Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Fri May 25 2018 Valid 12Z Mon May 28 2018 - 12Z Fri Jun 01 2018 ...Subtropical Storm Alberto expected to bring heavy rainfall to portions of the Gulf Coast and Southeastern states... ...Overview/Synoptic Assessment... Progressive flow across the northern Pacific Ocean will become a bit less progressive across the CONUS, with a number of features showing blocking tendencies. Negative height anomalies are expected to persist across northeastern Canada/Greenland through the period, with ridging dominant off the southeast U.S. coast and across the central U.S. The track of Subtropical Storm Alberto will be initially follow a weakness between the two ridges, before coming under increasing influence of northern stream westerlies by mid to late next week, with a number of model/ensemble solutions suggesting the potential for the remnant low to move across the Ohio Valley/Appalachians and perhaps the Mid-Atlantic. Please refer to products issued by the NHC for the latest information and forecast for Subtropical Storm Alberto. Elsewhere, upper low initially in place across the Great Basin/Rockies on Mon is expected to move northeast into the northern plains by Tue and the Upper Midwest on Wed as it comes under increasing influence of the northern stream, and height falls digging into western Canada. The system is also expected to gradually lose amplitude as it encounters broad anticyclonic flow across the Great Lakes. Finally, guidance continues to suggest trough amplification along the West Coast by Tue-Wed, with heights falling across the western U.S. as the trough moves inland by Thu-Fri. Additional shortwave energy may approach the Northwest by next Fri. ...Guidance/Predictability Assessment... Model consensus continues to improve with respect to the track of Subtropical Storm Alberto. The ECMWF has been very consistent with the track of the system for a few runs now, and was also closest to the official NHC forecast track. Thus, the 00Z ECMWF was weighted heavily in the WPC forecast, with the 06Z GFS/GEFS and 00Z ECENS also included. The GFS was excluded after day 5 (Wed) as the model appeared perhaps too quick to weaken the remnant low and quickly move the mid-level system east. Farther north, model differences with the energy exiting the Rockies into the northern plains by mid-week were primarily related to amplitude, with some timing differences emerging by Thu night-Fri as the wave deamplifies across the Great Lakes. The aforementioned blend appeared to represent a good consensus solution here. Finally, along the West Coast, while all solutions agree on the arrival of troughing by mid-week, deterministic solutions continue to differ as to the exact character of the system. The ECMWF showed a more fragmented trough with two embedded shortwaves, while the GFS suggests a slightly more phased features and amplified trough. Weighting of ensemble means was increased a big by the middle to end of next week to account for these differences, with the blend showing a trough axis reaching the West Coast Wed night and moving into the Great Basin by Fri. ...Weather/Threats Highlights... Subtropical Storm Alberto is expected to pose a significant heavy rainfall threat across portions of the southern/southeastern states by early next week, with multi-inch rainfall totals looking increasingly likely for portions of the Gulf Coast States. Drier air wrapping around the western side of the system may result in a sharp gradient/cutoff of precipitation on the western side of Alberto, although confidence in the specific location of such a gradient is low at this point. Additional impacts along the Gulf Coast such as tropical storm conditions and storm surge are possible, although it is too soon to specify the exact location and magnitude of these impacts. Additional heavy rains will be possible across much of Florida and the Southeast as feeder bands traverse the region east of Alberto. Ensemble guidance continue to suggest the potential for a rainfall max across the southern Appalachians and foothills, from portions of north/northeast Georgia into western portions of the Carolinas, likely due to upslope enhancement of precipitation. By mid-week, as the remnants of Alberto move northward, the heavy rainfall threat may extend into portions of the Ohio Valley/central Appalachians and possibly the Mid-Atlantic. Farther north, locally heavy rains will be possible across portions of the central/northern plains and Midwest as a surface frontal boundary lingers in the region early next week and upper-level traverses the region. Above average temperatures are expected early next week across the central U.S. and Midwest, where highs could reach 10 to 20 deg F above average. A number of record high temperatures could be in jeopardy across the central U.S. next week. 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