Extended Forecast Discussion NWS Weather Prediction Center College Park MD 137 PM 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... The guidance offers reasonable clustering on days 3-4 (Thursday and Friday), with forecast spread and uncertainty increasing significantly over the weekend and into early next week. Accordingly, the WPC medium range progs used a mostly operational GFS/ECMWF blend early, transitioning to a totally ensemble based blend by the end of the period. This offers a forecast very close to that of the previous shift. The main area in question continues to be out in the Pacific and along the west coast days 5-7. System evolutions and stream interactions seem particularly sensitive to guidance data ingest as the majority of models and ensembles continue to show significantly high run to run variability. Particularly, the ECMWF has shown distinct diurnal variance with 12 UTC runs and 00 UTC runs on totally separate pages. Today and yesterdays 00z runs indicate more general troughing with embedded vorts lingering along or off the Pacific Northwest coast, while yesterday and Saturday's 12z runs have shown a deep closed low meandering in the west Pacific. While the GFS has shown a slightly better consistency the past few runs, it's still less than stellar. It appears spread with this system amongst the various ensemble members of both the GEFS and EC is high as well. ...Weather/Threats Highlights... Satellite and guidance trends offer quite favorable support for an inland lifting eastern Gulf of Mexico low and anomalously deep tropical moisture on its east side likely spreading a significant threat of excessive rains from the Southeast to up the Appalachians and Mid-Atlantic/Northeast later this week and 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 northern Rockies/plains and Midwest to the Northeast will support and periodically reinforce a persistent and wavy frontal zone 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 California 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 surface boundary and dry line to fire some sort of storm threat. 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