Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat Jun 09 2018 Valid 12Z Tue Jun 12 2018 - 12Z Sat Jun 16 2018 ...Pattern Overview... Latest guidance continues to show higher latitude blocking, the details of which will affect forecast details across the northern half of the contiguous U.S. The most common theme seen in the models/ensembles is that some energy from a southern Canada closed low and flow to its south and east will progress rapidly eastward under an upper high that closes off just northwest of Hudson Bay while remaining energy may retrograde back into western Canada. This evolution should lead to eastern Canada/U.S. troughing by the latter half of the period. Meanwhile a strong consensus has an upper trough moving into the West by Thu, anchored by an upper low most likely to track over Vancouver Island. By the latter half of the forecast there is still significant spread regarding details of flow within the overall trough. Interestingly the end result by next Fri-Sat could be another high amplitude trough-ridge-trough configuration albeit with meaningful detail differences from the same overall pattern taking shape this weekend. Farther south, an upper ridge should persist over the southern Rockies/Plains but with some spread and day to day variability for position. ...Model Guidance/Predictability Assessment... The overall forecast philosophy remains intact, following a blend of primarily operational guidance (06Z GFS/00Z ECMWF and less 00Z UKMET/CMC) early with a day 5 Thu transition toward a late period blend consisting of 40 percent 00Z ECMWF mean and the rest equal parts 00Z ECMWF, 06Z GEFS mean, and WPC continuity. Today there is somewhat greater clustering toward the idea of a more even splitting of initial southern Canada closed low energy and surrounding flow, with latest ensemble means reflecting some operational guidance that has some of the energy retrograding back into western Canada. What energy progresses eastward should support a surface system that brushes New England around midweek and trailing front that reaches as far south as the central latitudes of the eastern U.S. before stalling. There is decent continuity in principle toward the combination of ejecting energy and eastern Canada flow yielding an eastern North America trough by the latter half of the period, though with a slightly faster trend in the means at most valid times. Into day 5 Thu guidance has gravitated toward a common solution fairly well regarding the upper trough heading into the West. After that time models/ensembles diverge fairly rapidly. Once again the ensemble means provide a good intermediate solution between operational model extremes--the progressive ECMWF/CMC and slower/deeper GFS that closes off a low over or northeast of California. As was the case yesterday, teleconnections relative to the upstream ridge's positive height anomaly center recommend more modest West Coast troughing than forecast by the GFS (but perhaps with an axis not far from the GFS), while offering potential that the northern tier states could see somewhat broader troughing aloft than seen in the means. This seems to argue for holding onto a small ECMWF component in the forecast. It remains to be seen whether there will be faster trends over a multi-day time frame as was seen with the preceding system. The area extending from the western Caribbean into the Gulf of Mexico continues to exhibit fairly high forecast uncertainty. Once again the 06Z GFS has hedged slower/westward with its tropical development versus the 00Z run while the GEFS and NAVGEM are about the only other solutions to show GFS-like potential. If any type of weak feature were to exist, the combination of low level steering and most common guidance location of any upper level weakness would more likely tend to direct it into the western Gulf. Late in the period a system likely to develop in tropical eastern Pacific (to the east of Aletta) may reach close to southern Baja California. This system will bear watching especially with respect to the potential flow of moisture ahead of the western U.S. trough. ...Weather Highlights/Hazards... A favored axis for highest rainfall totals for the 5-day period should extend from the northern Plains through the Midwest and into the central Appalachians, as a frontal boundary heading into the East stalls from the Mid Atlantic into the Plains while the western part of the front eventually lifts up as a warm front ahead of Rockies/Plains low pressure. The Southeast will see periods of showers/thunderstorms as well with some locally heavy amounts possible. The strong upper trough moving into the West will bring scattered light to locally moderate precipitation to some areas. By late next week/weekend the potential flow of moisture from the tropical East Pacific will have to be monitored, as it may begin to enhance rainfall over portions of the Four Corners states. The most prominent temperature anomalies will be associated with warm to hot readings expected to spread/progress from California and the Great Basin/Southwest into the Plains over the course of the period. Expect decent coverage of min and/or max readings of 10-15F above normal within this area. The upper troughing moving into the West will bring a cooling trend first to the Northwest and then to a higher percentage of the West by Fri-Sat. Rausch 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