Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Thu May 23 2019 Valid 12Z Sun May 26 2019 - 12Z Thu May 30 2019 ...Record/Dangerous Heat for the Southeast will continue into next week... ...Heavy rainfall possible for portions of the Plains and Mid-Upper Mississippi Valley... ...Overview... Through the first half of next week an amplified upper pattern featuring a Gulf/Southeast ridge and deep western trough will promote hot/dry conditions focused over the Southeast and at the same time unseasonably cool temperatures from California into the northern Plains. A mean frontal boundary and one or more waves between the two contrasting air masses will promote a continue threat for heavy/excessive rainfall over parts of the Plains and Mississippi Valley. Although some residual western troughing appears likely, ejection of the initial deep western trough should ultimately lead to a less amplified pattern over the Lower 48 by the latter half of next week. ...Guidance Evaluation and Preferences... Today's forecast uncertainties continue to involve similar issues--details within the initial western trough, flow heading into East Pacific mean ridging, and Canadian troughing that may descend far enough south to reach the northern tier of the U.S. Latest guidance is placing greater emphasis on the second feature dropping into the overall western trough, a compact upper low currently near the southern coast of mainland Alaska, while ejecting leading energy into the Plains. This adjustment in shortwave details has led to two discrete surface features, a Plains to Great Lakes wave Mon-Tue associated with the leading ejecting energy followed by a Plains through Great Lakes/eastern Canada wave Tue-Thu supported by the deep upper low/trough ejecting from the West. It remains to be seen if the first wave will be as strong as shown by some operational models once it reaches the Great Lakes/Northeast given a fairly flat appearance of mean flow aloft over that region. A model blend will help to tone down the more extreme solutions. Meanwhile by Tue onward multi-day trends still suggest leaning toward maintaining some degree of eastern Pacific into western Canada mean ridging. Some GFS runs (including the 06Z version) continue to be on the aggressive side in pushing height falls toward the West Coast and the 06Z GEFS mean has trended in that direction, but for forecasts valid on day 5 Tue most GEFS members have trended notably higher for eastern Pacific heights compared to two days ago. This should allow for a lingering weak trough aloft over at least the southern half of the West as part of a developing split flow pattern over western North America. At the same time guidance has been very inconsistent and diverse for the specifics of southern Canada troughing that could affect the second wave lifting northeast from the Plains. The 00Z ECMWF/GEFS/CMC means provide the best intermediate and stable cluster for the location and depth of the upper trough. The 06Z GFS and to some degree 06Z GEFS mean keep the trough farther north than established consensus thus far. The updated forecast starts with a blend of operational models to reflect the emerging detail clustering for individual waves in the Sun-Tue time frame. Preferred guidance transitions to a blend among the models and ECMWF/NAEFS means (the latter to capture both 00Z GEFS and CMC ensemble data) for day 6 Wed and then mostly the means with a small lingering 00Z ECMWF component for day 7 Thu. ...Weather Highlights and Hazards... Over the Southeast expect widespread high temperatures in the upper 90s to low 100s F for most of the period. Such readings will likely challenge daily if not monthly records. The combination of daily highs and duration of the event may be extreme for this early in the season. Some locations over the interior Southeast may see at least a 40 percent probability of seeing maximum heat indices exceed 105F on one or more days. Meanwhile proximity of the upper high should limit rain chances. Areas farther north should see more variable temperatures depending on day-to-day frontal position with potentially strong contrasts between the cool and warm/hot side of the mean front. From California into the northern Plains the coverage of minus 10-20F anomalies for highs should be greatest during Sun-Tue with relaxing of the upper pattern favoring a moderating trend over this area Wed-Thu. Over the West expect areas from northern-central California through the Great Basin and northern-central Rockies to see highest precipitation totals during the period. Some precip could fall as snow in highest elevations. Uncertainty in details of flow aloft leads to low confidence in the forecast over the Pacific Northwest late in the period. Current forecast preferences lean toward the lighter half of the rainfall spread corresponding to longer maintenance of at least some eastern Pacific mean ridging aloft. A fairly broad area encompassing the north-central to south-central Plains through the Midwest into the western Great Lakes will see the potential for one or more episodes of heavy rainfall with some multi-inch totals possible for the five-day period. Already wet conditions will make flooding/flash flooding a significant concern that will have to be monitored closely as specifics of timing/intensity/duration come into clearer focus. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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