Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun May 13 2018 Valid 12Z Wed May 16 2018 - 12Z Sun May 20 2018 ...A convectively active and quite wet pattern for much of the country... ...Weather/Threats Highlights... Energetic nrn stream flow dug to the U.S./Canadian border from the Upper Midwest to the Northeast will support and periodically reinforce a persistent and wavy frontal zone underneath that will tends to shift slowly southward across the nrn third of the nation. Hot and increasingly moist warm sector conditions to the south will offer stark contrast. Mid-latittude impulses running over this frontal zone and a more mid-latitude stalled front will further act to focus instability and moisture pooling on each to support multiple rounds of heavy convection. It remains unclear the effect of widespread heavy convection within the warm sector will effect the overall distribution to the north, but the gradually weakening but quite lingering effects from a Gulf of Mexico upper low and anomalously deep moisture meandering up into the srn/southeastern U.S. will in itself be quite the focus for widespresd heavy convection over the region and with time up the Appalachians to the Mid-Atlantic as enhanced from terrain and slowed cell translations. In this pattern, record high temps may persist into mid-later this week over the mid-lower MS valley where there is a relative break from heat breaking thunderstorms. Record minimum temps will be more widespread during that period, northeastward across the Mid-South to the Mid-Atlantic. Meanwhile...the West will also be quite active as an amplified mid-upper level trough and associated surface frontal system slowly works inland from CA midweek to the Great Basin/Southwest later week and into the Rockies by next weekend. The greatest precipitation threat will be along and north of system track as enhanced by terrain/deformation. Ample but uncertain energies/height falls also ejecting out across the Rockies into the plains next weekend also offers a threat for afternoon/nocturnal convective development/clusters along/north of the lingering fronts, with warm sector meso-boundaries, and a dryline. ...Guidance and Predictability Assessment... Model and ensemble mass field guidance offers average clustering days 3-5 (Wed-Fri), with forecast spread/uncertainty increasing significantly next weekend on days 6/7. Accordingly, a composite model and ensemble mass field blend was used to derive the WPC medium range surface fronts/pressures and 500mb progs days 3-5 before turning to a more simple blend of the reasonably compatable 12 UTC NAEFS/ECMWF ensemble means days 6/7. WPC continuity was mixed due to effects from significant differences with the local focus of heavy rainfall between guidance in a widespread convective pattern. WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation forecasts, and winter weather outlook snow/sleet probabilities are 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 Schichtel