Extended Forecast Discussion NWS Weather Prediction Center College Park MD 249 AM EDT Wed May 16 2018 Valid 12Z Sat May 19 2018 - 12Z Wed May 23 2018 ...Guidance and Predictability Assessment... Model and ensemble clustering has improved some with the last few cycles at medium range time scales and a composite blend of the latest GFS/ECMWF and GEFS/ECMWF ensembles seems to provide a reasonable basis for the WPC product suite. Embedded systems still offer timing/emphasis variance, but overall uncertainty has recovered to near normal levels. ...Weather/Threats Highlights... Weekend srn stream shortwave trough energy/height falls ejecting northeastward from the Rockies to the N-Central U.S. will interact with nrn stream trough energy digging to the lee of a persistent wrn Canadian upper ridge. The wavy frontal boundary will focus some heavier local convection on the leading edge of cooling post-frontal high pressure spilling southward over the Plains. Upstream, guidance is in better agreement that an amplifying ern Pacific trough should dig into CA and the s-central Great Basin Mon-Wed to form a closed low aloft to support modest widespread precipitation mainly along/north of the system. Downstream...anomalously deep moisture from the Gulf of Mexico and western Atlantic will likely spread showers and storms from the Southeast to up the Appalachians and Mid-Atlantic/Northeast into the weekend. The models forecast widespread showers when the moisture intercepts the front across Long Island and adjacent southern New England/nearby islands. The wavy frontal zone will persist and be periodically reinforced from the Plains to the MS Valley, OH Valley, and Northeast. Hot and moist conditions persist to the south of the front. Late week record high temps are expected in parts of Texas to the mid-lower MS valley before the airmass modifies early next week. Record warm minimum temps are possible for the Mid-Atlantic in the warm, humid airmass. Instability and moisture pooling along the front supports showers and storms with potential for locally heavy downpours. Other showers/storms should develop in the TX/OK panhandles near the dryline and return low level southerly flow advects moisture into the region. Also, FL should experience diurnal showers/storms as 2 inch precipitable water values develop to fuel locally heavy downpours where best sea/lake breeze convergence occurs. 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 Schichtel