Extended Forecast Discussion NWS Weather Prediction Center College Park MD 240 AM EDT Tue Jun 26 2018 Valid 12Z Fri Jun 29 2018 - 12Z Tue Jul 03 2018 ...Overview and guidance evaluation/preferences... Energy flowing around a strong eastern Pacific ridge will help to eject an amplified trough forecast to be over the West as of day 3 Fri. Cyclonic mean flow across the Northern Tier states will trend broader and less amplified as this occurs. By the first half of next week guidance agrees in principle that the Pacific ridge will begin to retrograde and set the stage for downstream trough reloading that should align farther west than with the preceding trough. Farther east/south, a strong eastern ridge aloft will likely contain a high center that should track from the Ohio Valley northeastward into or near southern New England. Ridging across the South will likely build across the southern Plains/Rockies early next week as the Pacific/West Coast pattern adjustment begins. Guidance trends over the past day have dramatically lowered the spread for what happens to the initial western trough. The ECMWF/ECMWF mean have backed off a bit on the timing of height falls reaching the Great Lakes region by early next week but a faster progression in the GFS/GEFS/CMC mean played a much greater role in achieving the current clustering. In spite of this better agreement in principle there is still meaningful spread for details of the ejecting energy and to a greater extent for upstream flow feeding into the mean trough. In addition guidance still shows a variety of possible solutions for an upper low that is currently near the Aleutians. Most solutions indicate this feature will become the core of the trough that reloads near the West Coast around the end of the period. As has been the case in recent runs the ECMWF is on the fast side of the spread (but at least with the 12Z run slower than its predecessor) with the Aleutians low. GFS runs are keeping the low offshore through day 7 Tue while the CMC has adjusted from a compromise in the 12Z run to being closer to the GFS in the new 00Z run. An average of the ensemble means offers a reasonable intermediate approach given current guidance spread. Once the retrograding Pacific ridge settles into place, teleconnections with the associated positive height anomalies suggest the axis of the developing West Coast trough should ultimately drift a little offshore. The forecast remains ambiguous to the south of the upper high over the East. There continues to be a general signal that some energy flowing around the high may reach the Southeast but with poor agreement/consistency for specifics. The 12Z ECMWF was stronger than most other guidance with this energy by late in the period. For days 3-5 Fri-Sun guidance agreement was sufficiently good to allow for an operational model blend (18Z GFS/12Z ECMWF and lesser weight of the 12Z UKMET/CMC). Decreasing confidence in varying details of the operational runs led to incorporating 50-70 percent total weight of the 18Z GEFS and 12Z ECMWF/NAEFS means by days 6-7 Mon-Tue. The primary adjustment in the forecast was farther eastward progression of the Plains front into the upper MS Valley and Great Lakes mid-late period. ...Weather threats/highlights... Low pressure tracking northeast from the Plains into Canada from Fri onward along with its associated fronts and upper shortwave support will likely produce convective episodes capable of producing areas of heavy rainfall. Best signal for highest totals is across the northern Plains/Midwest/upper MS Valley/upper Great Lakes. Southward progression of the cold front may bring convection into the central Plains. Some activity may be strong to severe as well. Check latest SPC outlooks as details become more clear in coming days. Expect showers/thunderstorms to extend farther eastward later in the period but likely with lesser amounts as best upper dynamics lift into Canada. Meanwhile there should be diurnally favored convection over portions of the South/Florida through the period with some locally heavy amounts. It remains a question mark to what extent upper energy/surface feature(s) flowing around the deep layer high to the north may enhance this convection. Within the large area of hot and humid weather over the central/eastern U.S., highest min/max temperature anomalies of plus 10-20F should extend from the Midwest through the Great Lakes/Ohio Valley and Northeast. The heat/humidity will gradually erode from the west over the course of the period as a Plains cold front moves into the Great Lakes. There are many locations that may see one or more days of min temps at least close to daily record warm values. Daily record highs should be less numerous and most likely confined to the Great Lakes/Ohio Valley/Northeast during Sat-Mon. The humidity will contribute to high heat index values. Below normal temperatures will be most common over the West and High Plains, with the northern Rockies/High Plains likely to see one or more days of highs 10F or more below normal Fri-Sun. Ejection of the initial western trough aloft will help readings trend to moderately above normal levels over the central/southern parts of the West by early next week. Rausch WPC medium range forecasts of 500 mb heights, surface systems, weather grids, quantitative precipitation, and winter weather outlook probabilities are 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