Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sun May 13 2018 Valid 12Z Wed May 16 2018 - 12Z Sun May 20 2018 ...A convectively active and quite wet pattern for much of the country... ...Weather/Threats Highlights... Energetic northern stream flow swinging across/through the U.S./Canadian border from the Upper Midwest to the Northeast will support and periodically reinforce a persistent and wavy frontal zone from the southern Plains through the Ohio Valley to the Northeast. Hot and juicy warm sector conditions to the south will offer stark contrast to the cool/continental air mass to the north. Additional mid-latitude impulses running over this frontal zone will further act to focus instability and moisture pooling which will support multiple rounds of heavy rain and embedded quasi-disorganized convection. It remains unclear how the convection within the warm sector will affect the overall rainfall distribution to the north, but the gradually weakening (and lingering) effects from a Gulf of Mexico upper low and anomalously deep moisture on its east side will likely lift northward through the southeastern U.S. into the mid-Atlantic which will spread at least modest to potentially locally heavy rain up the Appalachians to the Mid-Atlantic later this week into this weekend. In this pattern, record high temps may persist into mid-later this week over the mid-lower MS valley to the west of the Gulf upper low with possible record warm mins farther east in a rather humid air mass. Meanwhile...the West will also be quite active as an amplified mid-upper level trough and associated surface frontal system slowly work inland from CA midweek to the Great Basin/Southwest Thu-Fri and into the Rockies by the weekend. The greatest precipitation threat will be along and north of the upper system track as enhanced by terrain/deformation. Weaker ejecting shortwaves out of the Rockies into the plains will help initiate scattered showers/storms ahead of this weakening upper low, which will likely exit onto the Plains in weakened state late Sat into next Sun. SPC does not show a risk area for severe weather through the period but enough instability/moisture should meander around the sfc boundary and dry line to fire off at least some storms through the period. ...Guidance and Predictability Assessment... Upper highs over British Columbia and near Bermuda will mostly hold in place leaving the lower 48 wavering between zonal and quasi-blocked flow. Model and ensemble mass field guidance offers good clustering days 3-5 (Wed-Fri), with forecast spread/uncertainty increasing significantly by the weekend on days 6/7. Accordingly, a mostly deterministic model blend was used to derive the WPC medium range surface fronts/pressures and 500mb progs days 3-5 before turning to a more simple blend of the reasonably compatible 06 UTC GEFS / 00Z UTC ECMWF ensemble means days 6/7 due to northern stream uncertainties across Canada and a breakdown of Pacific flow agreement. WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation forecasts, and winter weather outlook snow/sleet probabilities are 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 Fracasso/Schichtel