi Valley) and then lifting back north as a warm front ahead of another system emerging into the Plains. Central and southern parts of the East may see areas of diurnally-favored convection within a humid and warm to hot airmass. Upper level energy from a combination of the tail end of the initial East Coast trough and what tracks underneath the western Atlantic ridge may enhance convection over parts of the Southeast and southern Mid-Atlantic. Specific details of this energy have low predictability though. Through midweek very warm to hot temperatures will prevail from the central-southern Plains through much of the East. The axis of warmest anomalies should be from the central Plains/Midwest into the Northeast with some readings 10-15F above normal, and more coverage of such anomalies for morning lows versus daytime highs. Some daily records for warm lows may be challenged even away from the areas with highest anomalies. Expect highs to moderate after midweek but very warm lows are likely to persist over many areas through the end of the week. A cold front should bring cooler weather to the Northeast by the start of the weekend. The highest heat index values during the period should be through midweek over the Mid/Lower Mississippi Valley and along the Gulf Coast where max values may locally exceed 110F. Westward expansion of upper ridging will promote increasing coverage of highs 5-10F above normal over southern parts of the West and then across much of California later in the week. Some of this heat may reach western Oregon by next Saturday. The Northern Plains region will be consistently cool during the period with some highs up to 10-15F below normal, mainly during Tuesday-Thursday. 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