Extended Forecast Discussion NWS Weather Prediction Center College Park MD 126 AM EDT Fri Jun 08 2018 Valid 12Z Mon Jun 11 2018 - 12Z Fri Jun 15 2018 ...Pattern Overview... Upper flow across North America during the medium range is expected to show a large degree of blocking, particularly at the higher latitudes. An anomalous upper ridge axis is expected to initially be in place on day 3 (Mon) from near the Great Lakes north across Hudson Bay. Over the following days, this features is forecast by most guidance to transition to a highly anomalous blocking upper high centered north of Hudson Bay. Energy traversing the westerlies will be forced south of this feature, resulting in a relatively active storm track across the U.S. northern tier. Additionally, the upper ridge along with negative height anomalies located across eastern Canada will be favorable for the transport of cooler air from high latitudes southward, enhancing thermal gradients across the northern U.S. Farther south, upper ridging is likely to remain strong across the southern plains and Mississippi valley. Lastly, upper troughing is likely to amplify along the West Coast by mid to late next week, although the exact nature of the trough/upper low remains unclear at this time. ...Model Guidance/Predictability Assessment... Models/ensembles have performed very poorly with respect to the aforementioned upper ridge/blocking high now forecast to develop across Canada during the medium range. Teleconnections associated with a few of the stronger height anomalies around the northern hemisphere do little to aid in the forecast of the Canadian block. Further, the significant run-to-run variability among the guidance surrounding this feature suggests a highly unstable pattern with low predictability. The effects of this reverberate to other aspects of the forecast, including significant variability even by days 3-4 with respect to short wave/frontal timing across the Mid-Atlantic region. A thorough look at several of the most recent model cycles seemed to perhaps reveal slightly more run-to-run consistency in the GFS relative to other guidance, especially from day 4 onward, with perhaps the least consistency shown by the ECMWF. The position and strength of the blocking Canadian ridge affects the timing and amplitude of shortwave energy crossing the northern plains/Midwest Tue-Wed. Given these considerations, the forecast for days 3-4 was based on a multi-model deterministic blend including the 12Z GFS/UKMET/ECMWF, with the ECMWF weighted least of the three - components of the 12Z ECENS/GEFS means were also included. From day 5 (Wed) onward, there is at least a general consensus among the guidance that a relatively vigorous shortwave/upper low will approach the Pacific Northwest, and potentially amplify as it reaches the coast (although the 12Z CMC kept a weaker system and quickly moved it inland). While there is agreement on the existence and general timing of this feature, there is once again a high degree of run-to-run variability as to the exact character of the feature (open trough vs. closed low). Additional complications arise from Wed onward with the GFS persistently developing and area of low pressure in the Gulf of Mexico which it then moves northward toward the Gulf Coast, while the ECMWF and CMC show no such system. As a result, the forecast was trended toward ensemble (12Z ECENS/GEFS) means from day 5 onward. ...Weather Highlights/Hazards... A surface frontal boundary and upper-level shortwave are likely to bring scattered to numerous showers and storms with areas of heavy rainfall to portions of the Southeast Mon-Tue. Lingering moisture across the Southeast through the end of next week should result in continued afternoon/evening convection with locally heavy rains. The upper trough and relatively strong surface frontal system crossing the northern plains/Midwest Mon-Tue is expected to result in areas of locally heavy convection, with this activity spreading eastward to the Northeast as well as the Ohio/Tennessee Valleys by Wed-Thu. Meanwhile, much of the western U.S. will be dry. The arrival of the upper trough and associated surface front across the Northwest by next Wed-Thu should bring an increase in shower activity, reaching the northern Rockies by Thu-Fri as the system moves inland. Continued upper ridging across the south central U.S. will result in persistent warm/hot temperatures, with highs expected to range from 6 to 12 deg F above average through much of the forecast period from portions of the southern/central plains to the lower/middle Mississippi Valley. Ryan 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