Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Thu May 17 2018 Valid 12Z Sun May 20 2018 - 12Z Thu May 24 2018 ...Guidance and Predictability Assessment... A model and ensemble composite seems to provide a reasonable mid-larger scale pattern evolution over medium range time scales. However, the focus of embedded mid-smaller scale systems still offers quite a bit of timing/emphasis variance, especially over the n-central U.S. Latest model trends there support less progressive srn stream impulse/surface wave moisture and instability focus in a wetter convective pattern threat now more separated from nrn stream influence early next week. The 12/00 UTC UKMET seem to best support this. The 00 UTC ECMWF drastically slowed compared to its prior 12 UTC run and recent GFS runs have been slower. ...Weather/Threats Highlights... An amplifying ern Pacific trough should dig into CA and the Great Basin next week to form a closed low aloft to support modest widespread precipitation mainly along/north of the system. Downstream, srn stream shortwave trough energy is now expected to slowly eject northeastward from the Rockies to the N-Central U.S. A wavy frontal boundary will focus anomalously deepened pooled moisture/instabilty to fuel some heavier local convection on the leading edge of cooling post-frontal high pressure spilling southward over the Plains. A wavy frontal zone will meanwhile 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. Instability and moisture pooling near the front supports showers and storms with a threat of local downpours. 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 Reinhart