Extended Forecast Discussion NWS Weather Prediction Center College Park MD 206 AM EDT Thu May 16 2019 Valid 12Z Sun May 19 2019 - 12Z Thu May 23 2019 ...Guidance/Predictability Assessment... The initial cyclone this weekend is forecast to cross from the upper MS Valley to the Great Lakes Sunday and across the northeast and into the Canadian Maritimes Mon into Tue. The overall trend was for slightly faster motion than prior forecasts. Given good agreement among the 12z ECMWF and its ensemble mean, the progs gave more weighting to these solutions. The 12z UKMET was the southern most solution and given the least weighting. Model and ensemble solutions continue to show above normal forecast spread with respect to northeastern U.S. flow through this weekend. The anticipated upper trough moves onshore Sun in the Pacific northwest/CA, with the surface reflection crossing the Great Basin Sun and Mon. As a wave emerges as the system crosses the southern Rockies, lee cyclone development occurs Mon with the new system moving across the southern and central Plains Mon into Tue, continuing north northeast across the Plains to the upper MS Valley by Wed 22 May. The 12z ECMWF/18z GFS offer good forecast agreement with their ensemble means and Canadian global tracking the mid-upper low further northwest. The next in the series of 500 mb wave may move onshore into CA Thu 23 May, according to the 12z ECMWF/GFS/Canadian Ensemble Means. Downstream from the series of cyclones developing within the western US trough, a developing mid level anticyclone is forecast over the southeast US, which gradually becomes stronger in the middle of next week. Excellent clustering of solutions exist, so confidence is above normal in the developing anticyclone. The WPC product suite for temperatures, dewpoints, etc. was derived from a blend giving more weight to the 12z ECMWF/ECMWF Ensemble Mean with lower weighting to the 18z GEFS Mean and 18z operational GFS. ...Weather Highlights and Hazards... Several rounds of showers/storms are expected in the central US, with Sat-Sun targeting the eastern Southern to central Plains to the Upper MS Valley , then into the Upper Great Lakes and lower MS Valley Sun. Locally heavy rain is possible in these areas. The next system entering the southern Plains Mon triggers the next round of showers/storms. See the Storm Prediction Center severe weather outlooks for each day's severe convective potential. As the system moves north the activity spreads from the central into northern Plains Tue and into the MS Valley before moving downstream into the upper Great lakes Wed 22 May. Upstream, a dynamic mid-upper level trough will produce rainfall in CA/Pacific northwest to the Great Basin over the weekend. On Monday the slow moving frontal system targets the ranges of eastern NV across UT/CO/WY/ID with valley showers and mixed rain/snows at higher elevations. On Tue the precipitation is focused in the pacific northwest to northern CA inland into the northern great Basin and northern Rockies. On Wed the focus of valley showers and mixed precip at higher elevations moves into the northern Rockies and high Plains, then gradually spreading into the northern Plains and upper MS Valley. The building mid level ridge in the southeast leads to growing confidence about hot temperatures developing in the southeast. The first forecast 100 degree readings popped up in both the 12z ECMWF and ECMWF Ensemble Mean in southern GA next Wed 22 May and Thu 23 May, marking an early season developing heat wave. There is potential for record setting heat in the southeast and southern Appalachians the latter half of next week. Along with the heat comes a dry spell in AL/GA and the Carolinas, with below normal chances of rain. Persistent cold is forecast in the west to the northern Plains this weekend thru the middle of next week. Large areas of -10 to -20 degree anomalies for daily high temperatures remain forecast across the Great Basin and northern Plains Sun, continuing into the early and middle portions of next week. Overnight minimum anomalies are not as high a magnitude but still below normal over large portions of the west to the northern Plains. Petersen Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml