Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM 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 during the forecast period. Flow across southern Canada and the northern contiguous U.S. will push along 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. Recent consensus has been showing a West Coast/Northwest upper troughing departing toward the Plains, allowing a southern tier ridge to bridge across California and join with an East Pacific ridge--supporting another West Coast heat wave from late week through the weekend. Some new 00Z guidance is casting some doubt on this scenario though. At the same time slow retrogression of a western Atlantic ridge may ultimately reinforce ridging over the East. ...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. For the leading system tracking eastward from central North America mid-late week, the 12Z CMC became the most suspect solution by day 5 Friday and beyond--closing off an upper low near Nova Scotia instead of carrying the core of the upper trough well northward. The new run is closer to consensus. Meanwhile some guidance has trended a little more amplified over eastern Canada in a partial nod to some recent GFS runs. The upstream trough expected to track from the Northwest into the Plains still exhibits various trends and differences in character due in part to involvement of multiple pieces of shortwave energy. Heading into yesterday there had been somewhat of a faster trend but 12Z/18Z consensus had become a bit slower and more amplified. The new 00Z GFS and ECMWF have added a new wrinkle, trending out of sync with other guidance (including the GEFS mean) for northeastern Pacific flow by the latter half of the period. This leads to a shortwave dropping into the Northwest instead of feeding into the Plains trough. It will be interesting to see if/how much the 00Z ECMWF mean follows this idea. The 12Z/18Z GFS runs compared much better to the means (and also somewhat better versus the 12Z ECMWF) late in the period. 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 for Sunday depicts extrapolation of yesterday's tropical wave for Saturday that was coordinated with the National Hurricane Center. A blend of 12Z/18Z operational models represented consensus well for days 3-4 Wednesday-Thursday. The questionable 12Z CMC over/near the Northeast required phasing out its solution after Thursday and increasing detail uncertainties led to increasing 18Z GEFS/12Z ECens input by next weekend. The 12Z/18Z GEFS and 12Z ECMWF provided some continued operational model weight through the weekend. ...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. Rausch/Kong 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