Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Tue May 15 2018 Valid 12Z Fri May 18 2018 - 12Z Tue May 22 2018 ...An excessive rainfall threat over the East and active Western to Central U.S. weather upstream... ...Guidance and Predictability Assessment... Guidance offers reasonable clustering days Friday and Saturday, but forecast spread and uncertainty increases significantly later weekend and into early next week. Accordingly, the WPC medium range progs were primarily derived from a GFS/ECMWF/UKMET blend early, then quickly transition to an ensemble blend. ...Weather/Threats Highlights... Anomalously deep moisture from the Gulf of Mexico and western Atlantic will likely spread a significant threat of excessive rains from the Southeast to up the Appalachians and Mid-Atlantic/Northeast later week into the weekend. In this pattern, record high temps persist mid-later week for the mid-lower MS valley west of the main rainfall area, with record warm mins possible for the Mid-Atlantic in the humid airmass. The West will meanwhile also be quite active as an amplified mid-upper level trough and frontal system slowly work over the Great Basin/Southwest Fri and Rockies by the weekend. Widespread precipitation will develop along and north of upper system track as enhanced by terrain lift and deformation. Ejecting shortwaves out of the Rockies into the plains will help initiate scattered showers/strong convection ahead of this weakening upper system, that then further shears over the east-central U.S. Overtop...Energetic nrn stream flow also digs to the U.S.to also periodically reinforce a persistent and wavy frontal zone from the Plains to the Northeast. Hot and moist warm sector conditions to the south will offer stark contrast to the cool/continental air mass to the north. Instability and moisture pooling along the front will support multiple rounds of heavy rain with embedded convection. 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 Santorelli/Schichtel