Extended Forecast Discussion NWS Weather Prediction Center College Park MD 124 AM EDT Sat Jun 09 2018 Valid 12Z Tue Jun 12 2018 - 12Z Sat Jun 16 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 is expected to be in place on day 3 (Tue) from the Upper Great Lakes north across Hudson Bay. As shortwave energy traversing the westerlies across North America undercuts this ridge, the main anomaly center should focus near/just northwest of Hudson Bay by the middle of next week. As with any highly anomalous blocking pattern, a significant question exists as to how long the setup will persist, and model solutions continue to vary in that respect. As long as the block persists, a relatively active flow regime will exist from the Pacific Northwest eastward into the Midwest, Great Lakes, and Northeast, south of the blocking upper high. A couple such shortwaves are expected to cross the Midwest/Great Lakes/Northeast Tue-Fri. Additionally, models show consensus that an upper wave should reach the Pacific Northwest by next Wed, with some degree of height falls pressing into the interior western U.S. through the end of next week. Meanwhile, an upper ridge is also expected to remain anchored across the south central U.S., varying in strength throughout the period. ...Model Guidance/Predictability Assessment... The WPC forecast was initially based on a heavily deterministic blend including the 12Z ECMWF/GFS during days 3-4, with increasing weight placed toward the 12Z ECENS/NAEFS ensemble means from day 5 onward. Models showed some significant differences and run-to-run variability throughout the period. The 18Z GFS was much weaker with a shortwave initially across the Mid-Atlantic on day 3, and was rejected in favor of the 12Z run which was more similar to the consensus including the ECMWF and the ensemble means. With shortwave energy crossing the CONUS northern tier days 3-5, solutions vary widely due to complex interactions with Arctic energy rounding the eastern side of the anomalous ridge. The GFS has been persistent in amplifying a trough across the Northeast by days 6-7, while the ECMWF shows a similar scenario but is a bit weaker and quicker to move the feature east. The 12Z GFS was probably the most similar to the ECMWF in that respect. With the upper energy reaching the West on day 4, the ECMWF also a bit weaker and quicker to push the wave into the interior West by day 6. The GFS has been rather variable with the 18Z run even keeping the system along the West Coast as a retrograding upper low into day 7, while the 12Z run seemed to be somewhat of a reasonable middle ground solution (and was relatively close to the ensemble mean based consensus). Given the unstable nature of this flow pattern with a high degree of blocking, predictability is low and no scenario can really be ruled out, especially by days 6-7. Finally, the GFS continues to develop an area of low pressure in the Gulf of Mexico by the middle of next week more so than any other deterministic solutions. The ECMWF and CMC both continue to show a much weaker and broad area of disturbed weather drifting westward across the Gulf. At this time, there is a lack of support for a GFS-like solution among ECENS and CMCE ensemble members. ...Weather Highlights/Hazards... Scattered to numerous showers and thunderstorms are expected Tue-Wed from the Mississippi Valley east to the Eastern Seaboard along a lingering surface frontal boundary and ahead of an approaching cold front moving from the Midwest on Tue to the Northeast on Wed. Areas of heavy rain appear possible, but confidence in the location of any heavy rain is low. As the cold front moves off the East Coast by Thu, the trailing frontal boundary will linger across the Ohio Valley and central plains, where it will continue to focus areas of convection through the end of next week. The potential shortwave crossing the northern Rockies by next Fri-Sat may further enhance the shower/thunderstorms potential across the northern plains. Temperatures are expected to be well above average through the middle of next week from the Great Basin east across the central U.S., where high temperatures may range from 5 to 15 deg F above average. The arrival of lower heights into the western U.S. by Fri-Sat will bring temperatures down closer to average values for much of the West, but expect hot conditions to continue for much of the central U.S. as the upper ridge persists. 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