Extended Forecast Discussion NWS Weather Prediction Center College Park MD 105 AM EDT Sat May 05 2018 Valid 12Z Tue May 08 2018 - 12Z Sat May 12 2018 ...Pattern overview... Persistent negative height anomalies in the north central Pacific will support anticyclonic upper flow across western North America through much of the medium range. Downstream of the ridge, negative height anomalies are expected to persist in the vicinity of Hudson Bay, with broad anticyclonic flow across areas from the Midwest to the Mid-Atlantic/Northeast. The result will be a series of Pacific surface fronts that generally weaken as they move into the interior western U.S., and the potential for low pressure development east of the Rockies across the central/northern plains, tracking eastward across the Midwest/Great Lakes. ...Guidance and uncertainty assessment... A multi-model deterministic blend including the 12Z ECMWF/UKMET/GFS served as a starting point for the WPC forecast early in the medium range, with increasing weight placed toward the 12Z ECENS/NAEFS means after Day 5 given quickly increasing uncertainty. Consensus has improved somewhat early in the period (Tue-Wed) surrounding the eastern U.S. trough. The ECMWF continues to be the deepest solution and the slowest to lift the feature out, while the GFS and UKMET have moved toward a deeper solution compared to this time last night. A middle ground approach continues to be favored per the latest ensemble means, supporting the idea of a multi-model deterministic blend early in the forecast period. Elsewhere, consensus is a bit better with respect to shortwave energy crossing the northern plains/Midwest Tue-Thu and the Northeast by Thu night. Differences surrounding this system are largely timing related, with the ECMWF bringing the upper wave and surface frontal system east a bit faster than the consensus based on the latest ensemble means. Meanwhile, models show two potential shortwave undercutting the western ridge and moving into the northwestern U.S. The first of these begins as an upper low off the Northwest coast on Tue, which should begin to weaken as it approaches the coast and the broad area of ridging. Models show some significant differences as to how quickly the features weakens, which in turn impacts timing as the feature moves inland. The GFS continues to lead the pack with weakening the wave/moving it inland fastest, and a slower solution is preferred at this time based on a consensus surrounding the 12Z ECMWF/NAEFS means. The 12Z ECMWF was much closer to this consensus than was the GFS. Models are in general agreement on a second shortwave reaching the Northwest late Thu, but deterministic solutions differ substantially. The last couple runs of the ECMWF (along with the 12Z CMC) really amplify this feature across the Northwest, while the GFS is much weaker moves the wave inland fairly quickly. Given that this feature does not originate as a deep upper low offshore like the first, and the time scale under consideration, confidence is low as to the specific evolution by next Fri, and an ensemble approach was favored, representing a compromise solution. ...Sensible weather/threats highlights... While some uncertainty remains as to the specific impacts of the eastern U.S. trough Tue-Wed, the highest probability scenario is for most significant activity to remain just offshore. Slight potential remains for moisture to wrap back as far as the coast or slightly inland. Farther west expect moisture initially over the High Plains to progress along with surface low pressure/frontal system into and through the northern/central Plains and eventually reach the East. Some Plains into Great Lakes/Ohio Valley convection could be locally heavy and/or strong. Meanwhile the system arriving from the eastern Pacific will bring a period of enhanced rainfall to the Pacific Northwest/northern Rockies from mid to late next week. The Interior West and portions of the High Plains should see the warmest temperature anomalies during the period, plus 10-20F and locally higher. There may be some daily records for highs/warm lows. Eastern states will see near to slightly below readings Tue-Wed followed by a modest rebound to warmer conditions. The arrival of a cold front across the Great Lakes/Northeast by Fri-Sat should usher another round of cooler temperatures into those regions, although the magnitude of this cooler air remains a bit murky at this point. 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