Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EDT Tue May 28 2019 Valid 12Z Fri May 31 2019 - 12Z Tue Jun 04 2019 ...Overview and Guidance Evaluation/Preferences... Split flow is expected to persist across North America through most of the medium range. The subtropical ridge is expected to remain in place across the Gulf of Mexico as it begins a very slow retrogression westward. Flow associated with an active North Pacific upper-level jet will split as it approaches the West Coast of North America where a ridge axis will be in place initially, before gradually shifting eastward to the central U.S./Canada by late in the weekend into early next week. As northern stream energy periodically encounters a persistent upper low in the vicinity of Hudson Bay, the energy will amplify in the broad cyclonic flow with shortwaves digging into the north central and northeastern U.S. Meanwhile slower progressing southern stream energy will move from the Great Basin/Rockies to the central U.S. and Great Lakes, where interaction with amplifying northern stream energy is possible. Model consensus was sufficient during the first part of the medium range to use a multi-model deterministic blend (including the 12Z ECMWF/CMC and 18Z GFS) during days 3-4 (Fri-Sat). The CMC was dropped from the blend by day 5 (Sun) as some significant differences began to emerge with how that solution handled northern stream shortwave energy across east central Canada. A wave of low pressure is expected to quickly cross the Upper Midwest/Great Lakes/Northeast during this time period, with a trailing frontal boundary extending south into the Ohio/Mississippi Valleys and the Plains. Meanwhile, a northern stream shortwave is expected to move into western Canada, weakening as it moves into the ridge, with a cold front moving into the Pacific Northwest. By days 6-7 (Mon-Tue), models show decent consensus that the northern stream ridge axis should shift eastward to the Northern Plains/Midwest, allowing stronger height falls to reach the Pacific Northwest. Meanwhile, the subtropical ridge should gradually retrograde to the western Gulf of Mexico, keeping a warm and moist air mass feeding northward into the south central states. Leading shortwave energy crossing the northern Rockies Sun should bring a low pressure system into south central Canada by early next week with the associated frontal system moving across the Northern Plains and Midwest. Spread among deterministic solutions was higher for this system given the time range, but ensemble means painted a somewhat more consistent picture in terms of timing/placement of the surface low. Thus, majority weight was shifted to ECENS/GEFS ensemble means in the forecast for days 6-7. ...Weather Highlights and Hazards... Areas of showers and thunderstorms with locally heavy rainfall are possible across portion of the Southern and Central Plains Fri-Sat north of a weakening surface front along the Rio Grande Valley. Showers and thunderstorms may become more widespread across these areas Sun-Tue, with continued heavy rain potential, as another cold front arrives from the north, providing additional focus/enhancement of convection. 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. This activity should be a bit more widespread Fri-Sat before becoming more sparse Sun-Tue as the most significant shortwave energy moves east and heights gradually rise across the region. Above average temperatures are expected to continue across the Southeast, although not to the extent seen in recent days. High temperatures are forecast to be 5 to 10 deg F above average across this area into early next week. 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. 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