Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 AM EDT Fri May 25 2018 Valid 12Z Mon May 28 2018 - 12Z Fri Jun 01 2018 ...Potential heavy rainfall event for portions of the eastern Gulf Coast and Southeastern states late this weekend into early next week... ...Overview/Guidance Preferences... The WPC forecast was based on a blend of the 12-00Z ECMWF/12Z GFS and respective 12z ECMWF and 18z GEFS Ensemble Means Days 3-5, switching to heavy weighting towards the 2 ensemble means days 6 Thu 31 May and Day 7 Fri 01 Jun. The deep layer ridge building from Texas across the mid MS Valley and weak upstream flow leads to slow forward motion of the prospective cyclone developing in the Gulf of Mexico later this weekend and slowly moving onshore into the Gulf coast and then moving inland into the southeast early next week. See the National Hurricane Center's tropical weather outlooks and discussions for more details on the system evolution. The forward motion is forecast to pick up by Wed-Thu in response to the shortwave moving across the northern plains/Midwest. Once this system moves towards the mid Atlantic coast next Fri, phasing with the system over the Great Lakes could lead to extratropical wave development, with a forming warm front moving towards the northeast US. A cutoff upper low initially across the Great Basin this weekend will eject slowly northeast across the Rockies and then northern high plains by Tue morning and crossing the northern Plains and deamplifying as it moves into the upper MS Valley and Great Lakes mid week. The model agreement breaks down after the system crosses the Lakes with lots of timing and phasing differences regarding the southern stream system merger come Fri 01 June. The models are in good agreement on the next mid-upper level trough reaching the West Coast by next Tue night/Wed morning 30 May, continuing slowly inland Wed-Fri. The 12z ECMWF falls out of phase by deamplifying the upper trough while the 12z ECMWF and 18z GEFS ensemble means and 12z Canadian/18z GFS were in better agreement on keeping an amplified trough. With timing differences between the faster GFS and slower Canadian, the preference was to use the means from the 12z ECMWF and 18z GEFS ensembles. ...Weather/Threats Highlights... The Gulf of Mexico low pressure will bring the potential for a heavy rainfall event to portions of the Gulf Coast and Southeast states. Deterministic ECMWF solutions show the potential for multi-inch rainfall totals from Sun onward. The two areas of particular concern are along the Gulf Coast where the system comes onshore Sun-Tue east of the mouth of the Mississippi and another area Wed-Thu across the southern Appalachians/foothills, where upslope enhancement along with deep tropical moisture inflow may enhance rainfall amounts. Additional moderate to locally heavy rains will be possible Sun-Mon from portions of the northern Great Basin/northern Rockies east into the High Plains, as deep moisture flux ahead of the upper low focuses precipitation along a quasi-stationary frontal boundary. PWs across portions of Wyoming/Montana east into the northern High Plains are near max values on percentiles, with values near +3 standard deviations possible. Thus, this appears to be another area of potential heavy rainfall near the persistent surface front in the vicinity. Convection developing ahead of the central Plains trough leads to increased shower and thunderstorms mid week from the central to northern Plains. Temperatures are expected to be well above average across much of the Plains states and upper-mid MS Valley with readings 10 to 15 deg F above average from late in the weekend through early next week. Parts of west Texas will have multiple consecutive days over 100 degrees early to middle portions of next week. Well above normal temperatures next Fri 01 Jun spread into the OH/TN Valleys and lower MS Valley, with potential to reach 100 degrees in non-coastal Texas and southern/western OK to the ARKLATEX. Petersen/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