Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 AM EDT Mon May 14 2018 Valid 12Z Thu May 17 2018 - 12Z Mon May 21 2018 ...Inundating Rainfall Threat for the East... ...Guidance and Predictability Assessment... Guidance offers reasoable clustering days 3-4 (Thu-Fri), with forecast spread/uncertainty increasing significantly over the weekend. Accordingly, a composite model/ensemble blend was used to derive the WPC medium range surface fronts/pressures and 500mb progs days 3/4 before transitioning to a blend of modestly compatable ensemble means days 5-7. Forecast spread from the Pacific to the west coast is particularly egregious days 5-7 in a pattern with much below normal predictabilty. System evolutions and stream interactions seem quite sensitive to guidance data ingest and most models and ensembles have shown less than stellar run to run variance. Trends from at least the last 7 runs of the ECMWF have also shown a distinct diurnal variance with 12 UTC runs on one page and 00 UTC runs on quite another. This modeling phenomena has been highlighted in this sensitive flow pattern, but I've noticed this occurance in varied degrees over the past few years. ...Weather/Threats Highlights... Satellite and guidance trends offer quite favorable support in that an inland lifting ern Gulf of Mexico low and anomalously deep tropical moisture on its east side will likely spread a significant threat of excessive rains from the Southeast to up the Appalachians and Mid-Atlantic/Northeast later this week into this weekend. In this pattern, record high temps persist mid-later week for the mid-lower MS valley just to the west of the main rainfall area, with possible record warm mins farther east in the humid airmass. Overtop...energetic northern stream flow swinging across/through the U.S./Canadian border from the nrn Rockies/plains and Midwest to the Northeast will support and periodically reinforce a persistent and wavy frontal zones from the Rockies/Plains 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. 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 says enough instability/moisture should meander around the sfc boundary and dry line to fire some storm threat through the period. 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