Extended Forecast Discussion NWS Weather Prediction Center College Park MD 125 AM EDT Sun Jun 10 2018 Valid 12Z Wed Jun 13 2018 - 12Z Sun Jun 17 2018 ...Pattern Overview... Models/ensemble show improved consensus that a blocking upper high initially in place northwest of Hudson Bay on day 3 (Wed) should begin to break down by next weekend, allowing somewhat more progressive flow to take hold across Canada. As this more progressive flow takes shape, amplification of upper troughs across the Northeastern U.S. and the northern Great Basin/northern Rockies appears probable by next weekend. Meanwhile, upper flow affecting much of the remainder of the CONUS may become less progressive, with the potential for a re-intensification of a blocking upper high across the south central U.S. by next weekend, with another similar feature possibly developing off the Pacific Northwest coast. ...Model Guidance/Predictability Assessment... A multi-model deterministic blend including the 12Z ECMWF/UKMET and 18Z GFS served as a basis for the WPC forecast during days 3-5 (Wed-Fri). This blend appeared to favorably resolve, and represent a consensus solution for shortwave energy crossing the Great Lakes/Northeast and the Pacific Northwest/northern Rockies, and the associated surface frontal systems, during that time period. The greatest differences among the guidance for the day 3-5 time frame were surrounding the trough across the Northwest, with some question as to whether the wave will be slower with more amplification (as shown by the 18Z GFS/12Z UKMET) or slightly slower to amplify and quicker to move inland (as shown by the 12Z ECMWF). For days 6-7 (Sat-Sun), increased weighting was placed on the 12Z ECENS/NAEFS ensemble means. Model/ensemble spread increased during this time period as the upper flow across the U.S. trends toward a blockier setup. What is clear is that as upper ridging likely strengthens across the south central U.S., and across the Northeast Pacific, the pattern should favor slower progression and amplification of upper troughs across the West and along the East Coast. In the West, some solutions are more aggressive and quick to cut off an upper low over the Great Basin, some even retrograding it westward beneath the upper ridge (18Z GFS/GEFS), while other solutions keep a slightly more progressive pattern with an trough that attempts to move eastward across the Rockies, but struggles to make quick progress due to the expanding ridge (12Z ECMWF/00Z GFS). At this time, given the overall pattern, and a trend among many ensemble members toward the slower/more amplified solution, the forecast was weighted at least slightly more on the NAEFS due to the more amplified GEFS/CMCE members. Farther east, similar timing/amplitude differences exist with the trough digging across the Great Lakes/Northeast/Mid-Atlantic Fri-Sat. An ensemble-heavy approach was suitable for this feature as well given large model-to-model and run-to-run variability. With the flow across the north Atlantic a bit more progressive compared to the north Pacific and CONUS, it is less clear whether a slower/more amplified or faster/less amplified scenario is preferred. Finally, the GFS continues to develop a more well-defined area of low pressure from a broader area of disturbed weather reaching the Gulf of Mexico by Thu. Given continued lack of support for this scenario among ensemble members (other than the GEFS), the WPC forecast maintained a broader trough/area of disturbed weather drifting generally westward across the Gulf Fri-Sun. ...Weather Highlights/Hazards... Scattered to numerous showers and thunderstorms with locally heavy rains will be possible from the central plains/mid-Mississippi Valley to the Southeast/Mid-Atlantic through much of the period as a cold front passes and then stalls from the plains to the Appalachians. Convection with areas of heavy rain may flare up across the northern plains by Thu-Fri as an upper shortwave/surface frontal system approach, with this activity likely spreading east into the Midwest and south into the central plains Sat-Sun in association with the cold front. Uncertainty remains surrounding the expected broad area of disturbed weather crossing the Gulf of Mexico late in the week, but based on the current forecast, potential for more widespread showers and thunderstorms could begin to increase by next weekend across the western Gulf Coast states. Across the Desert Southwest and southern Great Basin, shower and thunderstorm activity may be on the increase by late in the week into next weekend as Pacific moisture associated with Tropical Storm Bud streams northward toward the region. Confidence is low at this time given the time range, but areas of heavy rain are possible. Temperatures are expected to be above average across much of the U.S. through the medium range. The greatest temperature anomalies should focus across the Great Basin/central Rockies Wed-Thu, shifting east into the central plains and mid-Mississippi Valley by Fri-Sun. High temperature anomalies of +5 to +15 deg F are possible in these areas. Given the time of year, however, record high temperatures appear unlikely at this time. 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