Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1148 AM EDT Tue Jun 12 2018 Valid 12Z Fri Jun 15 2018 - 12Z Tue Jun 19 2018 ...Tropical moisture to bring potentially heavy rains to portions of the Southwest and western Gulf Coast... ...16Z update... 00Z models/ensembles have mostly stayed the course since the previous issuance, and a refresh of the latest guidance (including the 00Z GFS/ECMWF/ECMWF ens mean and 06Z GFS/GEFS) offered a reasonable starting point with few changes to the fronts/weather grids. This included the northward progression of hurricane Bud in the Pacific into northwestern Mexico and into Arizona (likely just its moisture by then), the lingering of troughing in the Great Basin, and the ridging in the Midwest/southeast/mid-Atlantic. 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 Mississippi Valley. This will favor a relatively amplified and slowly-evolving flow pattern across the CONUS, with a persistent trough/upper low across the interior West and near/off the Eastern Seaboard. The western trough will transport moisture associated with the remnants of Hurricane 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 (Fri-Sun). Models seem to have a significantly better handle now on the timing 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, and a slower eastward progression (as was the thinking over recent days). Farther east, models continue to show variation with the depth of an upper low forecast to develop near/off the Mid-Atlantic coast after a shortwaves dives southeastward across the Great Lakes/Northeast Fri-Sat. Timing of the features is similar among the various models, however, thus a blend should should represent a consensus solution in terms of depth at this time. 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 (Mon-Tue) the pattern remains largely stable with very little in the way of significant changes. Models now show a general consensus that the western upper low may linger across the Four Corners region or Great Basin into early next week while the upper ridge over the central U.S. intensifies and then migrates eastward toward the Mississippi Valley, and more progressive flow persists with another shortwave crossing the Great Lakes/Northeast. Some discrepancy exists as to the evolution of the upper low off the East Coast, with the ECMWF eventually absorbing the features back into the northern stream flow by Tue, while the GFS drifts the feature farther south toward the Bahamas. At this time, given the pattern, across most of the CONUS, would expect the upper low to remain separated from the northern stream flow for a longer period of time (as was even indicated by previous ECMWF runs). Otherwise, models show typical timing/intensity variations with these features as would be expected for almost a week out. 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 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 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. 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 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. 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. 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 stationary 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 across the Mississippi Valley. High temperature anomalies of +10 to +18 deg F are expected Fri-Sun across areas from the central plains to the Mid-Mississippi Valley, with highs approaching 100 degrees for some areas. Forecast highs and lows are expected to approach but mostly fall short of record warm/hot values through the period. By Mon, the greatest temperature anomalies will shift a bit east from the Mid-Mississippi Valley into the Ohio Valley. 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