Extended Forecast Discussion NWS Weather Prediction Center College Park MD 153 AM EDT Wed Jun 20 2018 Valid 12Z Sat Jun 23 2018 - 12Z Wed Jun 27 2018 ...Guidance and Predictability Assessment... Guidance overall shows above normal agreement with the flow pattern evolution for the medium range, bolstering forecast confidence. Nrn stream flow features a persistent central Canada upper ridge bookended by amplified wrn Canada/NW U.S. and ern Canada/NE U.S. cooling troughs. A modestly progressive pattern underneath over the CONUS has some tendency for system amplification. Energy crossing from the Rockies to the central U.S. shows potential for deepening and/or closing off, moving through a region of relatively weak upper flow beneath the Canadian ridge. Some recent guidance including the 18/00 UTC GFS runs and yesterdays 00 UTC Canadian show potential for closing off a Northeast trough Mon/Tue as per nrn stream energy split. However, the last two Canadian runs backed off from flow separation/cyclogenesis and the last few ECMWF/UKMET runs and ensembles support more modest amplitude. The amplified central Canadian ridge position offers some 18/00 UTC GFS support, but they are less likely scenarios. The 12 UTC ECMWF on the other hand may be too progressive late period given upstream amplitude. Accordingly, the WPC medium range product suite was primarily derived from a composite blend of reasonably clustered guidance from 12 UTC GFS/ECMWF and GEFS/ECMWF ensemble means. This maintains excellent WPC continuity. ...Weather Highlights and Threats... Lead OH Valley trough/low energy Saturday shears northeastward over the Northeast Sunday. Post-frontal high pressure will nudge a moderating trailing front far down over the ern U.S. next week. Upstream, two main amplifying shortwave troughs dig from the NW U.S. southeastward over the Rockies to the north-central states. These systems produce organized surface lows along a main frontal boundary from the Plains to the MS/OH/TN valleys and the East. Convective complexes/heavy rain focus along the front and into the warm sector fueled by pooled moisture. A late season cooling front/modest pcpn shows increased promise to work more ernestly over the NW U.S. next week with supporting mid-upper level trough amplification. Schichtel WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook 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