Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Sat May 25 2019 Valid 12Z Tue May 28 2019 - 12Z Sat Jun 01 2019 ...Record/Dangerous Heat expected across the Southeast with heavy rain possible for portions of the central U.S... ...Overview and Guidance Evaluation/Preferences... Guidance continues to agree well for the large scale evolution through next Sat but embedded detail questions remain. A split flow pattern will settle over western North America during the latter half of next week. Within this pattern a northeastern Pacific ridge will build into western Canada to replace an initial ridge that collapses southeastward, while eastern Pacific shortwave energy will drop into a persistent Great Basin/Southwest upper trough. The trough will not be as deep as the one crossing the area in the short range time frame. Farther east expect the mean flow to flatten with time as the northern Plains through Northeast come under greater influence from cyclonic flow to the south of an upper low near Hudson Bay, and East Coast ridging anchored by an eastern Gulf/Florida upper high through midweek loses amplitude. Over central/eastern North America days 3-5 Tue-Thu the primary forecast uncertainties continue from prior cycles. There is still some variability with the amplitude of cyclonic flow reaching the northern tier states, affecting the associated front that drops south from Canada Tue-Thu. However the past couple cycles or so have become a little more stable for timing of system tracking through the Upper Midwest. The 00Z CMC is in the minority with a faster solution. By Thu the latest GFS runs are somewhat faster than the majority scenario (including the 06Z GEFS mean) with weak trailing energy--ultimately affecting cold front progression over the East. Meanwhile confidence remains low for exact wave details along the leading front that extends through the Northeast during the first half of the period. The 00Z UKMET was in the minority at some valid times over this region. Based on these considerations the updated forecast started with the 00Z ECMWF/ECMWF mean and 06Z GFS/FV3 GFS/GEFS mean with about 2/3 weight toward the operational runs. The result tilted the blend away from the more questionable GFS details over the East by Wed-Thu. By days 6-7 Fri-Sat there was reasonable overall agreement among the models and means. Thus the only adjustment from the earlier portion of the forecast was to increase ensemble weight to 40-60 percent while maintaining the same components. Note that the 00Z GFS appeared overdone with height falls nearing the West Coast by day 7 Sat. Meanwhile consensus shows another front pushing into the northern tier states from Canada by Fri-Sat and weakening of the front that reaches the East/South. The southern Plains part of the front may lift back as a warm front late in the week. ...Weather Highlights and Hazards... High temperatures should be 10-20 deg F above average across much of the Southeast Tue-Thu with readings likely nearing or surpassing 100 deg F for many areas along/below the fall line. These temperatures will combine with moderate humidity levels to produce dangerous heat index values that could at least briefly exceed 105 deg over some areas. There will be multiple days with potential for record highs/warm lows. In less extreme fashion some of this heat should spread northward into portions of the Mid-Atlantic through midweek. A front will provide a cooling trend later in the week. Meanwhile the upper trough initially ejecting from the West will lead to cool conditions with high temperatures 10-20 deg F below average from California through the Great Basin/north-central High Plains Tue. Temperatures should gradually trend closer to average mid-late week. At the same time upper ridging will be close enough to the Northwest to provide above normal temperatures (generally plus 5-15 deg F anomalies) for most of next week. Heavy rainfall will be a threat across portions of the central U.S. during the next week. The low pressure system crossing the central U.S. Tue-Wed and trailing front could produce multiple rounds of potentially heavy rainfall and severe thunderstorms across portions of the Plains and the Midwest, with multi-inch rainfall totals possible. Some enhanced rainfall could extend into the Great Lakes/Northeast with a leading frontal wave, and then later in the week as the front extending from the main Plains system reaches the area. Later in the week the trailing end of the front should linger for a time across the Southern Plains and Lower Mississippi Valley where it could continue to focus areas of showers and storms with at least locally heavy rainfall. The next shortwave and developing low pressure system crossing the northern Plains as well as northward return of the front settling over Texas could once again enhance the rainfall potential for portions of the central U.S. by Fri-Sat. Across the West, the upper low reaching the central Rockies by early Tue will generate focused precipitation (possibly some snow at highest elevations) in its vicinity very early in the period. Persistent troughing aloft could favor periods of diurnally favored activity through the rest of the week into the weekend. Rausch/Ryan 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