Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1144 AM EDT Wed Jun 13 2018 Valid 12Z Sat Jun 16 2018 - 12Z Wed Jun 20 2018 ...Tropical moisture to bring potentially heavy rains to portions of the Southwest and northwestern Gulf Coast... 16Z update... 00Z/06Z models/ensembles remain in good agreement from the previous cycle and overnight WPC forecast. A refresh with a blend of the latest GFS/ECMWF and their ensemble means paired well with the demise of (current) Tropical Storm Bud in northwestern Mexico and the general trough/ridge pattern over the lower 48. Questions lingering in the forecast lie mostly in the west regarding how far south the upper low may sink into California next Tue/Wed (ECMWF and its ensemble members were farther south than the GFS/GEFS). For now, a blended solution was prudent given a lack of trend and a tendency for the models to falter in blocky patterns. Fracasso ...Pattern Overview... The breakdown of a blocking ridge across Hudson Bay will allow for more progressive northern stream flow to take hold from the Canadian prairies into the Great Lakes and Northeastern U.S. Meanwhile farther south, the flow will become more amplified and less progressive as an anomalous upper ridge axis builds off the North America West Coast, and subtropical ridging migrates eastward from the southern/central plains to the Ohio/Tennessee Valleys and the Southeast. This will favor a relatively amplified and slowly-evolving flow pattern across the CONUS, with a persistent and perhaps retrograding trough/upper low across the interior West. The western trough will transport moisture associated with the remnants of now Tropical Storm Bud into the southwestern U.S. and eventually into the Plains, where a lingering stationary frontal boundary will meander through most of the medium range period. Additionally, a slow-moving trough/area of disturbed weather moving west across the Gulf of Mexico is expected to approach the western Gulf Coast states by Sat-Sun, potentially lingering for a few days due to the blocky pattern surrounding the feature. ...Model Guidance/Predictability Assessment... The WPC forecast was initially based on a heavily deterministic multi-model blend, including the 12Z ECMWF/UKMET and 18Z GFS during days 3-5 (Sat-Mon). Models consensus has continued to improve with respect to the timing of the breakdown of the blocking upper high across Hudson Bay, and the developing progressive higher latitude flow in its wake. Farther south, as the ridge amplification occurs off the West Coast, models have continued a slow trend toward more trough/upper low amplification across the West. Farther east, models have trended toward a more progressive solution with respect to shortwave energy expected to amplify off the East Coast on Sat, with most solutions now taking this feature east and out to sea more quickly than in recent days. Additionally, models show good consensus that the upper ridge initially centered across the lower/mid-Mississippi Valley should show a slow migration eastward toward the Ohio/Tennessee Valleys. Improved model consensus and resultant forecast confidence during the day 3-5 time frame was sufficient to justify majority use of deterministic solutions. During days 6-7 (Tue-Wed) the pattern remains largely stable with very little in the way of significant changes. Models (with the exception of the 12Z CMC) have shown a trend over the past day toward retrograding the western U.S. upper low by Tue-Wed, bringing it to California by the middle of next week (a solution that the GFS showed led the other models on). Meanwhile, the slow eastward migration of the Mississippi Valley upper ridge should continue, and more progressive flow should persist farther north with another shortwave crossing the Great Lakes/Northeast. Models show a range of solutions as to how far south into the ridge the northern stream flow/height falls will be able to push across the Mid-Atlantic and Ohio Valley by the middle of next week, with the GFS a bit more aggressive with this relative to the ECMWF. Nonetheless, given the evolution toward a seemingly stable upper pattern, predictability seems slightly above average for the time frame. A majority of ensemble means (12Z ECENS/NAEFS) was used as a forecast basis for days 6-7, but a substantial component of the deterministic ECMWF/GFS was continued through the end of the forecast period. ...Weather Highlights/Hazards... A weakening surface front and moist air mass will support scattered 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 west-northwest to northwest across the Gulf of Mexico is also expected to bring widespread and potentially heavy rains to portions of TX and LA from the weekend through early next week, with moisture potentially spreading north across the southern plains as well. Farther north, a shortwave, surface low pressure system crossing the northern plains and Upper Midwest (along with a potential influx of moisture from the eventual remnants of Hurricane Bud) will produce areas of convection with potentially heavy rainfall. The frontal boundary appears likely to stall for a period of time from the central plains to the Upper Midwest, resulting in several days with shower/thunderstorm potential, and potential flooding threat for some areas. Meanwhile, moisture associated with the remnants of Hurricane 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 across the central and northern Rockies. Temperatures will also become a bigger story during the next week as the upper ridge drifts east across the Mississippi Valley toward the Ohio/Tennessee Valleys and the Southeast. High temperature anomalies of +10 to +18 deg F are expected Sat-Sun across areas from the central plains to the Ohio Valley and Great Lakes. By Mon-Tue as the ridge shifts east, these temperatures anomalies will reach the Eastern Seaboard. Widespread record high temperatures are not anticipated, many locations will near record values, and a few records are possible. High dew points in addition to the hot temperatures will make for dangerous heat index values for some areas, particularly for urban areas where warm overnight lows will remain warmer than surrounding rural 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