Extended Forecast Discussion NWS Weather Prediction Center College Park MD 108 AM EDT Mon Jun 11 2018 Valid 12Z Thu Jun 14 2018 - 12Z Mon Jun 18 2018 ...Pattern Overview... A transition still looks in order for the for the flow pattern across North America over the next week. A blocking upper high initially in place northwest of Hudson Bay should break down, with somewhat more progressive flow taking hold by early next week from the Canadian prairies/U.S. northern plains to New England. Farther south, however, blocking is expected to increase, with anomalous upper ridging likely to develop by the weekend near the coast of British Columbia, and ridging shifting east from the southern/central plains toward the Ohio Valley. This pattern will favor development of an amplified and slow-moving upper trough across the interior West by late in the week. Height falls digging into the Northeastern U.S. by late in the week should show some degree of amplification as well, but the speed of eastward progression into the Atlantic Ocean is somewhat unclear at this time as the feature will lie at the interface between more progressive northern stream flow and the blocking tendencies farther south. ...Model Guidance/Predictability Assessment... The WPC forecast during days 3-5 (Thu-Sat) was based on a multi-model deterministic blend including the 12Z ECMWF/GFS/UKMET. This blend represents a consensus solution, favorably resolving relatively minor timing/amplitude differences with shortwave energy digging across the Northeast/Mid-Atlantic, and with the upper trough beginning to amplify across the Pacific Northwest during that time period. The GFS has continued to hang onto the potential for low pressure development in the Gulf of Mexico by Sat, but has trended toward the ECMWF (less organized and farther west) with each consecutive run. From day 6 (Sun) onward, spread begins to increase with the two areas of upper trough amplification - across the interior west and along/off the Mid-Atlantic/Northeast coast. Significant question remains as to how deep a potential upper low across the Great Basin will be, and if/how quickly the energy will move east across the Rockies by day 7 (Mon). Ensemble spaghetti plots for the past couple runs show a trend toward more amplification and a slower progression, and this seems to be supported conceptually by the evolution toward more flow blocking and the anomalous ridge amplifying off the West Coast. The 12Z ECENS mean was less progressive than the deterministic ECMWF, as was the GFS. Farther east, there is a general consensus that the Northeast/Mid-Atlantic upper trough should move offshore, but confidence is low in the degree of amplification that will occur. The 12Z ECMWF was the most amplified deterministic solution, closing off an anomalous upper low east of New England that persists into day 7. Due to lower confidence in this area, a mix of ensemble means is preferred. Majority weighting toward the 12Z ECENS/NAEFS ensemble means was used in the forecast for days 6-7 to account for increased spread, and to reflect a somewhat more amplified and slower progression of the western trough than shown by the deterministic 12Z ECMWF. ...Weather Highlights/Hazards... A weakening surface front and moist air mass will support scattered to numerous showers and thunderstorms through the weekend, especially during the afternoons and evenings, across the Southeastern states, with the potential for locally heavy rainfall. A broad trough/area of disturbed weather moving westward across the Gulf of Mexico is also expected to bring the potential for more widespread rains to portions of the western Gulf Coast states by the weekend. Farther north, a shortwave, surface low pressure system crossing the northern plains and Upper Midwest will produce areas of convection with potentially heavy rainfall. The frontal boundary may slow or stall for a period of time Sat-Sun from the central plains to the Upper Midwest, resulting in several days with shower/thunderstorms potential, and potential flooding threat for some areas. Confidence is low at this time as to the specifics of what areas will see the heaviest rain, however. Finally, moisture associated with Tropical Storm Bud should begin to spread into the southwestern U.S. and the Rockies by the weekend, bringing the potential for areas of heavy rainfall. Interactions with a stationary frontal boundary draped from the Great Basin into the Central Rockies could lead to an enhancement of heavy rainfall potential. Some model solutions even suggest the possibility that the moisture from Bud could spread east into the central/northern plains, further enhancing heavy rain potential along the frontal boundary. Temperatures will also become a bigger story during the next week as the upper ridge expands across the central U.S. and then drifts east into the Ohio Valley. High temperature anomalies of +10 to +18 deg F are expected Thu-Sat across areas from the central plains to the Mid-Mississippi Valley, with highs approaching 100 degrees for some areas. By Sun-Mon, the greatest temperature anomalies will shift a bit east from the Mid-Mississippi Valley into the Ohio Valley. A few record high temperatures are possible. High dew points in addition to the hot temperatures will make for dangerous heat index values for some areas. 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