Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1200 PM EDT Sat Jun 16 2018 Valid 12Z Tue Jun 19 2018 - 12Z Sat Jun 23 2018 ...Pattern overview and Guidance/Predictability Assessment... The medium range period (Tue-Sat) starts out with a closed low over the western U.S., with the guidance now showing a faster weakening of the upper low and ejection of short wave energy into the Plains and the Mississippi Valley by the end of the week. Strong ridging over the Pacific Northwest will retreat eastward as a shortwave enters the West Coast by Friday. Meanwhile, the northern periphery of the persistent downstream ridging across the East/Southeast is finally eroded by northern stream cyclonic flow across the Great Lakes and the Northeast, which should suppress the oppressive heat and humidity expected this weekend and early next week. Overall, the models and ensembles show a reasonably similar larger scale pattern over the next week, but smaller mid-scale embedded features still offer a bit of forecast spread. This cycle of the WPC medium range suite used a blend of the latest runs of the GFS/ECMWF with their ensemble means. Significant contribution from deterministic runs were included days 3-4, with increasing ensemble contribution by the end of the forecast period to try and de-emphasize the less predictable smaller scale features which still remain highly uncertain. The solution maintains reasonably good WPC continuity. The biggest shift is at the end of the period with a slightly faster progression of troughing into the Plains/MS Valley which may help spin up a wave along the boundary, with the nearly stationary boundary across the Plains now beginning to drop slowly south/eastward. ...Weather Highlights/Threats... A long standing and wavy frontal boundary remains quasi-stationary from the central Plains to the Ohio Valley and Mid-Atlantic. Periods of locally excessive convection will focus along this boundary and in the warm sector aided by early period tropical moisture influx from the Gulf of Mexico. WPC medium range 48-hour QPF shows the heaviest potential along the immediate TX coast due to a potential tropical wave/disturbance. Moisture streaming northward intersects the wavy boundary to the north to produce possibly several inches of rainfall across the central Plains and back into the northern/central Rockies. As the trough and front shifts slowly eastward days 5-7, the best rainfall will focus along the surface boundary and to the north of any potential wave development across the upper Midwest. Temperature wise, ridging across the Southeast should weaken in response to cyclonic flow in the northern stream and the western U.S. trough sliding eastward. Afternoon highs above normal are still likely heading through next week across much of the East, but not nearly as extreme as the late weekend/early week heat expected. Just a handful of near record max temps are expected Tuesday, but theres a higher probability of near or record breaking high minimums, as overnight temps hover in the 70s. Well above normal temps expected across the Pacific Northwest as well with daytime highs well into the 90s across interior regions, which would come close to records. Across the Intermountain West and Central states, temps should be below normal underneath upper level troughing and cloudy/rainy conditions. Santorelli 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