Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1205 PM EDT Sat May 26 2018 Valid 12Z Tue May 29 2018 - 12Z Sat Jun 02 2018 ...Subtropical Storm Alberto expected to bring heavy rainfall to portions of the Gulf Coast and Southeast... ...Overview/Synoptic Assessment... The flow pattern across North America will initially be somewhat blocky early next week. Active/more progressive flow across the northern Pacific will gradually translate downstream, however, with the northern stream flow across the U.S. and Canada becoming at least somewhat more progressive by later next week. Subtropical Storm Alberto is forecast to be onshore across the Gulf Coast states by day 3 (Tue), with the surface low moving north to the Tennessee Valley by Wed, and the Ohio Valley by Thu. Please refer to advisories issued by the National Hurricane Center for the latest information and forecast for Subtropical Storm Alberto. Farther north and west, a shortwave is expected to move from the central/northern Rockies Tue morning into the northern plains/Upper Midwest by Wed morning. By Thu as this shortwave crosses the Great Lakes, some degree of interaction may occur with the remnant circulation from Alberto across the Ohio Valley. By next Fri-Sat, it remains unclear whether Alberto will be fully absorbed into the northern stream or not, an outcome which will likely depend heavily on the specific amplitude of the northern stream wave. Models show some possibility for the combination of northern stream energy and remnants of Alberto to form a cutoff low near the Eastern Seaboard by next Fri-Sat. An upper trough is expected to reach the West Coast by Wed-Thu, with height falls reaching the Great Basin and perhaps the northern Rockies by Thu-Fri. Additionally, an upper low traversing the north Pacific/Gulf of Alaska is expected to move southeastward toward the Pacific Northwest by next Fri-Sat. While the northern stream becomes a bit more active by late next week, guidance suggests with some degree of consensus that a ridge should begin to build across the southern plains. Despite the trend toward a more progressive northern stream, this appears a bit tenuous, with a number of solutions showing the potential to for the flow to move back toward a blockier setup by next weekend, dependent to some degree of the amplitude of the ridging that can develop across the central U.S., and whether an upper develops near the East Coast. ...Guidance/Predictability Assessment... The WPC forecast was initially based on a heavily deterministic multi-model blend, including components of the ECMWF/GFS/CMC. The 00Z ECMWF solution seemed to fit most closely with the official NHC forecast track for Alberto. Ensemble weighting was increased from day 5 onward given some uncertainties in the amplitude of shortwave energy moving from the northern plains to the Great Lakes, and the degree of interaction with Alberto. Uncertainty increases further by days 6-7 as model solutions differ significantly as to what happens with the western U.S. trough. The GFS/CMC have been persistent in developing either a highly amplified trough or cutoff low across the Great Basin/Southwest, while the ECMWF has been persistent in keeping the system more progressive and quickly moving the energy east toward the northern Rockies and northern plains. The 00Z ECENS/06Z GEFS means seems to represent a reasonable compromise between the two scenarios, and a middle ground solution is preferred at this time. Thus, weighting of these ensemble means was boosted further by next Fri-Sat. ...Weather/Threats Highlights... Subtropical Storm Alberto is expected to produce widespread heavy rains generally near and east of the forecast track. Models continue to show multi-inch rainfall totals possible during the medium range across portions of the Gulf Coast states, Southeast, and Tennessee Valley. ECENS/GEFS ensemble members continue to suggest two precip maxima during the Tue-Thu time frame, on extending from portions of Mississippi and Alabama north to Tennessee and portions of Kentucky, and another across the southern Appalachians, upslope flow along with the deep tropical moisture could enhance rainfall rates. The northern extent of heavy rainfall potential is a bit uncertain at this time, and will depend on the exact track of Alberto and the degree of interaction that occurs with the northern stream system. Additionally, the northern Rockies and central/northern plains could see the potential for a couple rounds of convection and locally heavy rain through the period, with a couple shortwaves potentially traversing the region, and a surface frontal boundary lingering in the vicinity. Above average temperatures are expected through much of the medium range across the central U.S. Max temps on Tue could reach 10-15 deg F above average from the Upper Midwest to the Great Lakes. From Wed onward, the southern plains will become the focus for the greatest temperatures anomalies, with highs potentially reaching 10 to 20 deg F above average, and a number of record high temperatures potentially in jeopardy. Ryan WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities can be found at: http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4