493 FXUS02 KWBC 250602 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 202 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... Models/ensembles showed broad agreement at large scales through the medium range. A broad upper-level low is expected to persist in the vicinity of Hudson Bay, with persistent positive height anomalies across the Southeast U.S. associated with the subtropical ridge. Farther west an upper ridge axis is expected to persist from the Pacific Northwest into western Canada, with Pacific shortwave energy undercutting the ridge continuing to reinforce a mean trough across the Southwest/Great Basin before progressing east into the central U.S. This pattern will keep the focus for frequent areas of low pressure and precipitation across portions of the Plains and the Mississippi Valley - unwelcome news given the extremely wet conditions and flooding concerns across those areas in recent weeks. Agreement among the guidance with respect to some of the forecast details has improved somewhat over the past day, with at least a couple waves of low pressure expected to traverse a frontal boundary across the central and northeastern U.S. days 3-5 (Tue-Thu), with a relatively strong secondary front entering the Midwest/Great Lakes by Thu. A blend of deterministic guidance resolved many of the remaining differences with respecting to track/timing of these features, and represented a good consensus. Thus, the forecast for days 3-5 was based on a blend of the 12Z ECMWF/UKMET and 18Z GFS. By Fri-Sat (days 6-7) a vigorous shortwave should dig into the Midwest/Great Lakes with a leading cold front spreading across the Northern Plains. The arrival of additional southern stream shortwave energy should once again spark the development of a surface wave of low pressure across the Central Plains by Fri night-Sat. Models/ensembles generally agree on this overall scenario, but with some significant detail differences given the time range. Thus, use of deterministic guidance was tapered in favor of majority ECENS/GEFS ensemble means in the forecast during days 6-7. ...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 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. early-mid week 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 trailing from the main Plains system reaches the area. Later in the week the trailing end of the front should linger across the Southern Plains and Lower/Mid-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 across the central U.S. could once again enhance the rainfall potential for portions of the Southern/Central Plains by Fri-Sat. Across the West the most-focused precipitation should be associated with the upper low tracking out of the across the Rockies Tue while residual troughing aloft could favor periods of diurnally favored activity through the rest of the week. 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 $$