Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1158 AM EDT Fri Jun 08 2018 Valid 12Z Mon Jun 11 2018 - 12Z Fri Jun 15 2018 ...Pattern Overview... The higher latitudes of North America in particular are likely to see a significant degree of blocking during the period, with the evolution of embedded/surrounding flow providing some significant forecast challenges for specifics by the mid-late part of next week. There is currently a majority scenario that has strong western North America trough/upper low energy progressing underneath an anomalously strong northern Canada upper high that closes off (highest standard deviations from normal likely to be around Tue-Wed). The southern Canada/northern U.S. energy may combine with eastern Canada flow to yield an overall trough covering parts of eastern Canada and U.S. by late next week. Elsewhere, expect an upper trough to reach the West Coast around midweek with some degree of inland progression possible thereafter while ridging that builds over the southern Rockies/Plains during the first half of the week should persist into Fri with a hint of southeastward drift. ...Model Guidance/Predictability Assessment... Even though there has been some pronounced trending with some aspects of the forecast from the perspective of multi-day continuity, the ensemble means have provided less run to run variability than other individual solutions. In addition the likely closure of an upper high for a time over northern-central Canada lends itself to low predictability for forecast details across much of southern Canada and northern contiguous U.S. As a result the overall forecast preference places the greatest focus on the ensemble means. An operational model blend (06Z GFS/00Z ECMWF and to a lesser degree 00Z CMC/UKMET) compared well to the means while providing added detail days 3-4 Mon-Tue so the early part of the forecast did not use the means per se, but subsequent divergence of operational runs led to exclusive use of the means (more 00Z ECMWF mean than 06Z GEFS) and some continuity by days 6-7 Thu-Fri with a model-mean blend for Wed. Most guidance as a whole has shown faster multi-day trends for the greatest proportion of energy that should progress eastward along the U.S.-Canadian border into midweek. However the 06Z GFS differs from the majority in holding a greater part of closed low energy over western Canada, while some solutions like the ECMWF offer potential for energy to linger over central longitudes for a time before continuing eastward. The faster trend has led to earlier flattening of New England troughing aloft after Mon, while keeping the compact upper low over eastern Canada--which itself has trended noticeably slower versus previous consensus--well north of Maine. Beyond that there is the question of how much eastern Canada flow could retrograde and interact with flow coming from the west. Typical biases suggest leaning away from the side of the envelope that shows the most retrogression. Note that CMC/CMC ensemble runs have offered very different scenarios that seem more reflective of ideas from multiple days ago, but at least they provide some added reservations about any specific deterministic forecast. So after Tue overall confidence rapidly decreases regarding details of surface lows/fronts affecting southern Canada and the eastern half of the U.S. Regarding the western U.S. trough, the most notable aspect of the forecast is that compared to 24 hours ago the GFS and ECMWF have essentially flipped, with the 00Z/06Z GFS now showing a deep/slow solution that the 00z ECMWF had yesterday. Ensemble means have been more consistent over recent days and arguing for an intermediate solution, though the 06Z GEFS mean has trended somewhat deeper in deference to its operational counterpart. Strength/sharpness of the upstream ridge supports a fair degree of amplitude but teleconnections relative to the ridge's positive height anomaly center suggest the western trough should become a little weaker and more progressive than the GFS scenario at or shortly after the end of the forecast. GFS/GEFS guidance still appears overdone with the surface development/moisture heading into the Gulf of Mexico from the western Caribbean but has been gradually trending toward other guidance that has more consistently signaled a more suppressed pattern. ...Weather Highlights/Hazards... A cold front progressing eastward from the Plains, with the trailing end stalling over the south-central Plains and eventually lifting back north as a warm front, will likely provide a multi-day focus for showers and storms over the central-eastern U.S. with some rainfall locally heavy. SPC outlook currently shows potential for severe weather over parts of the Midwest early in the week. Other convection may be on the strong side as well. Diurnally favored convection over the Southeast may be enhanced early in the week by a leading front over the East and an upper level weakness. The system moving into the West mid-late week should bring clouds and scattered precipitation. Most activity should be on the lighter half of the spectrum but a few pockets of at least moderate totals cannot be ruled out. The strong upper system departing from western North America early in the week will bring chilly temperatures to the northern Rockies and vicinity on Mon followed by a warming trend. Ridging aloft over the southern Rockies/Plains will promote a broad area of warm to hot weather over the western-central U.S. during the week. Highest anomalies for min and/or max temperatures in the plus 10-15F range should expand from California through much of the Intermountain West and then portions of the Rockies/Plains. On Mon there will also be an axis of well above normal readings just ahead of the Plains cold front. 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