Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 AM EDT Fri May 24 2019 Valid 12Z Mon May 27 2019 - 12Z Fri May 31 2019 ...Record/Dangerous Heat expected across the Southeast... ...Heavy rainfall possible for portions of the Plains and Mid-Upper Mississippi Valley... ...Overview and Guidance Evaluation/Preferences... An amplified flow regime is expected to persist during the medium range from the North Pacific across North America. The most persistent large scale features will be a strong subtropical ridge anchored near the Southeast U.S., a mean upper trough across the West (likely becoming cutoff over time), and a persistent upper low near Hudson directing an active northern stream upper jet into the north central states. While models and ensembles showed generally good agreement at large scales even through day 7, fairly significant differences emerged even relatively early in the forecast period with respect to the details of individual systems. Energetic shortwave energy moving from the Rockies into the central U.S. days 3-4 (Mon-Tue) should spark development of a surface low pressure system across the Central/Northern Plains and Midwest, with the potential for at least a couple waves of low pressure along the front. As a result, models showed fairly significant timing/placement differences for these areas of low pressure across the north central U.S., which grew further by day 5 (Wed) as the main surface low crosses the Great Lakes and lifts north into Canada. A blend of deterministic solutions was used as a starting point during this time frame, using solutions most closely clustered with the ensemble means. A blend of the 12Z ECMWF/UKMET/18Z GFS were used during days 3-4, with the UKMET dropped on day 5 in favor of increasing weight on ensemble means. By days 6-7 (Thu-Fri), the main disagreement among the guidance was exactly how much heights fall across the Midwest/Great Lakes/Northeast as an energetic shortwaves moves around the larger upper low near Hudson Bay. The 12Z ECMWF was much more amplified with this feature than the GFS (and relative to previous ECMWF runs). The ECENS and GEFS ensemble means, on the other hand, showed very similar setups by day 7, with the GFS seemingly the closes deterministic solution to the means. This idea keeps lower heights confined farther north in Canada with a period somewhat zonal flow across much of the central/eastern U.S. as the upper ridge weakens (at least somewhat). Given the greater uncertainty, the ECENS/GEFS means were weighted more heavily by days 6-7, along with some continued use of the GFS. ...Weather Highlights and Hazards... High temperatures are expected to be 10-20 deg F above average across much of the Southeast Mon-Thu, with temperatures expected to near or surpass 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 in excess of 105 deg for some areas. Meanwhile, the upper trough across the Great Basin/Rockies early next week will be accompanied by cool conditions and high temperatures 10-20 deg below average. Elsewhere, heavy rain will pose a threat across portions of the central U.S. through much of next week. The low pressure system across the central U.S. could produce multiple rounds of potentially heavy rainfall and severe thunderstorms Mon-Tue across portions of the Plains and the Midwest, with multi-inch rainfall totals possible. By Wed-Thu the Northern Plains and much of the Midwest should dry out as the surface front moves east, but the trailing end of the front is expected to 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. 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