Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Thu May 17 2018 Valid 12Z Sun May 20 2018 - 12Z Thu May 24 2018 16 UTC Update... Model guidance has started to stabilize with the timing of an impulse of shortwave energy ejecting out of the central Rockies/Plains on Sunday, although there is still some uncertainty with the details of an associated weak surface wave, especially as it lifts out of the middle Mississippi Valley and towards the lower Great Lakes and Ohio Valley Sunday night. In regards to the upper low expected to amplify over the western U.S. early next week, the majority of 00Z model guidance shifted towards a weaker low closing off near California on Monday, and the updated WPC medium range forecast reflects this weaker shift in the guidance. The updated WPC medium range forecast stayed fairly close to ensemble means and continuity late in the forecast period in order to account for the wide range of solutions and large run-to-run variability with the evolution of flow over the eastern Pacific during the later half of next week. Gerhardt ...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 Schichtel