Extended Forecast Discussion NWS Weather Prediction Center College Park MD 138 AM EDT Wed May 29 2019 Valid 12Z Sat Jun 01 2019 - 12Z Wed Jun 05 2019 ...Overview and Guidance Evaluation/Preferences... A split flow pattern is expected to be in place across North America on day 3 (Sat). Flow associated with an active North Pacific upper-level jet will split as it approaches the West Coast of North America where a persistent broad upper-level low will be in place near California. An upper ridge axis initially across western Canada is expected to shift eastward to central Canada and the north central U.S. by Sun-Mon. Farther east, northern stream energy will periodically encounter a persistent upper low initially in the vicinity of Hudson Bay, and gradually drifting eastward through the forecast period. These shortwaves will amplify as they dig into the Great Lakes and Northeast, traversing the broad cyclonic flow. Meanwhile, slower progressing southern stream energy will periodically eject from the California upper low, reaching the central U.S. and potentially interacting with amplifying northern stream energy. Meanwhile, the upper-level ridge which has been persistent across the Southeast and eastern Gulf of Mexico as of late will begin a slow retrogression westward this weekend into next week. This will not significantly change the feed of warm and moist air northward into the central U.S., but it will allow for temperatures across the Southeast to return much closer to seasonal norms. A blend of the 12Z ECMWF/UKMET and 18Z GFS solutions was used as a basis for the forecast during days 3-5 (Sat-Mon). These solutions showed some timing/amplitude differences with the shortwave/low pressure system crossing the Great Lakes/Ohio Valley/Northeast during this time period, but the blend represented a good consensus. Differences also exist in how shortwave energy is handled traversing the North Pacific jet, reaching British Columbia and the Pacific Northwest by Mon. During days 6-7 (Tue-Wed), a rapid succession of shortwaves traversing from the North Pacific along the U.S./Canada border becomes the primary forecast concern. Given the relatively high frequency nature of this flow and amplifying differences/inconsistencies among solutions, a lean heavily toward ECENS/GEFS ensemble means was preferred by days 6-7. This approach supported the idea of a frontal system moving into the north central U.S. by Tue and into the Great Lakes by Wed, with the trailing cold front crossing the Plains and stalling across the Rockies/Great Basin. ...Weather Highlights and Hazards... Areas of showers and thunderstorms with locally heavy rainfall are possible across portion of the Southern and Central Plains Sat north of a weakening surface front across south Texas. Showers and thunderstorms may become more widespread across these areas Sun-Tue, with continued heavy rain potential, as another frontal boundary arrives and stalls, providing additional focus/enhancement of convection. This same frontal boundary will also bring scattered to numerous showers and thunderstorms to areas from the Midwest and Ohio Valley to the Mid-Atlantic and Northeast Sat-Sun, with locally heavy rainfall a possibility. The arrival of yet another frontal system across the north central U.S. Tue-Wed may bring another round of increased showers and thunderstorms to the Northern and Central Plains. Farther west, periodic showers are expected from the Great Basin to the central/northern Rockies as a couple weak southern stream shortwaves traverse the region Sat-Sun. Temperatures across much of the Southeast are expected to remain above average, although not to the extent seen in recent days. High temperatures are forecast to be several degrees above average into early next week, before perhaps falling close to seasonal norms by Wed. Farther west, the Pacific Northwest will initially be quite warm, with highs expected to be 10 to 15 deg above average with the upper ridge in the vicinity. These temperatures should moderate slightly by early next week, but above average temperatures are expected to continue as northern stream shortwave energy fails to lower heights substantially across the region. Across the lower Great Lakes, Mid-Atlantic, and Northeast, passage of a cold front may bring a brief period of below average temperatures Sun-Tue, with high temperatures across portions of those areas several degrees below average. 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