Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM 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 Southeastern states... ...Overview and Guidance Assessment... The models continue to forecast Subtropical Storm Alberto to move onshore into the Gulf coast and move north and gradually recurve under increasing influence of northern stream westerlies by mid to late next week. A number of model/ensemble solutions suggests potential for the remnant low to move across the Ohio Valley/Appalachians and reform as an extratropical low over the Mid-Atlantic. Please refer to products issued by the NHC for the latest information and forecast for Subtropical Storm Alberto. The 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 and was also closest to the official NHC forecast track. Thus, the 12z ECMWF and ecmwf Ensemble Mean was weighted heavily in the WPC forecast. The 18z GFS was not included as its timing was faster than the NHC track. Elsewhere, upper low initially in place in Wyoming 12z Tue 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 crosses the Great Lakes. The trough also may phase with the remnants of Alberto as its upper trough moves across the Ohio Valley, maintaining timing and phasing differences that lower confidence in the forecast and lead to more weighting on a multi-model and ensemble consensus until better agreement develops. Guidance continues to suggest trough amplification along the West Coast Tue night-Wed, with heights falling across the western U.S. as the trough moves inland by Thu-Fri. Model agreement breaks down as there are timing differences regarding the next upper trough to eject east from the Aleutians across the north Pacific towards the Pacific northwest. The 12z ECMWF was on the amplified and fast side of the suites of ECMWF model/ensemble solutions, as well as other operational models. This faster solution in turn ejected the Great Basin trough faster east Late Thu through Fri. Weighting of ensemble means was increased Thu-Sat 02 Jun to account for these differences, with a more persistent western trough than the operational 12z ECMWF the result of the preference. ...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. See the latest NHC advisories for the complete list of hazards. Heavy rains will be possible across much of the Southeast and lower MS Valley as Alberto tracks inland. 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 rainfall is expected to expand into portions of the Ohio Valley/central Appalachians and possibly the Mid-Atlantic. 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 trough traverses the region. Above average temperatures are expected next week across the mid MS Valley to the southern Plains and lower MS Valley, where highs are expected to reach 10 to 20 deg F above average. A number of record high temperatures could be in jeopardy next week in these areas. Highs from 100 to 110 degrees are expected the latter half of next week in west to central portions of Texas. Petersen 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