Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Tue May 29 2018 Valid 12Z Fri Jun 01 2018 - 12Z Tue Jun 05 2018 ...Record heat expected across the southern Plains... ...Guidance and uncertainty assessment... Model and ensemble guidance offer a reasonably well clustered range of solutions in the day 3/4 (Fri-Sat) time frame in a pattern with near average expected predictability. Increasing model to model run inconsistency and overall guidance forecast spread into the days 5-7 time frame suggests a transition to below normal predictability for smaller-mid scale systems, but some common themes remain to support the overall larger scale flow evolution. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of the quite compatable 06 UTC GFS/GEFS mean and 00 UTC ECMWF/ECMWF ensemble mean days 3/4. Transitioned to much higher ensemble mean weighting days 5-7 amid growing forecast uncertainty. Leaned a bit more on the ECMWF then ECMWF ensemble mean overall in these blends in an effort to maintain as much WPC continuity as possible consistent with the expected larger scale flow evolution. ...Weather Highlights and Threats... An upper low/trough will likely drop southeast into the Mid-Atlantic States and Carolinas, setting the stage for wet weather for the Mid-Atlantic and Southeast states. A lingering lead/wavy surface front over the from the southern plains through the southeastern U.S. into the western Atlantic will also provide a focus for deeper moisture pooling and some heavier local convective downpours. Upstream, a general trough over the Northwest composed of a few active shortwaves should generate some late season rainfall over northern parts of the West late this week and locally heavy convection over the Plains well into next week. Ejecting western U.S. impulse energy will play an uncertain role in rainfall distribution/intensity downstream over the central and eastern U.S. through the period. The dominant focus of the temperature forecast will be on a multi-day southern Plains heat wave whose influence may extend at times westward into the Rockies and eastward into the lower MS Valley and western Gulf coast. Hot afternoon readings in the 100-110F range should be most common in the southern High Plains/lower Rio Grande, with Saturday setting up to be the hottest day. It is likely that record values for max/warm min will be reached/exceeded on multiple days, with some of the record-breaking heat also extending for a time farther north/northeast. There should be some southward suppression of the heat in Texas with time from Sunday into Monday as a front settles southward, though it should rebound by Tuesday. On the other hand, mean troughing aloft will tend to keep highs near to below normal near the West Coast and by the weekend/early next week over the East. Schichtel 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