Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Sun May 26 2019 Valid 12Z Wed May 29 2019 - 12Z Sun Jun 02 2019 ...Record/Dangerous Heat expected across the Southeast with heavy rain possible for portions of the central U.S... ...Overview and Guidance Evaluation/Preferences... Models/ensembles showed broad agreement at large scales through the medium range, with some significant differences remaining with respect to the details. A broad upper-level low is expected to persist in the vicinity of Hudson Bay, with persistent positive height anomalies centered across the northeastern Gulf of Mexico associated with the subtropical ridge. Farther west an upper-level ridge axis is expected to persist from the Pacific Northwest into western Canada, with Pacific shortwave energy undercutting ridge reinforcing a mean trough across the Southwest/Great Basin before progressing east into the central U.S. This pattern will keep the focus for the development of areas of low pressure and resultant precipitation across portions of the Plains and the Mississippi Valley. A blend of the 12Z ECMWF/UKMET/18Z FV3 was used as a basis for the forecast during days 3-5 (Wed-Fri). Differences emerged fairly early on with northern stream shortwave energy rotating around the broader Hudson Bay upper low, and its interaction with southern stream energy crossing the Midwest/Great Lakes Wed-Thu. The GFS and CMC were significantly slower with northern stream energy, with a delayed interaction with the southern stream feature resulting in a significantly slower timing for the surface cold front across the Great Lakes/Northeast through Fri - and those solutions were thus excluded from the forecast due to a lack of sufficient ensemble support. Additional northern stream shortwave energy is forecast to reach the Northern Plains on Fri, with the development of a wave of low pressure expected along a secondary frontal boundary as it continues to move east during days 6-7 (Sat-Sun). Timing and amplitude differences with this wave translate to fairly significant differences with respect to the track/intensity of any surface wave(s) of low pressure along the front. Finally, shortwave energy riding an active upper jet across the North Pacific should push a cold front into the Pacific Northwest by next Sun. Models showed somewhat better consensus on this feature relative to features farther east, with ensembles and deterministic solutions all showing similar timing for the upper wave, and some amplitude differences. The preference at this time was to stick fairly close to the ensemble means during this time period given the spread. The 12Z ECMWF and 18Z FV3 solutions continued to appear somewhat well-centered relative to the means through this time period. Thus, the forecast during days 6-7 was based on a blend of majority ECENS/GEFS ensemble means, with minority components of the ECMWF/FV3 continued through the end of the forecast period. ...Weather Highlights and Hazards... High temperatures should be 10-20 deg F above average across much of the Southeast Wed-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. 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 10-15 deg F below average from the Great Basin to the High Plains on Wed. Temperatures should gradually trend closer to average by late week. At the same time upper ridging will be close enough to the Northwest to provide above average temperatures (generally plus 5-15 deg F anomalies) for most of the week. Heavy rainfall will be a threat across portions of the Southern/Central Plains and perhaps the Mid-Mississippi Valley during the upcoming week. A trailing cold front from the system crossing the Great Lakes Wed-Thu could produce widespread showers and thunderstorms with potentially heavy rainfall and severe weather across portions of the Southern Plains, with localized heavier rainfall amounts possible northeast along the front into the Mid-Mississippi Valley. The next shortwave along with the northward progression of a weakening warm front could once again enhance the rainfall potential for portions of the Southern/Central Plains Fri-Sat. Across the West, largely terrain focused precipitation is expected from the Great Basin to the central/northern Rockies through much of the week with the passage of several upper-level shortwaves during the period 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