Extended Forecast Discussion NWS Weather Prediction Center College Park MD 108 AM EDT Tue May 08 2018 Valid 12Z Fri May 11 2018 - 12Z Tue May 15 2018 ...Pattern overview... Ridging is expected to remain prevalent along the North American west coast through much of the period, periodically flattened or undercut by Pacific shortwave energy. Meanwhile, a broad area of negative height anomalies will persist through the medium range northeast of Hudson Bay. The area of broad cyclonic upper flow between the large scale western ridge and the Hudson Bay trough will be favorable for amplification of any Pacific shortwave energy that can break through the ridge, and this appears to be the case from Fri onward across the Great Basin/Rockies. Meanwhile, the western Atlantic subtropical ridge will attempt to exert its influence northward into the Southeast and portions of the Mid-Atlantic states. ...Guidance/uncertainty assessment/preferences... The amplifying trough/upper low across the West will be the most significant forecast problem during the medium range. Models/ensembles begin the medium range with a reasonably good consensus as the system is beginning to amplify across the northern Great Basin on Fri. From Sat onward is when some significant predictability issues emerge, primarily related to whether the system lingers for longer as a closed upper low across the Great Basin, or begins to more quickly eject east into the plains. By Sun morning, deterministic solutions range from the 12Z CMC which still has the upper low entirely over the Great Basin, to the 12Z ECMWF/UKMET which were over the Rockies. The deterministic guidance does not tell the full story, however. Ensemble spaghetti plots reveal a substantial number of ECENS members that have the system over the central plains or even the Mississippi valley by the same time (as was shown by the 00Z ECMWF run last night). A closer look at the ECENS members seems to reveal a relatively rare bimodal distribution of members, with a number of members also keeping the upper low over the Rockies or even the Great Basin Sun morning (making the mean likely unrealistic). The differences in ECENS members only grow with time from Sun onward, and the more quickly progressive members result in phasing with northern stream energy and much lower heights by Mon-Tue across the Great Lakes/Northeast. Meanwhile, the overwhelming majority of GEFS and CMCE members fall closer in line with the deterministic guidance, showing a slower progression, and a resultant stronger ridge maintaining itself in the eastern U.S. by early next week. Finally, additional Pacific shortwave energy appears likely to reach the Northwest or western Canada by next Tue, once again eroding (to some degree) the ever resilient western ridge. Model solutions continue to vary widely as to the exact character of this energy, ranging from a consolidated upper low to a more elongated trough. Preferred to stay close to the ensemble means at this time. Given these considerations, the WPC forecast was initially based on a heavily deterministic multi-model blend, including the 12Z ECMWF/CMC and 18Z GFS during days 3-4. From day 5 onward, weighting of deterministic solutions was gradually reduced, with significant weight placed toward the 12Z NAEFS mean by days 6-7. ...Sensible weather/threats highlights... An often stationary frontal boundary extending from the central Rockies and adjacent plains to the Great Lakes/Northeast Fri-Mon will serve as a focus for scattered to numerous showers and thunderstorms over a large area. Models continue suggest the potential for areas of heavy rain across portions of the central Rockies Fri-Sat as the vigorous upper trough amplifies to the west, and a surface low develops along the front across the central plains, enhancing low-level easterly upslope flow. A number of deterministic solutions show the potential for localized multi-day totals of at least a few inches, likely in favored upslope areas. Farther east, broad southerly flow across the plains will transport moisture north and lift it across the frontal boundary, and models show potential for multiple rounds of convection along the front Fri-Sun across portions of the Midwest/Great Lakes and perhaps even into portions of the Northeast, with the potential for areas of heavy rainfall. Confidence in the specific locations of any heavy rain is relatively low at this time, and would likely be determined by mesoscale/convective processes. Farther south, a dissipating frontal boundary will focus increasingly widespread shower and thunderstorm activity across the southern/central Florida Peninsula from late this week into early next week. Areas of heavy rainfall will be possible given the multi-day nature of the event and high Pwat values (+2 to +3 standard deviations). High temperatures are expected to be 10 to 20 deg F above average across much of the southern tier on Fri, with the center of greatest anomalies shifting east to the lower Mississippi valley, Southeast, and Mid-Atlantic by Sun-Mon (+10 to + 15 deg F anomalies). A few record high temperatures are possible. 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