Extended Forecast Discussion NWS Weather Prediction Center College Park MD 140 AM EDT Mon May 27 2019 Valid 12Z Thu May 30 2019 - 12Z Mon Jun 03 2019 ...Episodes of heavy rain possible for portions of the central U.S... ...Overview and Guidance Evaluation/Preferences... Split flow is expected to persist across North America during the medium range. The subtropical ridge is expected to remain entrenched across the Gulf of Mexico, keeping a feed of warm and moist air into the central and southeastern states. 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 through much of the period 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 fro the Great Basin/Rockies to the central U.S. and Great Lakes, where interaction with amplifying northern stream energy is a possibility. The 12Z ECMWF/CMC were the preferred deterministic solutions through much of the medium range forecast period, and a blend of these solutions was used heavily in the forecast for days 3-5 (Thu-Sat). The GFS and FV3 solutions differed from the start on Thu with the system crossing the Midwest/Great Lakes. The GFS/FV3 were deeper with southern stream energy crossing the Midwest on Thu, and slower with amplifying northern stream shortwave energy south of Hudson Bay relative to the ECMWF/CMC. The result was a more significant and slower wave of low pressure along the surface front across the Midwest on Thu in the GFS relative to the ECMWF/CMC. The UKMET sided with the ECMWF/CMC but had its own issues and seemed too weak with the frontal wave and too fast with pushing the cold front eastward. The ECENS/CMCE ensembles provided a significant degree of support to their deterministic runs, lending to the preference of those solutions over the GFS even on days 3-4. By days 6-7 (Sun-Mon), ensembles show good consensus that the upper ridge axis initially across the Pacific Northwest and western Canada should propagate eastward to the north central U.S. and central Canada, allowing height falls to reach the northwestern U.S. as a Pacific jet/shortwave arrive. The ECMWF and CMC showed similar solutions to the ensemble means with respect to his evolution, as well as with another shortwave rounding the base of the Hudson Bay upper low into the Great Lakes, with an associated frontal wave moving from the Midwest on Sat to the Great Lakes/Northeast Sun-Mon. Detail differences and run-to-run variability do increase among the deterministic guidance by late in the forecast period. As a result, ECENS and NAEFS ensemble means were boosted to comprise a majority of the forecast for days 6-7, with continued use of minority components of the ECMWF/CMC. ...Weather Highlights and Hazards... Areas of showers and thunderstorms with locally heavy rain are possible Thu-Fri from the Southern Plains to the Ohio Valley and Northeast along a surface cold front and around the periphery of the subtropical ridge. Another round of more significant and widespread convection and heavy rainfall is possible across the Southern/Central Plains Sat-Sun as the trailing end of the frontal boundary lifts north as a warm front and weakens, and another cold front dives south across the central U.S. before stalling by the weekend and providing additional focus for areas of showers and thunderstorms. 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. Hot temperatures will persist into Thu across portions of the Southeast and southern Mid-Atlantic, where highs may reach 10-18 deg F above average. A gradual cooling trend is expected from Fri into early next week as the upper ridge retreats a bit into the Gulf of Mexico allowing heights to fall across the Southeast - although any cooling will be relatively modest with highs expected to remain 5-10 deg above 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