Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Sun Aug 22 2021 Valid 12Z Wed Aug 25 2021 - 12Z Sun Aug 29 2021 ...Upper Midwest heavy rain potential late week into the weekend... ...Overview... Guidance maintains a fairly typical late-summer pattern next week and into the weekend. The prevailing jet stream across southern Canada into the northern U.S. will support a couple rainfall-focusing waves/frontal systems, with the Upper Midwest region likely to see the heaviest activity. Meanwhile, a broad upper ridge over the South and parts of the East will maintain hot and humid conditions. The most oppressive heat/humidity combination is likely to continue into midweek or so over the Mississippi Valley and vicinity. Toward the end of next week, recent model guidance has lowered the chance of another heat wave to build along the West Coast as the deterministic guidance indicates the possibility of an upper trough digging down across the Pacific Northwest. Along the East Coast, slow retrogression of a western Atlantic ridge may ultimately reinforce ridging. ...Guidance Evaluation and Preferences... Most models and the ensemble means have been identifying two primary features within the southern Canada/northern U.S. flow but with some detail differences and run-to-run variability. The 00Z and 06Z GFS backed off on the degree of frontal wave development across the northern Plains midweek in disagreement with both the ECMWF and CMC. The 12Z GFS switched back to a solution more in line with the ECMWF, CMC, and WPC continuity, however. Therefore, more of the 06Z GEFS instead of the 06Z GFS was incorporated into the WPC blend to handle this system. There is a noticeable discrepancy between the QPF axis from the ECMWF and GFS as well. A blend leaning more toward the EC mean was used. Toward the end of next week, recent deterministic guidance indicates a higher probability for an upper trough to dig down across the Pacific Northwest, most notably in the 00Z ECMWF solution. Recent runs from the GFS have followed suit with a similar scenario by abandoning the idea of extending an upper ridge into the western U.S. The ensemble means generally show a weaker version of this scenario. In the tropical realm, attention will turn to the northwestern Caribbean through the western Gulf of Mexico where guidance is suggesting at least a wave if not a defined tropical system. CMC runs had been on the strong and northern side of the spread though the new 00Z run has trended southward, a sensible adjustment given the prevalence of upper ridging to the north. The manual forecast next weekend shows tropical wave positions in the Gulf in coordination with the National Hurricane Center. 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... Surface waves and associated fronts will cross the northern U.S. and southern Canada, producing episodes of showers and thunderstorms that could be locally heavy. Guidance continues to highlight the best potential for heavy rainfall over the Upper Mississippi Valley and Midwest from late this week into the weekend. 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 whose trailing cold front will move into the area during next weekend. Parts of this region have been dry recently so some of the rain may initially be beneficial. However a multi-day period of significant rainfall could end up being problematic over some areas. 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 South and/or southern Mid-Atlantic. Specific details of this energy have low predictability. Rainfall may increase along the central/western Gulf Coast next Sunday as a possible wave reaches the western Gulf of Mexico. Very warm to hot temperatures from the central-southern Plains through much of the East as of Wednesday (plus 10F or so anomalies for highs from the Mid Mississippi Valley into Northeast) should gradually moderate with time, especially with highs trending to within a few degrees of normal by next Sunday. The highest heat index values should be on Wednesday over the Mid/Lower Mississippi Valley and along the Gulf Coast where max values may locally reach 110F or so. Morning lows will be more persistently above normal with areas from the Midwest into Great Lakes/Ohio Valley tending to see readings stay close to or a little above 10F above normal through the period. Some daily records for warm lows may be challenged even away from areas with the highest anomalies. A cold front and trailing high pressure will bring the Northeast a break from the warmth late week into the weekend. Based on recent guidance consensus, expansion of the upper ridge across southern parts of the West would support increasing coverage of highs 5-10F above normal over southern parts of the West and then across much of California later in the week, followed by up to plus 10-15F highs reaching western Oregon next weekend. More western troughing aloft in the new 00Z GFS/ECMWF would significantly curtail this warming trend though. The Northern Plains region will be consistently cool during the period. Some 10-15F below normal highs are possible from Wednesday into Saturday. 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