Extended Forecast Discussion NWS Weather Prediction Center College Park MD 321 AM EDT Fri May 04 2018 Valid 12Z Mon May 07 2018 - 12Z Fri May 11 2018 ...Pattern overview... The large scale flow pattern across North America during the medium range looks to return to a ridge in the west/trough in the east pattern which has been typical for much of the past several months. The amplitude of the pattern will be less than in recent months, however, more typical of mid to late spring. Characteristics of this pattern will include weakening Pacific frontal boundaries moving into the western U.S. as the associated upper-level shortwaves encounter a broad area of anticyclonic upper flow. Meanwhile, the central U.S. to the Great Lakes/Northeast will be the preferred storm track east of the Rockies. Additionally, a lingering surface front off the Eastern Seaboard shows at least some potential among the guidance for surface wave development. ...Guidance and uncertainty assessment... Models show widely varying solutions for an upper shortwave initially across the Ohio/Tennessee valleys, with the 12Z ECMWF the most aggressive with amplifying this feature and keeping an upper low in place across the Southeast even into Day 5 (Wed). The ECMWF was supported to some degree by the 12Z CMC. Meanwhile, the 12Z/18Z GFS along with the UKMET were much weaker. Ensemble means seemed to support a middle ground solution, with the GEFS/NAEFS means both deeper than the GFS/UKMET. The evolution of this features will also significantly influence the development of a potential wave of low pressure along a lingering surface front off the Eastern Seaboard. The WPC forecast took a compromise approach, not as deep as the 12Z ECMWF, but more so than the weaker solutions. Elsewhere, models showed typical timing/amplitude differences with additional shortwave energy crossing the Midwest Tue-Wed, as well as with an upper low approaching the Pacific Northwest during the same time frame. The 18Z GFS was likely the strongest outlier with respect to the latter, seeming to weaken the system and move it inland too quickly. Ensemble means tended to favor a slower/deeper solution as shown by the ECMWF/CMC/UKMET. By Day 7 (Thu), solutions differ as the energy cuts into the upper ridge across western North America. The 12Z ECMWF suggested some potential for interaction/phasing with higher latitude energy rotating around a broad area of negative height anomalies center over Hudson Bay, while the GFS/CMC keep a stronger ridge in place, keeping the Pacific energy more separated from the northern stream. Given significant run-to-run variability by late in the period, confidence is very low in the details, and the forecast was trended heavily toward the ECENS/NAEFS means. ...Sensible weather/threats highlights... The aforementioned variations among the guidance with respect to the shortwave across the eastern U.S. on Days Mon-Tue have significant implications for if/where precipitation will occur. A stronger ECMWF-like solution would result in the potential for heavy rainfall across portions of the Southeast. The compromise approach favored for this system would temper the heavier precip shown by the ECMWF somewhat, although the potential for at least locally heavy rainfall is there. Meanwhile, showers and thunderstorms will be possible from the central/northern High Plains to the Midwest Tue-Thu as a surface frontal wave develops east of the Rockies. The building ridge across the West will most of the region dry, with above average temperatures (high temps 10-15 deg F above average). The Pacific Northwest will be the only area to see potentially wet conditions, showers increasing by mid-week as a Pacific front/upper low move onshore. Ryan Associated WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation forecasts, and winter weather outlook snow/sleet 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