Extended Forecast Discussion NWS Weather Prediction Center College Park MD 149 AM EDT Thu May 30 2019 Valid 12Z Sun Jun 02 2019 - 12Z Thu Jun 06 2019 ...Overview/Weather Highlights and Guidance Evaluation/Preferences... An active northern stream will flow pattern is expected to continue, with a strong North Pacific upper-level jet splitting as it approaches the West Coast of North America. An upper-level low will persist in the vicinity of California through much of the extended period, only very slowly moving eastward. Meanwhile, the subtropical ridge will migrate westward and reposition over the south central U.S. Farther north, an upper ridge axis initially across central Canada should slowly move east and weaken, with some potential for ridging to become reestablished across western Canada and the northern U.S. Rockies by the middle of next week. Downstream, a persistent upper low across eastern Canada will cause incoming northern stream shortwaves to periodically amplify as they dig into the Great Lakes and Northeast, traversing the broader anticyclonic flow. These digging waves along with the westward migration of the subtropical ridge will allow for heights to fall across much of the Great Lakes/Ohio Valley and the Mid-Atlantic/Northeast, ushering in a more comfortable air mass with high temperatures near or several degrees below average through much of the period. Relatively slow-progressing southern stream shortwaves will periodically eject eastward from the upper low, reaching the central U.S. and potentially interacting with amplifying northern stream energy. These waves along with return flow across a persistent baroclinic zone may continue to support areas of increased convective potential from the Southern/Central Plains into the Mid-Mississippi Valley, with the potential for at least locally heavy rainfall/flash flooding as well as strong thunderstorms. The WPC medium range product suite was initially derived from a blend of the 12Z ECMWF/CMC/18Z GFS solutions during days 3-5 (Sun-Tue). These solutions handled features similarly and any differences were favorably smoothed out by the blend. By days 6-7 (Wed-Thu), more significant differences emerged with respect to whether upper ridging becomes reestablished across the northern Rockies/Great Basin and western Canada (GFS/GEFS), or whether stronger Pacific energy keeps the ridge broken down, with stronger height falls moving inland by Thu. At this time, chose to lean heavily toward the GEFS, and show at least some reestablishment of the western ridge, and a slower arrival of Pacific energy by the middle of next week. This decision was informed by an analysis of teleconnections based on some of the stronger hemispheric 500 hPa height anomalies persistent throughout the medium range, including a blocking ridge across north central Asia, and negative height anomalies north of the British Isles. Teleconnections associated with both of these anomaly centers supported the idea of ridging from the northwestern U.S. into western Canada, with mean troughing across eastern Canada. Thus, the forecast for days 6-7 was based mostly on the GEFS/GFS solutions, with a minority component of the ECENS mean included as well. 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