Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Sat Aug 21 2021 Valid 12Z Tue Aug 24 2021 - 12Z Sat Aug 28 2021 ...Remnants of Henri forecast to depart New England on Tuesday... ...Late-week heavy rain possible across the upper Midwest.... ...Overview... The medium-range period should begin with the remnants of Henri weakening further while departing New England. In the wake of Henri, a typical late-summer pattern is expected to prevail into next weekend as a couple of frontal waves are forecast to move across the northern half of the country where heavy rain is possible across the upper Midwest. Meanwhile, as a warm ridge aloft remains anchored across the South, heat and oppressive humidity are expected to continue into midweek near/along the Mississippi Valley. Another upper ridge building in from the eastern Pacific will likely bring another heatwave into the West Coast during the latter portion of next week. ...Guidance Evaluation and Preferences... Latest model guidance has come into better agreement that the remnant circulation of Henri will be exiting New England on Tuesday after having interacted with an upper-level low and meandering over New England during the short-range period. Behind Henri, ensemble means show very good agreement on the pattern evolution across the U.S. through the end of next week. The GFS tends to emphasize the development of a frontal wave over the northern Plains on Tuesday, giving rise to uncertainty in the placement of the QPF axis across the northern Plains. However, agreement is much better regarding the next wave moving into the northern Plains late next week. Therefore, a composite blend of the 06Z GFS/GEFS, 00Z ECMWF/EC mean, and a smaller proportion from the 00Z CMC/CMC mean was used to compose the WPC medium range package. This yielded a solution quite compatible with the previous WPC forecast package. ...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. Kong/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